Effects of Snow Melt and Rain Event on Transport of Radiocesium in the Tone River After the Fkushima Dai-Ichi Nuclear Power Plant Accident

6th Asia-Pacific Symposium on Radiochemistry September 17 ~ 22, 2017 • ICC Jeju • Jeju Island, Korea

EFFECTS OF SNOW MELT AND RAIN EVENT ON TRANSPORT OF RADIOCESIUM IN THE TONE RIVER AFTER THE FKUSHIMA DAI-ICHI NUCLEAR POWER PLANT ACCIDENT

Seiya Nagao1, Toshiki Morokado2, Shu Tadou3, Kyuma Suzuki4, Shinya Ochiai5

1Low Level Radioactivity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University: O24, Wake- machi, Nomi, Ishikawa 923-1224, Japan and [email protected] 2School of Chemistry, College of Science and Engineering, Kanazawa University: Kakuma-machi, City, Kanazawa City, Ishikawa 920-1192, Japan and [email protected] 3 School of Chemistry, College of Science and Engineering, Kanazawa University: Kakuma-machi, City, Kanazawa City, Ishikawa 920-1192, Japan and [email protected] 4Gunma Prefectural Fisheries Experimental Station: 13 Shikishima, Maebashi City, Gunma 371-0036, Japan and suzuki- [email protected] 5 Low Level Radioactivity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University: O24, Wake- machi, Nomi, Ishikawa 923-1224, Japan and [email protected]

The radioactivity of 134Cs and 137Cs in river water from the upper Tone River was determined at Maebashi in Gunma Prefecture, Japan in a period of February 2012 to November 2016. The radioactivity of 137Cs ranged from 1.8 to 8.8 mBq/L under normal flow condition. On the other hand, snow-melting period, radiocesium concentration gradually increased together with the increase in water level. In the rain events, the radioactivity increased by 205 mBq/L. Maximum concentration was observed on 16 October 2013 after rain event during snow-meting season. The results indicate that rain event is main factor controlling transport of radiocesium in the Tone River system.

I. INTRODUCTION

A nuclear accident at the Fukushima Dai-ichi Nuclear Power Plant (NPP) occurred after the 2011 Tohoku Earthquake and Tsunami. About 15 PBq from both 134Cs and 137Cs were released from the Fukushima Dai-ichi NPP1,2 because of venting operations and hydrogen explosions. Surface deposition results reveals significant external radioactivity in a zone extending northwest from the NPP. However, a mountainous area in Gunma Prefecture, located about 220 km from Fukushima Dai-ichi NPP south of Fukushima Prefecture, shows similar accumulation of 134Cs and 137Cs with the south of Fukushima Prefecture3. To estimate short–term and long–term impacts of the radiation dose in Japan, it is important to understand the dynamics of 134Cs and 137Cs on river watershed environments. This paper reports transport behavior of 134Cs and 137Cs in the upper reaches of Tone River system in Gunma Prefecture, Japan during February 2012- November 2016.

II. MATERIALS AND METHOD

Experimental filed was selected at the Tone River system, with watershed area of 16840 km2 and river length of 322 km. River research was conducted at the monitoring site, Maebashi monitoring site, in the Tone River Fig. 1 Sampling location in this study in Gunma Prefecture from February 2012 to November 2016.

1 6th Asia-Pacific Symposium on Radiochemistry September 17 ~ 22, 2017 • ICC Jeju • Jeju Island, Korea

The radioactivity of 134Cs and 137Cs in the river waters was measured using gamma-ray spectrometry with ammonium molybdophosphate (AMP)/Cs compound. The AMP/Cs compound was dried at room temperature and weighed to determine the weight yield of AMP/Cs compound. The dried AMP/Cs compound was packed into a polyethylene bag (3.5 cm × 7.0 cm). The radioactivity of 134Cs and 137Cs was measured for the AMP/Cs compound using gamma-ray spectrometry with low background Ge detectors equipped with a multichannel analyzer at the Low Level Radioactivity Laboratory and the Ogoya Underground Laboratory of Kanazawa University during 1–3 days. Gamma emission peaks were used for calculating activity at 605 keV and 795 keV for 134Cs and 661 keV for 137Cs. The cascade summing effect was corrected for 134Cs using a contaminated soil sample from Fukushima. Decay correction of radioactivity for 134Cs and 137Cs was done at each sampling date.

III. RESULTS AND DISCUSSION

The 134Cs and 137Cs radioactivity in the river waters ranges from 0.8 mBq/L to 36.3 mBq/L and 1.8 mBq/L to 205 mBq/L, respectively, for the upper Tone River. In snow-melting season, the 137Cs radioactivity is 6-30 times higher than the normal flow condition. There is a positive correlation 137 with Cs radioactivity and water level. On the other hand, after rain events, Fig. 2 Variation of 137Cs radioactivity in the 137Cs radioactivity increased to 205 mBq/L. 137 river waters from the upper Tone River as a Figure 2 shows relationship between Cs radioactivity versus turbidity function of turbidity. in river waters. The four river water samples collected after rain events (20 June, 1 October 2012, 16 October 2013) and snow-melting + rain event (24 April 2012) period are plotted above the regression line for the other samples. These results indicate that riverine suspended solids with higher radiocesium content are transported by rain event. Therefore, rain event is an important factor controlling the transport of radiocesium in the upper river system.

IV. CONCLUSIONS

This study was investigated on the transport of radiocesium in the upper Tone River during 2012-2016. The effects of rain event and snow-melting are significantly affected for the export of radiocesium. The suspended solids with higher radioactivity are transported during rain events.

REFERENCES

1. Nuclear Emergency Response Headquarters (NERH), Government of Japan, Report of the Japanese Government to the IAEA Ministerial Conference in Nuclear Safety – The accident at TEPCO’s Fukushima Nuclear Power Stations, 2011. 2. M. CHINO et al., Preliminary estimation of release amounts of 131I and 137Cs accidentally discharged from the Fukushima Daiichi Nuclear Power Plant into the atmosphere. J Nucl Sci Technol, 48, 1129 (2011). 3. Ministry of Education, Culture, Sport and Science, http://radioactivity.mext. go.jp/en/contents/5000/4901/24/1910_1216.pdf, 2011.