Nuclear Energy ECEG-4405

Size: px

Start display at page:

Download "Nuclear Energy ECEG-4405"

Derek Knight
6 years ago
Views:

1 Nuclear Energy ECEG-4405

2 Today s Discussion Technical History and Developments Atom Nuclear Energy concepts and Terms Features Fission Critical Mass Uranium Fission Nuclear Fusion and Fission Fusion Fission Nuclear Reactor 2

3 Technical History and Developments Developments Prior to and During WW : discovery of radioactivity 1911: discovery of the nuclear atom (Ernest Rutherford). 1911: Rutherford noted the enormous amount of energy associated with nuclear reactions compared to chemical reactions. 1932: discovery of neutron (James Chadwick ). 1938: discovery of nuclear fission. 1939: researchers recognized that enough neutrons were released during fission reactions to sustain a chain reaction (in a pile of uranium and graphite). A chain reaction requires the release of two neutrons (or more) for every neutron used to cause the reaction. 3

4 Technical history (Cont d) 1942 (Dec. 2): demonstration of the first operating nuclear reactor (200 Watts) 1943 (Nov.): 1 MW reactor put into operation at Oak Ridge, Tennessee 1944 (Sept.): 200 MW reactor put into operation at Hanford, Washington for the production of plutonium. This reactor was built in only 15 months 1944 (Sept.): nuclear reactor for electricity generation proposed, using water for both cooling and neutron moderation. Essentially, this is the birth of nuclear energy for civilian use 7

5 Developments after Ww-2 Technical history (Cont d) 1946: AEC (Atomic Energy Commission) established to oversee both military and civilian nuclear energy 1953: Putman report/book, a thoughtful analysis of the case for nuclear energy for electricity production 1953: US Navy began tests of the PWR (pressurized water reactor) 1957: 60 MW reactor at Shipping-port, PA began to generate electricity for commercial use. The plant was built by the AEC, though Navy leadership played a predominant role : exploratory period: 14 reactors built, of many different designs, all but 3 under 100 MW size. 8

6 : only 5 reactors built Technical history (Cont d) : main period of ordering of nuclear reactors in the US. Size was much larger, many reactors of 600 to 1200 MW size 1974: honeymoon over nuclear energy no longer highly valued by the public : fall off in orders, with no US orders after : cancellation of orders, over half of orders were canceled, or construction never brought to completion Most reactors ordered prior to 1970 were built and brought on line Many reactors ordered after 1970 never came on line they were canceled 9

7 Technical history (Cont d) : most of US s reactors brought on line for commercial operation, indicating that most US reactors are 7 to 27 years old, or have 13 to 33 years of operation left, assuming a 40 year operating life 1979: Three Mile Island accident. Reactor shut down 1986: Chernobyl accident Early 1990s: 7 nuclear reactors shut down, including 3 of early design and 4 of marginal performance. These shutdowns do not necessarily mean that a steady stream of reactors will be shut down before their nominal life of 40 years is reached 1990s: Shoreham (Long Island) reactor shut down for good by public protest 10

8 The Atom An atom a relatively heavy (positively charged) nucleus and a number of much lighter (negatively charged) electrons in various orbits around the nucleus. The nucleus consists of sub particles, called nucleons. Nucleons neutrons (electrically neutral), and proton (positively charged) The atom as a whole is electrically neutral One atom may be transformed into another by losing or acquiring some of the above sub particles resulting in a change in mass A m and release (or absorb) large quantities of energy E 11

9 The Atom (Cont d) According to Einstein s law: c the speed of light in vacuum and g c. conversion factor applies to all, physical, chemical, or nuclear processes in which energy is released or absorbed If it is associated with changes in the atomic nucleus, it is classified as nuclear 12

10 Features Nuclear Energy Concepts & Terms 1. Heat energy source is fission of radioactive material, (U-235) 2. Two typical plant designs: a. Pressurized water reactor (PWR) (U.S.) b. Boiling water reactor (BWR) (Russian) 3. Fuel pellets are in a large number of tubes (fuel rods) 4. Water circulates through core 5. Water converted to steam drives turbine 6. Turbine turns generator electricity 13

11 Fission Nuclear Energy Concepts (Cont d) Unstable (radioactive) elements spontaneously split (radioactive decay), emitting high energy particles. Collision of particles with other atomic nuclei can trigger further nuclear decompositions Einstein equation: E = mc 2 conversion of mass to energy E = energy, m = mass converted, c = speed of light 15

12 Critical Mass Nuclear Energy Concepts (Cont d) There is threshold mass of a radioactive isotope at which the flux density of radioactive particles will sustain a chain reaction: Controlled nuclear reactor, can be basis of an electric power plant Uncontrolled result is an atomic bomb explosion Some of the radiations Homework: Read about each! 16

13 Nuclear Energy Concepts (Cont d) Uranium fission U-235 fission in the presence of U-238 causes the conversion of part of the U-238 into Plutonium-239 which can be concentrated to make an H-Bomb Half life, T Time for half the atomic nuclei to spontaneously split The amount decays exponentially N = N o exp ( t/t) N = Amount of radioactive material, No = Initial amount, t = Elapsed time ZX A The number of protons in the nucleus is called the atomic number Z. The total number of nucleons in the nucleus is called the mass number A. 17

14 Nuclear Fusion and Fission Nuclear reactions of importance in energy production are fusion, fission, and radioactivity Fission a heavy nucleus is split into two or more lighter nuclei Fusion two or more light nuclei fuse to form a heavier nucleus. In both, there is a decrease in mass resulting in exothermic energy 18

15 Fusion Energy is produced in the sun and stars by continuous fusion reactions The heat produced in these reactions maintains temperatures of the order of several million degrees in their cores and serves to trigger and sustain succeeding reactions. On earth, although fission preceded fusion, the basic fusion reaction was discovered first, in the 1920s Fusion reactions are called thermonuclear because very high temperatures are required to trigger and sustain them n neutron p proton D deuterium T tritium 19

16 Fusion (Cont d) The reaction that is, by far, the easiest to ignite is: To estimate the energy released, the mass lost is calculated The mass of ion is: = kg Note: mass of electron is subtracted from mass of tritium atom. The mass of the deuterium ion is: = kg, so that the mass of the left side equation is kg Right hand side: mass of helium ion & neutron is kg, deficit of kg When multiplied by c 2 yields an energy of joules per deuterium/tritium pair The reaction yields 337 TJ per kg of tritium/deuterium alloy or 562 TJ per kg of tritium. 20

17 Fission Fission can be caused by the neutron (electrically neutral), which can strike and fission the positively charged nucleus at high, moderate, or low speeds without being repulsed and keep the reaction going When Otto Hahn, demonstrated uranium fission in 1939, it became immediately obvious that a sustained chain reaction might be achievable all that was needed was to cause one of the emitted neutrons to split a new uranium atom 21

18 Fission (Cont d) There are at least four fissile elements of practical importance: 233 U, 235 U, 239 Pu, and 241 Pu Of these, only 235 U is found in nature in usable quantities 233 U, 239 Pu, and 241 Pu must be created by transmutation of fertile materials, respectively, 232 Th, 238 U and 240 Pu Western world has kg reserves of uranium oxide (U 3 O 8 ) But, only kg are fissile, corresponding to an available energy of 2600 EJ(Compare this with the 40,000 EJ of available coal energy). 22

19 Fission (Cont d) Nuclear fission reaction (with a corresponding release of energy) occurs when a fissile material interacts with neutrons. Consider 235 U: The resulting 236 U decays with the emission of alpha particles (lifetime 7.5 seconds) More importantly, the uranium also suffers spontaneous fission, i.e., absorbs a neutron, and splits into smaller nuclei releasing, on average, 2.5 neutrons and about j of energy: Per kg of the energy released is: 23

20 Fission (Cont d) However, the situation is somewhat more complicated than suggested by the equation above because more energy and additional neutrons are produced by the radioactive decay of the fission products, which are called delayed neutrons. Note that using natural uranium, the so called chain reaction is difficult because of the small percentage of the fissile 235 U. The emitted neutrons have a much greater probability of being absorbed by the abundant 238 U the reaction simply dies out The solution is to enrich the uranium by increasing the percentage of 235 U. This is a complicated and expensive process If the enrichment is carried out far enough, a nuclear bomb can be built 24

21 Nuclear Reactor The Parts 1. Nuclear Fuel 2. Moderator 3. Control Rods 4. Reflector 5. Reactors Vessel 6. Biological Shielding 7. Coolant 25

22 Nuclear Fuel Nuclear Re (Cont d) should be fissionable material to undergo nuclear fission by nuclear bombardment and to produce a fission chain reaction can be one or all of the following U 233, U 235 and Pu 239 Natural uranium found in earth crust contains three isotopes namely U 234 (Trace), U 235 (0.7% and most usable) and U 238 (99.3%) The U 235 is most unstable and is capable of sustaining chain reaction and has been given the name as primary fuel. U 233 arid Pu 239 are artificially produced from Th 232 and U 238 respectively and are called secondary fuel. Homework: Read more! 26

23 Moderator Nuclear Re (Cont d) In the chain reaction the produced neutrons move fast becoming far less effective in causing the fission of the U 235 and trying to escape from the reactor. To improve the utilization of these neutrons their speed is reduced by colliding them with the nuclei of other material called moderator, which is lighter, does not capture the neutrons but scatters them Graphite, heavy water and beryllium are generally used as moderator Reactors using enriched uranium do not require moderator (But costly) Homework: Read more! 27

24 Control Rods Nuclear Re (Cont d) The furnace of the reactor is fed continuously and the heat energy is controlled by regulating the fuel feed and the combustion air Fuel consumption and power level of the reactor depends upon the neutron flux in the reactor core Energy produced during chain reaction is so much that if it is not controlled the entire core and the surrounding structure may melt and radioactive fission products may come out of the reactor The control rods (cylindrical or sheet form) are of boron or cadmium make and are moved in and out in the reactor core absorbing neutrons and damping down when in and absorbing less neutrons when out The shifting of the rods may be done manually or automatically 28

25 Nuclear Re (Cont d) Reflector The neutrons produced during the fission process will be partly absorbed by the fuel rods, moderator, coolant or structural material etc. Those left unabsorbed will try to leave the reactor core never to return to it and will be lost To minimize such losses the surroundings of the reactor core is made of a material called reflector which will send the neutrons back into the core to cause more fission reflector is made up of graphite and beryllium Reactors Vessel It is a strong walled container housing the cure of the power reactor containing moderator, reflector, thermal shielding and control rods. 29

26 Biological Shielding Nuclear Re (Cont d) Shielding the radioactive zones from possible radiation hazard is essential to protect the operating men from the harmful effects During fission, alpha and beta particles, deadly gamma rays and neutrons are produced Thick layers of metals or plastics are sufficient to stop the alpha and beta particles while thick layers of lead or concrete are provided around the reactor for stopping the gamma rays Coolant Coolant flows through and around the reactor core The coolant either transfers its heat to another medium or if it is water it gets converted into steam and is directly sent to the turbine Homework: Read more about coolants and the cycles! 30

27 31

Nuclear Physics and Nuclear Reactions

Slide 1 / 33 Nuclear Physics and Nuclear Reactions The Nucleus Slide 2 / 33 Proton: The charge on a proton is +1.6x10-19 C. The mass of a proton is 1.6726x10-27 kg. Neutron: The neutron is neutral. The