Abstract
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 8.1E+9 and 5.7E+10 Bq/cm2 from the measured peak count rate of 0.662 MeV photons. If Cs-137 was distributed on the surfaces of the gaps with radius 6m and with the averaged concentration of 5 points, 2.6E+10 Bq/cm2, total amount of Cs-137 is estimated to be 30 PBq.
Highlights
Ambient dose equivalent rates at the operation floor of Fukushima Daiichi Nuclear Power Station Unit 3 were very high
It was estimated that about 5 mm of concrete surface was removed by cutting and sucking
The collimator consists with a 30 cm x 30 cm x 30 cm lead block
Summary
Ambient dose equivalent rates at the operation floor of Fukushima Daiichi Nuclear Power Station Unit 3 were very high It was order of Sv/h at maximum at 5 m from the floor surface. If this estimation is correct, the main component of the photon field above the shield plug is scattered photons within 60 cm concrete. Measurements of pulse height distribution were performed using a CdZnTe detector inside a lead collimator with enough shielding
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