Modelling of pool stratification and mixing induced by steam injection through blowdown pipes

Containment overpressure is prevented in a Boiling Water Reactor (BWR) by condensing steam into the pressure suppression pool. Steam condensation is a source of heat and momentum. Competition between these sources results in thermal stratification or mixing of the pool. The interplay between the sources is determined by the condensation regime, steam mass flow rate and pool dimensions. Thermal stratification is a safety issue since it limits the condensing capacity of the pool and leads to higher containment pressures in comparison to a completely mixed pool with the same average temperature. The Effective Heat Source (EHS) and Effective Momentum Source (EMS) models were previously developed for predicting the macroscopic effect of steam injection and direct contact condensation phenomena on the development of stratification and mixing in the pool. The models provide the effective heat and momentum sources, depending on the condensation regimes. In this work we present further development of the EHS/EMS models and their implementation in the GOTHIC code for the analysis of steam injection into containment drywell and venting into the wetwell through the blowdown pipes. Based on the PPOOLEX experiments performed in Lappeenranta University of Technology (LUT), correlations are derived to estimate the steam condensation regime and effective heat and momentum sources as functions of the pool and steam injection conditions. The focus is on the low steam mass flux regimes with complete condensation inside the blowdown pipe or chugging. Validation of the developed methods was carried out against the PPOOLEX MIX-04 and MIX-06 tests, which showed a very good agreement between experimental and simulation data on the pool temperature distribution and containment pressure.