Numerical investigation on improved spray system for efficient aerosol removal during the decommissioning of Fukushima Daiichi nuclear power plants

During decommissioning of the Fukushima Daiichi (1F) nuclear power plants, retrieving the solidified debris inside the reactors is a critical issue. Debris must be segmented using cutting techniques for retrieval by robots. However, these processes will generate submicron radioactive aerosol particles. Water spray system is considered as a useful technique for removing these aerosols. To investigate the optimization of the spray system for 1F decommissioning, this study conducts numerical simulations using Euler-Lagrange approach in STAR-CCM +. The evaluations focus on aerosol removal processes within our UTARTS facility under varying spray configurations. Our simulations consider the particle-laden gas and spray droplets in the continuous Eulerian phase and dispersed Lagrangian phase, respectively. Numerical models for aerosol scavenging through various mechanisms, including inertial impaction, diffusion, and interception, are developed and implemented. The characteristics of droplets and gas flow during spraying are investigated. With validation from experimental results, simulations incorporating newly designed virtual spray nozzles are analyzed for improved scavenging performance. The study suggests that an optimized combination of spray configurations may exist. The results from this study are expected to be beneficial for the improvement of spray system designs to efficiently scavenge aerosols during 1F decommissioning, thereby mitigating radioactivity release and minimizing the production of contaminated water.