Cobalt application in repair tools for maintenance and modernisation of NPP equipment

Abstract

Relevance. The research discusses the actual risks that can arise from the formation of the radioisotope cobalt 60Co from the stable isotope 59Co during radiation neutron capture, which is specific to repair work at a nuclear power plant, but these risks are poorly understood. Aim. The research aims to analyse the possibility of additional exposure to ionising radiation for the repair personnel of a nuclear power plant due to the use of a repair tool that may contain cobalt. Methodology. The physical-theoretical and analytical approaches are used in the study. Results. A certain risk has been identified as a result of the use of drills or other tools for cutting metal containing cobalt among the repair tools. The calculation (analysis) of the activation of a cobalt-containing drill bit showed that a drill bit weighing 500 g containing 8% of 59Co in the tool steel acquires a partial activity (i.e., only cobalt activity) of 5.397 × 10-8 Ci, which leads to gamma radiation with an exposure dose rate of 0.024 mSv/year. The calculations of dose rates did not consider gamma radiation from the nuclear isomeric transition 60mCo→60Co, which could only worsen the radiation risk picture (when considering other examples). Conclusions. The use of a drill bit with the cobalt content specified in the calculations (or the location of the drill bit in the vicinity of the personnel), one way or another, contributes to the personnel exposure. It is proved that the calculation was not conservative, but rather liberal, because gamma radiation from the 60mCo isomer was not considered, otherwise, the obtained numerical values would have been higher. A practical recommendation to prohibit or reduce the use of tools containing cobalt during repairs on nuclear power plant equipment was made, theoretically, given that the analytically justified need to study the issue of radiation safety is relevant and vital for the safety of NPP maintenance personnel