Uranium Refining and Conversion Plant Decommissioning Project

Abstract

The uranium refining and conversion plant (URCP) at Ningyo-toge was constructed in 1981 for the purpose of demonstrating on refining and conversion process from yellow cake (or uranium trioxide) to uranium hexafluoride by way of uranium tetrafluoride. For 20 years, 385 tons of natural uranium hexafluoride and 336 tons of reprocessed uranium hexafluoride (approximately) was produced. There are two different type of refining processes in the URCP. One is the wet process by convertig the natural uranium and the other is the dry conversion process for the reprocessed uranium. The dismantling of the dry process facilities began in March, 2008. It was found the large amount of uranium residuals such as wet slurry and powder uranium inside the vessels and pipes. Therefore, we have to take care of the spread of the contamination during dismantling works. The basic strategy concerning plant dismantling were the optimization of the total labor costs and the minimization of the radioactive wastes generated. The dismantling procedure is shown below; i) measuring doserate by using high sensitivity surveymeters, and nuclide identification by using gamma ray spectrometry, ii) estimating uranium mass inventory, iii) planning work force distributions with radiological survey staffs, iv) deciding dismantling methods concretely, v) decontaminating schematically if required, vi) collecting detailed data of working conditions, vi) measuring and classifying contaminated materials, vii) managements of radioactive waste drum and non-contaminated equipment, viii) control for personal exposures. Almost all equipment will be decontaminated except building decontamination it by around 2013FY. In addition, the secondary wastes were also yielded. Few thousands man-days were necessary for this project. The measurement data have not showed the high environmental radiation doserate, generally less than 0.3μSv/h. However, by the trace of the reprocessed uranium, the trans-uranium nuclides such as uranium-232 progenies, Th-228 and Tl-208 were observed. The accumulation of the nuclides which emit high energy gamma rays such as Tl-208 caused radiation exposure. As for the waste disposal, the determination of uranium content must be necessary. We have been now developing the uranium measuring systems with better accuracy. The further tasks imposed by our experiences are summarized the followings; i) minimization and reduction of radioactive wastes, ii) decontamination for the buildings and utilities, iii) wastes disposal. We have to work hard toward the final decommissioning.