NUCLEAR REACTOR CORE PROBLEMS
Even though fission product swelling is less with oxide fuels, this irradiation-induced volume
increase has been observed in UO2 and mixed-oxide fuels for a number of years. This swelling
of the fuel has generally been attributed to both gaseous fission-product bubble formation and the
accumulation of solid fission products. Swelling can cause excessive pressure on the cladding,
which could lead to fuel element cladding failure. Swelling also becomes a consideration on the
lifetime of the fuel element by helping to determine the physical and mechanical changes
resulting from irradiation and high temperature in the fuel and the cladding. Fuel element life
or core burnup, which indicates the useful lifetime of the fuel in a reactor, is also determined by
the decrease in reactivity due to the decrease in fissile material and the accumulation of fission-
product poisons. Under operating conditions, fuel pellets undergo marked structural changes as
a result of the high internal temperatures and the large temperature gradients. Thermal stresses
lead to radial cracks and grain structure changes. These structural changes tend to increase with
the specific power and burnup of the fuel.
The important information in this chapter is summarized below.
Nuclear Reactor Core Problems Summary
Fuel Pellet-Cladding Interaction (PCI)
PCI may lead to cladding failure and subsequent release of fission products
into the reactor coolant.
Expansion of the fuel pellets due to high internal temperatures, cracking due
to thermal stresses, and irradiation-induced swelling may lead to contact of
the fuel with the cladding.
Design features to counteract PCI include:
An increase in the cladding thickness
An increase in the clad-pellet gap, with pressurization to obviate
The introduction of a layer of graphite or other lubricant between the
fuel and the cladding
Operational limitations to reduce PCI
Plant procedures limit the maximum permissible rate at which power
may be increased to lessen the effect of PCI.