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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