Chapter 32 Applied Nucleonics

Transcription

1 Chapter 32 Applied Nucleonics GOALS When you have mastered the content of this chapter, you will be able to achieve the following goals: Definitions Define each of the following terms, and use each term in an operational definition: fission thermonuclear reaction chain reaction plasma critical mass "magnetic bottle" moderator tracer breeder reactor biological half-life neutron activation effective half-life fusion Fission Reactors Explain the operation of a nuclear fission reactor. Radiation Detectors Compare different nuclear radiation detectors in their uses. Radiation Problems Solve problems involving radiation application using biological half-life and dosage data. Tracer Applications Outline the use of radioactive tracers in medical applications. PREREQUISITES Before beginning this chapter you should have achieved the goals of Chapter 21, Electrical Properties of Matter, Chapter 27, Quantum and Relativistic Physics, Chapter 30. X Rays, and Chapter 31, Nuclear Physics. 258

2 Chapter 32 Applied Nucleonics OVERVIEW Our modern society places an ever-increasing demand on energy acquisition. During the 1980's, nuclear energy from the processes of both fission and fusion will surely provide additional energy for your use. The uses of nuclear energy will no doubt continue to be important; i.e., medical and tracer applications. In this chapter you will read about the conversion of nuclear energy into electrical energy and some of the problems which complicate the process. In addition, the problem of containing new and used nuclear materials will be presented. SUGGESTED STUDY PROCEDURE When you begin to study this final chapter, you should be familiar with the following chapter Goals: Definitions, Fission Reactors, Radiation Problems and Tracer Application. A further discussion of the terms listed under Definitions can be found in the Definitions section of this Study Guide chapter. Next, read chapter sections and In addition to the important energy issue, be sure to note the contribution nuclear energy has made in other areas like medicine. Tables 32.1 and 32.2 will highlight some specific uses of radioactive materials. Now, turn to the end of the chapter and read the Chapter Summary and complete Summary Exercises 1-7 and 10. Next, do Algorithmic Problem 1 and Exercises and Problems 1, 2, 7, and 8. For more work with the concepts of this chapter, now turn to the Examples section of this Study Guide chapter. Now you should be prepared to attempt the final Practice Test provided in this Study Guide. If you have difficulties with any part of the test, you should refer to the appropriate text or section for assistance. This study procedure is outlined below Chapter Goals Suggested Summary Algorithmic Exercises Text Readings Exercises Problems & Problems Definitions 32.1,32.3,32.4 1,2,3,4, 5,6 Fission Reactors Radiation ,7,8 Problems Tracer Applications Radiation 32.6,32.7, Detectors 32.9,

3 DEFINITIONS FISSION - The dividing up of heavy nuclei to obtain energy. This process, which creates heavy radioactive waste products, is the basic process in all nuclear power generation. CHAIN REACTION - The fission reaction sustained by neutrons from preceding nuclear fission reactions. The basis of nuclear fission reactions and bombs. In some fission processes excess neutrons are emitted which can be used to trigger the fission of nearby nuclei, if there are enough nearby nuclei. CRITICAL MASS - The necessary mass of fissionable material to sustain a chain reaction. Enough nearby nuclei to continue a fission process is called a critical mass. MODERATOR - The material used in fission reactors to slow down fast neutrons so that a controlled chain reaction can be maintained. BREEDER REACTOR - A nuclear fission reactor that produces additional fissionable fuel material in addition to usable nuclear energy. At this time breeder reactors are still not widely used in the generation of electrical energy. NEUTRON ACTIVATION - The process resulting in induced radioactivity of samples subjected to neutron bombardment. Neutron activation analysis has lead to the solution of some spectacular crimes, such as in the sale of fraudulent paintings. FUSION - Combining light nuclei to obtain energy. This appears to be the fundamental natural source of all the energy from the sun and stars. THERMONUCLEAR REACTION - The name given to all fusion reactions and the energy source of stars. The fusion process is one of intense heat, more than 100,000,000 degrees celsius. PLASMA - The state of matter consisting of completely ionized atoms and electrons as characterized by fusion reactions. Plasma physics is now a widely recognized research specialty. Plasma physicists have made use of very high power pulsed lasers beams as energy sources. MAGNETIC BOTTLE - The magnetic field confinement system used to contain plasma in fusion reactors. Have you ever thought of having a container for particles with no "walls"? This is a spectacular example of interaction-at-a-distance. 260

4 TRACERS - Radioactive tracers are samples that contain radioactive isotopes of biochemically active materials. Their activity can be traced in a biological system. (See Table 32.1) BIOLOGICAL HALF-LIFE - The time required for one-half the biochemically active sample to be eliminated by natural processes. Since about 20% of the air in your lungs is expelled in each breath the biological half-life of the stale air in you lungs is about the time required for you to take three breaths, (4/5) 3 1/2. EFFECTIVE HALF-LIFE - The resultant of radioactive and biological half-life. The magnitude of the effective half-life of a tracer is always less than the smaller of its biological or its radioactive half-life. ANSWERS TO QUESTIONS FOUND IN THE TEXT SECTION 32.1 Introduction As the fossil fuel supply continues to be used at an ever increasing rate the use of nuclear fuels becomes more and more common. What are the fundamental processes of nuclear fuels? What are the benefits? the risks? These are questions that all of us consider. We will have to decide on the energy sources of the future. SECTION 32.2 Nuclear Fission Since plutonium-239 only has a half-life of 24,000 years and the older rocks on the surface of the earth date back at least a million years, that's almost 42 halflives of plutonium-239, or one part in 3 1/2 trillion parts of any original plutonium-239 would still be around. It seems rather unlikely we would find any natural occurring plutonium-239. EXAMPLES RADIATION PROBLEMS 1. The isotope P passes through the liver with a half-life of 18 days. The effective half-life of P in the liver is 8.0 days. What is the radioactive half-life of P? What data are given? T b = 18 days; T = 8.0 days What physics principles are involved? The biological and radioactive half-lifes of an isotope may be combined to yield an effective half-life. What equation is to be used? 1/T = 1/T b + 1/T r (32.1) Solution We need to solve Equation 32.1 for T r 1/8.0 days = 1/18 days + 1/T r T r = 14.4 days Thinking about the answer Notice that the effective half life is always less than even the biological or the radioactive half-life. 261

5 2. Assume that a man has incorporated within the bones of his body 10µCi of the alpha emitter, 276 Ra which has a radioactive half-life of 1622 years. The energy of the primary alpha particle is 4.79 MeV. Assume a uniform distribution of the radium in 7.0 kg of bone. What is the total dose per day from the alpha particles of 226 Ra? What data are given? Activity = 10µCi = 10 x 3.70 x 10 4 dis/sec. = 3.7 x10 5 dis/sec Energy = 4.79 MeV / alpha = 4.79 MeV/dis. Mass = 7.0 kg = 7000 gm. What data are implied? The dose unit is the rad which is equivalent to the absorption of 10-5 J/gm or 6.25 x ev/gm = 6.25 x 10 7 MeV/gm. What physics principles are involved? All of the energy of the alpha particles will be absorbed by the bone. We only need to compute the total alpha particle kinetic energy and divide by the bone mass. What equations are to be used? Dose = Energy Absorbed / Unit Mass in rad Solution Dose = (4.79 MeV / alpha) x (3.70 x 10 5 alphas/sec.) = 1.77 x 10 6 MeV/sec Daily Dose = 1.77 x 10 6 MeV/sec x 60 sec/min x 60 min/hr x 24 hr/day = 1.53x10 11 MeV/day To convert the daily dose of 1.53 x MeV/day radiation unit of rads, we need to divide it by the mass of the absorber Daily Dose / Mass = (1.53 x MeV) / 7000 = 2.19 x 10 7 MeV/gm But 1 rad = 6.24 x 10 7 MeV/gm Daily Dose in rads = (2.19 x 10 7 MeV/gm) x (1rad / (6.25 x 10 7 MeV/gm)) Daily Dose = rads Thinking about the answer Such a person will receive an annual dose of about 125 rads. Recall that the relative biological effectiveness (RBE) of 5 MeV alpha particles is 20; then this person will get a radiation dose of 2500 rem per year. Look at Table 30.1 and you will see that this is much larger than typical background radiation levels. 262

6 PRACTICE TEST 1. A nuclear fission reactor produces electrical energy from release nuclear energy. a) Make a block diagram showing the three main parts of the fission reactor generating station needed to convert the nuclear energy into electrical energy. b) Inside the reactor, uranium fuel divides (fissions) and forms a chain reaction with other atoms. How does this reaction remain "self-sustaining? How is the reaction controlled? 2. The effective half-life of K 42 used for radioactive muscle tracing is.5 days. The radioactive half-life is also.5 days. Find the biological half-life of K A doctor wishes to study a defective thyroid by using a radioactive tracer. Please name the isotope which would be used and outline the procedure explaining the important steps in making this analysis. ANSWERS: 1. Each fission produces two slow neutrons. If the fuel density is large enough to make it highly probable that these two neutrons will hit and "split" two additional atoms, each of these will in turn produce 2 slow neutrons, etc. 2. Infinite I After an injection of 131 I, the thyroid gland is monitored with a geiger counter. The rate of increased activity is related to the activity of the gland. 263