Delayed Equilibrium Model (DEM) of Flashing Choked Flows Relevant to LOCA and Implementation in System Codes

Abstract

In the context of nuclear reactor safety, a pipe breach in the primary circuit is the initiator of a Loss of Coolant Accident (LOCA). The calculation of leak rates involving the discharge of water and steam mixtures plays an important role in the modeling of LOCA’s for both GEN II and GEN III reactors, and also for the Supercritical Water Reactor of GEN IV. Indeed, the flow though the breach determines the depressurisation rate of the system and the time to core uncover which in turn are of major concern for when and how different mitigation auxiliary systems will be initiated and be efficient. This paper deals with the new development of the DEM model focused on thermodynamic non-equilibrium conditions, which prevail in the flashing flow process near the critical section. This model, developed at the University of Louvain (UCL), is the 1-D Delayed Equilibrium Model (DEM) for choked or critical flow rate in steady state or quasi-steady state conditions. The DEM are assessed against experimental data such as Super Moby-Dick and BETHSY experiments done in CEA during the eighties. The DEM model has been recently implemented in the WAHA code, which is based on a two fluid 1D six equations model. The methodology can be applied to other system code (CATHARE, RELAP, etc.).