Development of uncertainty assessment methodology for in-vessel corium retention by external vessel cooling

Abstract

An uncertainty assessment methodology is proposed to evaluate the feasibility of in-vessel retention of the molten corium through the external reactor vessel cooling (IVR-ERVC) during severe accidents of the pressurized water reactors. Important assumptions and approaches adopted in formulating the methodology include several steps: utilization of the core damage frequencies from the level 1 probabilistic safety assessment results, assumption of the thermal failure occurrence of the reactor pressure vessel if the wall heat flux exceeds the critical heat flux (CHF) on any location of the vessel external surface, calculation of the wall heat flux using integrated severe accident code calculations with complementary calculations for obtaining the limiting wall heat flux, determination of the CHF on the external reactor vessel wall from available experimental data, and uncertainty assessments in dealing with wall heat flux and critical heat flux. The success probability of IVR-ERVC for each scenario is determined by comparing the distributions of wall heat flux with the critical heat flux at the limiting location and time using the Monte Carlo simulation. Finally, the overall success probability can be obtained by the weighted sum of the success probability for each scenario. The practicability of a proposed methodology is demonstrated by a preliminary application of the proposed methodology to the 3 LOCA scenarios of the Korean Standard Nuclear Power Plant (KSNP). The CHF and the external vessel cooling performance are affected by the plant specific design of IVR-ERVC such as reactor cavity geometry and global flow circulation paths. Currently, the SULTAN CHF correlation would be most useful for the CHF assessment, since it properly incorporates the design characteristics of the IVR-ERVC. The effects of the IVR-ERVC design parameters on the CHF and success probability are evaluated by the SULTAN correlation for the wide range of mass velocity, subcooling, decay po...