Characterization measurements of fluental and graphite in FiR1 TRIGA research reactor decommissioning waste

The objective of this article is to study the activities and special characteristics of graphite and Fluental neutron moderator material in FiR1 TRIGA Mark II type research reactor in Finland. Quantifying the nuclide vector and total activities of decommissioning waste is a legal requirement and provides input to many other aspect of decommissioning project. Because earlier calculations of decommissioning waste nuclide inventories used partially assumed input data e.g. material compositions, measurement data is needed to validate the calculated results. Graphite and Fluental are especially important in long term final disposal of the waste, because graphite contains volatile radionuclides (¹⁴C, ³H, ³⁶Cl) that spread rapidly in final disposal conditions and the corrosion of aluminium in Fluental will result in hydrogen pressure generation, which can accelerate the diffusion of radionuclides in the bedrock. Moreover, these types of materials are not used in Finnish power reactors and their behaviour in final disposal conditions have not been studied so thoroughly earlier. This study reports composition and activity measurements from graphite, activation and leakage of tritium in Fluental and reviews main phenomena that could occur in final disposal environment. Measured activities were compared to estimates calculated with a point-depletion code modelling the reactor irradiation history. Results are used to validate the calculated estimates of total activities and the scaling matrix method that will be used to classify the packed decommissioning waste. Due to limited number of samples, numerical results still contain variations. However, developed methods are still valuable in future analyses and measured data also rules out unexpected activities.