NUCLEAR REACTOR CORE PROBLEMSDOE-HDBK-1017/2-93Plant MaterialsNUCLEAR REACTOR CORE PROBLEMSMaterial problems in a nuclear reactor plant can be grouped into at least twocategories, one concerning the nuclear reactor core and one that will apply to allplant materials. This chapter discusses specific material problems associated withthe reactor that include pellet-cladding interaction, fuel densification, fuel-cladding embrittlement, and effects on fuel due to inclusion and core burnup. EO 1.12STATE nuclear reactor core problems and causes associated withthe following:a.Pellet-cladding interactionb.Fuel densificationc.Fuel cladding embrittlementd.Fuel burnup and fission product swellingEO 1.13STATE measures taken to counteract or minimize the effects of thefollowing:a.Pellet-cladding interactionb.Fuel densificationc.Fuel cladding embrittlementd.Fission product swelling of fuel elementsFuelPellet-CladdingInteractionFuel pellet-cladding interaction (PCI) may lead to cladding failure and subsequent release offission products into the reactor coolant. PCI appears to be a complex phenomenon that tendsto occur under power ramping conditions. Expansion of the fuel pellets due to high internaltemperatures, cracking due to thermal stresses, and irradiation-induced swelling may lead tocontact of the fuel with the cladding. Thermal, chemical, and mechanical interactions may thenoccur that, if not appropriately accounted for in the design, may lead to cladding failure. Designfeatures to counteract PCI include the following.a.an increase in the cladding thicknessb. an increase in the cladding-pellet gap, with pressurization to prevent cladding collapsec.the introduction of a layer of graphite or other lubricant between the fuel and the claddingMS-05Page 22Rev. 0
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