Engineering Fundamentals CBT

Transcription

1 Engineering Fundamentals CBT Printout of CBT Content for Reference Purposes Only Reference CBT: Chemistry V

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3 Engineering Fundamentals CBT: Printout of CBT Content for Reference Purposes Only Reference CBT: Chemistry V December 2008 EPRI Project Manager Ken Caraway ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California PO Box 10412, Palo Alto, California USA askepri@epri.com

4 DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT EPRI NOTE For further information about EPRI, call the EPRI Customer Assistance Center at or askepri@epri.com. Electric Power Research Institute, EPRI, and TOGETHERSHAPING THE FUTURE OF ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc. Copyright 2009 Electric Power Research Institute, Inc. All rights reserved.

5 Summary PRODUCT DESCRIPTION This document provides a printout of the CBT content for use as a reference document only. Students are encouraged to use the CBT as animations, flash video, and interactive features are intended to enhance their learning experience. NOTE: The CBT should be used to validate information as errors may have been introduced when converting the graphics, equations, etc. Abstract The Chemistry module of Engineering Fundamentals, EF Chemistry Version 1.0, provides a basic overview of this topic, applicable to users in all engineering disciplines who are beginning their career in the nuclear power industry. Description The EF Chemistry module covers basic terms and concepts as well as their applications in nuclear power plants. This course will help new engineers understand how their work might impact and/or be impacted by chemistry in a nuclear power plant and how coordination can help assure high equipment reliability and system integrity. This module is intended for use as orientation training for new engineering support personnel. Platform Requirements The following hardware and software are required: Windows 2000 / XP GB RAM, 120 GB Microsoft Internet Explorer 6.0 Windows Media Player 10 Adobe Flash Player 9.0 CD-ROM This personal computer software is designed to: 1) Run directly from the CD-ROM. 2) Be installed and run on a dedicated server where it can be accessed by multiple users from other computers. Application, Value and Use EF-Chemistry, Version 1.0: Allows engineering support personnel to review the content when they desire and at their own pace. Uses interactive features and graphics to illustrate key concepts and enhance training. iii

6 Keywords Training Chemistry Corrosion Materials Fundamentals

7 ACKNOWLEDGEMENTS EPRI would like to acknowledge the following individuals for their active participation and significant contributions toward the development of this training course: Ken Caraway Tim Eckert Lisa Edwards Scott Hayes Ken Johnson Charlie McIlwain Mike Mosley David Perkins Chris Prigmore Mike Stuart EPRI EPRI EPRI Omaha Public Power District Duke Energy Corporation Duke Energy Corporation SCANA Energy EPRI Handshaw, Inc. ]Dominion v

8 CONTENTS 1 INTRODUCTION TO CHEMISTRY FUNDAMENTALS CHEMISTRY BASICS CORROSION CONTROL WATER QUALITY RADIOCHEMISTRY SPECIFIC SYSTEM AND EQUIPMENT CONSIDERATIONS

9 1 INTRODUCTION TO CHEMISTRY FUNDAMENTALS Introduction Welcome to the Chemistry Fundamentals course. During the course of your career in the nuclear industry, you could easily find yourself working in Design, Plant Support, Operations, Licensing, Maintenance, or Training. Regardless of where you ultimately reside, one fundamental topic that you need to understand is power plant chemistry. When you have completed this lesson, you will be able to explain the importance of a properly controlled power plant chemistry program. 1-1

10 Chemistry in the Plant Most of the major mechanical systems in a nuclear power plant are filled with water, steam, or some combination of the two. Each system has unique characteristics that require the careful management of specific chemistry control strategies to prevent corrosion. These major systems include the following: The Reactor Coolant System Steam Generators Feedwater Component Cooling Water Service Water Systems In the remainder of this course, you'll learn more about chemistry control strategies and how they help protect plant equipment.

11 Corrosion As you learned on the previous page, chemistry control strategies are used to prevent corrosion in major plant systems. Corrosion is a process by which a material is attacked and broken down due to reactions that occur between the metal and its environment. Any time metal components are exposed to water, corrosion will occur. Think about the various plant systems as parts of the human body. The fluid running through these systems is like the blood in your body. Corrosion is like a disease that can result in damage and even death, in some instances. You'll learn more about corrosion in Lesson

12 Operating Experiences A properly controlled power plant chemistry program will: Protect the primary system pressure boundary integrity Protect fuel cladding integrity and fuel performance Minimize out-of-core radiation fields Minimize corrosion of all secondary plant and auxiliary system components Throughout this course, you will have the opportunity to view operating experiences related to chemistry in nuclear power plants. These operating experiences are true stories related to the lesson, which demonstrate the importance of chemistry and how your role affects chemistry in the plant. A fatal accident occurred at a nuclear plant as a result of corrosion. A steam pipe in the turbine building failed. The section of pipe had corroded from nearly half an inch to a thickness a little greater than that of metal foil. The 180º steam injured 11 workers who were in the building at the time of the accident. Four of the injured workers died from this corrosion-related failure. The impact to plant personnel and the public would have been far worse if the system that failed contained reactor coolant. This operating experience illustrates the need to be aware of corrosion mechanisms and proper chemistry controls. An engineer at a nuclear power plant can be involved in all facets of power plant design and operation, and the decisions they will make on a daily basis can play an important role in maintaining equipment reliability and system integrity.

13 Conclusion It is important for you, as a power plant engineer to understand that you play a key role in helping to maintain proper chemistry controls. We all have to work together to protect the safety of the public and plant personnel, maintain the reliability of plant systems and equipment, and minimize outage time and equipment maintenance costs. In the following lessons, you will be introduced to: Chemistry Fundamentals Corrosion Controls Water Quality Radiochemistry System and Equipment Chemistry Considerations These lessons will help you to understand what is involved in properly controlling plant chemistry. 1-5

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15 2 CHEMISTRY BASICS Introduction Power plant chemistry is relevant to any fluid-filled system within the plant, especially those containing water, and to fluids released to the environment. In this lesson, we'll review some basic chemistry concepts that you will need to understand before we discuss how chemistry applications will affect you. When you have finished this lesson, you will be able to: Explain atomic structure, chemical valence, chemical bonding, and chemical reactions Explain the oxidation and reduction process Explain the factors affecting reaction rates and chemical equilibrium Explain ph, acids, and bases Describe how solids interact in solutions Explain the sources and impact of impurities Describe how gases interact in solutions Explain the boiling process and effects 2-1

16 Matter All matter is composed of chemicals. These chemicals can be elements or compounds. Elements Elements are listed on the Periodic Table. Compounds Compounds are composed of elements combined together by transferring (ionic bond) or sharing (covalent bond) electrons. Most of the chemicals you are likely to experience are compounds; the remainder are elements (e.g., oxygen, neon, gold).

17 Atomic Structure Atoms are the fundamental units of elements, of which all materials are made. They consist of a dense nucleus containing protons and neutrons surrounded by a diffuse cloud of electrons in discrete orbits, as pictured in the graphic at right. In order to remain electrically neutral, an atom must have the same number of protons and electrons. Protons are positively charged, while electrons are negatively charged. If the atom does not remain neutral, it will become an ion with a net positive or negative charge based on the excess numbers of protons or electrons, respectively. Positively charged ions are called cations. Negatively charged ions are called anions. 2-3

18 Valences The orbits of electrons are naturally arranged in groups called shells, as pictured in the graphic at right. When two different types of atoms have the same number of electrons in their outermost shell, they will have similar chemical properties. Atoms have a natural tendency to combine in ways that complete their outermost electron shells. This tendency determines the number and strength of the bonds that an atom can form with other atoms. With the exception of the first shell, which requires only two electrons to be filled, shells require a total of eight electrons for completion. Numerical values, called valences, can be assigned to each atom. Valences indicate the number of electrons that an atom can give up to leave a completed outermost shell or take in to complete the outermost shell. Positive valences indicate that electrons can be given up, while negative valences indicate that electrons can be taken in.

19 Periodic Table Elements with the same number of valence electrons share similar chemical properties. These similarities are the basis for the arrangement of the periodic table. Click each numbered blue circle on the graphic below to learn more. 1 - Each vertical column is numbered according to the number of electrons in the outermost shells of the elements shown. The elements included in a vertical column are known as a group. 2 - Horizontal rows represent successive series of elements with increasing mass numbers and progressively more of the electron shells being filled. The elements included in a horizontal row are known as a period. 3 - Elements in the first column are called alkali metals and have a strong tendency to give up their one extra electron. 4 - Elements to the left of the zigzag separation shown are classified as metals and tend to give up electrons when bonding. 5 - Elements to the right are classified as nonmetals and tend to take in electrons when bonding. 6 - Elements in the seventh column are called halogens. They need only one electron to reach completion and are therefore strongly reactive. 2-5

20 Balancing Chemical Reaction Expressions The process of chemical bonding is governed by the rules for combining orbital electrons. The number of bonds required and the ratios of atoms being combined can be determined and written as reaction expressions. Since chemical reactions do not involve the destruction of the reacting materials, the reaction expressions must balance. In other words, all reactants must be accounted for in the resulting products. Subscripts are used to the right of the chemical symbols to represent how many atoms of that type are present in the indicated material. The graphic below shows examples of balancing expressions.

21 Oxidation and Reduction Chemical reactions can also be described as redox reactions. Redox is a shortened combination of the terms Reduction and Oxidation. Redox reactions are commonly used to describe corrosion processes. Reduction Reactions A reduction reaction is one in which a substance gains electrons. The substance that gains these electrons is said to have been reduced. The agent that effects this change (i.e. supplies the additional electrons) is therefore described as a reducing agent. Oxidation Reactions An oxidation reaction is one in which a substance loses electrons. The substance that loses the electrons is said to have been oxidized. The agent that effects this change (i.e. takes away the lost electrons) is described as an oxidizing agent. The mnemonic described in the graphic will help you remember the difference between the two types of redox reactions. 2-7

22 Oxidation and Reduction Half-Cell Reactions Oxidation and reduction are complementary processes because they represent the reverse of each other. Therefore, we can say that an oxidizing agent is reduced by its reaction with a material (since the oxidizing agent has picked up additional electrons) and a reducing agent is oxidized by its reaction with a material (since the reducing agent has lost some of its electrons). So-called half-reactions can be written to show electron transfers into and out of individual reactants and products. The expressions below show the half-reactions that represent the oxidation and reduction that take place when sodium and chlorine combine to form salt. Describing processes in terms of oxidation and reduction half-reactions is particularly useful for analyzing the reactions experienced by ions in water solutions. The ions will act as independent dissolved particles that can give up or receive electrons individually.

23 Reaction Rates - Temperature The rate at which a chemical reaction occurs depends on a number of variables, including temperature, the presence of a catalyst, and the concentration of reacting materials available. A simplified model for explaining the factors affecting reaction rates is the collision theory, which postulates that the chemical combination of atoms occurs when they physically collide with sufficient energy for bonding to occur. Following this model, anything that increases the number or energy of collisions between reactants will increase their rate of reaction. Since an increase in temperature will increase the molecular energy of the reacting atoms, it will also typically increase their reaction rate. 2-9

24 Reaction Rates - Catalyst Another factor affecting reaction rates is the presence or absence of a catalyst. A catalyst is a material that is not used up in a chemical reaction, but whose presence increases the rate at which that reaction occurs. The catalyst works by forming an intermediate product with one or more of the reactants, which results in a lower activation energy for the other reactant(s). Since this intermediate product breaks up, releasing the uncombined catalyst and the combined products, the catalyst is never used up. An example is shown below. A real-life example is a catalytic converter in your vehicle, which decomposes toxic compounds such as nitrous oxides, carbon monoxide, and residual hydrocarbons. Catalysts may be used as addition chemicals in power plant systems. For example, Activated Carbon is used as a catalyst in the Water Treatment Process to break down chlorine, which is used in the production of demineralized water. The chlorine must be removed to prevent damage to the Reverse Osmosis Filter Membranes, another step in the purification process.

25 Reaction Rates - Concentration Another important factor affecting reaction rate is the concentration of reacting materials available. This is especially true for reactants, which are dissolved in water or other solvents. Increasing the concentration of a given reactant will result in a greater chance of the other reactant(s) colliding with the original reactant in a given time period. Therefore, the rate of a chemical reaction is directly proportional to the concentrations of its reactants. A good analogy is a crowd of people: the more crowded the group, the greater the probability of reactions, such as elbowing and increased tempers. The increase in reaction rate due to increased concentrations is the primary reason for minimizing impurities in power plant systems. 2-11

26 Review Question - Reaction Rates Which of the following factors would increase reaction rate? A. The addition of a catalyst B. An increase in temperature C. The addition of fluid to the system D. An increased concentration of one or more of the reactants Anything that increases the number or energy of collisions between reactants will increase their rate of reaction.

27 Chemical Equilibrium In analyzing chemical reactions and their rates, it is important to consider cases where a given reaction may also have a reverse reaction. The products of one reaction may have a natural tendency to recombine to re-form the original reactants. Reversible reactions are often shown with a double arrow to indicate that the reaction can proceed in either direction. Two examples of reverse reactions are shown in the graphic at right. Reverse reactions are constantly occurring. Equilibrium is reached when the rate of product formation by the original reaction is exactly balanced by the rate of product removal by the reverse reaction. Equilibrium can be mathematically expressed with an equilibrium constant (K). This constant is based on a given temperature. 2-13

28 Water Equilibrium An important example of a reversible equilibrium reaction is the self-ionization of water. To a limited extent, pure water will naturally disassociate to form hydronium ions (H 3 O + ) and hydroxyl ions (OH - ), as shown in the graphic below. Due to the electromagnetic attraction between the bipolar water molecule's negative pole and a hydrogen ion's positive charge, hydrogen ions (H + ) in a water solution will naturally become associated with a water molecule and form one hydronium ion (H 3 O + ). A reverse reaction in which the hydronium and hydroxyl ions recombine to form water is also constantly occurring. Because of this, an equilibrium is reached when the rate of ionization is exactly balanced by the rate of recombination. This natural equilibrium with equal numbers of hydronium and hydroxyl ions can be shifted by the presence of increased amounts of these ions resulting from the presence of acids or bases in the water solution.

29 Acids and Bases Acids and bases are particularly important because of their effect on the chemistry of water solutions in the plant. Either extreme can be destructive to metals or flesh. Acids An acid can be defined as a compound that increases the concentration of positively charged hydrogen ions (and thus hydronium ions) when dissolved in water. Hydrogen ions and hydronium ions can be represented in symbols as H + or H 3 O +, respectively. Bases A base can be defined as a compound that increases the concentration of negatively charged hydroxyl ions when dissolved in water. Hydroxyl ions can be represented in symbols as OH

30 ph An important measure of the acidity or alkalinity of a water solution is provided by ph. ph is the negative of the common (or base 10) logarithm of the hydronium ion concentration (when given in moles per liter); ph = -log [H + ]. Since the natural concentration of hydronium ions in pure water is 10-7 moles per liter, then applying the definition will show that the ph of pure water is 7. When an acid is added to water, it causes the formation of additional hydronium ions and a resulting lower concentration of hydroxyl ions (in order to keep the equilibrium constant unchanged at ). This will result in ph values less than 7. When a base is added to water, it causes the formation of additional hydroxyl ions and a resulting lower concentration of hydronium ions (again to keep the equilibrium constant unchanged at ). This will result in ph values greater than 7.

31 Salts Salts are the compounds formed from the ions that remain when hydronium and hydroxyl ions are separated from acids and bases. When acids and bases are combined, they will react to form water (the combination of the hydronium and hydroxyl ions), a salt, and varying amounts of thermal energy, depending on the particular reactants. If the acid and base are combined in the correct proportions, the resulting solution will have a ph of 7. The resulting solution is then known as neutral. The process of mixing acids and bases to form neutral solutions is known as neutralization. 2-17

32 Solutions Many of the process streams in a power plant are mixtures of water with various other substances. The ways in which these mixtures interact with the materials that contain the process stream are important to the safe and efficient operation of the power plant. A solution is a mixture of a liquid and other material, comprised of the solvent and solute. Solvent The solvent is the liquid into which the solute is dissolved in (typically water) to form the solution. Solute The material being dissolved or suspended is referred to as the solute. When in solution, dissolved solids are not larger than single molecules (or ions). There are two general types of dissolved impurities we deal with in power plant applications: gases and solids.

33 Suspended Solids Suspended solids are relatively large groups of molecules in a liquid solution that are carried along by the motion of the fluid flow but will tend to settle out of the solution when the fluid flow stops. Common suspended solids are dirt, sand, and corrosion products, such as rust. Suspended solids can readily be removed through a mechanical process like filtration. Dissolved solids may become suspended solids under the correct conditions. For example, if you dissolve a large amount of sugar in hot coffee or tea, then let it cool, some of the dissolved sugar will precipitate out. This equilibrium is based on the solubility, or the ability of a solute to dissolve in the solution. Salts readily dissolve in water, but oils do not. Typically the higher the temperature, the higher the solubility. However, some solutes, such as cobalt and nickel, become less soluble at higher temperatures. Depending on the solute, other chemical parameters can affect solubility. For example, oxygen and ph are two major parameters that affect corrosion products. 2-19

34 Gas Solubility Gases readily dissolve into solutions. The ability of a gas to dissolve into a solution is called gas solubility. Gas solubility can be affected by two things: temperature and pressure. Temperature: The temperature of the solution has a major impact on how much gas can be dissolved in the solution; near freezing or boiling gases are less soluble. For example, consider a carbonated beverage. When you open a bottle or can that is hot or nearly frozen, large amounts of carbon dioxide come out of the solution. Pressure: Gas pressure will also affect gas solubility and can be defined by various gas laws. If you increase the gas pressure, large amounts of carbon dioxide escape when the container is opened. This same effect has caused gas binding in safety injection systems.

35 Impurities Most dissolved and suspended solids are undesirable in plant systems due to their potentially damaging effects. These are considered impurities, or unwanted material in the water systems. Impurities can originate from many different sources, such as corrosion products, material design, leaks, makeup water impurities, chemical addition impurities, and maintenance activities such as poor foreign material exclusion or improper use of chemicals. Impurities are maintained As Low As Reasonably Achievable (ALARA). Impurities may cause many potential problems, including: Corrosion Fouling Plugging Mechanical binding Radiation dose These problems will be discussed in greater detail later in this course. 2-21

36 Review Question - Impurities Which of the following problems may be caused by impurities in plant systems? A. Mechanical binding B. Fouling C. Corrosion D. Plugging All of these problems may be caused by the presence of dissolved or suspended solids in plant systems.

37 Boiling in Power Plants Impurities exist in all our water systems. Some of these impurities move with the steam in the systems, but most stay in the liquid phase of the boiling process. Boiling is a natural process we regularly use in power plant systems. Boiling is the change of state of water from a liquid to a vapor. In power plants, we boil water to make steam, and then use the steam to produce electricity with a turbine generator. Boiling Water Reactors (BWR) In a BWR, the boiling occurs in the reactor core. Pressurized Water Reactors (PWR) In a PWR, the boiling occurs in the steam generators. In PWRs, some boiling may also occur in the reactor core; this is minimized and called Sub-Cooled Nucleate Boiling (SNB). Without boiling, power production would be difficult. The problem with impurities staying in the liquid phase is that they concentrate to levels high enough to cause problems, such as scale, corrosion, and reactor flux tilt. Thus, we need to maintain impurities ALARA, and remove them before they cause problems. 2-23

38 Conductivity One easy means of detecting impurities is conductivity. Many dissolved solids are ions, and thus can readily conduct electricity. Measuring the ability of the solution to conduct electricity gives an approximation of the impurities present and possible problems with power plant systems.

39 Conclusion You've completed the Chemistry Basics lesson. You now have a better understanding of how the chemistry in a power plant is related to the water in any systems within the plant. You also learned some basic chemistry concepts that will form the foundation of later lessons. Now that you have completed this lesson, you can: Explain atomic structure, chemical valence, chemical bonding, and chemical reactions Explain the oxidation and reduction process Explain the factors affecting reaction rates and chemical equilibrium Explain ph, acids, and bases Describe how solids interact in solutions Explain the sources and impact of impurities Describe how gases interact in solutions Explain the boiling process and effects 2-25

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41 3 CORROSION CONTROL Introduction Engineers need to be aware of the types and causes of corrosion, as well as what affects the rate of corrosion. This awareness ensures that they can effectively monitor their systems and equipment to maintain integrity. Mechanical systems and equipment raise obvious corrosion concerns. In addition, structural supports, equipment enclosures, and electrical connections are also vulnerable to corrosion problems and warrant attention to ensure integrity and reliable service. After completing this lesson, you will be able to: Explain what causes corrosion Identify factors that influence general corrosion rates Identify the ways in which general corrosion can be minimized Identify the types of localized corrosion Identify the factors that influence localized corrosion rates Identify the ways in which localized corrosion can be minimized Please note that the Nuclear Power Plant Materials course is considered a pre-requisite to this course. If you have not previously completed that course, you may want to exit now and complete it before continuing with this lesson. 3-1

42 Operating Experience The consequences of corrosion can be serious, as described in the following operating experience example. A manual reactor trip was initiated due to increasing steam generator sodium levels. Sodium levels at the plant were increasing as a result of impurity ingress from a tube plug leak in the Condenser Air Removal System cooler. The high sodium concentration in the steam generators could have caused caustic conditions on the steam generator tubing, leading to a failure of the tubing from caustic induced intergranular stress corrosion cracks (IGSCC). The direct cause was a failed tube plug in the "D" Condenser Air Removal (AR) System seal water cooler. The composition of the brass screw on the tube plug was identified as ~65% Cu (Copper) and ~35% Zn (Zinc). No evidence of mechanically-related fracture was present. The absence of any mechanically-related cracking, such as fatigue, overload, or stress-corrosion-cracking strongly indicates that this failure was caused by corrosion. This brass alloy is one of the least corrosion-resistant brass alloys made. The root cause was inadequate questioning attitude and technical rigor by Engineering during the tube plugging. The consequences of installing material with a potential for corrosion was not adequately assessed. This operating experience was classified as a knowledge-based error with respect to inadequate assessment of: Corrosion of materials Operational configurations of the Air Removal and Condensate Systems

43 How Corrosion Happens The basic concept of how metals corrode is fairly simple. Metal atoms have more electrons than they really want. They can give up a few electrons to leave a complete outer shell. Most metals in their natural state are found as an ore (normally a metal oxide), such as Fe 2 O 3 (hematite). Prior to commercial use, the metal oxide is refined to its pure elemental state by removing the oxygen. After refining, the metal is in an excited, unstable state. The instability occurs because the metal atoms (Fe) are sharing electrons in the outer shell with other metal atoms (Fe + Fe) rather than with oxygen atoms. The metal - oxygen bond (Fe + O) results in a more stable electron arrangement. The refined metal will try to reach stability. Corrosion is the inevitable process of the metal finding something that will accept its unwanted electrons, so it can happily turn back into dirt. 3-3

44 Electrochemical Potential Since corrosion involves the transfer of electrons, it is a redox reaction, where the corroding metal is oxidized by another substance that is reduced. This reaction can be viewed as an electrical process where electrons flow through the metal from the corroding anode to the cathode where reduction occurs. In order to complete the electrical circuit, there has to be an electrolyte (such as water) with mobile ions. Mobile ions in the electrolyte allow cations to move toward the cathode and anions to move toward the anode to balance the charges. The quantity of electrolyte doesn't have to be large; moisture from air can be enough to support corrosion. Voltage (or potential) is the measure of the driving force for electrons to flow. The driving force for corrosion reactions is expressed as a voltage or the electrochemical potential (ECP). The potential for each half of a redox couple can be expressed as the voltage relative to a standard hydrogen electrode. The difference between the potential for the two half-cells is the net driving force for total reaction.

45 General Corrosion General Corrosion is the uniform attack on a metal surface by its environment. That is, the corrosion rate is uniform over the entire surface of the metal. A simple corrosion reaction is the interaction of a pure metal (such as iron) and pure water, as depicted in the graphic at right. Pure water ionizes to form H + and OH - ions. Iron atoms on the surface can give up electrons to the H + ions, forming hydrogen gas and a ferrous (Fe +2 ) ion: Fe + 2H + Fe +2 + H 2. In this reaction, the iron is oxidized by the H + ions from the water itself. Once the ferrous ions reach a high enough concentration, the Fe +2 ions can combine with the OH - ions and precipitate as ferrous hydroxide Fe(OH) 2. In most situations, a stronger oxidizer than hydronium ions (such as dissolved oxygen) will allow the ferrous oxide to lose additional electrons. At higher temperatures and lower dissolved oxygen concentrations, magnetite or black iron oxide (Fe 3 O 4 ) is formed. At lower temperatures and at more oxidizing potentials, hematite or red iron oxide (Fe 2 O 3 ) forms. As the corrosion product builds up on the surface, it will tend to isolate the underlying iron from the water, slowing the corrosion rate. Using the electrical model, the buildup of the corrosion product increases the resistance in the corrosion circuit, decreasing the current. 3-5

46 Factors Influencing General Corrosion Rates There are several factors that increase general corrosion rates. Increasing Temperature Increasing Dissolved Oxygen Decreasing ph Temperature is a measure of the motion of atoms or molecules. As the temperature increases, corrosion rates generally increase. The increase is in part due to the higher probability of the reacting atoms coming in contact in a manner that allows a reaction to occur. Oxygen is the most readily available oxidizing agent, and increases the electrochemical potential. Decreasing ph increases the availability of H + ions that can accept electrons. Increasing Flow Decreasing ph also decreases the availability of OH - ions. OH - ions can combine with dissolved metal cations to form an insoluble corrosion product that can protect the underlying metal. However, depending on the metal, OH - ions at very high concentrations can interfere with the stability of the protective corrosion product layer. Increasing the flow rate of electrolytes over metal surfaces can increase the general corrosion rate because: The transport of oxidants to the corroding surface is increased The proper formation of a passive metal oxide layer is prevented or the oxide film is removed Shocks and Stresses The metal surface is eroded Increased corrosion can also occur due to shocks or stresses that remove the protective layer and allow corrosion to proceed. Such types of shocks or stresses include: Chemical Shock Thermal Shock Hydraulic Shock Mechanical Shock or Stresses We'll take a closer look at these shocks and stresses on the next page.

47 Review Question - Corrosion Factors Which of the following factors increase the rate of general corrosion? A. Decreasing the ph of the electrolyte B. Decreasing the flow rate C. Increasing dissolved oxygen D. Increasing the temperature The rate of general corrosion rate can be increased by: Increased temperature Increased dissolved oxygen Decreased ph Increased flow rate Shocks or stresses 3-7

48 Shocks and Stresses Shocks and stresses are among many factors that can increase general corrosion rates. Corrosion is increased because the shock or stress removes the protective layer. Chemical Shock A chemical shock, such as a sudden change in the ph level, can dissolve the passive layer or add H + ions to increase the corrosion rate. Thermal Shock A thermal shock is a temperature change that causes the metal to expand or contract creating defects in the passive layer. Hydraulic Shock A hydraulic shock is caused by fluid forces that may remove the passive layer. Mechanical Shock or Stresses Mechanical shock or stress may also remove the passive layer, exposing more base metal to the environment.

49 Alloy/Metal In addition to metals, alloys are used in power plant applications. Alloys are a mixture of a metal with one or more other elements, and can be thought of as a base metal with impurities that are intentionally introduced in order to alter the properties of the metal. Stainless steel is an example of an alloy designed to improve the corrosion resistance of iron through the addition of chromium and nickel. As stainless steel corrodes, the iron and nickel chromite corrosion products that form on the surface are particularly insoluble and protective of the inner base metal. In addition to the carbon steel and stainless steel ferrous based alloys, other types of alloys are used throughout the plant: Copper-based alloys used in some heat exchanger tubing Nickel-based alloys (Inconels) used in steam generator tubing Cobalt-based alloys (Stellites) used in valve seats Zirconium alloys (Zircalloys) used in fuel cladding Each material may need its own set of ideal environmental conditions to minimize its general corrosion. If multiple materials are used in the same system with the same water flowing through the different components, then the chemistry control may have to be a compromise. For example, the optimum ph for minimizing corrosion of ferrous materials is much higher than the ph for minimum corrosion of copper alloys. If a new material is introduced, great care must be exercised to ensure that its corrosion performance will be acceptable. 3-9

50 General Corrosion Control The rate of general corrosion is normally minimized by controlling the chemical environment (the chemistry of the electrolyte solution). The chemical environment can be controlled in a number of ways. Controlling Dissolved Oxygen Content Maintaining Neutral to Slightly Basic ph Using Corrosion Inhibitors Dissolved oxygen is controlled by the addition of hydrogen in a reactor coolant system or hydrazine in secondary systems. These chemicals react with oxygen to form water, thereby preventing the oxygen from reacting with metal surfaces. Generally, oxygen concentrations are kept as low as practical; however, there are situations where conditions that are too reducing can lead to the formation of a less stable protective film. Proper ph is maintained by adding lithium hydroxide in the primary system or an amine in the secondary system. Since amines and hydrazine are volatile chemicals, they vaporize with the water in the steam generator and increase the ph of the steam. This allows treatment of the condensate/feedwater systems, the S/Gs, and the steam system with the same chemicals. This type of chemistry control is often referred to as all volatile treatment. Corrosion inhibitors generally contain a base to elevate ph, oxygen scavengers to control dissolved oxygen concentrations, or chemicals to provide a protective coating on metal surfaces. Chemicals used for copper corrosion control include nitrogencontaining organic compounds (azoles). Corrosion inhibitors are normally used in closed cooling systems and typically cannot be used in systems that operate at elevated temperatures.

51 Localized Corrosion As you learned earlier, general corrosion is the most common and familiar type of corrosion, where the entire surface of a metal corrodes at a fairly uniform rate, and therefore the expected life of equipment can be estimated with some accuracy. Localized corrosion is the rapid attack on a metal in a small area. This type of corrosion is a more treacherous type of metal deterioration, since it can lead to a sudden failure with only slight metal loss in a very short period of time. The types of localized corrosion of particular concern in nuclear power plants are: Galvanic Corrosion Crevice Corrosion Pitting Stress Corrosion Cracking and Intergranular Attack Microbiologically Influenced Corrosion (MIC) Flow Accelerated Corrosion (FAC) Boric Acid Corrosion On the next few pages, we ll take a closer look at these types of localized corrosion. 3-11

52 Galvanic Corrosion When two dissimilar metals are placed in electrical contact in a conductive environment, the difference in electric potential between the two materials causes a voltage difference between the two metals. 1. The more noble metal attracts the electrons and becomes the cathode. 2. The metal with the lower attraction for electrons becomes the anode. 3. The anode is oxidized by giving up electrons that are conducted through the electrical contact between the two metals. 4. Positively charged metal ions are left behind, which go into a solution. 5. The solution in which the ions are immersed completes the circuit, allowing the positively charged metal ions to migrate through the solution to the cathode. The rate of corrosion is generally greatest at the interface between the two dissimilar materials. Galvanic corrosion control methods include preventing the electrical contact between dissimilar metals and minimizing the conductivity of the electrolyte. Engineers should exercise caution when using dissimilar metals in the same system. The tendency of galvanic corrosion to attack only the anode material can actually be used to provide a corrosion protection benefit. Sacrificial anodes consisting of materials known to have a lower attraction for electrons than the protected materials are installed immersed in system fluid that is in good electrical contact with components being protected. Zinc has a lower attraction for electrons than most metals in common use, and is frequently used for this purpose. Since the protected materials are forced to become the relatively uncorroded cathode of a galvanic corrosion cell, this practice is known as cathodic protection.

53 Crevice Corrosion Crevice corrosion is a form of electrochemical attack on a small area of metal in the vicinity of a crack or crevice, as depicted in the graphic at right. It is similar to galvanic corrosion except that the difference in potential is created by local differences in the composition of the electrolyte solution rather than two dissimilar metals. Due to little mixing of the electrolyte inside and outside the crevice, locally low concentrations of oxygen or locally high concentrations of ions can be created. This concentration difference causes the crevice to become an anode with respect to the bulk material of the metal, resulting in dissolution of the metal inside the crevice and growth of the crevice deeper into the metal. To limit crevice corrosion, the most important environmental factors to control are dissolved oxygen concentration and water impurities, especially salts or other materials forming ions in solution. Methods to remove ions will be discussed in the next lesson. Crevice corrosion can also occur on the outside of piping. It is therefore important to quickly identify and correct any leaks. 3-13

54 Pitting Corrosion Pitting corrosion is a type of localized electrochemical attack occurring in the vicinity of local irregularities in a metal that form a localized anodic area. Once started, the pits tend to grow by the formation of metal oxides accentuating the original local irregularities. The resulting pit tends to maintain a width about as wide as it is deep, while getting larger and larger. Since pitting can start from relatively small irregularities (and does not require a crevice), it can occur at almost any location. As with crevice corrosion, pitting can be limited by controlling dissolved oxygen concentration and water impurities. Pitting often occurs where the water is stagnant, making it desirable to maintain flow through components at all times. If flow cannot be maintained, consider placing the component in service on a regular basis.

55 Review Question - Localized Corrosion Which of the previously discussed types of corrosion occurs in the vicinity of local irregularities in the metal? A. Galvanic B. Crevice C. Pitting D. Stress corrosion cracking Pitting corrosion is a type of localized electrochemical attack occurring in the vicinity of local irregularities in a metal that form a localized anodic area. 3-15

56 Stress Corrosion Cracking and Intergranular Attack Failures in materials that occur by environmentally-induced crack propagation are known as stress corrosion cracking (SCC). This crack propagation is caused by the synergistic interaction of temperature, tensile stress, a corrosive environment, and susceptible materials. Transgranular cracking occurs when the corrodent attacks the bulk material. Intergranular cracking (IGSCC) occurs when the grain boundary is more susceptible. Intergranular Attack (IGA) occurs when a corrodent attacks the grain boundaries without sufficient stress to cause cracking. The magnitude of stress necessary to cause failure depends on the corrosive medium and on the structure of the base metal. In some cases, the combination of stress, susceptible microstructure, and temperature are sufficient for cracking to occur even in pure water environments with no specific corrodent. This is known as primary water stress corrosion cracking (PWSCC). For BWRs, conductivity (primarily from chloride and sulfate ions) is a strong factor for IGSCC.

57 Alloys and Stress Corrosion Cracking Cracking has been observed in many alloy and environment combinations. The most common alloy and environment combinations that are likely to cause cracking are summarized in the table at below. 3-17

58 Microbiologically Influenced Corrosion (MIC) Microbiologically Influenced Corrosion (MIC) is the result of the formation of a crevice underneath a bacteria colony and a harsh chemical environment. MIC occurs in part due to the chemical excretions of living organisms on a metal surface. The harsh chemical environment under the colony accelerates corrosion as the organisms produce metabolic by-products that are corrosive to structural metals. Some bacteria even concentrate halides, which result in severe, localized corrosion of ferrous metals. Sulfate reducing bacteria (which grow in the absence of oxygen) increase iron dissolution at the anode. Microbes produce chemicals that oxidize iron, destroy protective coatings, and can even metabolize corrosion inhibitors. Untreated water normally contains bacteria and the nutrients to sustain them. High purity water can also contain significant numbers of microbes and sufficient nutrients to sustain growth. MIC is most likely to occur under stagnant, low, or intermittent flow conditions. Standby and redundant systems in nuclear power plants are particularly susceptible to MIC due to the stagnant conditions that exist in the piping for extended periods of time. Methods for preventing or mitigating the effects of MIC include: Avoiding stagnant conditions Eliminating nutrient source(s) Periodically cleaning systems to remove organic deposits (mechanical and/or chemical dispersants) Using biocides to kill bacteria (when discharge limits allow) In some cases, larger organisms such as Zebra Mussels or Asiatic Clams can cause crevice conditions leading to corrosion.

59 Flow Accelerated Corrosion (FAC) Flow Accelerated Corrosion (FAC) is a localized increase in the corrosion rate caused by a relatively high flow rate at the corroding surface. FAC primarily affects carbon steel in slightly basic water under reducing conditions. The reducing conditions result in higher corrosion product solubility, and the high flow rates cause a more rapid removal of the protective oxide. Turbulent flow conditions increase the FAC rate. Due to the flow conditions and materials, FAC is of particular concern in the condensate, feedwater, steam extraction, and heater systems. These systems are generally accessible during operation, and fatalities have occurred due to ruptures of thinned pipe walls. In most plants, Engineering is responsible for monitoring susceptible piping and components. Stainless steel is not susceptible to this type of corrosion. The chemistry conditions that can reduce FAC are an increase in the ph and a small increase in the oxygen concentration. These increases lower the corrosion product solubility. 3-19

60 Boric Acid Corrosion Boric acid is used in the primary systems of PWRs to control reactivity during normal plant operation. There is little concern with corrosion inside the primary systems, since the oxygen concentrations are low and all boric acid-containing systems are stainless steel (which is resistant to boric acid). If borated water leaks from primary systems through gaskets, mechanical joints, valve packing, or cracks, significant corrosion problems can develop. The water will become oxygenated, and the boric acid can concentrate as the water boils off or evaporates. These factors can increase the corrosion rate of exposed carbon steel from less than inches per year to as much as 10 inches per year. The corrosion type of greatest concern due to leakage of borated water is wastage, a form of uniform corrosion. Plants now have Boric Acid Corrosion Control programs to ensure that all boric acid leakage is identified, the problem is documented, the source is located, and the condition is dispositioned. BWRs monitor for boron/lithium leaking from cracked control blades, but have not had IGSCC cracking issues due to borate in the reactor water. The following lesson learned is courtesy of the U.S. NRC. Reactor pressure vessel heads of PWRs have penetrations for control rod drive mechanisms and instrumentation systems made from nickel-based alloys. Conditions in PWR plants can cause PWSCC of the alloys and metals used in the welds. A plant began an outage that included remotely inspecting the vessel head penetration nozzles from underneath the head focusing on the control rod drive mechanisms (CRDM). Three CRDM nozzles had indications of through-wall axial cracking. A visual examination of the area was conducted that identified a large cavity in the RPV head on the downhill side of one of the CRDM nozzles. The corrosion was caused by borated water that leaked from the reactor coolant system onto the vessel head through cracks in the nozzle and the weld attaching the nozzle to the RPV head. The remaining thickness of the RPV head was found to be approximately 3/8 inch (the thickness of the stainless steel cladding on the inside surface of the RPV head). Stainless steel cladding is resistant to corrosion by boric acid, but cannot provide structural integrity to the vessel. Failure of the stainless steel cladding would have resulted in a LOCA and activation of the emergency systems.

61 Conclusion Corrosion is an inevitable process. The chemistry controls for plant systems can be effective in slowing corrosion rates enough to get adequate service life. Knowing what causes and influences corrosion will help you make appropriate choices in your day-today activities to support the corrosion control efforts in your organization. Now that you have completed this lesson, you can: Explain what causes corrosion Identify factors that influence general corrosion rates Identify the ways in which general corrosion can be minimized Identify the types of localized corrosion Identify the factors that influence localized corrosion rates Identify the ways in which localized corrosion can be minimized 3-21

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63 4 WATER QUALITY Introduction Welcome to the Water Quality lesson. Water is essential for the operation of a nuclear power plant. It is used to transfer heat, moderate the reactivity of the reactor, and act as a carrier for chemical treatment within the systems. But water can also carry harmful impurities that can damage plant components and systems, cause failure or reduced performance of components, lead to increased dose to workers, and prolong outages. Therefore, it is essential that the purest water possible be maintained in these plant systems. When you complete this lesson, you will be able to: State the reasons that pure water is essential for power plant operations State the sources of impurities entering the systems and the problems they can cause Describe the methods used to remove impurities from the systems State the reasons for chemical treatment of system water Describe the three action levels and the reasons for having them 4-1

64 Role of Engineering There are many ways that engineering can have a direct impact on chemistry. System Modifications A system modification may change temperatures, pressures, and/or flow rates, which could lead to changes in filtration and ion exchanger performance. These changes could also affect how the plant behaves in an abnormal operating condition. It is important to note that any change will alter the equilibrium of the system, thereby creating other changes as a result. Coatings and Cleaning Materials An engineer may also be involved in the choice of coatings or cleaning materials that can have an impact on station chemistry. Material Selection Perhaps the most critical choice that may face an engineer in terms of station chemistry is that of material selection. When selecting materials, an engineer must take many things into consideration including cost, strength, durability, availability, and most important to chemistry: corrosion resistance. Operating Experience: In the 1990s, a nuclear plant experienced a loss of off-site power due to a tornado, causing the power plant to trip offline. When the plant was starting up, a slug of secondary resin was sent to the steam generators, resulting in contaminant concentrations many times greater than action level 3 values. Action levels will be discussed in more detail later in this lesson. While this caused no immediate damage to the steam generators, the long-term reliability of the steam generators was affected. This situation could have been avoided if an engineer had recognized the importance of installing a check valve to prevent resin intrusion in the event of loss of off-site power.

65 Impurities As you learned earlier, impurities in the water can damage plant components and systems. Not only do the impurities adversely impact equipment reliability, but considerable resources and expenses can be consumed in detection, evaluation, source identification, water treatment, and clean up. Impurities can come from a variety of sources, but typically from: Makeup water sources, such as lakes, rivers, or oceans Leaks between system interfaces, such as heat exchangers Foreign material that entered the system during maintenance, such as packaging material, tools, chemicals, or even sweat Degradation of components due to corrosion or wear Different materials used in replacement parts and equipment Impurities in the water can lead to many problems, including: Clogging of filters and/or small pipes Fouling of heat exchangers Higher radiation levels, due to activation of the impurities in the reactor core Accumulation of sludge in steam generators Corrosion 4-3

66 Water Purity Even very small amounts of impurities can have an impact on plant components and systems. For example, the typical specification limit for sodium in makeup water is one part per billion (1 ppb). The typical value of sodium in makeup water is far less, at about 0.01 ppb. How small is one part per billion? Consider an average-sized swimming pool, which contains approximately 30,000 gallons of water. Now, consider a packet of salt like the one you would find in a cafeteria. How many packets of salt do you think you would need to add to a swimming pool to exceed a typical specification limit of 1 part per billion? A salt packet contains about 1 gram of salt. Since salt is about 40% sodium by weight, this would correlate to about 3.5 parts per billion. As you can see, just one packet of salt in a swimming pool would exceed the typical specification of makeup water by a factor of 3.5! To put this in perspective, modern sodium analyzers can detect as little as a single drop of sweat in a swimming pool.

67 Purification Processes Preventing the ingress of impurities in a system should be a primary focus of engineers. You can contribute to this effort by: Giving chemistry issues due consideration in the selection of materials Coordinating closely with the Chemistry department to ensure related problems are avoided or minimized Reinforcing foreign material exclusion (FME) practices any time work is done on a system that could allow ingress of impurities Despite these efforts, impurities can build up in a system, so it is important to have ways to purify the water. Pure water is provided and maintained in the plant by three major processes. Letdown/Makeup Filtration In a closed system, any impurities that find their way into the system tend to concentrate over time, particularly in components such as the steam generator. One way to remove impurities from the system is to remove some of the impure water and replace it with pure water. This process is referred to as letdown (or blow down ) and makeup. Filtration is used to remove solid material from the water. Filters can be in the form of screens to remove bulk materials from water, such as fish from plant makeup water or rust particles from condensate or feedwater. Ion Exchange Filters can also be in the form of sub-micron filtration to remove microscopic particles from the system water. Some impurities will dissolve in the water and cannot be removed by filters. Many of these impurities carry an ionic charge (either positive or negative). Ion exchange is a type of purification that exchanges impurities that carry an ionic charge. We'll look at ion exchange in more detail on the following pages. 4-5

68 Review Question - Purification Processes Which of the following processes may be used to provide and maintain pure water in a plant? A. Activation B. Filtration C. Ion exchange D. Letdown/makeup Filtration, ion exchange, and letdown/makeup are all processes used to provide pure water and maintain water purity in the plant. Activation is a radiochemical reaction.

69 Ion Exchange Ion exchangers typically utilize a material called resin, which gives up one ion (such as hydronium (H+) or hydroxyl (OH-)) for another ion. Note: Hydronium is actually H 3 O+, but for the sake of simplicity we will speak of it in terms of a hydrogen ion, H+. Ion exchangers that remove positive ions are called cation beds. Ion exchangers that remove negative ions are called anion beds. When these two types of resin are mixed together, they are called mixed beds. 4-7

70 Cation Resin Let s look at an example of how this works. When salt is dissolved in water, it forms positive sodium ions and negative chloride ions. When this solution is passed through cation resin, the positively charged sodium ions (Na+) are removed and replaced by hydronium ions (H+). Remember: Cation resin captures a + (positive) ion: captures a + ion = cation An important consideration is that the resulting solution contains H+ ions and Cl- ions. This forms a weak hydrochloric acid solution, which causes the ph to go down (i.e., become more acidic).

71 Anion Resin Anion resin works similarly to cation beds, except it typically exchanges OH- for negatively charged ions. When a sodium chloride solution is passed through anion resin, the negatively charged chloride ions (Cl-) are removed and replaced by hydroxyl ions (OH-). Remember: Anion resin removes a - (negative) ion: a negative ion = anion Again, it is important to note that the resulting solution contains Na+ and OH- ions. This forms a weak sodium hydroxide solution, which causes the ph to rise (i.e., become more basic). 4-9

72 Mixed Resin In order to balance the ph effects, we generally use a mixture of cation and anion resin in ion exchangers. This filters out both anion and cations without having much effect on ph.

73 Ion Exchanger Effects Take a look at the partial periodic table below. The elements on the left side of the chart form positively charged ions and those on the right form negatively charged ions. As you can see, an ion exchanger can also remove boron, which is important for reactivity control. It is very important to remember that improper use of an ion exchanger can cause a reactivity event. 4-11

74 Ion Exchanger Effectiveness In order to maintain the purity of system water, while preventing problems such as reactivity events, you must understand the parameters that can affect the effectiveness of ion exchangers. Ion exchangers can be affected by several parameters. Flow Rate Temperature Water Chemistry Fouling Saturation The effectiveness of an ion exchanger is affected by contact time. Temperature can cause the resin to have greater affinity for some ions and less affinity for others. Changes in water chemistry, such as ph, can change the behavior of an ion exchanger. If an ion exchanger becomes fouled by impurities, it cannot do its job. Once an ion exchanger has depleted its ion exchange sites, it must be regenerated or replaced.

75 Process Monitoring Many plant systems have inline analyzers in place to continuously monitor certain chemical parameters on each system. Among these are ph, conductivity, oxygen, as well as specific ion monitors to monitor for impurities such as sodium and chloride. Iron is also monitored to provide an assessment of the effectiveness of water chemistry control or to determine if the equilibrium of the system has been affected Some of these monitors provide continuous readout in the chemistry office and control room as an early indicator of potential chemistry problems. Chemicals are often added to a system to: Control reactivity Adjust ph Scavenge oxygen Inhibit corrosion These chemicals may react with materials, such as gaskets or metals used in the fabrication of components. In order to avoid any compatibility issues, you should familiarize yourself with your site s chemical treatment strategy to determine the types of chemicals used to achieve these goals. 4-13

76 Action Levels The Chemistry department continuously monitors key system parameters to establish trends and triggers for taking action to address out of specification conditions. The detection level of chemistry analysis equipment is so sensitive that any change in the system, such as reactor power level or starting or stopping a pump, can affect impurity levels. While water purity is essential, we must balance that with operational flexibility. Since a power station provides electricity, we can't just shut down at the first signs of water impurities. Action levels allow us to diagnose and fix water quality issues while giving operations the flexibility to continue plant operations. Action Level 1 represents the plant response to the detection of a small impurity ingress. While it is important to determine the source of the impurity, a reduction in power is not warranted, since the consequences of operation are low. In fact, system operational changes may actually mask the problem, making it more difficult to determine the cause. Action Level 1 typically calls for increased monitoring to isolate the cause of the problem. Action Level 2 represents the plant response to the detection of a larger impurity. These amounts can begin to endanger the plant systems and may require a reduction in power after a certain time if the cause cannot be identified and corrected. At these levels, the source of the impurity is usually large enough that it will not be masked by changes in power level. Action Level 3 represents the plant response to the detection of high levels of impurities. These amounts could result in significant degradation of plant equipment. At these levels, the problem is large enough that its cause should be obvious. Since chemical reactions tend to be magnified by heat, the only course of action left at this point is to shut down the plant.

77 Basis for Action Levels It is important to note that Action Levels are designed to ensure that the plant will operate reliably for the duration of its license. Think about plant chemistry in terms of healthy eating. While it is unlikely that a highcholesterol snack will cause an immediate heart attack, it is not advisable to make it a habit. If you are going to enjoy retirement, your heart needs to last as long as the rest of your body does. Similarly, operating under adverse chemistry conditions does not necessarily mean that a component failure is imminent. But it does mean that long-term equipment reliability may be impacted. 4-15

78 Review Question - Source of Impurities Which of the following are typical sources of water impurities in plant systems? A. Activation in the reactor core B. Degradation of components due to wear C. Leaks between system interfaces D. Fouled heat exchangers Leaks between system interfaces and the degradation of components are both typical sources of water impurities in plant systems. Activation in the core and fouled heat exchangers are possible consequences of impurities in the systems.

79 Conclusion You've completed the Water Quality lesson. Now you have a better understanding of the importance of pure water in power plant operation. You also know how Action Levels, prevention, purification processes, and chemical treatments contribute to water quality. Engineers need to be aware of the impact of changes to materials or operational parameters, and must be vigilent in preventing leaks and the intrusion of foreign material. Now that you have completed this lesson, you can: State the reasons that pure water is essential for power plant operations State the sources of impurities entering the systems, and the problems they can cause Describe the methods used to remove impurities from the systems State the reasons for chemical treatment of system water Describe the three action levels and the reasons for having them 4-17

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81 5 RADIOCHEMISTRY Introduction Welcome to the Radiochemistry lesson. When you have finished this lesson, you will be able to: State the two major categories of reactor coolant radionuclides Describe how fission products enter the reactor coolant and give examples of common fission products Describe how activation products are produced in the reactor coolant system and give examples of common activation products Explain the radiolysis and recombination of water in the reactor coolant Explain the significance of tritium production Explain how radioactive isotopes affect the dose to plant personnel and the public Describe how isotopic data can be used as an engineering diagnostic tool for failed fuel monitoring and degraded equipment monitoring 5-1

82 Why Radiochemistry? Radiochemistry is a branch of chemistry that deals with radioactive substances and phenomena. Radiochemical parameters are regularly monitored to ensure the optimal operation of nuclear plant systems. There are several specific objectives achieved by performing radiochemical analyses. The following objectives apply to various areas of nuclear plant chemistry and are invaluable to plant engineering: To establish baseline data for reactor coolant activity so that abnormalities may be detected To assess the condition of the fuel cladding and determine the extent of fuel failure To control coolant activity levels to ensure plant Technical Specifications are not exceeded (i.e. control on-site and off-site dose within limits) To control liquid and gaseous releases within limits To detect and quantify any primary to secondary leakage in PWRs

83 Review Question - Why Radiochemistry Which of the following objectives are accomplished by performing radiochemical analyses? A. Assess the condition of the fuel cladding B. Establishing baseline data for reactor coolant activity C. Establishing liquid and gaseous release limits D. Detect and quantify leakage in PWRs These are all objectives that are accomplished by performing radiochemical analyses. 5-3

84 The Two Major Categories After a nuclear reactor has been in operation, its coolant becomes radioactive. Two major sources of coolant radioactivity are: Fission Products When uranium fission occurs, the atom does not split into two equal fragments. Instead, the two fragments tend to have atomic weights of about 95 and 140 mass units. Occasionally, fission results in three fragments. When this occurs, the third fragment is a tritium nucleus. Fission products can enter the reactor coolant by three mechanisms. The first is inherent in the fuel fabrication process/material, the second is a natural process, and the third is the result of fuel clad failure. Activation Products Activation products are produced in a nuclear reactor by particle bombardment of stable atoms. The radioisotopes are produced by particle bombardment of RCS water, dissolved impurities, and system corrosion products as the RCS water passes through the high flux of an operating reactor core.

85 Nuclear Fuel Assemblies As shown in the figures below, nuclear fuel assemblies are metal structures designed to hold fuel rods in a matrix such that fluid flow can remove the reactor heat. Fuel rods are metal tubes (cladding) that contain ceramic uranium fuel pellets where the heatproducing fission reaction occurs. The fuel rod cladding is the primary fission product barrier, with the primary system boundary and containment serving as the other two fission product barriers. Excellent fuel integrity practices prevent breaches in fuel cladding. However, if breaching is not prevented, fission products are released into the plant s reactor coolant system, potentially resulting in plant contamination and/or increased worker radiation exposure. PWR Fuel Assembly BWR Fuel Assembly 5-5

86 How Fission Products Enter the Reactor Coolant Fission products can enter the reactor coolant by three mechanisms. Tramp Uranium Diffusion Cladding Defects Tramp uranium is uranium which is either on or very close to the surface of a fuel pin after manufacture and is available for fission. The most noticeable result of tramp uranium fission products in the coolant is the presence of baseline iodines and noble gas isotopes (krypton and xenon). Diffusion is the second means for fission products to enter the coolant. While we commonly think of metals in the fuel cladding as being a solid barrier, it is porous. There are spaces between the atoms through which other atoms can migrate (diffuse). How quickly an atom can migrate through the fuel cladding is dependent on temperature and the size of the atom. Fission gases like xenon and krypton can diffuse relatively quickly. Fission products in the gap region can also escape through cladding defects, such as: Imperfect welds (the end caps of the pins are welded onto the pin tubing) Minute cracks (due to thermal stresses or chemical attack) Larger holes (typically the result of chemical attack) Mechanical damage (due to hydraulics or the introduction of foreign material into the reactor coolant system)

87 Common Fission Products There are four general categories of fission products: fission gases, radioiodines, soluble fission products, and insoluble fission products. The specific product can provide an indication of where a problem may lie. Fission gases are comprised of radionuclides of chemically inert krypton and xenon noble gases. Noble gas isotopes above the baseline established by diffusion and tramp will be early indicators of fuel defects. There are five radioiodines that are routinely analyzed in the coolant. Iodine ratios will indicate the size of defects in the coolant system. Radionuclides of Cesium are soluble fission products. Cesium ratios can indicate which cycle of fuel is leaking. Common Fission Products Barium: Ba 140 Iodine: I131, I 132, I 133, I 134, I 135 Zirconium: Zr 95, Zr 97 Xenon: Xe 131 m, Xe 133, Xe 133 m, Xe 135, Xe 137, Xe 138, Xe 139 Strontium: Sr 89, Sr 90 Krypton: Kr 85 m, Kr 85, Kr 87, Kr 88, Kr 89 Cesium: Cs 137, Cs 138 Niobium: Nb 95 Rubidium: Rb 88, Rb 89 Molybdenum: Mo

88 Common Activation Products The other major source of coolant radioactivity is activation products. The two subcategories of activation products are coolant activation products, which generally result from a nuclear reaction involving water or low levels of impurities in the reactor coolant, and activated corrosion products. Chromium-51 Cobalt-58 Cobalt-60 Manganese-54 Manganese-56 Iron-59 Silver-110m Copper-64 T ½ = 27.7 days This isotope is produced by an n-γ reaction with Cr-50, and may be an indicator of whether or not sufficient hydrogen is being maintained in the reactor coolant system. The source is steel and high nickel alloys. T ½ = 71 days This isotope is produced by an n-p reaction with Ni-58, and may be used to identify sampling problems or a change in the boron-lithium ratios in the reactor coolant. The source is steel and high nickel alloys. T ½ = 5.27 years This isotope is produced by an n-γ reaction with Co-59, and is the major source of radiation after the reactor has been shutdown for a month. The source is stellite. The stellite alloys contain high percentages of cobalt and are used as hard-facing surfaces on valves to prevent wear. T ½ = 312 days This isotope is produced by an n-p reaction with Fe-54, and may be used to identify sampling problems or a change in the boron-lithium ratios in the reactor coolant. The source is steel and high nickel alloys. T ½ = 2.58 hours This isotope is produced by an n-γ reaction with Mn-55, as well as an n-p reaction with Fe-56. A sudden increase in this isotope is an indication of a loose part in the reactor coolant system. T ½ = 45 days This isotope is produced by an n-γ reaction with Fe-58, and is a major source of radiation during the first month of reactor shutdown. The source is steel and high nickel alloys. T ½ = days Along with Cd-115 and In-115m, Ag-110m is an indication of control rod wear. Ag- 110m is produced by an n-γ reaction with Ag-109. Cd-115 is produced by an n- γ reaction with Cd-114. In-115m is produced by beta decay of Cd-115, as well as n-n or p-p reactions with In-115. A significant increase in Ag-110m indicates control rod cladding failure. Cu-64 is produced by an n-γ reaction with Cu-63. The source is Admiralty Brass.

89 Iron-55 Nickel-63 Nickel-65 Zinc-65 Iron-55 is produced by an n-γ reaction with Fe-54. The source is Stainless Steel. Ni-63 is produced by an n-γ reaction with Ni-62. The source is Stainless Steel and Nickel based alloys. Ni-65 is produced by an n-γ reaction with Ni-64. The source is Stainless Steel and Nickel based alloys. Zn-65 is produced by an n-γ reaction with Zn-64. The source is Admiralty Brass and injection of non-depleted zinc. 5-9

90 The CRUD Cycle Reactor components are constructed of a number of different alloys. The metallic surfaces in contact with the coolant can go into solution as soluble ions or be physically taken into the coolant as particles, commonly referred to as "CRUD" Surfaces Corrode Outside of Core Accumulations of corrosion and wear products (i.e. CRUD) occur on the inner surfaces of nuclear reactor systems. Corrosion Products Released Into Coolant CRUD deposits are released into the reactor coolant. Particles Deposited in Neutron Flux Area CRUD deposits are carried to the core surface, an area with high neutron flux, by the reactor coolant. Corrosion Products Become Neutron-Activated Once present in the core, CRUD deposits can form on external fuel-rod cladding surfaces and become activated by neutrons and protons. Irradiated Metal Oxides Released The radioactive CRUD deposits, which typically consist of metal oxides, may be released back into the RCS as a result of a "CRUD burst". CRUD bursts will be explained in further detail on the next page. Radioactive Materials Deposited Outside of Core The activated CRUD can then deposit in low-flow regions on surfaces outside of the core, creating a significant radiation hazard to personnel.

91 Causes of CRUD Bursts Typically, the levels of radioactive CRUD in the core increase until a protective film builds up over the exposed component surfaces. Thereafter, an equilibrium condition develops between production, deposition, resuspension, and removal of CRUD by plant purification systems. However, this equilibrium may be altered by a "CRUD burst." A CRUD burst is caused by a shock to the reactor system, releasing activated CRUD back into the reactor coolant. Mechanical shock Addition of chemicals Thermal shock Hydraulic shock A disturbance of the CRUD layer resulting from vibrations or physical movement of RCS components. An example is a reactor trip. A reaction of the corrosion film surface in the RCS to a change in the coolant chemistry. A chemical shock can be a result of the addition of a chemical additive or by an accident, such as air injection or the injection of organic solvents. Such shocks may produce adverse chemical reactions, including the formation of nitric acid, a decrease in ph, or an increase in oxygen content. In a BWR, tripping hydrogen injection is the major chemical shock that can occur. A disturbance of the CRUD deposits resulting from differences in the expansion or contraction rates between RCS components and the CRUD layer. This type of shock is likely to occur during startup or shutdown. A disturbance of the CRUD layer resulting from a large variation in the system flow rate. An example is the starting and stopping of pumps. The mnemonic device MATH is helpful for remembering the four general causes of CRUD bursts. 5-11

92 Review Question - CRUD Cycle Listed below are the steps in the CRUD cycle. Place the steps in the correct order. Corrosion products become neutron-activated Corrosion products released into coolant Irradiated metal oxides released Radioactive materials deposited outside of core Surfaces corrode outside of core Particles deposited in neutron flux area Answer: 1. Surfaces corrode outside of core 2. Corrosion products released into coolant 3. Particles deposited in neutron flux area 4. Corrosion products become neutron-activated 5. Irradiated metal oxides released 6. Radioactive materials deposited outside of core

93 Radiolysis Water is a vital component in a nuclear power plant. The purity of the water and its chemistry will affect equipment life, fuel integrity, and radiation levels. In the reactor core, some water molecules undergo radiolysis and are broken down into oxygen and hydrogen. These dissolved gases are impurities that may affect the reactor water quality. Ionizing radiation from the reactor can induce coolant water to break apart into hydrogen and oxygen radicals. While the actual reactions involved are complex, the dissociation of water into hydrogen and oxygen gas is shown below. There is a minimum hydrogen concentration level needed to suppress the radiolysis of water in the core (the reaction is an equilibrium reaction). The oxygen present is directly proportional to the N16 formed. By adding hydrogen to the coolant system, we can control/suppress the oxygen levels and the corrosion rate. The reaction below illustrates the oxygen from the water becoming activated, thereby producing the radioactive isotope Nitrogen-16 The N16 produces a high energy gamma, which is useful for monitoring the main steam lines for evidence of primary to secondary leakage in a PWR. 5-13

94 Tritium Production A final concern regarding radioactivity in nuclear power plant systems is the production of tritium. Tritium, hydrogen-3, is a radionuclide produced by neutron activation and fission in nuclear reactors. It may also be produced in the core as a ternary fission product. Tritium is a biological concern since it can be inhaled, ingested, and absorbed by the human body. Tritium typically exists as tritiated water. In this form, it can migrate anywhere accessible by water in the human body. Ternary Fission Neutron Activation of Lithium-6 Neutron Activation of Boron-10 Neutron Activation of Hydrogen-2 In ternary fission, three fission fragments are produced, including Xe-140, Rb-91, and H-3. Tritium is the third fission fragment, occuring in about 1 in 12,500 U 235 fission events. While most of this tritium remains trapped within the fuel and fuel pins, enough diffuses into the coolant to serve as the predominant source of tritium late in the core cycle. This reaction (Li 6 + n H 3 + He) can produce large amounts of tritium. However, the use of 99.99% Li 7 lithium hydroxide minimizes the production of tritium by this reaction. This reaction (B 10 + n Li 7 + He or B 10 + n H 3 + 2He) is the predominant source of tritium during the first half or two-thirds of the core cycle. Using burnable poison rods along with the presence of boron inside some fuel pins, much of the tritium produced by this reaction remains within the fuel pins. Deuterium, H 2, exists in nature (about 1 in 6500 hydrogen atoms), but not of a sufficient quantity to be a significant contributor to tritium production.

95 How Isotopes Affect Us Remember that changes in the radiochemistry of plant systems can be directly correlated to events happening within the plant. The following problems are a few changes that may alter radiological conditions of the plant. Resin sluice activities Crud bursts Fuel failures PWR Primary to Secondary tube leaks Addition of hydrogen to BWRs for IGSCC mitigation at moderate to high concentrations We are required by regulations to control the inventory of activity in the coolant system to protect personnel and the public from adverse doses of radiation during normal operations and accident conditions. Also, accurate monitoring of radiochemical parameters may aid in diagnosing problems in reactor systems. For instance, if the internals of a coolant pump wear excessively, we would expect to see a corresponding increase in the activation products as the metal particles pass through the core and become activated. The increase in noble gas activity (Xe 133 and Xe 135 ) in the coolant system can key us early on to very small fuel defects. Efficient teamwork and collaboration between engineering and chemistry personnel will ensure that radiochemistry indications are used as effectively as possible in managing equipment reliability and plant performance. 5-15

96 Conclusion You have completed the Radiochemistry lesson. Now that you have finished this lesson, you can: State the two major categories of reactor coolant radionuclides Describe how fission products enter the reactor coolant and give examples of common fission products Describe how activation products are produced in the reactor coolant system and give examples of common activation products Explain the radiolysis and recombination of water in the reactor coolant Explain the significance of tritium production Explain how radioactive isotopes affect the dose to plant personnel and the public Describe how isotopic data can be used as an engineering diagnostic tool for failed fuel monitoring and degraded equipment monitoring 6 SPECIFIC SYSTEM AND EQUIPMENT CONSIDERATIONS Introduction Welcome to the Specific System and Equipment Considerations lesson. Engineers need to understand the impact of chemistry controls on long term equipment reliability and plant dose rates. A lack of chemistry controls or understanding of chemistry processes can have an adverse impact on over-all system performance and equipment reliability, forcing longer outages or an increased number of forced outages. When you have finished this lesson, you will be able to: Understand the impact and relationship of chemistry controls on system performance

97 Identify specific system functions and the associated relationships with chemistry controls Identify the factors that impact system performance related to chemistry controls

98 Primary Chemistry Controls Primary chemistry controls address three priorities: 1. Ensuring primary system pressure boundary integrity 2. Ensuring fuel-cladding integrity and achievement of design fuel performance 3. Minimizing out-of-core radiation fields Often, and in order to achieve optimized programs, the chemist is required to weigh the advantages and disadvantages of various programs. For example, in PWRs, operation with higher ph may have benefits related to fuel deposits, but this must be balanced with material issues. For BWRs, operation with elevated reactor water zinc for shutdown dose control must be balanced with fuel performance concerns Primary chemistry control programs are documented in the plant's Primary Strategic Water Chemistry Plan, which is reviewed and approved by Senior Plant Management. 6-3

99 Primary Systems: Chemical Volume and Control System The main function of the Chemical Volume and Control System (CVCS) related to chemistry control is to control the reactor coolant system water chemistry conditions, activity level, soluble chemical neutron absorber concentration (boron), and water inventory. The CVCS provide the plant with the ability to maintain and balance system ph in PWRs with long term primary chemistry goals and objectives, including maintaining primary system ph 6.9 during power operation and ascension. Chemistry is controlled by passing the flow through demineralizers that remove ionic impurities and through a filter that removes solids. A makeup tank provides: A location for the addition of makeup water A means and location for changing boron and dissolved gas concentration Nitrogen or hydrogen gas can be admitted to the tank gas space or vented to the radioactive waste gas system in order to maintain purity and allow for reactor coolant system degasification.

100 Primary Systems: Maintaining ph For PWRs, primary system ph is maintained 6.9 during operation to minimize corrosion release rates and deposition. Three particular areas of concern include corrosion of structural (non-fuel) materials, corrosion product transport, and corrosion product deposition on core surfaces, which inhibit nuclear heat transfer. For BWRs, the ph is not controlled as the plant is operated in the neutral ph range. System ph is maintained by adjusting lithium concentration based on boron concentration. These programs can be defined as modified, coordinated, elevated lithium, and coordinated chemistry at elevated ph, where ph T is maintained as close to constant and as high as practical, throughout the cycle, as pictured in the graphic. 6-5

101 PWR Primary Systems: Hydrogen Controls Operation Hydrogen is added to the RCS to maintain reducing conditions. This addition minimizes general corrosion and risks of stress corrosion cracking (SCC) by adjusting volume control tank or makeup tank gas space overpressure in order to maintain RCS hydrogen between 25 and 50 cc/kg. Research data shows that increasing operating bulk coolant hydrogen towards the upper end of the normal operating band (50 cc/kg at STP) results in an improvement in PWSCC mitigation without adverse conditions on other plant materials and systems. Research groups and EPRI continue to evaluate and understand the role of bulk coolant hydrogen on PWSCC crack growth rates (CGRs) in an effort to minimize PWSCC. These data demonstrate a correlation between the coolant hydrogen relative to the nickel-nickel oxide transition. However, considerations for operating with elevated (>50 cm 3 /Kg at STP) or reduced (<25 cm 3 /Kg at STP) RCS hydrogen to mitigate PWSCC must be tempered against collateral issues including fuel impact, corrosion product impact, and system limitations. Shutdown In order to de-gas the reactor coolant system for shutdown maintenance, the gas spaces are alternately charged with nitrogen and vented to the gaseous waste system. The letdown water will exchange its dissolved hydrogen for nitrogen over time, reducing the reactor coolant system hydrogen gas concentration. During degasification, volume control tank pressure is maintained as low as possible (but above 10 psig for seal concerns) in order to ensure the maximum reduction in hydrogen concentration. Hydrogen is removed because of flammability concerns when opening up the system.

102 BWR Primary Systems: Hydrogen Controls During operation, Hydrogen is added to the RCS via the feedwater system to maintain reducing conditions. This addition minimizes general corrosion and risks of stress corrosion cracking (SCC). In BWRs, hydrogen water chemistry (HWC) can be implemented in two ways. Method 1 Hydrogen is injected into the feedwater to achieve an ECP of <-230 mv (SHE) of reactor internals and piping, usually requiring feedwater concentrations between 1.0 and 2.0 ppm. Method 2 The second method of reducing ECP in BWRs involves HWC with Noble Metal Chemical Application (NMCA). This method combines reduced hydrogen injection concentrations (typically <0.4 ppm) with noble metal treated surfaces on the reactor and piping systems. The noble metal provides catalytic surfaces for the recombination reactions of hydrogen and oxidants at a reduced hydrogen injection rate. Hydrogen injection in BWRs is practiced continuously throughout the cycle with availability rates generally greater than 90%. 6-7

103 Primary Systems: Primary Zinc Injection PWRs Zinc injection has been used in PWRs to mitigate the occurrence of PWSCC of Alloy 600 and to reduce plant dose rates. Zinc is injected into the primary systems via letdown or charging systems to reduce plant dose rates and mitigate PWSCC. Zinc is incorporated into the oxide films of wetted surfaces in an operating PWR, changing the morphology and composition of oxide films, thereby changing the corrosion characteristics. BWRs Zinc injection is used in BWRs to reduce radiation exposure and shutdown dose rates. Zinc is injected through the feedwater system. The mechanism of the zinc ion effect is two-fold. 1. Measurements have shown that the zinc ion promotes the formation of a more protective corrosion film on stainless steel, especially when reducing conditions are present. 2. Both cobalt and zinc will incorporate into fuel deposits where activation takes place. When the zinc ion concentration exceeds the cobalt concentration, fuel deposits will contain (and release) lower concentrations of activated corrosion products, such as Co-60. Consequently, the reactor water will have reduced Co- 60 concentrations.

104 PWR Secondary Systems: Steam Generators Steam generator corrosion is affected by temperature, stress, materials of construction, and water chemistry factors. Water chemistry factors are primarily associated with ph, electrochemical potential (ECP), and specific species that can accelerate corrosion of materials. Intergranular attack and stress corrosion cracking (IGA/SCC) are strongly affected by localized ph and increasing ECP, or indications of oxidizing conditions. Independent of ph and ECP, some chemical species can interfere with surface oxides and accelerate corrosion of tube surfaces. These species can be in the form of reduced sulfur and lead, which both appear to interfere with formation of the protective oxide films. Steam generator and secondary chemistry control is often referred to as "all volatile chemistry control," as the amines and hydrazine that vaporize in the steam generators are carried with the steam to the balance of the secondary systems. This increases the ph of the steam, allowing the condensate/feedwater systems, the steam generators, and the steam system to be treated with the same chemicals. Because of the boiling in the steam generator, nonvolatile species like sodium or chloride can concentrate up to high enough concentrations to form very caustic or acidic localized conditions. 6-9