Activation analysis of graphite as a moderator-reflector for RBMK-1500 type reactor

Decommissioning of nuclear installations after their service life and their dismantling are connected with the necessity of the disassembling, handling and disposing of a large amount of radioactive equipment and structures. The concrete used as a biological reactor shield and graphite as a moderator-reflector can contribute to the significant amount of waste. To reduce this undesirable volume to the minimum and to successfully plan the dismantling and flow of radioactive materials to the storage facilities, the activation of the structures should be correctly evaluated. In this context both calculations and experiments should be combined in order to determine without ambiguities the long-lived induced activity of concrete and graphite. The main objective of this study is to validate the calculational tools and data libraries to determine accurately the activation of the graphite present in the active reactor core and its surroundings. After this validation procedure is finished, one will be able to characterise the activation of graphite in the RBMK-1500 reactor that is presently operating in Lithuania and will be closed in 2005. For this purpose the isotopic composition of fresh RBMK graphite samples has been analysed by two different techniques at CEA Saclay: a) GDMS (mass spectrometry), and b) activation analysis based on gamma spectroscopy after irradiation in both thermal and fast neutron fluxes in the research rectors OSIRIS and ORPHEE. Finally, theoretical activation estimates were done using Monte Carlo techniques for the neutron transport and different data libraries, namely ENDF, JENDL, JEF, and EAF, for activation estimates. The results were compared with experimental measurements in the RBMK reactor. Preliminary results of this study will be reported. Similar approach and methodology as above is going to be applied in the case of analysis on activation of concrete and graphite for the research reactor RUS (Strasbourg, France) to be dismantled in the near future. Our results related to the modelling will be also presented, while on-site activation measurements will start only by the end of this year and a direct comparison of predicted results with experimental values will be available somewhat later. (authors)