Macroscopic modelling of heat removal to large water pools in the code AC2/COCOSYS and its application to experimental facilities simulations

• New models provided in AC 2 /COCOSYS code to simulate heat transfer to water pools like used in SMR safety concepts. • Enabling the filling up of containment rooms with a water pool in simulations. • Models extended for heat-up of water pool and temperature stratifications. • Models extended for boiling heat transfer and emptying of the water pool. • Calculations for experiments in THAI SMR and EU-PASTEL SACO facility. In the safety concepts of advanced nuclear power plants, the decay heat removal from the reactor core to large water pools located inside or outside the containment is provided for the prevention of postulated incidents and accidents and mitigation of their consequences. One example is NuScale’s SMR safety concept relying on external cooling of the containment by positioning it submerged in a large water pool. When analysing such scenarios with a code, the code must be capable to plausibly simulate a significant heat-up of large water pools including boiling conditions. The formulation of flow processes in established integral severe accident codes like MELCOR, ASTEC, or AC 2 /COCOSYS is based on one-dimensional transport equations, which poses a challenge for the codes with regard to the simulation of real 3D flow patterns with localized heat input into such water pools. In addition to that, for the case of AC 2 /COCOSYS, there were no models available for the simulation of heat release from submerged hot structures to water pools. With view to large water pools, three major modelling improvements in AC 2 /COCOSYS are addressed here. Firstly, the modelling of the water pool as a discretised geometrical volume is enabled. This allows to consider a subdivision of the pool into several COCOSYS zones, and a water circulation and temperature distribution within the pool. In addition to that, an extended model range for heat transfer between hot structures and water, including nucleate boiling, film boiling and corresponding transition regimes is added to COCOSYS. Finally, a simplified Plume approach to model an efficient upward heat transport due to boiling phenomena is presented. The implemented models are validated by application to experiments focused on heating of water in large pools (THAI SMR and PKL SACO experiments); post-test calculation results are outlined. Based on the experience made with the macroscopic modelling approach, conclusions are drawn for further potential or needs of improvements.