Calculation of neutron fluence distribution in components in the reactor vicinity and their validation on experiments as a basis for activity calculations

Due to the “German Energiewende”, all nuclear power plants (NPPs) in Germany will be shut down by the end of 2022. Consequently, the safe and sustainable decommissioning and dismantling of the NPPs will be an important challenge for the next decades.
The aim of the project EMPRADO (“Entwicklung einer Methode zur Pre-Aktivitäts- und Dosisleistungsberechnung von reaktornahen Bauteilen auf Basis von Neutronen-fluenzverteilungen” – “Development of a method for pre-activity and dose rate calculations of components in the reactor vicinity based on neutron fluence distributions”) is to develop a standardized method to calculate the specific and temporal progression of activation for reactor components and near-reactor concrete as well as construction elements based on the power history of a nuclear reactor. A non-destructive early radiological characterization in the region of the reactor core is thus possible, which is required for an optimal planning of the dismantling. This can make a significant contribution to minimize the radioactive waste and the radiation exposure of personnel during the decommissioning.
The exact 3D neutron fluence calculations are carried out by Monte-Carlo simulations using the international widely used program MCNP6. For validation of the calculation results neutron flux measurements on the basis of activation foils (monitors) are carried out at several German NPPs which are still in operation.
The monitors used in this project are thin metal foils (10 × 10 × 0.1 mm) of Ti, Fe, Ni, Cu, Zn, Nb, Pd, In, and Sn. They are placed during the annual revision of the NPP at different construction elements, for instance near the reactor pressure vessel and at the first concrete wall, the biological shield, at different heights. At these positions the monitors remain during one working cycle of the NPP; that is approximately one year. After recovery, the neutron activation products are analyzed qualitatively and quantitatively by gamma spectroscopy and liquid scintillation counting (LSC). The experimentally determined activities are compared with the calculated ones in order to optimize the simulation procedure.
First experimental results of a feasibility study carried out in NPP Grohnde will be presented and compared to the available literature.