Radon Progeny Size Distribution in and around Closed Uranium Mine

INTRODUCTION The Japan Nuclear Cycle Development Institute (JNC) has uranium mines in Japan in order to develop the technique of uranium exploration. The JNC has already finished developing the exploration. But, the waste rock from the exploration activities has been left at the mine sites around Ningyo-toge. The radiation monitoring around the mine sites has been carried out because the JNC has to maintain the waste rock safely according to the Mine Safety Law. Radon is one of the most important radioactivity for the monitoring. Radon progeny attaches with ambient aerosol particles immediately after it is decayed from radon. Therefore, there are attached and unattached fractions of radon progeny in air (see Fig.1). A radon progeny size distribution of the attached fraction is affected by ambient aerosol and it is one of the important factors for the dose estimation due to radon progeny in air. A size distribution of unattached fraction is not so much affected by surrounding condition, but the ratio of unattached fraction to total radon progeny is an important factor for the dose estimation. Therefore, the measurements of the size distribution of attached fraction and the ratio of unattached fraction were carried out in and around mine sites.