A study using RELAP5 on capability and instability of two-phase natural circulation flow under passive external reactor vessel cooling

Abstract

External reactor vessel cooling (ERVC) in the gap clearance between the reactor vessel wall and insulation material is an important management strategy for in-vessel retention (IVR). Because the process of two-phase natural circulation flow under ERVC condition owns a low pressure and high power boundaries in cavity flooding balance condition, the flow instability is considered as one of the important factors determining heat removal capacity of ERVC. A great number of experiments are performed to investigate the effect of configurable and thermal–hydraulic parameters of ERVC system on the flow in the gap, especially the latest Chinese full-scaled and full-height REPRC heating experiment. To complement the experiment effort, a simulation study utilizing RELAP5 program is performed to investigate the two-phase natural circulation flow capability and instability for various sets of initial condition such as pressures, subcoolings, flooding levels and heat powers. The partial results are compared with the corresponding REPRC experiment data from available articles. And empirical formulae and numerical calculation are used to investigate into and compare with the critical subcoolings and density wave instability obtained from RELAP5 calculation. Besides, instability boundaries under various pressures are found. The results show that the effect of the thermal–hydraulic parameters for the optimized ERVC system configuration on two-phase natural circulation flow is still great during the early stage after hypothetical accidents. The steady two-phase natural circulation flow cannot be established and intensive flow rate oscillation and back flow occur as the flooding water subcooling is below the critical value.