Effective Approaches to Simulation of Thermal Stratification and Mixing in a Pressure Suppression Pool

Pressure Suppression Pool (SP) is an important passive element in a Boiling Water Reactor (BWR) safety system. Containment pressure depends on the efficiency of steam condensation inside the SP. Thermo-gravitational stratification induced by steam injection at small flow rates can reduce steam condensation capacity of the SP and significantly increase pool surface temperature and steam partial pressure in the wetwell. Parameters which are important for the SP operation are time scales of (i) stratification development, and (ii) mixing of initially stratified SP. In the present work we discuss “Effective Heat Source” (EHS) and “Effective Momentum Source” (EMS) approaches to prediction of stratification and mixing time scales in the SP. The GOTHIC code is used as a computational platform. The data from the POOLEX tests STB-20 and STB-21 is used for validation. Analysis of the simulation results obtained with EHS and EMS models and comparison with experimental data confirms (i) reasonably good accuracy of the EHS model in predicting of temporal evolution of stratification in a large scale pool, and (ii) feasibility of the EMS approach for predicting of characteristic time scales of mixing in the pool.