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Preparatory tasks for decommissioning of nuclear power plant start with radiological characterization. Residual radioactivity inventory evaluation is a main part of the characterization. Reliable information on the inventory is important for specification for decommissioning plan. Japan Atomic Power Company (JAPC) has already started these tasks for Tsuruga Nuclear Power Plant Unit 1 (TS-1). We can optimize decommissioning plan using the information. To obtain the reliable information, we improved an evaluation procedure. The procedure is divided into two main steps. First step is neutron flux distribution calculation and second one is radioactivity distribution calculation. Radioactivity distribution is calculated using neutron flux distribution. In this work, we improved the evaluation procedure to obtain the reliable information on the inventory. Because of the limitation of computer resource, two-dimension (2D) approximation model was applied to radioactivity distribution around Reactor Pressure Vessel (RPV). We can calculate reliable 2D neutron flux distribution by having better understanding of neutron transport phenomena. Neutron flux was measured at 30 locations in TS-1 Primary Containment Vessel (PCV) using activation foils. And in order to understand the neutron transport phenomenon inside the PCV, we also calculated neutron flux distribution with the three-dimensional (3D) discrete ordinates method calculation (Sn) code. By consideration about the result of the measurement and 3D calculation, we could understand the characteristics of the neutron flux distribution inside the PCV. To simulate the neutron flux distribution well with 2D Sn code, neutron flux behaviors inside the PCV had been investigated with referencing the measurement values and with observing calculated 3D neutron flux distribution. 2D calculation model had been modified repeatedly until reliable calculation result was provided. After several model modifications, the reliable 2D calculation was accomplished and important neutron transport phenomena that are necessary to simulate the neutron flux distribution well was understood. Network-parallel-computing technique was applied to radioactivity distribution calculation. Using this technique, we could calculate radioactivity at all space mesh points that were used with 2D Sn code and we obtained the radioactivity distribution. By using this distribution, we can estimate a quantity of radioactivity around RPV more accurately and optimize dismantling designs.

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Tokai Power Station (graphite moderated, gas-cooled reactor, GCR) stopped its commercial operation in March 1998 and is decommissioning now. Since graphite blocks in Tokai reactor core are major low level wastes (LLWs), the realistic and reasonable method to estimate radioactivity of graphite blocks is required for final disposal and its licensing procedure. In general, LLWs, which were installed in or around a reactor core, have large radioactivity, theoretical calculations can be applied to the estimation of the radioactivity. This paper describes the concept of the method using statistical approach to determine the radioactivity of the graphite blocks in the reactor core. This method directly considers the variations of input calculation conditions, for example, compositions of impurity elements, irradiation neutron flux and irradiation period. In this paper, the variations of the compositions of impurity elements were statistically considered with the mean value and the standard deviation that were determined with elemental analyses. Many activation calculations were performed with the compositions that were determined with pseudorandom numbers, the mean value and the standard deviation. The calculated radioactivities distribute also statistically and a mean value and a standard deviation of radioactivity can be determined. The distribution of calculated radioactivities shows consistency to radiochemical analyses of graphite blocks from the reactor core and this shows that the method is applicable to the estimation of the graphite block radioactivity. Furthermore, this method can be considered to reduce over-excess estimation margin and can obtain reasonable radioactivity rather than using maximum or conservative values of all input conditions. This method is now being developed and approved as one of basic procedure for determining the radioactivity of wastes by Standards Committee of the Atomic Energy Society of Japan.

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