Material problems in a nuclear reactor plant can be grouped into at least two categories, one concerning the nuclear reactor core and one that will apply to all plant materials. This chapter discusses specific material problems associated with the reactor that include pellet-cladding interaction, fuel densification, fuelcladding embrittlement, and effects on fuel due to inclusion and core burnup.

EO 1.12 STATE nuclear reactor core problems and causes associated with the following:

a. Pellet-cladding interaction

b. Fuel densification

c. Fuel cladding embrittlement

d. Fuel burnup and fission product swelling

EO 1.13 STATE measures taken to counteract or minimize the effects of the following:

a. Pellet-cladding interaction

b. Fuel densification

c. Fuel cladding embrittlement

d. Fission product swelling of fuel elements

Fuel Pellet-Cladding Interaction

Fuel pellet-cladding interaction (PCI) may lead to cladding failure and subsequent release of fission products into the reactor coolant. PCI appears to be a complex phenomenon that tends to occur under power ramping conditions. 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. Thermal, chemical, and mechanical interactions may then occur that, if not appropriately accounted for in the design, may lead to cladding failure. Design features to counteract PCI include the following.

a. an increase in the cladding thickness

b. an increase in the cladding-pellet gap, with pressurization to prevent cladding collapse

c. the introduction of a layer of graphite or other lubricant between the fuel and the cladding

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.

Fuel Densification

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 cycle.

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.

a. 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 column

c. 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 cladding failure.