A past leak of tritium from the Fukushima Daiichi Nuclear Power Plant has unexpectedly provided the key to understanding how another, more dangerous radioactive isotope continues to enter the ocean. Researchers have now identified rainwater washing over the contaminated roofs of the plant’s reactor buildings as the primary source of ongoing radioactive cesium-137 releases into the facility’s harbor, solving a long-standing puzzle about fluctuating contamination levels.
For years, operators of the damaged plant have observed persistent low-level discharges of cesium-137, a major radioactive contaminant from the 2011 nuclear accident. Concentrations in the harbor water would inexplicably rise after rainfall and during the summer, even as extensive efforts were made to contain contaminated water. By using the previously leaked tritium as a tracer to follow water movement through the site, a team from the University of Tsukuba determined that more than half of the cesium flowing into the sea is being scoured from reactor building rooftops and funneled through a specific drainage channel. This breakthrough provides the first quantitative breakdown of the contamination pathways, offering crucial new insights for remediation.
An Accidental Tracer Solves a Mystery
Since the initial disaster, Tokyo Electric Power Company Holdings (TEPCO) has implemented numerous measures to control the vast amounts of contaminated water at the site. While these actions significantly reduced the discharge of radioactive materials, cesium-137 continued to appear in the plant’s harbor. Observations since 2016 showed a distinct pattern: cesium levels would increase after it rained and throughout the summer, then decrease in the winter. Officials had traced these fluctuations to an outlet known as the K drainage channel, which was redirected to the harbor in 2015, but the precise mechanism driving the release remained unknown.
The solution came from an unlikely source: a separate contamination event. During 2013 and 2014, tritium, a radioactive isotope of hydrogen, leaked from on-site storage tanks and seeped into the groundwater. Because tritium binds with oxygen to form radioactive water, it moves through the environment in exactly the same way as normal water. Recognizing this, University of Tsukuba researchers realized the leaked tritium could be used as a “natural hydrological tracer,” allowing them to precisely track the movement of water across the plant grounds and identify the routes it took to the K drainage channel and, ultimately, the ocean.
Rainwater from Reactor Roofs is the Main Culprit
The investigation, which analyzed data from 2016 to 2021, revealed that the dominant pathway for cesium contamination is “roof drainage.” This process begins when rainwater lands on the large, flat roofs of the reactor buildings, which remain contaminated with cesium-137 particles from the original 2011 meltdowns. The rainwater dissolves and captures these radioactive particles before flowing into gutters and pipes. This contaminated water is then funneled directly into the K drainage channel, providing a direct route to the harbor.
The study quantitatively demonstrated the significance of this pathway. According to the researchers’ analysis, this roof drainage mechanism is responsible for an estimated 53% of the total cesium-137 discharge into the port annually. This finding was crucial, as it pinpointed a major, previously underappreciated source of ongoing contamination that was directly linked to weather events like rainfall. The direct connection explains why harbor cesium concentrations would spike shortly after precipitation. The discovery shifts the focus of remediation efforts to the high, difficult-to-access surfaces of the reactor buildings themselves as the primary lingering source of mobile contamination.
Identifying Secondary Contamination Pathways
While roof drainage was the largest contributor, the tritium tracer also helped quantify two other significant sources of radioactive cesium that feed into the drainage system and the harbor.
Surface Runoff from Contaminated Ground
The second-largest pathway identified was rainwater surface runoff. This involves rain falling on the ground across the plant site—including paved areas, soil, and debris fields—that still holds residual cesium-137 contamination. As this water flows across the ground, it picks up radioactive particles before eventually making its way into the same K drainage channel. This runoff pathway was found to be responsible for approximately 31% of the cesium discharge observed during the 2016–2021 study period. Unlike the highly concentrated flow from the reactor roofs, this source is more diffuse, representing a broader, site-wide challenge for containment and cleanup operations.
Seasonal Groundwater Baseflow
A third, smaller pathway involves contaminated groundwater, described by the researchers as “baseflow.” This is a persistent, low-level seepage of groundwater into the drainage channel system. The study revealed that this baseflow contributes about 15% of the total cesium discharged into the harbor. Critically, the tritium tracer data helped explain the seasonal variations that had puzzled observers for years. The researchers found that the rate of this groundwater flow is influenced by air temperature. During the summer, higher temperatures cause changes in the groundwater dynamics, increasing the flow rate and carrying more dissolved cesium into the drainage channel. This finding directly accounts for the consistent rise in harbor cesium levels during warmer months.
New Focus for Future Cleanup Efforts
The findings, published in the scientific journal Water Research, provide the most detailed understanding to date of how radioactive cesium continues to escape from the Fukushima Daiichi site. By precisely identifying and quantifying the main pathways, the University of Tsukuba study offers a clear road map for more effective and targeted remediation. The discovery that over half of the cesium comes from reactor rooftops highlights the urgent need to address contamination on these elevated surfaces, either through decontamination or by engineering new systems to divert the rainwater before it enters the existing drainage network.
This research underscores the immense complexity of managing nuclear accident sites long after the initial crisis has passed. It demonstrates how radioactive contaminants can persist in the environment and find unexpected pathways to migrate, even when major containment structures are in place. The innovative use of a previous tritium leak as a hydrological tool not only solved a key environmental mystery at Fukushima but also established a methodology that could be applied at other complex industrial or contaminated sites to track the movement of pollutants. The results provide TEPCO and Japanese regulators with critical data to refine their strategies and prioritize actions that can finally cut off the lingering flow of radioactive materials into the marine environment.
