An estimation method for the total amount of 137 Cs contamination on the bottom of top shield plug in the Fukushima Daiichi nuclear power plant unit 2

The estimation of the total amount of 137Cs contamination on the bottom of the top shield plug in the Fukushima Daiichi Nuclear Power Plant Unit 2 is one of the key issues for pushing the decommissioning forward. Direct measurement of the 137Cs contamination amount is, however, difficult: the contamination is on the other side of a 61 cm thick concrete shield plug, i.e. opposite side of the surface of the shield plug on which ambient dose rate measurements were performed. This paper proposes a method for estimating the total amount of the 137Cs contamination on the bottom of the top shield plug using a dose rate meter in a lead collimator to define the contamination area from which gamma-rays are emitted. The proposed method is applied to the dose rates measured at 12 points on the shield plug surface. The estimated total amount of 137Cs contamination is about 30 PBq.