NUCLEAR REACTOR CORE PROBLEMS
a decrease in the clearance gap heat conductance between the pellets and the cladding.
This decrease in heat transmission capability will increase the energy stored in the fuel
pellet and will cause an increased fuel temperature.
To minimize the effects of fuel densification, plant procedures limit the maximum permissible
rate at which power may be increased to ensure that the temperature will not exceed 1200C
during a loss of coolant accident. This allows the fuel pellets to shift slowly, with less chance
of becoming jammed during the densification process, which in turn reduces the chance of
Fuel Cladding Embrittlement
Corrosion of zircaloy in water results in the release of hydrogen. A portion of the hydrogen
released, ranging from about 5% to 20%, diffuses through the oxide layer and into the metal.
This causes embrittlement of the base metal that can lead to cladding failure. The mechanism
of hydrogen embrittlement is discussed in Module 2, Properties of Metals. The zirconium alloy
zircaloy-2, which has been used extensively as a fuel-rod cladding, is subject to hydrogen
embrittlement, especially in the vicinity of surface defects. The alloy zircaloy-4 is, however, less
susceptible to embrittlement. As with metals in general, irradiation decreases the ductility and
increases the embrittlement of zirconium and the zircaloys. The magnitude of the radiation effect
depends upon the neutron spectrum, fluence, temperature, and microstructure (or texture) of the
material. Different fabrication processes yield products with different textures; therefore, the
radiation embrittlement of zircaloy is dependent on its fabrication history.
Irradiation at high temperatures can lead to brittle fracture of stainless steels used as cladding in
fast liquid metal breeder reactors. The effects of irradiation on metals is discussed in more detail
in a later chapter of this module.
Effects on Fuel Due to Swelling and Core Burnup
One of the requirements of a good fuel is to be resistant to radiation damage that can lead to
dimensional changes (for example, by swelling, cracking, or creep). Early reactors and some
older gas-cooled reactors used unalloyed uranium as the fuel. When unalloyed uranium is
irradiated, dimensional changes occur that present drawbacks to its use as a fuel. The effects are
of two types: 1) dimensional instability without appreciable change in density observed at
temperatures below about 450C, and 2) swelling, accompanied by a decrease in density, which
becomes important above 450C. Other reactors use ceramic fuels, with uranium dioxide being
the most common, have the advantages of high-temperature stability and adequate resistance to
radiation. Uranium dioxide (UO2) has the ability to retain a large proportion of the fission gases,
provided the temperature does not exceed about 1000C. Other oxide fuels have similar