Monitoring of radioactive substances in milk and feed in Tsukuba, Japan following the accident at the Fukushima Daiichi Nuclear Power Plant

Abstract

On March 11, 2011, the Great East Japan Earthquake and subsequent tsunami damaged the Fukushima Daiichi Nuclear Power Plant (FDNPP). In response to information of FDNPP accident, measurement of the γ-ray nuclide concentration in cow’s milk was started at National Institute of Livestock and Grassland Science (NILGS: Tsukuba, Japan, 36^o02’N, 140^o12’E) from March 12, 2011. On March 15, the concentration of ¹³¹I in milk was 155.2 Bq/L and reached the highest value of 244.8 Bq/L on March 23. After that,it gradually decreased to the lower limit detection value on May 26, 2011. On the other hand, the concentration of radioactive cesium increased later than ¹³¹I. On March 15, the concentrations of ¹³⁴Cs and ¹³⁷Cs were detected at 1.87 Bq/L and 2.24 Bq/L, respectively,and the concentration did not increase significantly until March 20. On March 21, the total level of (¹³⁴Cs and ¹³⁷Cs) was detected above 10 Bq/L. and thereafter, maintained above 10 Bq/L for three months. On the other hand, the concentration of ⁹⁰Sr in milk did not increase after the FDNPP accident and was maintained at 10 - 30 mBq/L before the accident. The survey of radioactive cesium concentration in total mixed rations (TMR) at NILGS started in November 2011. The maximum ¹³⁷Cs and ¹³⁴Cs activities, 13.44 and 7.78 Bq/kg (dry weight), respectively, were measured in the TMR obtained on November 2012. The radioactive cesium concentration level decreased to about 0.2 Bq/L for raw milk and about 2 Bq/kg (dry matter) for TMR, and then remained flat.