Application of NDA Techniques for Material Accountancy of Solid Waste at Rokkasho Reprocessing Plant (RRP)

摘要

In order to implement accurate material accountancy at large reprocessing plant like Rokkasho Reprocessing Plant (RRP), it is necessary to introduce accurate measurement systems not only for mainstream material, but also need appropriate measurement systems for solid waste materials, even very small nuclear material contained. RRP has generated various solid wastes for instance leached hulls, vitrified waste for high-level liquid waste and low-level solid waste. The plutonium quantity in these wastes cannot be measured directly nondestructive assay (NDA) techniques because the waste contains curium so that the neutron from spontaneous fissions is obscured by curium or plutonium. The most practical NDA technique is neutron counts emitted from curium in the wastes. In this sense, RRP has introduced several NDA systems in order to provide effective and efficient safeguards as well as the material accountancy at waste handling areas. Key NDA system for waste measurement is Waste Crate Assay System (WCAS). WCAS measures plutonium quantity in the solidified waste based on Add- a-source method and curium versus plutonium ratio method. WCAS consists of total 98 neutron detectors for high neutron efficiency and iron shield for gamma pile up are used. In order to determine the parameter setup for plutonium quantity in the waste, in active calibration using high-density dummy waste with Cf-source was carried out in phase I and phase II. RRP has performed active test using actual spent fuel since the end of March 2006 and has handled wastes since the end of May 2006. Actual wastes measurements were made optimization of parameters for plutonium quantity measurement and WCAS performance evaluation. This paper presents the experience of the NDA measurements and the evaluation results for the accountancy.