CLADDING AND REFLECTORS
DOE-HDBK-1017/2-93
Plant Materials
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
MS-05
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