Low 134Cs/137Cs ratio anomaly in the north-northwest direction from the Fukushima Dai-ichi Nuclear Power Station

Highlights

• We present new data of ¹³⁴Cs/¹³⁷Cs around Fukushima Daiichi Nuclear Power Station.

• The entire area of the low ¹³⁴Cs/¹³⁷Cs ratio anomaly around the FDNPS is revealed.

• The low ¹³⁴Cs/¹³⁷Cs ratio anomaly is coincident with a plume trace.

• The anomaly occurs in the area which had been contaminated before March 13, 2011.

Abstract

A low ¹³⁴Cs/¹³⁷Cs ratio anomaly in the north-northwest (NNW) direction from the Fukushima Dai-ichi Nuclear Power Station (FDNPS) is identified by a new analysis of the ¹³⁴Cs/¹³⁷Cs ratio dataset which we had obtained in 2011–2015 by a series of car-borne surveys that employed a germanium gamma-ray spectrometer.

We found that the ¹³⁴Cs/¹³⁷Cs ratio is slightly lower (0.95, decay-corrected to March 11, 2011) in an area with a length of about 15 km and a width of about 3 km in the NNW direction from the FDNPS than in other directions from the station.

Furthermore, the area of this lower ¹³⁴Cs/¹³⁷Cs ratio anomaly corresponds to a narrow contamination band that runs NNW from the FDNPS and it is nearly parallel with the major and heaviest contamination band in the west-northwest.

The plume trace with a low ¹³⁴Cs/¹³⁷Cs ratio previously found by other researchers within the 3-km radius of the FDNPS is in a part of the area with the lower ¹³⁴Cs/¹³⁷Cs ratio anomaly that we found.

Our result suggests that this lower ¹³⁴Cs/¹³⁷Cs ratio anomaly is the area which was contaminated before March 13, 2011 (UTC) in association with the hydrogen explosion of Unit 1 on March 12, 2011 at 06:36 (UTC) and it was less influenced by later subsequent plumes.

http://www.sciencedirect.com/science/article/pii/S0265931X17301947