Landside tritium leakage over through years from Fukushima Dai-ichi nuclear plant and relationship between countermeasures and contaminated water

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in 2011 released a large amount of radioactive materials, including tritium (³H), into the environment. While tritium is a low-energy beta emitter, its release necessitates monitoring due to its long half-life (12.3 years). Data on environmental tritium after the accident remains limited. Tritium is primarily produced in boiling water reactors like those at FDNPP through ternary fission. Estimates of the tritium inventory immediately after the accident vary significantly between sources. The paper focuses on investigating a less-studied release pathway: groundwater leakage. Three main pathways for tritium release from FDNPP are considered: ocean, atmosphere, and groundwater. Ocean and atmospheric releases have been documented, showing tritium detection in the Pacific Ocean and precipitation shortly after the accident. This study addresses the more complex issue of tritium leakage through groundwater, examining continuous groundwater samples collected from 2013 to 2019 from a location approximately 30 meters from the FDNPP site boundary. The investigation also uses ⁸⁷Sr isotope ratios as a natural tracer to analyze the hydrogeological origin of the groundwater and assess the impact of countermeasures implemented to prevent contaminated groundwater leakage.

Previous research has detailed tritium releases into the Pacific Ocean and atmosphere following the FDNPP accident. Studies have reported tritium concentrations in seawater and precipitation significantly exceeding natural levels shortly after the accident. However, studies focusing specifically on continuous land-side groundwater tritium leakage remain limited. Existing literature provides estimates of the tritium inventory at FDNPP, but these estimates vary considerably. Existing studies of groundwater contamination often focus on surface water rather than the deeper groundwater addressed in this paper. The use of strontium isotopes as tracers in hydrological studies provides an established method for examining water flow patterns and mixing. This study builds upon these previous investigations by focusing on the continuous monitoring of groundwater tritium and using stable isotopes to identify the source and pathways of the contaminated water. The paper references previous TEPCO reports on contaminated water management and groundwater bypass systems at the FDNPP site.

Environmental samples, including groundwater, were collected from Okuma Town, Fukushima Prefecture, near the FDNPP, from 2013 to 2019. Sump water samples were collected directly from a pipe embedded in a cliff at the plant boundary, minimizing soil or plant contamination. Tritium analysis was conducted using two methods: liquid scintillation counting (LSC) and noble gas mass spectrometry (using the helium-3 ingrowth method). LSC measured beta emissions, with detection efficiency determined using a self-absorption curve and quench correction. The helium-3 ingrowth method provided an independent confirmation of tritium concentrations. The ⁸⁷Sr/⁸⁶Sr isotope ratio analysis was performed using thermal ionization mass spectrometry (TIMS) on water samples collected from a cliff and the shoreline. This technique helped determine if the detected groundwater originated from the plant or elsewhere and provided insight into the groundwater's flow paths. The paper describes sample collection, preparation, analysis and data processing techniques in detail. Specific equipment and software used are also mentioned, including the LSC used for tritium analysis and the TIMS for strontium isotope analysis. The analytical methods used for tritium and strontium isotope analyses are outlined, including the steps to reduce any possible contamination during the process. The use of standards and controls to ensure the accuracy and reliability of the measurement is also noted. The detection limits for both tritium and strontium are provided. The paper also notes the availability of supplementary data for further details.

The study found continuous tritium detection in groundwater on the land side of the FDNPP from 2013 to 2019. The average tritium concentration in sump water was approximately 20 Bq/L, exceeding natural levels. Analysis of groundwater from wells installed as part of a groundwater bypass system showed significantly higher tritium concentrations (exceeding 3000 Bq/L at times), gradually decreasing over the study period. The highest tritium concentrations were observed in wells located near the south side of the plant. Strontium isotope ratio analysis revealed distinct differences between groundwater samples collected at the cliff and shoreline, suggesting different hydrogeological origins. This indicates that the groundwater flow pathways may be more complex than initially assumed. The data collected show the existence of a continuous path for groundwater to flow out of the plant to the outside. The analysis of tritium levels in the wells around the contaminated water tanks showed an increase in tritium activity during and immediately after the leak in 2013-2014. However, there was no clear correlation between tritium activity in wells in the H6 area and the leakage, suggesting that the contaminated water did not reach the wells in that area. Countermeasures to prevent contaminated water leakage, including the installation of frozen soil walls and impermeable walls, were implemented during the study period, but the study found continuous tritium leakage even after these barriers were installed. This suggests either the leakage has pre-existing pathways or that the barriers have not completely prevented the leakage.

The findings demonstrate a continuous, albeit low-level, tritium leakage from the FDNPP to the land side via groundwater. This leakage pathway, previously not well documented, poses a long-term environmental concern. The lack of a direct correlation between tritium concentrations and the implementation of countermeasures suggests the existence of established underground pathways for water movement before the implementation of those measures. The variability in tritium concentrations observed between nearby wells underscores the complexity of groundwater flow patterns, which are likely influenced by the pumping of water from the bypass wells. Three scenarios for future leakage are discussed: infiltration of surface water, leakage from contaminated water tanks, and groundwater spreading horizontally along an impermeable layer. The ⁸⁷Sr/⁸⁶Sr isotope ratio analysis suggests a complex groundwater flow pattern. While the detected tritium concentration is low, the continuous nature of the leakage and the potential for future events to increase the rate of leakage necessitate a stronger focus on land-side monitoring. The study's findings highlight the need for more comprehensive groundwater monitoring networks and improved understanding of the subsurface hydrology around the FDNPP site.

This research confirms continuous land-side tritium leakage from the FDNPP since 2013. The findings highlight the complex hydrogeology of the site and the limitations of current countermeasures. Strengthened monitoring systems on both the land and sea sides are essential to manage the long-term risk of tritium release. Future research could focus on more detailed hydrogeological modeling to better understand groundwater flow patterns and refine the prediction of potential future tritium leakage.

The study's analysis was limited to the specific sampling locations and time frame. While the study used multiple methods for tritium analysis, the observed tritium concentrations were relatively low, making precise quantification challenging. The data availability for flow rates from TEPCO limited the ability to fully quantify the absolute amounts of tritium being leaked.