Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

As the number of decommissioning projects increases worldwide, meticulous disposal strategies are needed for the growing quantity of potentially radioactive waste. Among these waste materials, structural components represent millions of tons of rubbles, of which over 90% is non-radioactive. The concrete biological shield surrounding the reactor pressure vessel absorbs neutrons during reactor operation. It is the structural element most prone to activation and classification as radioactive waste, presenting activity above the clearance levels. To reduce the time-consuming and costly experimental analyses for waste classification, numerical simulations are used to develop a reactor-specific model, predicting the activity distribution within the concrete structure. A precise model validation by a comprehensive radionalytical analysis is necessary. Further valuable insights are provided by a thorough structural characterization of the radionuclides within the cement structure.
Combined analysis and calculation campaigns were conducted on the unit 2 of the Greifswald nuclear power plant in Germany. Samples were taken from the biological shield in two positions of highest expected activity. A depth activation profile was constructed with gamma-spectrometry measurements. In parallel, calculations were run with a Monte-Carlo N-Particle code and the model was refined with precise geometry and composition information. For decommissioning, the relevant radionuclides within the biological shield include 60Co, 152Eu and 154Eu, exhibiting half-lives ranging from 5 to 20 years. 152Eu was found to be the limiting radionuclide for dismantling operations, showing activities above the German clearance level up to 35 cm of depth in the biological shield. The computational model demonstrated excellent agreement with the experimental measurements and accurate enough so that the activity distribution in the rest of the biological shield could be reliably predicted.
Additionally, we investigated the spatial distribution of radioactivity using a method combining digital autoradiography and Raman microscopy, revealing that feldspars are the predominantly activated mineral phases in the studied concrete matrix.
These results contribute to an effective and cost-saving decommissioning by minimizing the radioactive waste for final disposal and the radiation exposure of personnel during dismantling.