Role of Materials to Advanced Nuclear Energy

Abstract

Nuclear energy holds the promise to provide vast amounts of reliable baseline electricity at commercially competitive costs with modest environmental impact. However, the future of nuclear energy lies beyond the current generation of light-water reactors. Future reactors will be expected to provide additional improvements in safety, maintain high reliability, use uranium resources more efficiently, and produce lower volumes of less toxic solid wastes. Several advanced reactor concepts are under development to meet these demands. In most cases, these designs translate into higher operating temperatures and longer lifetimes, more corrosive environments, and higher radiation fields in which materials must reliably perform. The safe and economical operation of any nuclear power system relies to a great extent on the success of the fuel and the materials of construction. Materials used for fission and fusion-based nuclear engineering mainly include fuels, materials for fuel cladding, moderators and control rods, first-wall materials, materials for pressure vessels and heat exchangers. During the lifetime of a nuclear power system, the materials are subject to high temperature, corrosive environment, and damage from high-energy particles released during fission. The fuel which provides the power for the reactor has a much shorter life but is subject to the same types of harsh environments. This chapter will review and update nuclear energy reactors and the materials challenges that will determine the feasibility of these advanced concepts and define the long-term future of nuclear power.

Exercises

10.1.1 Part I: General Questions

• 10.1. Write the balanced nuclear equation for each of the following:

  • (a) α decay of gold-185.

  • (b) β- decay of actinium-228.

  • (c) proton emission of cobalt-56.

  • (d) β + decay of holmium-158.

• 10.2. The half-life of ¹⁴C is 5730 years.

  • (a) How old is a wooden bowl whose ¹⁴C activity is one-fourth of the activity of a contemporary piece of wood?

  • (b) At Stonehenge, a charcoal sample is dug up, presumably the remains of a fire; its ¹⁴C activity is 9.65 disintegrations per minute per gram of carbon. Living tissues have a ¹⁴C activity of 15.3 disintegrations per minute per gram of carbon. When were the fires of Stonehenge ignited?

• 10.3. Potassium-40 is often used to date minerals. How old is the rock if two-fifths of the original K-40 exists in it?

• 10.4. Describe three major environmental and security problems associated with nuclear power.

• 10.5. Plutonium is very damaging when inhaled as small particles because of the ionization of tissue by its emitted alpha particles. One microgram of plutonium is known to produce cancer in experimental animals. From its atomic weight (239) and half-life (24,360 years), calculate how many alpha particles are emitted by a microgram of plutonium over the course of a year? About how many ionizations do the particles produce?

• 10.6. A neutron generated by fission typically possesses a kinetic energy of 2 MeV. When such a neutron collides with a hydrogen atom 18 times, its energy is reduced to its thermal energy of 0.025 eV, that is, the kinetic energy it would possess by virtue of the temperature of its surroundings. The same neutron would have to collide with a sodium atom more than 200 times to reduce its energy by the same amount. Explain how these characteristics make water a suitable coolant in the pressurized light-water reactor that utilizes U-235 as the fuel, whereas sodium is the suitable coolant in the breeder reactor.

• 10.7. Why are the spent fuel rods from fission reactors more radioactive than the initial fuel rods?

• 10.8. How does a breeder reactor extend the supply of nuclear fuel? What are the problems associated with the breeder design?

• 10.9. From the standpoint of weapons proliferation, why is it more dangerous to fuel reactors with plutonium than with uranium in which U-235 is enriched to 2–3%?

• 10.10. Considering the ²³⁸U decay scheme, which of the daughters of uranium are likely to be most abundant in uranium-bearing soil, and why?

• 10.11. Any large-scale nuclear process produces radioactive waste. Compare the problems of waste from uranium fission reactors and from tritium fusion reactors.

• 10.12. What is the percentage of ²³⁵U in naturally occurring uranium and what is the rest made of? A nuclear fission reaction of an ²³⁵U atom caused by a neutron produces one barium atom, one Krypton atom, and three more neutrons. Evaluate approximately how much energy is liberated by this reaction.

• 10.13. Approximately how much percentage of energy is carried by the fission fragments (no calculation necessary for the last part of the question)?

• 10.14. What is the difference between fissile and fertile isotopes? Give two examples of each. What is the role of fertile isotopes in a breeder reactor?

• 10.15. Define a nuclear reactor? What is the basic difference between an atomic bomb and a power-producing reactor?

• 10.16. What are the prime differences between LWR and CANDU reactors (comment mostly on materials aspects)?

• 10.17. Describe the importance of control materials with respect to reactor safety and control. What are the primary requirements for a control material? Give at least four examples of control materials.

• 10.18. Categorize neutrons based on their kinetic energy. What is the major difference between a thermal reactor and a fast reactor?

• 10.19. Zirconium and hafnium both have crystal structures (HCP) in the general operating regimes of LWRs. Naturally occurring Zr always has some Hf (1–3 wt.%) in it. Why are Hf-containing Zr alloys very common in chemical industries but not in nuclear industries? What is the main application of Zr alloys in LWRs? What are the various functions of this reactor component? What are the reasons that make Zr alloys suitable for such use?

• 10.20. What are the two main zirconium alloys used in light-water reactors? Give their compositions. Name two recently developed zirconium alloys with their compositions.

• 10.21. What is neutron economy? What significance does it have? How much influence does it exert in the selection of materials used in nuclear reactors?

• 10.22. Define neutron flux and neutron fluence. What are their units?

• 10.23. Define neutron cross section? Briefly comment on the importance of neutron cross section from a reactor perspective.

• 10.24. Neutrons of 10 keV energy are incident on a light-water barrier. The neutron cross section for hydrogen (protium) at 10 keV is about 20 b and that of oxygen is only 3.7 b. Determine the half-value thickness of neutron attenuation for the water barrier (assume that neutron interaction with oxygen in water molecule is negligible). Find out the half-thickness value for 1 MeV neutrons traveling through the water barrier (neutron cross section for protium is 4.1 b for 1 MeV neutrons). Comment on the significance of the results.

10.1.2 Part II: Thought-Provoking Questions

• 10.25. Address characteristics of advanced nuclear energy. For new Generation IV systems or the extension of current technology, explain why should we be aware of the possibility of new phenomena due to irradiation, corrosion, or aging in both materials and fuels performance.

• 10.26. List the major differences of fission and fusion reactors, and address their advantages and disadvantages.

• 10.27. Address materials selection criteria for nuclear power applications, and give some samples to explain.

• 10.28. Describe materials selected for advanced nuclear fission reactor components. What are the current status and future trends?

• 10.29. Explain major factors to cause materials degradation in reactor cores.

• 10.30. What are the major considerations for materials selection in fusion reactors?

• 10.31. Explain the major steps of nuclear fuel cycles.

• 10.32. Discuss the potential applications of low-energy nuclear reactions in condensed matter.