Here, we summarize the criteria for materials selection for different nuclear components. Let us take the example of fuel cladding material for the LWR. As noted before, cladding materials are used to encapsulate the fuel and separate it from the coolant. The requirements for fuel cladding material are as follows: (a) low cross section for absorption of thermal neutrons, (b) higher melting point, adequate strength and ductility, (c) adequate thermal conductivity, (d) compati­bility with fuel, and (e) corrosion resistance to water. Following the first factor, we have discussed in Section 1.7 how different metals have different cross sec­tions for absorption of thermal neutrons. Although Be, Mg, and Al all have lower cross sections for absorption of thermal neutrons, other nonnuclear fac­tors become the impediment for their use in commercial power reactors. Even though Be has a high melting point (1278 °C), it is scarce, expensive, difficult to fabricate, and toxic. Mg has a low melting point (650 °C), is not strong at higher temperatures, and has poor resistance to hot water corrosion. Al has a low melt­ing point (660 °C) and poor high-temperature strength. Even though an Al — based alloy has been used as fuel cladding materials in reactors like ATR, and in the past a magnesium-based alloy was used in Magnox reactors, their use remains very limited. This leaves zirconium-based materials as the mainstay of fuel cladding materials for LWRs. Zirconium has various favorable features: (a) relatively abundant, (b) not prohibitively expensive, (c) good corrosion resist­ance, (d) reasonable high-temperature strength, and (e) good fabricability. Some of the properties could be further improved through appropriate alloying. More detailed discussion on the development of zirconium alloys is included in Appendix 1.A at the end of the chapter.

(Example:

The world’s first nuclear power plant was EBR-1.

It carried a coolant, an alloy of sodium (Na) and potassium (K), called Na—K (“nack”).

The following are the coolant characteristics:

• Stays liquid over a wide range of temperatures without boiling away.

• Transfers heat very efficiently taking heat away from the reactor core and keeping it cool.

• Allows neutrons from the reactor core to collide with U-238 in the breeding blan­ket and produce more fuels.)

1.9.3.5 Shielding Materials

1.10