Recent decreasing trend of personnel exposure in nuclear power plants in Japan and analysis of major factors contributing its reduction

The radiation exposure of workers at nuclear power plants in Japan has been decreasing steadily in both total and per capita average although the total number of such plants is increasing. The total and the average exposure greatly decreased from 131.27 person-sievert and 3.8 millisievert in 1978, which were largest in history, down to 87.39 person-sievert and 1.6 millisievert, respectively, in 1989. The present article first outlines statistics of radiation exposure of workers at major facilities throughout Japan. Then a breakdown by type of nuclear reactor used in the facilities (boiling water, pressurized water) is described and discussed. A separate section is devoted to statistics for power plants with boiling water reactors. As a typical example, various improvements made to decrease the radiation exposure at the Fukushima No. 1 Nuclear Power Plant, where six reactors have been installed, are described. These improvements include the introduction of high-performance shielding material for the containment vessels, efficient cleaning and decontaminating equipment and filters to prevent iron rust from entering the reactors. (N.K.)