Study of pool scrubbing events under jet injection regime

Abstract

Submerged jet hydrodynamics might have a significant role in the attenuation of radioactivity releases during nuclear power plant accidents. In particular, these studies are important in Steam Generator Tube Rupture accidents (SGTR accident) for Pressurized Water Reactors (PWRs), Station Black-Out (SBO events) in Boiling Water Reactors (BWRs) or in severe accidents, like the one occurred at the Fukushima Daiichi Nuclear Power Plant. Pool scrubbing has been habitually associated with globular discharges, i.e. at low injection velocities. Following this tradition, the SPARC90 code was developed to determine the trapping of fission products in pools during severe accidents, but only under these low injection velocity conditions. SPARC90 code assumes that the carrier gas enters the water pond at low or moderate velocities, forming a big bubble that eventually detaches from the injection pipe. However, there are a number of possible scenarios in which the capture of fission products in aqueous ponds might also occur under the jet injection regime, in which particle laden gases may enter the water at very high velocities resulting in a submerged gas jet. The present paper introduces the fundamentals, major hypotheses and code modifications developed in order to estimate particle removal during gas injection in pools under jet regimes. A simplified, yet reliable, approach to the submerged jet hydrodynamics was implemented based upon updated equations of jet hydrodynamics and aerosol removal, ensuring that both gas-liquid and droplet-particle interactions are correctly accounted for. The resultant code modifications were validated as far as possible, however, no suitable hydrodynamic tests were found in the literature. Hence, an indirect validation approach, based on data from pool scrubbing experiments, had to be employed. Moreover, validation was further limited by the scarcity of pool scrubbing tests under jet regimes (e., g., ACE, LACE, POSEIDON II and RCA experiments). This confrontation has been satisfactory, the experimental data and the simulations follow the same trends. We must highlight some main points, such as the capability of SPARC90-Jet to capture the increasing tendency of DF with both, aerosol diameter and pressure-submergency, catching not only the experimental trend but also the magnitude. Finally, emphasize the substantial improvement achieved with regard to the old SPARC90 code version, which has been clearly shown when comparing the SPARC90 and the SPARC90-Jet results against the available experimental data. But nevertheless, the work presented along this paper should be considered as a step towards an effective comprehension of the jet injection regime.