Study on restricted use of contaminated debris on Fukushima Daiichi NPS site, 1; Estimation of reference radioactive concentration for recycling materials

Shimada, Taro ; Miwa, Kazuji ; Takeda, Seiji 

Rubbles less than 5Sv/h of surface dose rate, which are stored in the Fukushima Daiichi NPS (1F) site, will be recycled and applied in a restricted use only within 1F site in the future. In this study, we suggested a concept for establishing the reference radioactive concentration of recycling material for the restricted use in the 1F site, and then calculated the reference radiocesium concentrations of the recycling material used for paved roads and the bases of concrete building. In order to prevent the working time from being decreased, it is necessary for the restricted use to suppress the additional dose to a level of value. Therefore, focusing on the current background air dose rate in the 1F site, we suggested the required condition that air dose rate increased by use of contaminated recycling material instead of non-contaminated did not exceed the variation range of the background dose rate. In addition, to validate the restricted use under the reference concentration, following three items should be confirmed; (1) additional dose for worker using the recycling material does not exceed 2mSv/y, 10% of worker dose limit; (2) the impact to air dose rate evaluated at the 1F site boundary does not exceed the 1mSv/y including direct and skyshine radiations from wastes stored in the site; (3) the impact of migrated radionuclides into groundwater does not exceed the operational target value. The reference radiocesium concentrations for recycling aggregate taken from contaminated concrete rubbles were evaluated for paved road and bases of concrete building.