Seven-year Temporal Variation of Cesium-137 Discharge Inventory from the Port of Fukushima Dai-ichi Nuclear Power Plant:

Continuous Monthly Estimation of Cesium-137 Discharge in the Period from April 2011 to June 2018

Abstract

　　After direct discharges of highly contaminated water from Units 2 and 3 of the Fukushima Daiichi Nuclear Power Plant (1F) from April to May 2011, Kanda suggested that relatively small amounts of run-off of radionuclides from the 1F port into the Fukushima coastal region subsequently continued, on the basis of his estimation method. However, the estimation period was limited to up to September 2012, and there has been no report on the issue since that work. Therefore, this paper focuses on the discharge inventory from the 1F port up to June 2018. In the missing period, the Japanese government and Tokyo Electric Power Company Holdings have continued efforts to stop the discharge, and consequently, the radionuclide concentration in seawater inside the 1F port has gradually diminished. We show the monthly discharge inventory of ¹³⁷Cs up to June 2018 by two methods, i.e., Kanda’s method partially improved by the authors and a more sophisticated method using Voronoi tessellation reflecting the increase in the number of monitoring points inside the 1F port. The results show that the former always yields overestimated results compared with the latter, but the ratio of the former to the latter is less than one order of magnitnde. Using these results, we evaluate the impact of the discharge inventory from the 1F port into the coastal area and the radiation dose upon fish digestion.