Estimated ¹³⁷Cs Radioactivity in the Gap Between the Top and Middle Covers at a Shield Plug in the Fukushima Daiichi Nuclear Power Station Unit 2

The contamination density of ¹³⁷Cs deposited in the gap between the top and middle covers of the shield plug in the Fukushima Daiichi Nuclear Power Station Unit 2 was estimated using three types of measurement results. Owing to the extremely high dose rate on the entire operation floor, including the top of the shield plug, only remotely measured dose rates, such as ambient dose equivalent rates, were obtained using robots. Based on three types of measurements, significantly higher concentrations of ¹³⁷Cs were observed than previously estimated. An estimation based on the measurements of the ambient dose equivalent rate inside the hole also demonstrated that the contamination density in the gap between the top and middle covers varied significantly at different positions of the cover. The results obtained will significantly aid in future decommissioning scenarios, and will be important for examining the progress of an accident.