Investigation of main radiation source above shield plug of Unit 3 at Fukushima Daiichi Nuclear Power Station

Hirayama, Hideo*; Kondo, Kenjiro*; Suzuki, Seishiro*; Tanimura, Yoshihiko   ; Iwanaga, Kohei*; Nagata, Hiroshi* 

Pulse height distributions were measured using a CdZnTe detector inside a lead collimator to investigate main source producing high dose rates above the shield plugs of Unit 3 at Fukushima Daiichi Nuclear Power Station. It was confirmed that low energy photons are dominant. Concentrations of Cs-137 under 60 cm concrete of the shield plug were estimated to be between 8.110 and 5.710 Bq/cm from the measured peak count rate of 0.662 MeV photons. If Cs-137 was distributed on the surfaces of the gaps of the shied plugs with radius 6 m and with the averaged concentration of 5 points, 2.610 Bq/cm, total amount of Cs-137 is estimated to be 30 PBq.