Research and Practice on Environmental Dose Reduction in Nuclear Power Plant Core Instrument Room

Qinshan Phase II Units 3 and 4 began to use the solvent cleaning technology for flux thimble tubes in 2017. In 2018, the environmental dose of the core instrument room increased significantly, resulting in higher radiation protection costs in the core instrument room during the overhaul and absorption dose of equipment maintenance person, etc. After the abnormality of the environmental dose of the core instrumentation room was found, the investigation into the reasons for the increase of the ambient dose in the core instrumentation room of Unit 3 found that because the room was only designed with an air inlet and no air outlet, a large amount of radioactive dust escaped from the room. The neutron flux detector was rusted for the first time and the material analysis of the radioactive dust found the activation products Fe-59 and Co-60. Aiming at this problem that has never occurred in the field of nuclear power, this paper firstly analyzes and studies the reasons for the increase of the environmental dose in the core instrument room. On this basis, the four environmental dose reduction measures adopted are described, including the improvement of the cleaning method of the flux thimble tubes, the decontamination of the equipment surface, the overall cleaning of the mechanical equipment, and all-round cleaning of room radioactive dust. From the practical results, the ambient dose in the core instrument room was reduced from nearly 1mSv/h to about 0.1mSv/h, and the room surface pollution and equipment radiation hot spots were almost eliminated.