Application of rapid in-vitro bioassay for determining internal contamination of radiation workers; To prepare for decommissioning in Fukushima Daiichi NPS

Ono, Masako*; Noguchi, Mizuki*; Nagaoka, Mika  ; Maehara, Yushi; Fujita, Hiroki

On March 11, 2011, a tremendous earthquake of a 9.0 magnitude occurred undersea off the northeastern coast, triggering a massive tsunami. Flooding from the tsunami caused the loss of core cooling functions, which led to the evaporation of water in the reactor pressure vessels of Units 1-3 at the Fukushima Daiichi Nuclear Power Station (FDNPS), resulting in an accident that damaged the reactor cores. After the accident, the decommissioning work is being carried out step by step, and it will now be entering the stage of taking on challenges in uncharted territory, on the way to full-scale decommissioning including retrieval of fuel debris. Radiation workers in decommissioning project who enters reactor buildings should conduct various kinds of tasks in working environment with relatively high concentration of radionuclides such as Strontium-90, Actinides, etc., is constantly present. Even if the workers wear appropriate protective equipment, intakes of radionuclides under unforeseen circumstances may result in internal exposure. Measurement of radioactive materials taken into the body in the event of an incident can be conducted by in-vivo bioassay using WBC and lung monitors, and in-vitro bioassay using analysis of feces and urine. In FDNPS, all workers who enter the controlled area are subjected to routine individual monitoring of internal exposure by WBC (for screening purposes). For alpha-emitting radionuclides such as actinides and beta-emitting radionuclides such as Strontium-90, in-vitro bioassay measurements are generally used. In case of an internal exposure event, it is necessary to establish a rapid bioassay for judgment of medical treatment and for information collections to develop a plan of action. This presentation will provide an overview of the decommissioning work at FDNPS and application of a rapid bioassay method for mixed nuclides of Pu, Am, Cm, U, and Sr.