Activation determination for decommissioning of nuclear power plants

Due to the “German Energiewende”, all nuclear power plants (NPPs) in Germany will have been shut down by the end of 2022. Consequently, a safe, economical, and efficient dismantling of the NPPs will be an important challenge for the next decades. This is not only valid for the very particular German case but will certainly foster progress with methods for optimal planning and implementation of decommissioning.
The decommissioning strategy for an NPP requires knowledge of the neutron activation and contamination levels, which have emerged during its operation. Such knowledge can significantly minimize the radioactive waste and contribute to the safety of the operating personnel and the public.
This study considered two strategies. In the first one, the radionuclide inventory in the material of an NPP already under dismantling was investigated. Among others, Co-60 and C-14 in steel samples from the reactor pressure vessel (RPV) and Eu-152, Ba-133 and C-14 in concrete drill cores from biological shielding were quantitatively determined. C-14 was measured by liquid scintillation counting (LSC) after oxidative combustion and the other RN by gamma spectrometry. In the second strategy, the neutron flux in NPPs still under operation was determined with activation monitors. Small metal foils, covering a broad neutron energy range (i.e. thermal, epithermal, and fast energy range), e.g., Ti, Fe, Ni, Cu, In, Sn, Zn, Ta, were placed near the RPV and at the interior and exterior biological shield during the regular annual revision and irradiated for one fuel cycle (approximately one year). After the activation monitors were removed and recovered, the activation of the metal foils was quantitatively determined by gamma spectrometry. The experimental data from both strategies were compared with results from Monte-Carlo simulations.
This work is funded by the German Federal Ministry of Education and Research (BMBF) under grant numbers 15S9412 (WERREBA) and 15S9409A (EMPRADO) and is carried out in cooperation with PreussenElektra GmbH and EWN Greifswald GmbH.