Numeric Determination and Validation of Neutron-Induced Radioactive Nuclide Inventories for Decommissioning and Dismantling of Light Water Reactors

The decommissioning of nuclear power plants requires project planning and budgeting both during the project and in advance, as well as the secured provision of financial and human resources. When a facility is free from irradiated fuel, the reactor pressure vessel with the nuclear components and the biological shield determine the activity inventory of the facility, which almost exclusively consists of activated radionuclides located in the respective structures. Knowledge of the activity distribution and nuclide vectors of the involved components is of vital importance for decommissioning planning. In this context, the development of a computation procedure is described coupling the Monte Carlo method for the determination of neutron flux densities and spectra with a procedure to perform activation calculations for the determination of nuclide vectors. For this purpose, detailed knowledge of the material composition, particularly the trace-element concentrations of nitrogen and cobalt in steel and additionally of europium and cesium in concrete structures, considerably impacts the accuracy of the calculated activities. Extensive validation using data collected from various nuclear power plants to be decommissioned, such as nuclide activities, neutron flux densities, and neutron and gamma dose rates, demonstrates the reliability of the computed nuclide distributions showing ratios of computed-over-measured values of typically between 0.9 and 3. The practicality of the developed method and the convenient use of the results have already been demonstrated analyzing several German boiling water reactors and pressurized water reactors and developing packaging strategies based on the produced results.