Three-dimensional simulation of a LBLOCA in an AP1000® containment building

Abstract

AP1000® is a Generation III+ reactor in which all safety systems relay on passive elements. AP1000 containment includes an In-containment Refueling Water Storage Tank (IRWST), a stainless steel liner that acts as a heat sink, and a Passive Residual Heat Removal System (PRHRS). In a containment Design Basis Accident (DBA) all these components are involved in the evolution of the accident and interact between each other. It is more difficult to model an AP1000 containment in comparison to other nuclear containments. The containment DBA modelling has been historically performed with Lumped Parameters Models (LPMs). The LPMs include several assumptions and hypotheses such as instantaneous mixing of the fluid inside a control volume, instantaneous contact of all thermal structures with the fluid inside a volume or neglecting three dimensional effects of the flow patterns, which can reduce the accuracy of the simulation in a complex containment like the AP1000. For this reason a three dimensional model of the AP1000 containment has been created with the GOTHIC code. A DBA is simulated in this model and results of pressure and temperature are obtained and analyzed. These results must be correctly treated and interpreted in order to compare them with LPM simulations. The results show that pressure distribution is homogeneous during the full transient, but temperature is not. These differences can be important enough to be a starting point for the developing of a new and needed three dimensional analysis methodology.