Application of imaging plate to the radiation protection in the MOX fuel fabrication facility

A large amount of unsealed plutonium (Pu) has been handled at mixed uranium and plutonium oxides (MOX) fuel fabricating facilities of the Japan Atomic Energy Agency (JAEA), Nuclear Fuel Cycle Engineering Laboratories. At these facilities, workplace monitoring has been performed to prevent possible internal exposure events due to Pu compounds. A conventional technique that visualize the Pu contamination by means of an autoradiography method has been employed on the site; however this technique has a distinct disadvantage in radioactivity quantifications of Pu or other artificial nuclides. An Imaging Plate (IP) is a passive, time-integrating, two dimensional radiation detector that is based on the photo-stimulated luminescence (PSL) phenomenon. The PSL intensity recorded in the IP has a good proportionality to the radiation intensity in a wide dynamic range. The IP has also excellent properties of the spatial resolution (50 μm squares at the minimum). The purpose of the present study is to develop a new algorism that implements both the identification and the quantification of each Pu contamination spot in various radiation control samples by using the IP. Characterization was conducted for PSL clusters induced by background radiations and those by Pu contaminants on an IP image of the samples taken at the MOX facilities in order to distinguish each other by several threshold items for the clusters. Conversion factors to the radioactivity from PSL values were also determined for different exposure periods, showing a significant fading effect. The proposed algorism showed a successful performance for Pu measurements, and would be a method for improving the radiation control at facilities handling Pu compounds. (author)