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
The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident released a large amount of radioactive materials into the environment since 2011. Most were in the gaseous state, released primarily through the atmosphere to the land of eastern Japan and to the north-west Pacific Ocean. The released amount was estimated to be approximately 520 PBq1, with radioactive iodine (mainly 131I), radioactive cesium (134Cs, 137Cs), and noble gases such as 133Xe accounting for most of the released amount. Tritium (3H, T1/2 12.3 y.) was an additional part of the radioactive materials released, but is considered as a “soft”, or low energy, beta emitter. The tritium beta energy is low (max 18.6 keV), and requires large quantities to deliver significant radiation doses, so that the measurement of other nuclear species was prioritized when considering human protection immediately following the accident. Therefore, data on tritium in the environment after the FDNPP accident are still limited in Japan2,3.
Tritium in a boiling water reactor is mainly produced by ternary fission. At FDNPP, 8.51 × 1013 Bq/month at 1.1 MW operation was produced by ternary fission4. Tritium is also produced in reactors by 10B(n, 2α)3H, 10B(n, α)7Li, 7Li(n, α)3H, or 6Li(n, α)3H, 2H(n, γ)3H5.
Cumulate 3H yields in the reactors at FDNPP have been estimated to range from 0.01% to 0.0108%6,7. According to estimates made immediately after the accident in 2011, there were reports that the inventory of 3H at the time of the accident was 1.81 × 1013 Bq8, but according to recent reports by Tokyo Electric Power Company Holdings (TEPCO), the inventory of 3H immediately after the accident was estimated to be 1.0 × 1015 Bq at Unit 1, 1.2 × 1015 Bq at Unit 2, and 1.2 × 1015 Bq at Unit 3, for a total of 3.4 × 1015 Bq4. As of March 24, 2016, 7.6 × 1014 Bq was in the storage tanks at the FDNPP site, 2.7 × 1013 Bq in the reactor building(R/B), and estimated 1.8 × 1015 Bq was released outside the reactor or in debris (Table 1)9,10.
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.
An estimated 0.1–0.5 PBq of 3H flowed into the north Pacific Ocean from the accident6,11. Tritium was detected in the north-west Pacific Ocean off the coast of Hirono town, Fukushima Prefecture 1 month after the accident12.
Investigation 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. A surface water concentration of 3H at 184 (± 2) Bq/L was detected in rice paddy fields at 1.5 km from the FDNPP plant12. Since both reports greatly exceeded the natural 3H level in Japan (1.1–7.8 TU, equivalent to 0.13–0.92 Bq/L) or 6 TU (equivalent to 0.71 Bq/L)2,14, there was no doubt that the 3H was from the FDNPP accident. Also, since the samples were collected approximately 1 month after the accident, the 3H on the ground most likely originated as precipitation from the atmosphere, not via groundwater.
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. To reveal the hydrogeological origin of the groundwater sources, Sr isotope ratio (87Sr/86Sr) was also measured as a natural tracer of water–rock interaction and ground water mixing patterns15,16,17,18.
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. In Japan, the limit for tritium release into the ocean is 6.0 × 104 Bq/L in a typical nuclear facility, but in the case of the FDNPP, 1500 Bq/L is the regulatory limit for tritium effluent19. Therefore, over 1.2 × 1012 L of water would be required for dilution.
Figure 1 shows a schematic diagram of the nuclear power plant site after the accident.
The land-side water impermeable wall (frozen soil wall) and the sea-side water impermeable wall (steel sheet pile) were installed to surround the circumference of the FDNPP and prevent 3H flow off site. Frozen soil walls block uncontaminated groundwater from getting close to reactors and buildings, while steel sheet piles block potentially contaminated groundwater from spreading into the ocean.
A series of wells were drilled at 35 m above sea level, upstream of FDNPP, to reduce the amount of groundwater flowing under the reactor building, and the well water was constantly pumped (Ground water bypass). The wells were drilled to a depth directly above an impermeable layer inside the plant’s grounds. Figure 2 shows the radioactivity of tritium in groundwater flowing through this bypass from June 2014 to June 2019. The ground water bypass system has 12 wells (No.1 to No.12)20, and the highest concentration of radioactivity was in No. 10 well on the south side. The concentration of 3H on June 2014 was 10 Bq/L, but it exceeded 3000 Bq/L in April 2016 and has been gradually decreasing since then to approximately 1400 Bq/L in 2019. No. 10 well is next to No.11, which also had levels of 3H higher than other wells, at 700 Bq/L as of June 11, 2019. No. 12 is the southernmost well, but unlike No. 10 and No. 11 wells, the tritium levels tended to decrease monotonically from a peak in April 201421.
Groundwater was estimated to flow into the ocean from the mountain side based on ground water flow modeling22.
It was not possible to determine from these data whether tritium-contaminated groundwater was still being released as tritium had already spread before the completion of the several barriers. Contaminated water may still be leaking from FDNPP site even after the barrier was completed23. 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.
The movement of groundwater may be impacted by the removal of the water from the wells. The amount of water removed from the wells has been changed in a timely manner in order to maintain appropriate groundwater level. If the water level was lowered too much, water flow would be induced from the reactor.
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.
3H radioactivity leakage
The concentration of 3H in the sump water collected at the sites indicated by asterisks in Fig. 3 is shown in Fig. 4. The 3H observed in sump water ranged from 15 to 31 Bq/L and was almost constant (average 20 Bq/L). The 3H exceeded the expected natural level (up to 7.8 TU(1 TU = 0.118 Bq/L), 0.92 Bq/L) of 3H, thus it is assumed that the 3H originated from FDNPP. Since the sump water were collected directly from cliffs, tritium in sump water would have passed under the ground of FDNPP site.
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). At the FDNPP, four measures have been taken to prevent surface water from infiltrating into groundwater25..
- 1. Grouting of surfaces (to prevent from soaking rainwater into the ground) (from Oct. 2014),
- 2. Pumping of water from the sub-drain (from Sep. 2015),
- 3. Frozen soil wall around the 4 nuclear plants (from Mar. 2016),
- 4. Sea-side impermeable wall (from Oct. 2015).
It was clear that there was no direct correlation with the radioactivity of tritium contained in the leachate compared with the respective construction periods.
No tritium above the natural level was detected in the flowing-wells about 500 m away from the nuclear power plant. (see supplementary data).
The flowing-well water tritium concentration ranged from 0.003 to 0.01 Bq/L and was measured using the ingrowth method. Natural level of 3H in Japan was ranged from 0.13–0.92 Bq/L. Meanwhile the radioactivity of tritium in flowing water was below 0.01 Bq/L. The radioactivity of the water was at least one eighth. It was considered to be at least three half-lives above conservative estimates. Therefore, it was estimated that the tritium in the groundwater from the flowing-well had an age of nearly 40 years.
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