1:45 PM - 3:15 PM
[MAG34-P08] Estimation of annual Cesium-137 influx from the FDNPP to the coastal water nearby
Keywords:Fukushima Daiichi Nuclear Power Station, radioactive cesium, direct release, box model
Due to the series of accident at the Fukushima Daiichi Nuclear Power Station (1F) in March 2011, radionuclides were introduced into the marine environment. One of the release pathways was the direct discharge of contaminated water into the ocean from the 1F (on-going direct release). There is concern about the impact of Cs-137, which has a half-life of 30 years and is considered to have a long-term impact on the environment. Direct discharge was mainly caused immediately after the accident (3-6 PBq (Takata and Kumamoto, 2021)), but even now, the on-going direct release is continuing because the concentration of radionuclides near the 1F is still higher than before the accident.
In this study, firstly we estimated the on-going direct release of Cs-137 from 1F over 10 years after the accident, using the Cs-137 monitoring results in the coastal area around 1F published by the Tokyo Electric Power Company Holdings (TEPCO). Secondly, change in the monitoring data related to regarding the decommissioning of 1F by TEPCO (e.g., construction of seaside impermeable wall), so their effects on the on-going direct release estimation were also discussed. The box model was set up to estimate the on-going direct release including both inside and outside of the port for the area around 1F. We estimated the amount of Cs-137 in the box (estimated value) from the model, assuming that there is no direct discharge. Second, the amount of Cs-137 in the box from the monitoring results (measured values). As a result, the measured value was larger than the estimated value, indicating that there is a direct discharge of Cs-137 from 1F to the seawater in the vicinity of 1F. Next, three-month moving averages of TEPCO's monitoring results were obtained to investigate the contribution of the measures for decommissioning to Cs-137 concentration in nearby seawater. The results were consistent with a decrease in observed Cs-137 concentration due to the following measures: 1) installation of the seaside impermeable wall (2015/10/26), 2) change in analytical method (monitoring stations T1 & T2: 2012/10/8, T0: 2014/10/11), and 3) change in the drainage channel at the north of Unit 5&6 Discharge outlet around 2019. Then, we estimated two different on-going direct release scenarios over a 10-year period: an on-going direct release scenario with no change in release rate from 2011 and a periodical measure considered scenario. From the comparison of the estimated direct release between two scenarios, total amount of Cs-137 released by the latter scenario was lower than that by the former scenario, implying that impact of direct release on the marine environment has been improved by the countermeasures.
In this study, firstly we estimated the on-going direct release of Cs-137 from 1F over 10 years after the accident, using the Cs-137 monitoring results in the coastal area around 1F published by the Tokyo Electric Power Company Holdings (TEPCO). Secondly, change in the monitoring data related to regarding the decommissioning of 1F by TEPCO (e.g., construction of seaside impermeable wall), so their effects on the on-going direct release estimation were also discussed. The box model was set up to estimate the on-going direct release including both inside and outside of the port for the area around 1F. We estimated the amount of Cs-137 in the box (estimated value) from the model, assuming that there is no direct discharge. Second, the amount of Cs-137 in the box from the monitoring results (measured values). As a result, the measured value was larger than the estimated value, indicating that there is a direct discharge of Cs-137 from 1F to the seawater in the vicinity of 1F. Next, three-month moving averages of TEPCO's monitoring results were obtained to investigate the contribution of the measures for decommissioning to Cs-137 concentration in nearby seawater. The results were consistent with a decrease in observed Cs-137 concentration due to the following measures: 1) installation of the seaside impermeable wall (2015/10/26), 2) change in analytical method (monitoring stations T1 & T2: 2012/10/8, T0: 2014/10/11), and 3) change in the drainage channel at the north of Unit 5&6 Discharge outlet around 2019. Then, we estimated two different on-going direct release scenarios over a 10-year period: an on-going direct release scenario with no change in release rate from 2011 and a periodical measure considered scenario. From the comparison of the estimated direct release between two scenarios, total amount of Cs-137 released by the latter scenario was lower than that by the former scenario, implying that impact of direct release on the marine environment has been improved by the countermeasures.