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).

Temporal variation of tritium concentration in monthly precipitation collected at a Difficult-to-Return Zone in Namie Town, Fukushima Prefecture, Japan.

This article discusses tritium concentrations in monthly precipitation in part of the Difficult-to-Return Zone in Namie Town during 2012-2021. The tritium concentrations, which were measured with a low background liquid scintillation counter after carrying out an enrichment procedure, fluctuated seasonally from 0.10 ± 0.02 to 0.85 ± 0.02 Bq L-1. This range of concentrations is concluded to not be unusual based on comparisons with the concentrations at other sites and estimates of the past range of the concentrations. Moreover, no significant variations in observed tritium concentrations were observed due to decommissioning work at the Fukushima Dai-ichi Nuclear Power Plant. These results contribute to understanding the background level of tritium concentration in precipitation before the oceanic discharge of treated water from the Fukushima plant. In addition, this article evaluates the amount of tritium supplied to the ocean by terrestrial rainwater pouring into the Pacific Ocean via Ukedo River, which flows through Namie Town; this information will contribute to the discussion on the impact of the oceanic discharge of treated water.

ANALYSIS OF 210PB DEPOSITION DISTRIBUTION CHARACTERISTICS IN WINTER AT ROKKASHO IN AOMORI BASED ON HIGH RESOLUTION ATMOSPHERIC TRANSPORT/DEPOSITION MODEL CALCULATION.

To evaluate 210Pb deposited on the surface of sea around Japan in detail, the performance of atmospheric transport/deposition model for 222Rn and its progenies was examined. To test the model's reproducibility of 210Pb deposition in winter, it was applied at Rokkasho in Aomori, where the model had been reported to significantly underestimate when the horizontal grid interval of 9 km was used. It was shown that the precipitation on the Pacific Ocean side and Mutsu Bay and hence the 210Pb deposition at Rokkasho were significantly improved with the grid interval of 3 km although the deposition was still underestimated by the observation. This underestimation was considered to be caused by model's neglect of horizontal drift of snow and rain. It was also pointed out that the deposited 210Pb was mainly contributed by the lower atmosphere up to 2-3 km.

DISTRIBUTION OF RADIOCESIUM IN BLACK PINE TREE FORESTS IN ROKKASHO, AOMORI, JAPAN.

Since the Fukushima Dai-ichi Nuclear Power Plant (FDNNP) accident in 2011, 134Cs had been detected in atmospheric deposition samples collected in Rokkasho and Aomori in Japan, located ~400-km north of the FDNNP, during March 2011-May 2015. To study the distribution of radiocesium (134, 137Cs) in a Japanese black pine tree forest, we measured the activity concentration of 134, 137Cs in whole black pine trees, including roots collected during 2017-19 along with soil samples in 2015 and 2017-19. The results show that most of the deposited 134Cs was retained in the forest floor in 2015, with depth distribution different from that of the weapon testing fallout. Proportions of the weapon fallout 137Cs in trunks, branches, needles and roots were 44, 17, 13 and 26%, respectively, indicating that >50% of 137Cs in above-ground part existed in the trunks. The total inventories of 134Cs and 137Cs in the forest were estimated to be 8.9 and 2.5·103 Bq m-2, respectively, and ~4.8% of 137Cs inventory was originated from the accident. Inventory of 134Cs in the forest-including the black pine trees-was 1.6 times larger than cumulative deposition outside of the forest due to the canopy effect.

LONG-RANGE ATMOSPHERIC TRANSPORT OF RADON IN EAST ASIA AND DEPOSITION OF ITS PROGENIES IN JAPAN.

A three-dimensional long-range atmospheric transport model for 222Rn and its progenies was applied to simulate 210Pb deposition in Japan and its adjacent waters. Monthly precipitation and 210Pb deposition simulated by the model for winter months were favorably compared with observational data although they were exceptionally underestimated in Aomori. The large monthly deposition along the Japan Sea coast lines during winter was successfully simulated by the model to be nearly 200 Bq m-2, which was contrasted by about 10-fold smaller deposition in other regions. It was also pointed out that the heavy deposition areas formed a narrow band structure along the coastline with width of several tens of kilometers and 210Pb deposition amount positively correlated with the height of mountains located leeward of the heavy deposition areas.

TRITIUM AND IODINE-129 IN WATER SAMPLES COLLECTED ADJACENT TO A SPENT NUCLEAR FUEL REPROCESSING PLANT IN ROKKASHO, JAPAN.

Between April 2006 and October 2008, tritium ( 3H) and iodine-129 ( 129I) were released into the atmosphere and ocean from a spent nuclear fuel reprocessing plant in Rokkasho, Japan. From 2005 to 2020, water samples were collected from water bodies around the plant, including a river, groundwater wells, a brackish lake, a fishing port and a coastal sea, to measure spatiotemporal changes in 3H and 129I concentrations. Water samples from the brackish lake and the fishing port between 2006 and 2008 occasionally had higher than background levels of 3H and 129I. Batched discharge of waste 3H and 129I was quickly diluted by advection-diffusion processes in the coastal sea, causing 3H and 129I from the plant to be indetectable. By contrast, concentrations of 3H and 129I that flowed into the brackish lake and the fishing port through various routes were high, as these water bodies are mostly closed systems.

TEMPORAL VARIATION OF POST-ACCIDENT 129I IN ATMOSPHERIC PARTICULATE MATTER COLLECTED FROM AN EVACUATED AREA OF FUKUSHIMA PREFECTURE, JAPAN.

To understand the behavior of atmospheric 129I that originated from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, activity concentrations of 129I in samples of atmospheric particulate matter (PM), comprising coarse (>1.1 µm) and fine (<1.1 µm) fractions (separated using a single stage impactor), were measured on a nearly monthly basis from October 2012 to October 2014 at a site in an area evacuated after the FDNPP accident. Total atmospheric 129I activity concentrations in PM at the site ranged between 0.15 and 2.17 nBq m-3. Specific activity concentration of 129I in total atmospheric PM ranged between 40.8 and 336 mBq kg-1, with a mean and standard deviation of 142 and 77.6 mBq kg-1, respectively. The specific activity in PM tended to be higher than not only the background values reported from soil collected before the FDNPP accident but also than those reported from the contaminated soil after the accident (range: 1.6-57 mBq kg-1; mean and standard deviation: 10.6 and 12.3 mBq kg-1, respectively). Total 129I/127I atomic ratios in PM ranged from 2.0 × 10-8 to 59.8 × 10-8, with a mean and standard deviation of 15.0 × 10-8 and 14.4 × 10-8, respectively. These ratios were generally lower than those of the contaminated soil collected after the FDNPP accident (range: 4.9 × 10-8-443 × 10-8; mean and standard deviation: 74.2 × 10-8 and 85.4 × 10-8, respectively). The 129I concentration and 129I/127I atomic ratio in atmospheric PM showed different characteristics from that of contaminated soils, suggesting that the presence of other atmospheric PMs plays a more important role as the host for 129I.

Ten-year radiocesium fluvial discharge patterns from watersheds contaminated by the Fukushima nuclear power plant accident.

After the Fukushima Dai-ichi nuclear power plant accident in March 2011, the fluvial discharge of 137Cs from watersheds to rivers was analyzed between 2011 and 2021. The concentrations of dissolved and particulate 137Cs were measured in river water samples collected from two rivers (the Hiso and Wariki rivers, mainly draining farmlands and forests, respectively) draining approximately 4-7 km2 watersheds in a montane area (the areal deposition of 134+137Cs; 1-3 MBq·m-2 in March 2011) of Iitate Village, Fukushima. Over the 10 yr analysis, the particulate 137Cs concentrations in the Hiso and Wariki rivers decreased by 70 and 50 times, respectively, and that of the dissolved form decreased by 150 and 130 times, respectively. Moreover, the apparent Kd (distribution coefficient) of 137Cs for water samples from both rivers have increased gradually over these periods. In 2011, the 137Cs discharge rates through the Hiso and Wariki rivers were 0.63% and 0.46% per year of the total amount of 137Cs deposited in their catchments, respectively; however, by 2021, these rates had decreased to 0.09% and 0.03% per year. The cumulative 137Cs discharge rates over 10 yr in the farmland- and forest-dominated rivers were 1.95% and 1.33%, respectively. These rates of the farmland-dominated river were ∼1.4-fold greater than those of the forest-dominated river. Moreover, ∼90% of the of the discharge occurred in particulate form while the remaining ∼10% was in the dissolved form. Thus, 137Cs deposited within these watersheds due to the accident was gradually discharged by the rivers over the 10-yr period; however, the majority remains stored in soils and litters etc. of the catchment area. These results indicate that 137Cs outflow from land-to-ocean will be limited in the future, as the river export of 137Cs is expected to continue decreasing.

Processes affecting land-surface dynamics of 129I impacted by atmospheric 129I releases from a spent nuclear fuel reprocessing plant.

Terrestrial environments impacted by atmospheric releases of 129I from nuclear plants become contaminated with 129I; however, the relative importance of each land-surface 129I-transfer pathway in the process of the contamination is not well understood. In this study, transfers of 129I in an atmosphere-vegetation-soil system are modeled and incorporated into an existing land-surface model (SOLVEG-II). The model was also applied to the observed transfer of 129I at a vegetated field impacted by atmospheric releases of 129I (as gaseous I2 and CH3I) from the Rokkasho reprocessing plant, Japan, during 2007. Results from the model calculation and inter-comparison of the results with the measured environmental samples provide insights into the relative importance of each 129I-transfer pathway in the processes of 129I contamination of leaves and soil. The model calculation revealed that contamination of leaves of wild bamboo grasses was mostly caused by foliar adsorption of inorganic 129I (81%) following wet deposition of 129I. In contrast, accumulation of 129I in the leaf due to foliar uptake of atmospheric 129I2 (2%) was lesser. Root uptake of soil 129I was low, accounted for 17% of the 129I of the leaf. The low root-uptake of 129I in spite of the 129I contained in the soil was ascribed to the fact that the most fraction (over 90%) of the soil 129I existed in "soil-fixed" (not plant-available) form. Regarding the 129I-transfer to the soil, wet deposition of 129I was ten-fold more effective than dry deposition of atmospheric 129I2; however, the deposition of 129I during the year represented only 2% of the model-assumed 129I that pre-existed in the soil; indicating the importance of long-term accumulation of 129I in terrestrial environments. The model calculation also revealed that root uptake of inorganic 129I can be more influential than volatilization by methylation in exportation of 129I from soil.

A SIMULATION STUDY OF DEPOSITION PARAMETERS FOR 129I DISCHARGED FROM THE ROKKASHO REPROCESSING PLANT.

The first commercial spent nuclear fuel reprocessing plant at Rokkasho in Japan discharged 129I from actual spent nuclear fuel into the atmosphere during its test operation from 2006 to 2008. Previously, we measured monthly atmospheric concentrations of gaseous and particulate 129I and atmospheric deposition rates of 129I from the campus of our institute, which is 2.6 km east of the main stack of the plant. In this study, we simulated the atmospheric concentrations and deposition rates of 129I using a combination of the Fifth-Generation Penn State/NCAR Mesoscale Model and the improved CG-MATHEW/ADPIC models, Version 5.0. Here, we report on the optimised deposition parameters of 129I used to simulate the measured values using 129I atmospheric discharge rates from the main stack.

Regional and global contributions of anthropogenic iodine-129 in monthly deposition samples collected in North East Japan between 2006 and 2015.

We measured the monthly atmospheric deposition flux of 129I at Rokkasho, Aomori, Japan-the location of a commercial spent nuclear fuel reprocessing plant-from 2006 to 2015 to assess the impact of the plant on environmental 129I levels. The plant is now under final safety assessment by a national authority after test operation using actual spent nuclear fuel. During cutting and chemical processing in test operations from April 2006 to October 2008, 129I was discharged to the atmosphere and detected in our deposition samples. 129I deposition fluxes largely followed the discharge pattern of 129I from the plant to the atmosphere, and most of the deposited 129I originated from the plant. In and after 2009, 129I deposition fluxes decreased dramatically to reach the background level; the 129I deposition fluxes at Rokkasho were almost the same as those at Hirosaki, where an additional sampling point was set up as a background site 85 km from the plant in 2011. The background 129I deposition fluxes showed seasonal variation-high in winter and low in the other seasons-at both Rokkasho and Hirosaki. The results of a backward trajectory analysis of the air mass at Rokkasho suggested that reprocessing plants in Europe were the origins of the high 129I flux in winter. The contribution of 129I released from the Fukushima Dai-ichi Nuclear Power Plant accident to the 129I deposition flux at Rokkasho in 2011 was small on the basis of the 129I/131I activity ratio.

Temporal variation of post-accident atmospheric 137Cs in an evacuated area of Fukushima Prefecture: Size-dependent behaviors of 137Cs-bearing particles.

The concentrations of 137Cs in the air, which were divided into coarse (>1.1 µm ϕ) and fine (<1.1 µm ϕ) fractions of particulate matter (PM), were measured from October 2012 to December 2014 in an area evacuated after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Total atmospheric 137Cs concentrations showed a clear seasonal variation, with high concentrations during summer and autumn related to the dominant easterly wind blowing from the highly radioactivity contaminated area. This seasonal peak was dominated by 137Cs in the coarse PM fraction. The 137Cs specific activity (massic 137Cs concentration) in the coarse PM was also found to increase significantly in summer, whereas that in the fine PM showed no variability during the year. These results show that coarse and fine 137Cs-bearing PM have different origins and behaviors in the resuspension process. The seasonal variation in atmospheric 137Cs concentration was well correlated with the mean 137Cs surface contamination (deposition density) around the observation site weighted by the frequency of wind direction, indicating that the atmospheric 137Cs concentration in the observation site was explained by the distribution of the 137Cs surface contamination and the frequency of different wind directions. We introduced a resuspension factor corrected for wind direction, consisting of the ratio of the atmospheric 137Cs concentration to the weighted mean 137Cs surface contamination, which evaluated the intensity of resuspension better than the conventional resuspension factor. This ratio ranged from 5.7 × 10-11 to 8.6 × 10-10 m-1 and gradually decreased during the study period.

Determination of Iodide, Iodate and Total Iodine in Natural Water Samples by HPLC with Amperometric and Spectrophotometric Detection, and Off-line UV Irradiation.

We developed a rapid, simple method for the iodine speciation analysis of water and applied it to natural water samples. Simultaneous determinations of I(-) and IO3(-) were achieved with an HPLC system with amperometric detection for I(-) and spectrophotometric detection after a postcolumn reaction for IO3(-). We determined the I(-) and IO3(-) concentrations in 20-µL water samples within 10 min. Total I concentrations in water samples were determined after the decomposition of organics by off-line UV irradiation for 30 min, followed by reduction to I(-). The analytical conditions were optimized by using test solutions rich in organic matter extracted from soils. We tested the new method with samples of groundwater, spring water, precipitation, soil percolate, stream water, and seawater as well as solutions extracted from soil. The method worked well, although the concentrations of some I species were below detection. This method is suitable for routine speciation analysis, which is important for studies of I behavior in the environment.

Tritium activity concentrations and residence times of groundwater collected in Rokkasho, Japan.

Tritium ((3)H) concentrations were measured in groundwater samples from four surface wells (4-10 m deep), four shallow wells (24-26.5 m deep) and a 150-m-deep well in the Futamata River catchment area, which is adjacent to the large-scale commercial spent nuclear fuel reprocessing plant in Rokkasho, Japan. The (3)H concentrations in most of the surface- and shallow-well samples (<0.03-0.57 Bq l(-1)) were similar to those in precipitation (annual mean: 0.31-0.79 Bq l(-1)), suggesting that the residence time of the water in those wells was 0-15 y. The (3)H concentrations in the samples from a 26-m-deep well and the 150-m-deep well were lower than those in the other wells, indicating that groundwater with a long residence time exists in deep aquifers and the estuary area of the catchment. It is not clear whether (3)H released during test operation of the plant with actual spent nuclear fuel affected the (3)H concentrations observed in this study.

Concentration of 129I in aquatic biota collected from a lake adjacent to the spent nuclear fuel reprocessing plant in Rokkasho, Japan.

The spent nuclear fuel reprocessing plant in Rokkasho, Japan, has been undergoing final testing since March 2006. During April 2006-October 2008, that spent fuel was cut and chemically processed, the plant discharged (129)I into the atmosphere and coastal waters. To study (129)I behaviour in brackish Lake Obuchi, which is adjacent to the plant, (129)I concentrations in aquatic biota were measured by accelerator mass spectrometry. Owing to (129)I discharge from the plant, the (129)I concentration in the biota started to rise from the background concentration in 2006 and was high during 2007-08. The (129)I concentration has been rapidly decreasing after the fuel cutting and chemically processing were finished. The (129)I concentration factors in the biota were higher than those reported by IAEA for marine organisms and similar to those reported for freshwater biota. The estimated annual committed effective dose due to ingestion of foods with the maximum (129)I concentration in the biota samples was 2.8 nSv y(-1).

Nuclear accident-derived (3)H in river water of Fukushima Prefecture during 2011-2014.

During 2011-2014, we measured (3)H concentrations in river water samples collected during base flow conditions and during several flood events from two small rivers in a mountainous area in Fukushima Prefecture, which received deposition of (137)Cs from the Fukushima Dai-ichi Nuclear Power Plant accident. (3)H concentrations above background levels were found in water samples collected during both base flow conditions and flood events in 2011. The (3)H concentrations during flood events were generally higher than those during base flow conditions. The (3)H concentrations in both rivers during base flow conditions and flood events decreased with time after the accident and reached almost background levels in 2013. We also measured (3)H concentrations in freshwater samples from 16 other rivers and one dam in eastern Fukushima Prefecture from 2012 to 2014 during base flow conditions. The measured (3)H concentrations were higher than the background level in 2012 and decreased with time. The (137)Cs inventory in the catchment area at each sampling point was estimated from air-borne monitoring results in the literature and compared with the (3)H concentrations. We found surprisingly good correlations between (137)Cs inventories in the catchment areas and (3)H concentrations in the water samples. Further studies will be necessary to clarify the reason for the good correlation.

Effects of radiocesium inventory on (137)Cs concentrations in river waters of Fukushima, Japan, under base-flow conditions.

To investigate the behavior of nuclear accident-derived (137)Cs in river water under base-flow conditions, concentrations of dissolved and particulate (137)Cs were measured at 16 sampling points in seven rivers of Fukushima Prefecture, Japan, in 2012 and 2013. The concentration of dissolved (137)Cs was significantly correlated with the mean (137)Cs inventory in the catchment area above each sampling point in both sampling years. These results suggest that the concentration of dissolved (137)Cs under base-flow conditions is primarily determined by the (137)Cs inventory of the catchment area above the sampling point. However, the concentration of particulate (137)Cs did not show a clear relationship with either the mean (137)Cs inventory or the dissolved (137)Cs concentration, thus indicating that particulate and dissolved forms do not effectively interact in rivers. To evaluate the contribution of the (137)Cs inventory within catchment areas, we analyzed relations between the (137)Cs concentration and the mean (137)Cs inventory over the area within certain flow path lengths that were traced along the river and slope above the sampling point. Coefficients of determination for dissolved (137)Cs concentrations were highest for the longest flow path, i.e., the whole catchment area, and lower for shorter flow paths. Coefficients of determination for particulate (137)Cs concentrations were only moderately high for the shortest flow path in 2012, whereas the values were quite low for all flow paths in 2013. These results suggest that dissolved (137)Cs can originate from a larger area of the catchment even under base-flow conditions; however, particulate (137)Cs did not show such behavior. The results also show that under base-flow conditions, dissolved and particulate (137)Cs behave independently during their transport from river catchments to the ocean.

Fluvial discharges of radiocaesium from watersheds contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident, Japan.

A large amount of radiocaesium, (134)Cs and (137)Cs, was released to the atmosphere and Pacific Ocean from the Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant (FDNPP) that was damaged by the tsunami caused by the Tohoku earthquake on 11 March 2011. Radiocaesium deposited on the ground is now the most important consideration in assessing the risk to people in the vicinity of the FDNPP and remediating the contaminated area. Transfer of radiocaesium from the ground through rivers is an important factor in the downstream contamination of irrigation waters, paddy fields, lakes, and the sea. We estimated the transport of radiocaesium through two small rivers, the Hiso River and Wariki River, that traverse mountainous areas in Fukushima Prefecture. Areal depositions of radiocaesium in their watersheds (catchments) were high (1-3 MBq m(-2)). Water samples were collected from the rivers twice during each of two baseflow and two flood stages in 2011 and analysed for radiocaesium in particulate and dissolved forms. The radiocaesium concentrations depended strongly on the rates of water discharge. Maximum activities of radiocaesium in the samples from the Hiso River and Wariki River when there was precipitation or flooding (July and September) were 25 ± 0.31 and 35 ± 0.25 Bq L(-1), respectively. Particulate radiocaesium during periods of flooding contributed over 90% of the total radiocaesium activity in the samples. The discharge of radiocaesium from the catchments during 2011 was estimated to be 0.5% and 0.3% of the total amount of radiocaesium deposited on the catchments of the Hiso River and Wariki River, respectively. It is considered that the most of the radiocaesium deposited in the catchment remains on the soil surface.

Concentration of (3)H in plants around Fukushima Dai-ichi Nuclear Power Station.

A large amount of radionuclides was released from the Fukushima Dai-ichi Nuclear Power Station (FDNPS) following the damage caused by the tsunami due to the Great East Japan Earthquake on 11 March 2011. Although many radionuclides in various environmental samples around the FDNPS have been measured, (3)H in the terrestrial environment has not yet been reported. We present here the first survey results of (3)H concentrations in plant samples collected around the FDNPS in 2011 from shortly after the accident. The free-water (3)H concentrations in herbaceous plant shoots and evergreen tree leaves were considerably higher than the previous background concentration, and diminished with distance from the FDNPS. Although reconstruction of atmospheric (3)H concentrations after the accident is difficult, a rough estimate of the radiation dose due to (3)H inhalation about 20 km from the FDNPS is on the order of a few microsieverts (µSv).

Effect of the counter anion of cesium on foliar uptake and translocation.

Direct deposition of radioactive material onto crops is one important pathway for safety assessment of radionuclides released from nuclear facilities. Foliar uptake of Cs by radish (Raphanus sativus L. cv. Redchim) was studied by applying droplets of Cs solution (CsCl or CsNO3) on an upper leaf surface. The uptake of Cs was strongly affected by counter anions of Cs in the applied solution. Approximately 80% of Cs was absorbed for CsCl solution, while only 20% was absorbed for CsNO3. The partition of absorbed Cs between leaf and root tuber was quite similar for both Cs compounds, which indicated that behavior of the absorbed Cs in radish was the same for both.