Plant MaterialsDOE-HDBK-1017/2-93NUCLEAR REACTOR CORE PROBLEMSEven though fission product swelling is less with oxide fuels, this irradiation-induced volumeincrease has been observed in UO2 and mixed-oxide fuels for a number of years. This swellingof the fuel has generally been attributed to both gaseous fission-product bubble formation and theaccumulation 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 thelifetime of the fuel element by helping to determine the physical and mechanical changesresulting from irradiation and high temperature in the fuel and the cladding. Fuel element lifeor core burnup, which indicates the useful lifetime of the fuel in a reactor, is also determined bythe 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 asa result of the high internal temperatures and the large temperature gradients. Thermal stresseslead to radial cracks and grain structure changes. These structural changes tend to increase withthe specific power and burnup of the fuel. SummaryThe important information in this chapter is summarized below.Nuclear Reactor Core Problems SummaryFuel Pellet-Cladding Interaction (PCI)PCI may lead to cladding failure and subsequent release of fission productsinto the reactor coolant.Expansion of the fuel pellets due to high internal temperatures, cracking dueto thermal stresses, and irradiation-induced swelling may lead to contact ofthe fuel with the cladding. Design features to counteract PCI include:An increase in the cladding thicknessAn increase in the clad-pellet gap, with pressurization to obviatecladding collapseThe introduction of a layer of graphite or other lubricant between thefuel and the claddingOperational limitations to reduce PCIPlant procedures limit the maximum permissible rate at which powermay be increased to lessen the effect of PCI.Rev. 0Page 25MS-05
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business