In the time since the earthquake, the tsunami, and the nuclear disaster in Japan in 2011, many in the ocean have improved from Fukushima Daiichi Nuclear Power Plant (FDNPP). Concentrations of cesium isotopes, some of the most abundant and long-lived pollutants released, are hundreds of thousands of times less than their peak in April 2011. Since mid-2015, none of the fish caught nearby has exceeded Japan’s strict limit of cesium 100 kr. / Kg (1, 2). However, huge challenges remain in shutting down reactors and cleaning the ground. Small, and sometimes unexpected, pollutant sources still enter the ocean to this day (3). Two of the biggest unresolved problems are what to do with more than 1,000 tanks on site containing polluted water and the effect of releasing more than a million tons of this water into the ocean.
Tank water is a mixture of reclaimed groundwater and intentional injected cooling water, both of which became contaminated when interacting with the nuclei of highly radioactive nuclear reactors. From the first months after the earthquake and tsunami, this water was contained in tanks to prevent further radioisotope release and treatment using several systems, most notably the Advanced Fluid Treatment System (ALPS). ALPS is designed to efficiently remove over 62 different pollutants. The installation also helped in an ice dam and other groundwater barriers, as well as diverting the flow of groundwater around the site, to reducing the daily accumulation of water from more than 400 to less than 200 metric tons per day.
Despite this effort, no decontamination system can remove 100% of all radioactive contaminants. Tritium, 3H, is difficult to remove, because it is a radioactive form of hydrogen that is part of the water molecule itself. Fortunately, tritium is relatively harmless because it emits a low-energy molecule that does little harm to living cells. As a result, tritium contains the lowest dose coefficient for these radioisotopes mentioned in tanks (4) and the highest permissible release limits (see Table). These properties do not detract from the possibility that large quantities of tritium may have adverse health effects.
The total amount of tritium in tanks is also important, which is said to be about 1 PBq (PBq = 1015 Bq) (5). This total is much less than 8,000 PBq of tritium that is still remaining from global nuclear experiments in the atmosphere in the 1960s or 2000 PBq from the natural interactions between the universe and nitrogen particles that make up tritium in the atmosphere. Additionally, all nuclear power plants emit tritium which, depending on the plant design, can be several PBqs per year, or higher, as in nuclear fuel reprocessing plans such as Cap de La Hague (6).
Release limits and risks
Different isotopes pose various environmental and health challenges.
However, this story is not just about tritium but what’s new in tanks. It was not until mid-2018 when TEPCO, the operator of FDNPP, released data detailing the quantities of the most dangerous isotopes, such as ruthenium 106, cobalt 60 and strontium 90 (7). Concentrations of these radioisotopes are orders of less magnitude than tritium but are very variable from one tank to the next (see figure). According to TEPCO’s own assessments, more than 70% of the tanks will need secondary treatment to reduce concentrations lower than those required by law to release them (7).
However, there are other important factors to consider. These radioisotopes act differently from tritium in the ocean and are more easily integrated into marine organisms or seabed sediments (see figure). For example, the biological concentration factors in fish are more than 50,000 to carbon-14 over tritium (8). Also, isotopes such as Cobalt 60 are 300,000 times more likely to ultimately be associated with seabed sediments (8). As a result, tritium behavior patterns in the ocean, with tritium dispersion and rapid dilution, cannot be used to assess the fate of these other potential pollutants.
Sorting what is in the tanks
One of the legacies of the Fukushima Daiichi nuclear disaster after the 2011 earthquake and Tohoku Oki is the accumulation of water with a variety of radioisotopes in tanks. An assessment of the risk of water discharging from these reservoirs to the ocean requires knowledge of the quantities of radioisotopes and their ability to concentrate in seabed sediments and biological tissues.
GPU: N. CARY / SCIENCE
To evaluate the results of tank release, full accounting is required after any secondary treatments for the isotopes remaining in each tank. This includes volume, not only for the nine isotopes currently reported but for a larger set of possible contaminants, such as plutonium. Plutonium may be present in FDNPP cooling water but was not released in large quantities into the atmosphere in 2011.
The public was told that there were few options other than ocean drainage. However, given the short half-life of known isotopes in tanks, time will help. With a half-life of 12.3 years, at 60 years, 97% of all tritium will break down, along with many other short-lived isotopes. In those overlapping years of on-site cleaning, about four times the current size will be generated. The risk of reservoir leakage – even if it is stored in earthquake-proof tanks, similar to what Japan is already doing for petroleum or LNG – must be balanced with the radiant amount of radiation after decay. The lack of space can be mitigated, and the urgency of ocean release, if tanks are stored outside current FDNPP limits.
Finally, general concerns should not be ignored as these decisions may have negative impacts on local fisheries that are now being rebuilt. Providing data is a good start (9) but not enough. Monitoring seafood and oceans should continue to involve local fishermen, and studies involving public participation in sampling will be an effective tool for improving public education and building confidence in the results (10).
The current focus on tritium in waste water retention tanks ignores other radioisotopes, but it is a solvable problem. The solution includes reducing concentrations of non-tritium pollutants, reporting after secondary treatment to independently verify the concentrations of all pollutants in each tank, and reviewing other storage options. If there is a release, the independent study of the oceans on multiple pollutants in seawater, marine plants and seabed sediments should be supported before, during and after. Although operators have promised some of this, actions will be more important than words. What needs to be added to the discussion is that the non-tritium isotopes in those tanks have a much different toxicity and fate in the ocean.
References and observations, radioactivity levels are measured in Becquerel (Bq) per unit volume or mass, with 1 Becquerel = one degradation event per second.
ICRP Publications 119, “Summary of Dosage Parameters Based on the Publication of 60 ICRP” (ICRP, 2010).
International Atomic Energy Agency, “Sediment distribution factors and concentrations of living organisms in the marine environment” (IAEA, 2004).
Acknowledgment: This work was supported by the Dearbrook Charitable Fund and the Center for Marine and Environmental Radioactivity. We also appreciate writing assistance by K. Kostel.
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