A critical review and update of modelling of treated water discharging from Fukushima Daiichi NPP.

Since the accident at the Fukushima Daiichi nuclear power plant (FDNPP) in March 2011 seawater is still needed to cool the reactor cores. This water, contaminated with radionuclides, has been collected in tanks and treated on the site of the FDNPP. In 2021, the Japanese government decided to gradually discharge treated water into the ocean, which started on the 24th of August 2023 and will continue for the next 30 years. This paper provides a critical analysis of the models that were used in the different radiological impact studies. Based on the analysis, a hydrodynamic and a compartment models with a harmonized setup were used to estimate the impact of the discharge on humans and biota. Doses obtained with these two models were within one order of magnitude for humans (<0.1 µSv/year) and for biota (<10-6 mGy/d) indicating that harmonization of the model parameters improved the reliability of the simulation results.

Estimate of Fukushima-derived radiocaesium in the North Pacific Ocean in summer 2012.

Distributions of radiocaesium (134Cs and 137Cs) derived from the Tokyo Electric Power Company (TEPCO) Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident in the North Pacific Ocean in the summer of 2012 were investigated. We have estimated the radiocaesium inventory in the surface layer using the optimal interpolation analysis and the subducted amount into the central mode water (CMW) by using vertical profiles of FNPP1-134Cs and mass balance analysis as the first approach. The inventory of the 134Cs in the surface layer in the North Pacific Ocean in August-December 2012 was estimated at 5.1 ± 0.9 PBq on 1 October 2012, which corresponds to 8.6 ± 1.5 PBq when it was decay corrected to the date of the FNPP1 accident, 11 March 2011. It was revealed that 56 ± 10% of the released 134Cs into the North Pacific Ocean, which was estimated at 15.3 ± 2.6 PBq, transported eastward in the surface layer in 2012. The amount of 134Cs subducted in the CMW was estimated to be 2.5 ± 0.9 PBq based on the mass balance among the three domains of the surface layer, subtropical mode water, and CMW.

The POSEIDON-R compartment model for the prediction of transport and fate of radionuclides in the marine environment.

A detailed description of the advanced version of compartment model POSEIDON-R for the prediction of transport and fate of radionuclides in the marine environment is given. The equations of transfer of radionuclides in the water and bottom sediment compartments along with the dynamical food chain model are presented together with dose module to assess individual and collective doses to the population due to the regular and accidental releases of radionuclides. The method for the numerical solution of model equations is also presented. The modelling results for the northeast Atlantic shelf seas were compared with measurements of 137Cs. •The three-dimensional compartment model POSEIDON-R describes the transfer of radionuclides and their daughter products in marine environment as a results of regular or accidental releases. This includes any transfer through the water column and sediments.•The model is complemented by a dynamic food chain model for transfer of radioactivity in pelagic and benthic food webs.•The dose module in the model calculates internal and external doses for humans and non-human biota.

Similarities and differences of 137Cs distributions in the marine environments of the Baltic and Black seas and off the Fukushima Dai-ichi nuclear power plant in model assessments.

The compartment model POSEIDON-R with an embedded food web model was used to assess 137Cs distributions in the Baltic and Black seas and off the Pacific coast of Japan during 1945-2020 due to the weapon testing and accidents at the Chernobyl and Fukushima Dai-ichi nuclear power plants. The results of simulations conducted with generic parameters agreed well with measurements of 137Cs concentrations in the water, bottom sediments, and in fish. In the Black and Baltic seas, salinity variations affected the transfer of 137Cs through the food web. The contamination of pelagic fish followed the water contamination with some delay, whereas demersal fish depuration was found to be related to decreasing 137Cs concentrations in the upper sediment layer. On the Pacific shelf off Japan, intensive currents and eddies caused the simulated depuration rates in fish to be one-two orders of magnitude larger than those in the semi-enclosed Black and Baltic seas.

Spatial and temporal distributions of (134)Cs and (137)Cs derived from the TEPCO Fukushima Daiichi Nuclear Power Plant accident in the North Pacific Ocean by using optimal interpolation analysis.

Optimal interpolation (OI) analysis was used to investigate the oceanic distributions of (134)Cs and (137)Cs released from the Tokyo Electric Power Company Fukushima Daiichi Nuclear Power Plant (FNPP1) accident. From the end of March to early April 2011, extremely high activities were observed in the coastal surface seawater near the FNPP1. The high activities spread to a region near 165°E in the western North Pacific Ocean, with a latitudinal center of 40°N. Atmospheric deposition also caused high activities in the region between 180° and 130°W in the North Pacific Ocean. The inventory of FNPP1-released (134)Cs in the North Pacific Ocean was estimated to be 15.3 ± 2.6 PBq. About half of this activity (8.4 ± 2.6 PBq) was found in the coastal region near the FNPP1. After 6 April 2011, when major direct releases ceased, the FNPP1-released (134)Cs in the coastal region decreased exponentially with an apparent half-time of about 4.2 ± 0.5 days and declined to about 2 ± 0.4 PBq by the middle of May 2011. Taking into account that the (134)Cs/(137)Cs activity ratio was about 1 just after release and was extremely uniform during the first month after the accident, the amount of (137)Cs released by the FNPP1 accident increased the North Pacific inventory of (137)Cs due to bomb testing during the 1950s and early 1960s by 20%.

Optimum interpolation analysis of basin-scale ¹³7Cs transport in surface seawater in the North Pacific Ocean.

¹³7Cs is one of the conservative tracers applied to the study of oceanic circulation processes on decadal time scales. To investigate the spatial distribution and the temporal variation of ¹³7Cs concentrations in surface seawater in the North Pacific Ocean after 1957, a technique for optimum interpolation (OI) was applied to understand the behaviour of ¹³7Cs that revealed the basin-scale circulation of Cs ¹³7Cs in surface seawater in the North Pacific Ocean: ¹³7Cs deposited in the western North Pacific Ocean from global fallout (late 1950s and early 1960s) and from local fallout (transported from the Bikini and Enewetak Atolls during the late 1950s) was further transported eastward with the Kuroshio and North Pacific Currents within several years of deposition and was accumulated in the eastern North Pacific Ocean until 1967. Subsequently, ¹³7Cs concentrations in the eastern North Pacific Ocean decreased due to southward transport. Less radioactively contaminated seawater was also transported northward, upstream of the North Equatorial Current in the western North Pacific Ocean in the 1970s, indicating seawater re-circulation in the North Pacific Gyre.