Speaker
Description
A growing number of nuclear power plants (NPPs) worldwide have been in operation for more than 40 years, and the need for efficient decommissioning strategies is ever-rising. As many nuclear countries lack a robust solution for long-term storage of radioactive waste, time and costs are crucial parameters when considering decommissioning strategies. Considerable amounts of structural waste are generated when dismantling a NPP, most of it inactive. Sorting this waste meticulously is essential to reduce the costs and occupancy rate within repositories. For this, a thorough radiological analysis of the structures is necessary. The recent developments in numerical simulations are tremendously helping the radionuclide inventory and activity mapping, and speeding up the decommissioning process. Trace elements present in concrete and steel are neutron-activated during reactor operation, and create a mix of radionuclides (RN) that remain in the structures after shutdown. To give reliable predictions on activation in the structures, the models must be refined and validated with the help of carefully selected experimental methods. Challenges rise from the difficulty of measuring some RN, namely beta-emitters, which require multi-step radiochemical separation procedures, and from the complexity of the concrete matrix.
In this study, a model of the German NPP Greifswald Unit 2 was developed with a Monte-Carlo N-Particle code. Targeted samples were taken in two positions where the highest activation is expected in the concrete biological shield surrounding the reactor pressure vessel (RPV). After building an activation depth profile of the gamma emitters relevant to the decommissioning timeframe in the concrete, namely 60Co, 152Eu and 154Eu, it was observed that 152Eu limits the release of the concrete up to 35 cm deep inside the concrete. The measured and calculated activity concentrations closer to the RPV were above the German unrestricted clearance threshold. The model was refined with precise composition measurements and the calculated and measured profiles matched commendably. The model can be considered reliable for gamma-emitters, which constitute the primary radiation concern for the workers and for the long-term storage of the waste.
An additional focus is set on difficult-to-measure radionuclides (DTM-RN), and different separation methods were developed and optimized. The main DTM-RN present in structural waste and relevant to decommissioning are 3H, 14C, 36Cl, 55Fe, 63Ni. The analysis of each RN presents its own challenges and this study aims for precise results to refine the computational model efficiently.
Beyond the determination of radionuclide inventories, their potential mobility during dismantling and storage represents an additional risk for the radiological safety and waste classification. In particular, changes in the concrete matrix over time may influence both radionuclide release and structural integrity. Therefore, a long-term leaching experiment was performed and the changes in porosity and microstructure within the concrete were monitored with computed microtomography.