Study On Severe Accident Sequences And Core Melt Process In IVR Evaluation For Npp

In-Vessel Retention (IVR) is an important and crucial measure forpreventing and mitigating consequence of severe accidents. And in IVReffectiveness evalution of nuclear power plant, it is a very important step todetermine the probability distribution function of three key uncertainparameters in the debris pool.In this thesis, severe accident sequences analysis and core melt processrelated research for IVR effectiveness evalution of one nuclear plant of ourcountry has carried out. This study chooses lots of severe accident sequenceswhich are related to IVR effectiveness evalution according to relevant criteria.And use of a serious accident analysis code caculates these sequences throughthe establishment of plant model. The change of state of debris pool in lowerhead has been shown from results of caculation. And a sensitivity analysis ofmodel parameters is obtained. Finally, according to numerous results ofcalculation, combined with the probability levels definition, several keyuncertainty parameters’ probability distribution functions are gotten.The results of this thesis from above research are shown below:①From the results of severe accidents which include LOCA, SBO andLOFW+ATWS which caculated by MAAP4and RELAP/SCDAP codesshown, both codes have a same development trend for severe accidents. But, the time of accumulator empty, debris relocation and coolant tempreturepredicted by both codes are different.②It is found that the influence of those uncertainty parameters which arerelate to IVR evaluation is not great by sensitivity analysis.③A large number of severe accident sequences are choosed andcaculated. The results shown that there is marked heat focusing effect on theheight of metal layer in debris pool.④It is found that three quasi-steady states will form before the finalsteady state. That is: when all UO2melt, the particulate layer in debris pooldisappear and the heat flux to lower head reaches peak value. Thesequasi-steady states will persist a long time, which will challenge to theintegrity of RPV. So, it is a must work to evaluate those impaction on IVReffectiveness.