​Activity Characterization Studies in FiR 1 TRIGA Research Reactor Decommissioning Project: Dissertation

During normal operation of a nuclear reactor neutrons cause fission reactions in the uranium fuel producing energy. In addition, some of the neutrons from reactor core are also absorbed into the construction materials close to the core causing activating nuclear reactions in these materials. Radioactive characterization is the process of determining the radiological properties of reactor fuel and activated structural materials. In the early phase of a decommissioning project characterization is performed with non-destructive computational methods and by studying material samples collected from low-active outer parts of a reactor. FiR 1 is a TRIGA Mark II type research reactor operated by VTT Technical Research Centre of Finland. The reactor has been used for various research and education purposes and producing radioisotopes for both medical and industrial use for years 1962-2015. This thesis consists of the activity characterization work carried out in the FiR1 TRIGA research reactor decommissioning project during the years 2014-2019. Radionuclides in the spent nuclear fuel were studied with two separate burnup calculation models. The calculated nuclide inventories were also tested by applying them in a computational dose rate model. The results were compared with dose rates that had been measured earlier from individual fuel elements. Activated construction materials were studied by building a three dimensional neutron transport calculation model of the reactor structures to calculate the neutron fluxes during reactor operation. This data was combined with reactor operation history and material-specific activating impurity concentrations to model the production of different radionuclides in these materials. Conservative assumptions have been used especially regarding material compositions and operational history modelling simplifications to provide slightly overestimated activities. Measurements from active samples support the calculated results and indicate that the calculations have been conservative as intended. The results from activity characterization are used in a nuclear decommissioning project as input data for estimating the amount of radioactive waste, choosing optimal dismantling methods, planning the practical radiation safety during dismantling and estimating the long-term safety of final disposal of decommissioning waste. The thesis also briefly describes applying the results in these parts of the FiR 1 reactor decommissioning project.