Katsumi Shozugawa, Mayumi Hori, Thomas. E. Johnson, Naoto Takahata, Yuji Sano, Norbert Kavasi, Sarata K. Sahoo & Motoyuki Matsuo
Abstract
There has been tritium groundwater leakage to the land side of Fukushima Dai-ichi nuclear power plants since 2013. Groundwater was continuously collected from the end of 2013 to 2019, with an average tritium concentration of approximately 20 Bq/L. Based on tritium data published by Tokyo Electric Power Company Holdings (TEPCO) (17,000 points), the postulated source of the leakage was (1) leaks from a contaminated water tank that occurred from 2013 to 2014, or (2) a leak of tritium that had spread widely over an impermeable layer under the site. Based on our results, sea side and land side tritium leakage monitoring systems should be strengthened.
Introduction
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There are three possible pathways for the release of 3H from FDNPP to the outside: ocean, atmosphere, and groundwater. Among them, direct releases to the ocean and releases to the atmosphere have been reported in detail.
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nvestigation of 3H in precipitation may be one of the easiest ways to confirm the release of 3H into the atmosphere. The highest tritium concentration in precipitation was estimated 10 days after the accident at 1342 TU (equivalent to 158 Bq/L)13.
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Leaking of 3H through groundwater is difficult to analyze. In this study, we report that 3H above natural levels has been detected continuously in groundwater sampled from 2013 to 2019 on land approximately 30 m from the FNDPP site boundary. A key aspect of this study is that the water examined was groundwater, not surface water. […]
From 2013 to 2019, several countermeasures have been taken at the FDNPP to prevent contaminated groundwater from leaking off site. The relevance will be discussed, including the results of detailed tritium measurements in the water collected inside/outside FDNPP site.
Results
Outflow of 3H into groundwater from FDNPP
Most of the tritium present in the FDNPP was assumed to have been produced by ternary fission. As long as no re-criticality occurs, no new tritium is produced. However, it is estimated that there is 1.8 × 1015 Bq of tritium that has not been identified in the turbine buildings and in contaminated water, in addition to the amount released outside after the accident or the amount in debris10.
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The fact that tritium has been continuously detected in groundwater from the bypass installed upstream of FDNPP even after the completion of the water barrier (frozen wall) does not mean that tritium in the groundwater flows to the sea. In addition, the radioactivity trends in the neighboring wells vary widely, indicating that groundwater is moving in a complex manner.
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In order to evaluate the absolute amount of tritium contained in well water, information such as flow rate would be required, but TEPCO has not disclosed flow rates publicly.
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In addition, the sump water also contained radiocesium (134Cs and 137Cs). The concentration of 137Cs ranged from 3 to 4 Bq/kg, and the ratio of 134Cs/137Cs radioactivity at the time of the accident was almost 1. This also suggests that the water originated from FDNPP site24.
Tritium deposited via the air in surface water is not expected to mix with ground water. No tritium exceeding natural levels was detected in the air and precipitation around the FDNPP during the study period (2013–2019).
Discussion
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Since the end of 2013, tritium originating from the FDNPP has been detected from the south side of the site. This is the first report of continuous tritium detection on the land side of the site. There are 2 possible causes of elevated tritium on the land side of the site: the leakage of contaminated water from the tanks in 2013 and 2014, or the leakage of tritium from the initial accident, which had already spread widely underground at the FDNPP site.
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It appears that an underground route for contaminated water has been established which could lead to future problems. It is necessary to strengthen surveillance of leakage on both the ocean boundary and also land boundary of FDNPP.
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