NUCLEAR REACTOR CORE PROBLEMS
Operational limitations such as rate of power increase and power for a given power ramp rate are
imposed to lessen the effect of PCI. PCI appears to be more likely to occur during initial power
increase and can be very costly if cladding failure occurs.
Some uranium dioxide (UO2) fuels have exhibited densification, the reverse of swelling, as a
result of irradiation.
Such behavior can cause the fuel material to contract and lead to
irregularities in the thermal power generation. The changes in fuel pellet dimensions have been
small because the changes are localized in the central region of the pellet and are somewhat
masked by other physical changes that occur at high temperatures during the early part of the fuel
Fuel densification increases the percent of theoretical density of UO2 pellets from a range of 90%
to 95% to a range of 97% to 98%. Densification apparently arises from the elimination of small
pores in the UO2 pellets. As densification takes place, axial and radial shrinkage of the fuel
pellet results and a 3.66 m column of fuel pellets can decrease in length by as much as 7.5 cm
or more. As the column settles, mechanical interaction between the cladding and the pellet may
occur, preventing the settling of the pellet and those above it on the column below. Once the
gap has been produced, outside water pressure can flatten the cladding in the gap region, resulting
in a flux spike. Because the thermal expansion of UO2 is greater than that of zircaloy, and the
thermal response time for the fuel during power change is shorter than that of the cladding, the
pellet temperature changes more quickly than the temperature of the cladding during a power
change. If creep (slow deformation) of the cladding has diminished the gap between the cladding
and the fuel pellets, it is possible for the difference in thermal expansion to cause stresses
exceeding the yield for the cladding material. Because irradiation reduces cladding ductility, the
differential expansion may lead to cladding failure. The process of fuel densification is complete
within 200 hours of reactor operation.
The problems of cladding collapse resulting from fuel densification and cladding creep have
occurred mainly with unpressurized fuel rods in PWRs. To reduce the cladding creep sufficiently
to prevent the formation of fuel column gaps and subsequent tubing collapse, the following
methods have been successful: pressurizing the fuel rods with helium to pressures of 200 psig
to 400 psig; and increasing fuel pellet density by sintering (bonded mass of metal particles
shaped and partially fused by pressure and heating below the melting point) the material in a
manner leading to a higher initial density and a stabilized pore microstructure.
There are three principle effects associated with fuel densification that must be evaluated for
reactors in all modes of operation.
an increase in the linear heat generation rate by an amount directly proportional to the
decrease in pellet length
b. an increased local neutron flux and a local power spike in the axial gaps in the fuel