Distribution of oceanic ¹³⁷Cs from the Fukushima Dai-ichi Nuclear Power Plant simulated numerically by a regional ocean model

Radioactive materials were released to the environment from the Fukushima Dai-ichi Nuclear Power Plant (1F NPP), operated by the Tokyo Electric Power Company (TEPCO), as a result of the reactor accident after the Tohoku earthquake and tsunami on 11 March 2011. The total estimated release amounts of ¹³¹I and ¹³⁷Cs from the 1F NPP reactors (No. 1, No. 2 and No. 3) to the atmosphere were 160 PBq (1.6 × 10¹⁷ Bq) and 15 PBq (1.5 × 10¹⁶ Bq), respectively (Nuclear Emergency Response Headquarters, Government of Japan, NERH, 2011). There are several potential pathways by which these materials might reach the ocean (Table 1). Some of the radioactive materials released to the atmosphere were introduced into the ocean by wet and dry atmospheric deposition. The total estimated amount of ¹³¹I, ¹³⁴Cs, and ¹³⁷Cs released directly to the ocean during 1–6 April, observed near the water intake of the 1F NPP No. 2 reactor, was 4.7 PBq (4.7 × 10¹⁵ Bq), with ¹³⁷Cs accounting for 0.94 PBq (9.4 × 10¹⁴ Bq) (NERH, 2011). In addition, an estimated 20 TBq (2.0 × 10¹³ Bq) of radioactive materials were released near the water intake of No. 3 reactor from 2:00 on 10 May to 19:00 on 11 May. The radioactivity in a planned release of low-level waste water was 42 GBq (4.2 × 10¹⁰ Bq). Thus, the direct release from 1F NPP No.2 reactor was the largest among these three direct release pathways from the 1F NPP reactors to the ocean. The estimates of the release rate from the 1F NPP No. 2 reactor to the ocean, however, have considerable uncertainty because they are not taken measured concentrations of radioactive materials in the ocean into account.

To distinguish the contributions of atmospheric deposition and direct release to the oceanic radioactive contamination, we examined ¹³¹I/¹³⁷Cs observed data. Although freshwater runoff (including both groundwater discharge and river runoff) of radioactive materials deposited at the 1F NPP site and subsequently washed out to sea by precipitation and drainage is another possible release pathway to the ocean, we ignored it in this study under the assumption that its contribution was small. In fact, ¹³⁷Cs deposited on land surface is difficult to be leached out by river runoff (Hirose et al., 1990). More detailed estimates may need to consider other potential release routes. We also ignored the planned release of low-level radioactive waste water in this study because the amount of radioactivity was far smaller than the estimated direct release from the 1F NPP No. 2 reactor.

Because it is more difficult to make observations from ships than to make similar observations on land, the available observations of the released radioactive materials in the ocean are too few to characterize the behavior of those materials in the ocean. Oceanic model simulations can interpolate and extrapolate sparse observations and thus can help us understand the oceanic behavior of radioactive materials (i.e., Tsumune et al., 2003a, Tsumune et al., 2003b, Tsumune et al., 2011). In addition, ocean circulation model simulations can provide useful information that allows efficient planning of future observations. Several institutions and agencies have performed simulations of the behavior of radioactive materials in the ocean, including the Japan Agency for Marine-Earth Science and Technology (JAMSTEC, 2011) and the Japan Atomic Energy Agency (JAEA, 2011) , and Toulouse University, France (Toulouse University, 2011). Intercomparison between simulation results obtained by multiple models will be useful to have better understanding of the behavior of oceanic tracer and improve ocean modeling. Moreover, by comparing observed concentrations of a radioactive tracer material with simulated results it is possible to assess the skill of an ocean circulation model. An accurate estimation of the release rate of ¹³⁷Cs from the 1F NPP reactors will be useful for intercomparison of models used for simulating the behavior of radioactive materials in the ocean.

In this paper, we describe the observation data and the regional ocean model in Section 2. In Section 3 (Results and Discussion), we analyze the observed data for ¹³¹I and ¹³⁷Cs to determine the amounts released directly and by atmospheric deposition. We also estimate the amount of ¹³⁷Cs released directly from the 1F NPP No. 2 reactor by comparing the observed data with simulated results. In addition, we simulate the behavior of ¹³⁷Cs released from the 1F NPP No. 2 reactor off Fukushima to understand the distribution of oceanic contamination. We verify the simulated result by comparison with the observed data. Finally, we compare the amount of oceanic contamination with ¹³⁷Cs between the Fukushima accident and past radioactive contamination events due to atmospheric weapons tests since 1945, the Chernobyl accident in 1986, and releases from the British Nuclear Fuels, Ltd (BNFL) reprocessing site at Sellafield, UK since 1952.