The Research On The Effects Of In-vessel Material Properties On The Severe Accident Progression

In case of the severe accident,different in-vessel materials demonstrate different behaviors under the same accident conditions due to their discrepancy in physical or chemical properties,which result in different effects on the severe accident progression.Therefore,the effects of the in-vessel material properties should be considered apart from the accident conditions in nuclear power plant safety analysis.An integrated safety analysis code called MELCOR1.8.5 is utilized to study the effects of in-vessel material properties on the progression of a severe accident that is assumed to occur in a 2^nd generation nuclear power plant.Firstly,a severe accident initiated by a large break LOCA is simulated under the assumption that none of the engineered safety features?ESFs?is activated except the safety injection tanks?SITs?.Secondly,the control rod guide tube,the control rod poison and the eutectics model are chosen.Different scenarios are simulated in comparison with the base case mentioned above in order to figure out the effects of these material properties on the severe accident progression,and reasons for these effects are analyzed according to the models describing these phenomena in MELCOR1.8.5.Last but not least,the sensitivity study is conducted on the parameters relevant to material properties such as gap release temperature,refreezing heat transfer coefficients and debris to lower head heat transfer coefficient.Conclusions below are drawn:1.Material properties like oxidation and eutectics have a significant effect on the severe accident progression.The severe accident progression is accelerated when the guide tube material is Zircaloy in comparison with stainless steel,with more oxidation heat rate generated in the core.The oxidation heat generation rate in the core is higher when boron carbide is taken as the control rod poison instead of the silver-indium-cadmium,and the composition of the in-containment atmosphere is different due to the reactions between boron carbide and steam producing several kinds of non-condensable gases.The accident progression is also accelerated when eutectics model is turned on;2.The severe accident progression is sensitive to the gap release temperature,the refreezing heat transfer coefficients and the debris to lower head heat transfer coefficient.The severe accident progression is decelerated when the gap release temperature is higher.The relationship between the refreezing heat transfer coefficients and the progression is not monotonic due to the interaction among models describing the phenomena during core melt in MELCOR1.8.5.The debris to lower head heat transfer coefficient affects not only the timing of lower head failure but also that of the containment failure.