Advantages and Challenges of using CFD for Pool Scrubbing Analysis

The retention of fission products in the form of aerosols and volatile species of iodine by bubbling them into liquid solution is known as pool scrubbing. It occurs frequently during accident scenarios such as SGTR (Steam Generator Tube Rupture), core meltdown and in BWR (boiling water reactor) suppression pool and FCVS (filtered containment venting system). Pool scrubbing is featured by complex hydrodynamics. The flow regime can be divided into injection, transition and bubble rise three zones. In each zone, different phenomena or mechanisms control the scrubbing of particles, e.g. globule breakup in the injection and transition zone, and swarm effect in the rise zone. Pool scrubbing codes were mostly developed in the 1980’s and 1990’s such as SUPRA, SPRAC and BUSCA, which have been widely used in the nuclear safety analysis. One major weakness of the lumped parameter codes is that they rely on various empirical correlations and assumptions, whose validity range is often limited. For example, the globule size in the jet regime and the rise velocity of large bubbles are found to be underestimated. CFD (computational fluid dynamics) has the advantage of resolving the gas-liquid interface and flow field inside the pool, capturing the coalescence and breakup of bubbles locally and physically. In the frame of IPRESCA project, several CFD studies on pool scrubbing have been published, using both VOF and two-fluid approaches. For three-dimensional large-scale pool simulations, the interface-tracking method is mainly restricted by high computational load and insufficient mesh resolution, while two-fluid method by the limitations in closure models. Reliable modelling of the processes, e.g. globule breakup in high-velocity injection zone, multiphase turbulence, interphase heat transfer, particle removal as well as bubble burst and entrainment at the free surface, remains a challenge.