Margin evaluation of in-vessel melt retention for small IPWR

In this paper, the safety margin of IVR (in-vessel melt retention) for small IPWR IP200 (integrated pressurized water reactor) is evaluated by numerical simulation. The thermal hydraulics and core degradation are modeled by SCDAP/RELAP5 code to provide initial conditions for the IVR. Meanwhile, the lower head behaviors are modeled by the finite element code COUPLE to present the dynamic evolution of molten pool with detailed temperature distribution. The transient analyses of thermal load firstly focus on the progression of corium solidification and external cooling, and then analogize to the IVR studies of other IPWRs to discuss the impacts which caused by the inherent design features of IP200. Besides, the differences between FIBS steady model and SCDAP transient model are discussed in terms of the heat transfer governing equation to summarize the benefits of transient calculation. In order to explain the calculation rationality, the result of maximum surface heat flux is also validated with the dimensionless data of related experiment. Finally, the mass composition of molten pool and the gap thermal resistance of inner wall are selected for parametric sensitivity analyses to assess the conservatism of maximum heat flux. For the quantitative conclusions, the low coolant storage of IP200 makes the core degradation very fast. The duration is about 9500s and the maximum temperature of liquid corium is close to 2900 K. The initial inner heat of molten pool is very high that the transient heat flux can get approximately 195% above the steady value. At the moment of maximum thermal load, the peak heat flux reaches 0.46 MW/m2, still lower than the local CHF. The lowest safety margin locates at the bottom angle and corresponding q/CHF is 0.65. The present work can provide references for the safety design of small IPWR.