Radiocarbon Concentrations in Environmental Samples Collected Near the Spent Nuclear Fuel Reprocessing Plant at Rokkasho, Aomori, Japan, During Test Operation Using Spent Nuclear Fuel

Carbon accumulation model for simulating 14C radioactivity in Chinese yam grown from a seed bulbil

Modeling carbon accumulation in crop plants is key to evaluating the transfer of atmospheric 14C into the edible parts of the plants growing near nuclear facilities. Chinese yam 'Nagaimo' (Dioscorea polystachya Turcz.) is a major crop cultivated near a spent nuclear fuel reprocessing plant in Rokkasho, Aomori, Japan. We developed a dynamic compartment model for assessing carbon and 14C accumulation in Chinese yam grown from a seed bulbil in the field. Light and temperature dependence of leaf photosynthesis and temperature dependence of respiration in leaves, stems and belowground parts (tuber and root) were incorporated into the model. Estimated amounts of carbon in the leaves, stems and belowground parts were good agreement with the measured data from the field. Simulation results of 14C accumulation using this model indicated that the accumulation of 14C in belowground parts at the harvest depends on the rate of photosynthesis on the day of exposure.

Radiocarbon in aquatic biota samples in a brackish lake adjacent to a reprocessing plant in Rokkasho, Japan, from 2006 to 2022

The commercial spent-nuclear-fuel reprocessing plant in Rokkasho, Japan, is scheduled for completion in 2024. Its fuel rods were cut into segments and chemically processed between 2006 and 2008, as part of the plant's test operations. During the test, 14C (4.4 TBq) were discharged from the plant's main stack into the atmosphere. At reprocessing plants, 14C has the largest contribution to the effective dose to the public among the nuclides released into the atmosphere, and it important to understand the concentration distribution and impact of 14C in the surrounding environment. This study conducts a continuous monitoring of 14C in aquatic biota samples around the plant from 2006 to 2022. The 14C concentration in most samples was determined at the background level, and the effect of the plant was slightly recognized in some samples. Moreover, the specific radioactivity of 14C decreased faster than the theoretical decay from 0.242 to 0.225 Bq-g C-1.

Tissue-free water tritium and non-exchangeable organically bound tritium concentrations in fish near coastline during and after operation of Japan's first nuclear fuel reprocessing facility

The risks from radioactive wastewater release from nuclear facilities into the ocean are a global concern. Radioactive contaminants, such as tritium (3H), in both forms of tissue free water tritium (TFWT) and non-exchangeable organically bound tritium (NE-OBT), can be incorporated into marine biota and cause radiation doses to biota and future consumers. However, no studies have been conducted to measure both forms of 3H in marine fish as well as evaluate the residence time in the vicinity of a nuclear fuel reprocessing facility. Here, fish from a brackish lake and from the Pacific Ocean coastline of Japan, which are near such a facility, were collected between 2006 and 2021. The reprocessing facility was operational between 2006 and 2009, during which time about 300 times more tritiated water was discharged per year into the ocean compared to the period when the facility was not operational. During operation the annual release was 30 times higher than the treated water released annually from Fukushima Daiichi. As expected, TFWT and NE-OBT concentrations increased in marine fish during operations and had peak values of 3.59 ± 0.03 and 0.56 ± 0.03 Bq/L, respectively. Total dose rates to the fish were 36,000 times lower than the 10 μGy h-1 benchmark. Concentrations gradually decreased to pre-operational levels as the facility was turned off with NE-OBT taking twice as long. Fish sampled from the brackish lake tended to have more incorporated TFWT and NE-OBT concentrations than ocean fish. This indicates that ocean tides might have contributed to the accumulation of discharged tritiated water in the lake via a narrow water channel, which highlights the importance of examining all marine ecosystems in future operations. In both marine environments, the estimated committed effective dose using the highest observed data through ingestion was well below public limits (91,000 times lower).

EXPERIMENTAL EVALUATION OF DISTRIBUTION OF 14C PHOTOASSIMILATED INTO CARBOHYDRATES IN DIFFERENT GROWTH STAGES OF FRUIT-BEARING APPLE SHOOTS USING A 13CO2 IN-SITU EXPOSURE SYSTEM

In this study, we aimed to investigate the photoassimilation process of 14CO2 into agricultural plants through determining the photoassimilated carbohydrate-13C in each part (leaves, current branch and fruit) of the fruit-bearing apple shoots exposed to 13CO2 in different growth stages (early and late fruit development stages). The carbohydrate content was assessed as soluble (ethanol-extracted fraction) and other (HCl-extracted and residual fractions) components. The total (i.e. sum of the three fractions) bulk carbohydrate concentrations in all parts of the shoots were statistically similar between different growth stages. The changes in the concentration of 13C-labeled soluble carbohydrate (i.e. ethanol-extracted fraction) to the total content between different growth stages were statistically unclear among all parts of the shoot. These results suggest that the distribution ratio of photoassimilated 13C in soluble and other components in the apple shoot was thereabout constantly independent of the growth stages.

VARIATION IN RADIATION DOSE RATES FROM RADIONUCLIDES DISCHARGED BY THE SPENT NUCLEAR FUEL REPROCESSING PLANT IN ROKKASHO UNDER DIFFERENT YEARLY WEATHER CONDITIONS

The radiation dose rate from radionuclides released by the spent nuclear fuel reprocessing plant in Rokkasho, Japan, was assessed for a year specified in the safety review during which the weather conditions were not significantly different from those of the other 10 y. However, the actual year-by-year variation in annual radiation dose rate was not examined. A model system for evaluating the dose rate from the radionuclides released into the atmosphere was constructed. In this study, the radiation dose rate in the weather conditions of 24 weather bins was estimated for a standard year by the model. The annual maximum dose rate from 1959 to 2012 was estimated using a simplified method that integrated the dose rates of each weather bin in the standard year by estimating the annual frequency of the bin in the target year. We obtained ~1.3 as the maximum/minimum ratio of the annual maximum dose rate.

RESEARCH ON THE ENVIRONMENTAL EFFECTS OF RADIONUCLIDES AT IES - AN OVERVIEW

This paper summarises the research works of the Institute for Environmental Sciences on the environmental behaviour of radionuclides related to the first commercial-spent nuclear fuel reprocessing plant at Rokkasho Village, Aomori Prefecture, Japan. The distribution and fluctuation in natural radiation in Aomori Prefecture were estimated as basic data. Radionuclides possibly released from the plant operation have been continuously measured using a variety of samples from different locations in Aomori Prefecture. During the test of cutting and chemical treatment of the spent fuel rods from 2006 to 2008, the concentration of 85Kr, 14C, 3H and 129I in the air increased, whereas that of 3H and 129I increased in several environmental samples. A numerical simulation model consisting of several sub-models was constructed for predicting the behaviour of released radionuclides in the environment and for evaluating the realistic radiation dose of residents around the facility.

Importance of root uptake of 14CO2 on 14C transfer to plants impacted by below-ground 14CH4 release

¹⁴C-labelled methane (¹⁴CH₄) released from deep underground radioactive waste disposal facilities can be a below-ground source of ¹⁴CO₂ owing to microbial oxidation of ¹⁴CH₄ to ¹⁴CO₂ in soils. Environmental ¹⁴C models assume that the transfer of ¹⁴CO₂ from soil to plant occurs via foliar uptake of ¹⁴CO₂. Nevertheless, the importance of ¹⁴CO₂ root uptake is not well understood. In the present study, below-ground transport and oxidation of ¹⁴CH₄ were modeled and incorporated into an existing land-surface ¹⁴CO₂ model (SOLVEG-II) to assess the relative importance of root uptake and foliar uptake on ¹⁴CO₂ transfer from soil to plants. Performance of the model in calculating the below-ground dynamics of ¹⁴CH₄ was validated by simulating a field experiment of ¹³CH₄ (as a substitute for ¹⁴CH₄) injection into subsoil in a wheat field in the UK. The proposed model simulation was then applied to ¹⁴C transfer in a hypothetical ecosystem impacted by continuous ¹⁴CH₄ input from the water table (bottom of 1-m thick soil), which simulated continuous release of ¹⁴CH₄ from a deep underground radioactive waste disposal facility. The contrast between the results obtained from the model calculation that assumed different distributions of roots (rooting depths of 11 cm, or 97 cm) and methane oxidation (characterized by e-folding depths of 5 cm, 20 cm, or 80 cm) in the soil provided insight into the relative importance of root uptake and foliar uptake pathways. In the shallowly rooted ecosystem with rooting depth of 11 cm, foliar uptake of ¹⁴CO₂ was significant, accounting for 80% of the ¹⁴C accumulation (as organic ¹⁴C) in the plant (leaf compartment). By contrast, in a deeply rooted ecosystem (rooting depth of 97 cm), where the root penetrated to depths close to the water-table, more than half (63%) the ¹⁴C accumulated in the plant was transferred via the root uptake pathway. We found that ¹⁴CO₂ root uptake (thus ¹⁴C accumulation in the plant) in this ecosystem depended on the distribution of methane oxidation in the soil; all ¹⁴C accumulated in the plant was transferred by the root uptake pathway when methane oxidation occurred at considerable depths (e-folding depths of 20 cm, or 80 cm) in the soil. The high level of ¹⁴CO₂ root uptake was ascribed to the oxidation of added ¹⁴CH₄ (i.e., production of ¹⁴CO₂) in the deep part of the soil and the subsequent high level of root uptake of the deep soil-water containing ¹⁴CO₂. These results indicate that ¹⁴CO₂ root uptake contributes significantly to ¹⁴CO₂ transfer to plants if ¹⁴CH₄ oxidation occurs at great depths and roots penetrate deeply into the soil. It is recommended that current environmental ¹⁴C models must be refined to consider the importance of the root uptake pathway to ensure that dose estimates of ¹⁴CH₄ release from deep underground waste disposal facilities are accurate.

Impacts of C-uptake by plants on the spatial distribution of 14C accumulated in vegetation around a nuclear facility-Application of a sophisticated land surface 14C model to the Rokkasho reprocessing plant, Japan

The impacts of carbon uptake by plants on the spatial distribution of radiocarbon (¹⁴C) accumulated in vegetation around a nuclear facility were investigated by numerical simulations using a sophisticated land surface ¹⁴C model (SOLVEG-II). In the simulation, SOLVEG-II was combined with a mesoscale meteorological model and an atmospheric dispersion model. The model combination was applied to simulate the transfer of ¹⁴CO₂ and to assess the radiological impact of ¹⁴C accumulation in rice grains during test operations of the Rokkasho reprocessing plant (RRP), Japan, in 2007. The calculated ¹⁴C-specific activities in rice grains agreed with the observed activities in paddy fields around the RRP within a factor of four. The annual effective dose delivered from ¹⁴C in the rice grain was estimated to be less than 0.7 μSv, only 0.07% of the annual effective dose limit of 1 mSv for the public. Numerical experiments of hypothetical continuous atmospheric ¹⁴CO₂ release from the RRP showed that the ¹⁴C-specific activities of rice plants at harvest differed from the annual mean activities in the air. The difference was attributed to seasonal variations in the atmospheric ¹⁴CO₂ concentration and the growth of the rice plant. Accumulation of ¹⁴C in the rice plant significantly increased when ¹⁴CO₂ releases were limited during daytime hours, compared with the results observed during the nighttime. These results indicated that plant growth stages and diurnal photosynthesis should be considered in predictions of the ingestion dose of ¹⁴C for long-term chronic releases and short-term diurnal releases of ¹⁴CO₂, respectively.

(14)C levels in the vicinity of the Fukushima Dai-ichi Nuclear Power Plant prior to the 2011 accident

A 50-year-old Japanese cedar (Cryptomeria japonica) from Okuma, ∼1 km southwest of the Fukushima Dai-ichi Nuclear Power Plant, was cored and each annual ring was analysed for (14)C. The (14)C specific activity values varied from 330.4 Bq kg(-1) C in the tree ring formed in 1971 to 231.2 Bq kg(-1) C in the 2014 ring. During the periods 1971-1976 and 2011-2014, the (14)C specific activities are indistinguishable from the ambient background values. However, compared with the ambient atmospheric levels, the (14)C specific activities between 1977 and 2010 are significantly elevated, clearly indicating (14)C discharges from the reactors during their normal operations. In addition, the specific activities are positively correlated with the annual electricity generation values. The excess (14)C specific activities were <36 Bq kg(-1) C, corresponding to an additional annual effective dose of <2 μSv via the food ingestion pathway in the study location. The primary wind direction is east-southeast/southeast with a frequency of ∼30%, in comparison to ∼20% frequency for the direction of the site under study (north-northeast/northeast). This would tend to indicate a similar magnitude of additional effective dose and consequently no significant radiological impact of atmospheric (14)C discharges from the FDNPP during the entire period of normal operations. Additionally, no (14)C pulse in activity can be observed in the year 2011 ring. This might be caused by a limited (14)C release from the damaged reactors during the accident or that the prevailing wind during the short period of release (11th-25th March 2011) was not in the direction of Okuma.

Estimation of 85Kr dispersion from the spent nuclear fuel reprocessing plant in Rokkasho, Japan, using an atmospheric dispersion model

The spent nuclear fuel reprocessing plant of Japan Nuclear Fuel Limited (JNFL) located in Rokkasho, Japan, discharged small amounts of (85)Kr into the atmosphere during final tests of the plant with actual spent fuel from 31 March 2006 to October 2008. During this period, the gamma-ray dose rates due to discharged (85)Kr were higher than the background rates measured at the Institute for Environmental Sciences and at seven monitoring stations of the Aomori prefectural government and JNFL. The dispersion of (85)Kr was simulated by means of the fifth-generation Penn State/NCAR Mesoscale Model and the CG-MATHEW/ADPIC models (ver. 5.0) with a vertical terrain-following height coordinate. Although the simulated gamma-ray dose rates due to discharged (85)Kr agreed fairly well with measured rates, the agreement between the estimated monthly mean (85)Kr concentrations and the observed concentrations was poor. Improvement of the vertical flow of air may lead to better estimation of (85)Kr dispersion.