INVESTIGATION ON AEROSOL SPRAY SCAVENGING BY USING A MULTI-HOLE NOZZLE WITH WATER MIST FOR DECOMMISSIONING OF FUKUSHIMA DAIICHI POWER PLANTS

During the decommissioning of Fukushima Daiichi nuclear power plants, it is important to consider a retrieval of the resolidified debris for both air and underwater configurations. For the subsequent retrieval of debris from the reactor building, resolidified debris needs to be cut into smaller pieces through various cutting methodologies. During the cutting process, aerosol particles in submicron scale are expected to be generated. It has been noted that such aerosols within the Greenfield gap (0.1-1 μm) are difficult to be removed effectively by traditional spraying methods. Therefore, aimed at improving the aerosol removal efficiency of the spray system, a new aerosol agglomeration method was recently proposed by injecting water mist to enlarge the sizes of aerosol particles before removing them using water sprays. The previous experiments using single-hole nozzles confirmed that the water mist could improve the aerosol spray scavenging efficiency due to the diffusiophoresis and different coagulation mechanisms which enhance the aerosol-mist agglomeration. However, in the actual Fukushima Daiichi decommissioning, due to the larger scale of containment, spray system with multiple nozzles or outlets will likely to be utilized, and its aerosol scavenging characteristics need to be examined. Therefore, this study performs experiments using a multi-hole (7 holes) nozzle with the injection of water mist to investigate the aerosol removal efficiency along with the spray characteristics. It is found that because of the different spray droplet characteristics (droplet velocity, size, etc.), the aerosol scavenging efficiency under a multi-hole nozzle spraying system is lower than the single-hole nozzle ones. The results obtained from the present study will be helpful for the optimization of spray system design for effective aerosol scavenging during Fukushima Daiichi decommissioning.