Nuclear fuels require surface protection to retain fission products and minimize corrosion. Also, pelletized fuel requires a tubular container to hold the pellets in the required physical configuration. The requirements for cladding material to serve these different purposes will vary with the type of reactor; however, some general characteristics can be noted. This chapter will discuss the general characteristics associated with cladding and reflectors.

EO 1.5STATE the four major characteristics necessary in a material used

for fuel cladding.

EO 1.6IDENTIFY the four materials suitable for use as fuel cladding material and their applications.

EO 1.7STATE the purpose of a reflector.

EO 1.8LIST the five essential requirements for reflector material in a thermal reactor.

Cladding

Cladding is used to provide surface protection for retaining fission products and minimizing corrosion. Cladding is also used to contain pelletized fuel to provide the required physical configuration.

Mechanical properties, such as ductility, impact strength, tensile strength, and creep, must be adequate for the operating conditions of the reactor core. Ease of fabrication is also important. It is desirable that ordinary fabrication procedures be applicable in fabricating the desired shape. The cladding must have a high corrosion resistance to its operating environment. It must have a high melting temperature to withstand abnormal operating conditions such as high temperature transients. Thermal conductivity should be high to minimize thermal stresses arising from temperature differences, and the coefficient of expansion should be low or well-matched with that of other materials. The cladding material should not be susceptible to radiation damage.

The nuclear properties of fuel cladding material must also be satisfactory. For thermal reactors, it is important that the material have a reasonably small absorption cross section for neutrons. Only four elements and their alloys have low thermal-neutron absorption cross sections and reasonably high melting points: aluminum, beryllium, magnesium, and zirconium. Of these, aluminum, magnesium, and zirconium are or have been utilized in fuel-element cladding.

Aluminum, such as the 1100 type, which is relatively pure (greater than 99%), has been used in low power, water-cooled research, training, and materials testing reactors in which the operating temperatures are below 100C. Magnesium, in the form of the alloy magnox, serves as cladding for the uranium metal fuel in carbon-dioxide cooled, graphite-moderated power reactors in the United Kingdom. The alloy zircaloy, whose major constituent is zirconium, is widely used as the fuel-rod cladding in water-cooled power reactors. The alloys in common use as cladding material are zircaloy-2 and zircaloy-4, both of which have mechanical properties and corrosion resistance superior to those of zirconium itself. Although beryllium is suitable for use as cladding, it is not used due to its high cost and poor mechanical properties.

The choice of cladding material for fast reactors is less dependent upon the neutron absorption cross section than for thermal reactors. The essential requirements for these materials are high melting point, retention of satisfactory physical and mechanical properties, a low swelling rate when irradiated by large fluences of fast neutrons, and good corrosion resistance, especially to molten sodium. At present, stainless steel is the preferred fuel cladding material for sodium-cooled fast breeder reactors (LMFBRs). For such reactors, the capture cross section is not as important as for thermal neutron reactors.

In 1977 the Carter Administration deferred indefinitely the reprocessing of nuclear fuels from commercial power reactors. This led the electric utility industry to conduct research on high-burnup fuels and programs that would allow an increase in the length of time that the fuel rods remain in the reactors. High integrity and performance of fuel cladding will become even more important as these high-burnup fuel rods are designed and programs for extended burnup of nuclear fuels are placed into operation.

Reflector Materials

A reflector gets its name from the fact that neutrons leaving the reactor core hit the reflector and are returned to the core. The primary consideration for selecting a reflector material is its nuclear properties. The essential requirements for reflector material used in a thermal reactor are:

Low macroscopic absorption (or capture) cross section to minimize loss of neutrons

High macroscopic scattering cross section to minimize the distance between scatters

High logarithmic energy decrement to maximize the energy loss per collision due to low mass number

Temperature stability

Radiation stability

In the case of a fast reactor, neutron thermalization is not desirable, and the reflector will consist of a dense element of high mass number.

Materials that have been used as reflectors include pure water, heavy water (deuterium oxide), beryllium (as metal or oxide), carbon (graphite), and zirconium hydride. The selection of which material to use is based largely on the nuclear considerations given above and the essential neuronic properties of the materials. Most power reactors use water as both the moderator and reflector, as well as the coolant. Graphite has been used extensively as moderator and reflector for thermal reactors. Beryllium is superior to graphite as a moderator and reflector material but, because of its high cost and poor mechanical properties, it has little prospect of being used to any extent. Beryllium has been used in a few instances such as test reactors, but is not used in any power reactors. Reactors using heavy water as the moderator-reflector have the advantage of being able to operate satisfactorily with natural uranium as the fuel material; enriched uranium is then not required. Zirconium hydride serves as the moderator in the Training, Research, Isotopes, General Atomic (TRIGA) reactor. The zirconium hydride is incorporated with enriched uranium metal in the fuel elements.

Summary

The important information in this chapter is summarized below.

Cladding and Reflectors Summary

Major characteristics required for cladding material:

Mechanical properties such as ductility, impact strength, tensile strength, creep, and ease of fabrication

Physical properties include high corrosion resistance and high melting temperature

High thermal conductivity

Nuclear properties such as small absorption cross section

Four materials suitable for cladding:

Aluminum is used for low power, water-cooled research, training, and materials test reactors in which temperatures are below 100C.

Magnesium is used for uranium metal fuel in carbon-dioxide cooled, graphitemoderated power reactors in United Kingdom.

Zirconium is used for fuel-rod cladding in water-cooled power reactors.

Beryllium is suitable for use as cladding but is not used as such due to its high cost and poor mechanical properties. It is, however, used as a reflector in some test reactors.

Reflectors are used to return neutrons leaving the reactor core back to the core.

Essential requirements for reflectors include.

Low macroscopic absorption cross section to minimize loss of neutrons High macroscopic scattering cross section

High logarithmic energy decrement due to low mass number Temperature stability

Radiation stability