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Fuel  Cladding  Embrittlement
Nuclear  Reactor  Core  Problems  Summary (Cont.)

Material Science Volume 2 of 2
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Plant Materials DOE-HDBK-1017/2-93 NUCLEAR REACTOR CORE PROBLEMS Even  though  fission  product  swelling  is  less  with  oxide  fuels,  this  irradiation-induced  volume increase has been observed in UO2 and mixed-oxide fuels for a number of years.   This swelling of the fuel has generally been attributed to both gaseous fission-product bubble formation and the accumulation 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 the lifetime  of  the  fuel  element  by  helping  to  determine  the  physical  and  mechanical  changes resulting from irradiation and high temperature in the fuel and the cladding.   Fuel element life or core burnup, which indicates the useful lifetime of the fuel in a reactor, is also determined by the 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 as a result of the high internal temperatures and the large temperature gradients.   Thermal stresses lead to radial cracks and grain structure changes.   These structural changes tend to increase with the specific power and burnup of the fuel. Summary The important information in this chapter is summarized below. Nuclear  Reactor  Core  Problems  Summary Fuel Pellet-Cladding Interaction (PCI) PCI  may lead to cladding failure and subsequent release of fission products into the reactor coolant. 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. Design features to counteract PCI include: An increase in the cladding thickness An  increase  in  the  clad-pellet  gap,  with  pressurization  to  obviate cladding collapse The introduction of a layer of graphite or other lubricant between the fuel and the cladding Operational limitations to reduce PCI Plant procedures limit the maximum permissible rate at which power may be increased to lessen the effect of PCI. Rev. 0 Page 25 MS-05







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