Dosimetry for Decommissioning of Nuclear Power Plants

In the framework of German NPP decommissioning studies, a 3D detailed Monte Carlo model of a German PWR has been developed based on detailed technical drawings. The model was used to estimate the reactor's neutron fluence characteristics (spectrum and distribution) necessary for the ensuing activation calculations. Experimental measurements were performed in active German PWRs to validate the developed model and ensure its neutron fluence prediction credibility. The validation of the model was based on metal foil-activation measurements. Several activation monitors, consisting of thin metal foils, were placed in key locations in two active PWRs. The monitors were irradiated in the reactors for approximately one year. After the monitors were removed and recovered, the activation of the metal foils was measured and compared against corresponding activation calculations. The calculated metal foils activation agreed well with the measured values. This fact indicates that the developed Monte Carlo model is reliable and can be used to calculate the neutron fluences in the components of German PWRs. In the next step of the study, it is planned to extend the reactor model and include the steam generators and other components and far-field reactor building structures. The neutron fluence levels resulting from the neutrons streaming from the reactor core to these rooms will be evaluated. Additional activation monitors will be placed in the active reactors' cavities and generator rooms to obtain additional measurement data for further validation, tuning, and optimization of the developed reactor model.