FIR 1 TRIGA activity inventories for decommissioning planning

The objective of the study has been to estimate the residual activity in the decommissioning waste of TRIGA Mark II type research reactor FiR 1 in Finland. Neutron flux distributions were calculated with Monte Carlo code MCNP. These were used in ORIGEN-S point-depletion code to calculate the neutron induced activity of materials at different time points by modelling the irradiation history and radioactive decay. The knowledge of radioactive inventory of irradiated materials is important in the planning of the decommissioning activities and is essential for predicting the radiological impact to personnel and environment. Decommissioning waste consists mainly of ordinary concrete, aluminium, steel and graphite parts. Results include uncertainties due to assumptions on material compositions and possible diffusion of gaseous nuclides. Comparison to activity inventory estimates of two other decommissioned research reactors is also presented. (authors)