Radioactive Aerosol Control and Decontamination in the Decommissioning of the Fukushima Daiichi Nuclear Power Station

The recent Nuclear Regulation Authority evaluation report suggests that at the Fukushima Daiichi Nuclear Power Station, the concrete shield plugs above the primary containment vessel (PCV) have exceptionally high radiation levels in Units 2 and 3, which may increase the risk of radiation exposure during decommissioning operations. During the cleaning and disassembly of such radiation hot spots, it is expected that a large amount of submicron-sized radioactive aerosol particles will be generated, which may influence the decommissioning operation. In the present study, laser cleaning experiments were conducted at the University of Tokyo Aerosol Removal Test Facility to simulate aerosol scavenging during the laser cleaning process. The facility can reproduce multiple phenomena expected in actual plant decommissioning, such as laser decontamination and simultaneous mist and spraying operations. Through the work, we have developed effective aerosol dispersion control methods and strategies based on the joint use of water mist and water spray to reduce radiation risk in either laser cutting or other means of decontamination methods. Preliminary laser cleaning experiments on various coated samples were conducted to assess the aerosol removal efficiency using water droplets and mist. It was verified that the proposed method effectively cleans the radiation hot spots during the decommissioning process.