DEVELOPMENT OF NEW MONITORING SYSTEM FOR ACTIVITY DISTRIBUTION ESTIMATION IN LARGE WASTE CONTAINER FOR LSA-II MATERIAL

Low-level radioactive waste generated from the operation of nuclear power plants is currently being stored in 200-L metal drums and transported to a disposal site in Japan. On the other hand, a great amount of low-level radioactive waste with larger dimensions will be generated from the decommissioning of nuclear power plants. When large radioactive waste is to be disposed of, it is desirable to package such waste in large containers instead of drums to reduce the cost of cutting it into small pieces, and also to protect workers and the public from additional radiation exposure. According to the IAEA Safety Standards Series Requirements No. TS-R-1 published in 2005, most of these wastes will be categorized as ‘low specific activity (LSA)-II material’, whose uniformity in the distribution of activity must be certified before transport. The following simple criterion of uniform distribution is suggested in the IAEA Safety Guide of ST-2:226.14, 15: differences between the specific activities of portions, which are defined as one-fifth or onetenth of the volume, should be within a factor of less than 10. Thus, we are investigating techniques for evaluating the activity distribution in large waste containers. In this study, metal pipes with different filling rates were taken to be radioactive waste in a large rectangular waste container (100 cm (W), 100 cm (H) and 300 cm (L)). First, a simple gamma-ray measurement method was examined using the Monte Carlo N-Particle Transport Code System (MCNP). The uncertainty of the activity distribution estimation of one-tenth the volume was calculated for various source positions and measurement positions. As a result, it was found that applying a simple gamma-ray measurement is inadequate to determine the distribution of activity when the filling rate is higher than approximately 10 % and the measurement position is 100cm from the center of the container. To overcome this limitation on the filling rate, a new conceptual design, which utilizes three-dimensional shape measurement, Monte Carlo calculation and radiation measurement, is proposed for the activity distribution monitoring system for waste with a higher filling rate.