NUCLEAR REACTOR CORE PROBLEMSDOE-HDBK-1017/2-93Plant Materialsc.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 fuelpellet and will cause an increased fuel temperature.To minimize the effects of fuel densification, plant procedures limit the maximum permissiblerate at which power may be increased to ensure that the temperature will not exceed 1200Cduring a loss of coolant accident. This allows the fuel pellets to shift slowly, with less chanceof becoming jammed during the densification process, which in turn reduces the chance ofcladding failure.FuelCladdingEmbrittlementCorrosion of zircaloy in water results in the release of hydrogen. A portion of the hydrogenreleased, ranging from about 5% to 20%, diffuses through the oxide layer and into the metal.This causes embrittlement of the base metal that can lead to cladding failure. The mechanismof hydrogen embrittlement is discussed in Module 2, Properties of Metals. The zirconium alloyzircaloy-2, which has been used extensively as a fuel-rod cladding, is subject to hydrogenembrittlement, especially in the vicinity of surface defects. The alloy zircaloy-4 is, however, lesssusceptible to embrittlement. As with metals in general, irradiation decreases the ductility andincreases the embrittlement of zirconium and the zircaloys. The magnitude of the radiation effectdepends upon the neutron spectrum, fluence, temperature, and microstructure (or texture) of thematerial. Different fabrication processes yield products with different textures; therefore, theradiation embrittlement of zircaloy is dependent on its fabrication history.Irradiation at high temperatures can lead to brittle fracture of stainless steels used as cladding infast liquid metal breeder reactors. The effects of irradiation on metals is discussed in more detailin a later chapter of this module.Effectson FuelDueto Swellingand CoreBurnupOne of the requirements of a good fuel is to be resistant to radiation damage that can lead todimensional changes (for example, by swelling, cracking, or creep). Early reactors and someolder gas-cooled reactors used unalloyed uranium as the fuel. When unalloyed uranium isirradiated, dimensional changes occur that present drawbacks to its use as a fuel. The effects areof two types: 1) dimensional instability without appreciable change in density observed attemperatures below about 450C, and 2) swelling, accompanied by a decrease in density, whichbecomes important above 450C. Other reactors use ceramic fuels, with uranium dioxide beingthe most common, have the advantages of high-temperature stability and adequate resistance toradiation. Uranium dioxide (UO2) has the ability to retain a large proportion of the fission gases,provided the temperature does not exceed about 1000C. Other oxide fuels have similarqualities. MS-05Page 24Rev. 0
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