Research Of Material Properties Of High Burnup Fuel Element Under Accident Conditions

For commercial operation nuclear reactors, in order to improve the marketcompetitiveness of power plants and minimize power generation cost, the average dischargeburnup of nuclear fuel components increased. A higher burnup of fuel assemblies bringshuge economic benefits to nuclear power plants, but it also causes many security problemsto reactor operation.In order to research the potential risk of high burnup fuel elements to reactor operation,hypothetical accidents of high burnup reactor are researched. This paper serve QinshanⅡreactor as the reference, a simulation model of RELAP5code is modeled, and anappropriate simulation model is determined after a detailed sensitivity analysis of theprimary loop system simulation. After modeled, large break loss of coolant accident（LBLOCA） as shear fracture of cold leg of the primary loop and rod ejection accident ofhigh burnup reactor and Qinshan II plant are studied. In LBLOCA research, this paper focuson the cladding temperature transients, the peak cladding temperature and the largestcladding oxidation share, and explore whether the new cladding material fail at a highburnup LBLOCA situation or not. In rod ejection accident analysis, we focus on thecladding and fuel temperature transients, check whether the maximum cladding temperatureand the maximum fuel enthalpy exceeded standards limit values or not, and analysiswhether there is PCMI failure in fuel element, finally evaluate on the integrity of newcladding material at high burnup rod ejection accident.The results show, the transverse flow between two channels, temperature feedbackcoefficient at transient operating conditions and the flow resistance coefficient of thesecondary side are sensitivity factors of primary loop system simulation by using RELAP5cod, but the amount of axial nods in a core channel and source types are not sensitivityfactors. Under LBLOCA situations: the peak cladding temperature lower than safetyguidelines limits1204℃and the maximum oxidation share of cladding thickness lowerthan standards requirements17%in high-burnup reactor and Qinshan II reactor, and there isno brittle fracture because of excessive cladding oxidation; New cladding materials arecapable of a high burnup LBLOCA. In rod ejection accident situation: peck fuel temperature in Qinshan II reactor is higher than in high-burnup reactor at hot zero power,because there is a big power peaking factor in Qinshan II reactor; but a lower fueltemperature in Qinshan II reactor at full power, because of a poor thermal conductivity inhigh burnup fuels; Peck fuel temperature are lower than the fuel melting temperature atcorresponding burnup, the maximum fuel enthalpy values, peck temperature of claddingand the largest share of oxidations are lower than the standards require in the two powerstates at two reactors; There occur PCMI failure in high burnup reactor in both power staterod ejection accident, and fuel elements maintain integrity in Qinshan II reactor; Newcladding materials at high elastic rod burnup accident can not keep intact.