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

DISTRIBUTION OF IODINE-127 IN MARINE ORGANISMS FROM COASTAL WATERS AROUND AOMORI, JAPAN

We measured the concentrations and determined the distribution of the stable isotope 127I in the marine organisms. Marine fish were collected from the Pacific Ocean near Aomori, Japan. The mean 127I concentrations in the muscle of marine fish ranged from 0.67 μg g-1-wet weight (ww) in cherry salmon to 0.84 μg g-1-ww in fat greenling. Among the tissues measured, the muscle showed the lowest levels of 127I in all fish species. The highest 127I concentration in the tissues of marine fish was observed in the ovary of fat greenling. The mean 127I concentration in the of Japanese scallop ranged from 0.06 μg g-1-ww in adductor muscle to 5.49 μg g-1-ww in mid-gut gland. The 127I concentrations in seaweeds were 67 μg g-1-dry weight (dw) in sea lettuce and 1783 μg g-1-dw in kombu. Thus, the distribution of 127I concentration in marine organisms varied considerably depending on the tissue.

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.

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.

Ultra-Sensitive Determination of Particulate, Gaseous Inorganic and Organic Iodine-129 and Iodine-127 in Ambient Air

Atmospheric iodine cycling is of significance in climate change and environmental and health impacts. To better explore speciation transformation of atmospheric stable and radioactive iodine, an ultra-sensitive analytical method was established for determination of ¹²⁹I and ¹²⁷I in particulate, gaseous inorganic, and gaseous organic species, which was conducted with a self-designed cascade sampling apparatus, followed by their separation with a pyrolysis system and accelerator mass spectrometry and ICP-MS measurements. Combustion protocols for three sampling matrices and NaOH concentration for iodine trapping were optimized to achieve a safe analytical procedure with a high chemical yield of iodine. Based on the lowest concentrations of ¹²⁹I and ¹²⁷I, a suitable activated carbon product for adsorption of gaseous organic iodine was carefully selected. The detection limits of the three species were 0.30-2.21 ng m^-3 for ¹²⁷I and 0.05-0.22 × 10⁵ atoms m^-3 for ¹²⁹I. This newly established method was successfully applied to analyze the levels and species of ¹²⁹I and ¹²⁷I in ambinet air from Xi'an, China, from May to August, 2020. Gaseous organic iodine was found to be the dominant species of ¹²⁷I and ¹²⁹I, accounting for about half of total iodine, and gaseous inorganic iodine and particulate iodine accounted for one-quarter each during the whole sampling period. Speciation variation of ¹²⁹I and ¹²⁷I indicates that speciation transformation apparently occurred at the turn of spring and summer, mainly between particulate and gaseous organic iodine. This study has implications on delicate tracing of the atmospheric behavior of iodine with long-lived anthropogenic ¹²⁹I.

Flux and pathway of iodine dissolution from brackish lake sediment in the northeast of Japan

Sediment is one of the most important entities controlling the environmental dynamics of iodine. We previously evaluated the dissolution flux of iodine from aquatic sediment to its overlying water in a brackish lake through an incubation experiment with a sediment core sample, reporting the regulation factors of this flux, such as the temperature and oxygenic conditions. In this study, factors controlling the seasonal variation in the dissolution flux of iodine were investigated via an incubation experiment using the sediment core samples collected in autumn and summer. The dissolution flux in this study was significantly smaller than that reported in our previous study. The iodine concentration detected in the overlying water of the sediment during incubation in this study was positively correlated with the concentrations of some inorganic ions, such as Na⁺, Cl^-, and SiO₄^2-, while these correlations were not confirmed in our previous study. As the dissolution of sedimentary iodine includes two pathways, which are the diffusion of sedimentary pore water and degradation of organic matter on surface sediment, correlations potentially indicate that the dissolution flux estimated in this study has a larger contribution from sedimentary pore water than that in our previous study. In addition, the higher flux estimated in our previous study was considered to be caused by the larger contribution from iodine derived from the degradation of phytoplanktonic organic matter on surface sediment. Assuming that the dissolution fluxes estimated in the previous and present studies are representative of the fluxes during and excluding the high productivity season in Lake Obuchi, respectively, we estimated the annual flux at 2.7 g y^-1 m^-2, which is comparable with our previous estimation.

Evaluation of radioiodine (129I) dissolution from sediment of a brackish lake beside a spent nuclear fuel reprocessing plant in Japan

Dissolution fluxes of stable (¹²⁷I) and radioactive (¹²⁹I) isotopes of iodine from a brackish lake sediment beside a spent nuclear fuel reprocessing plant in Japan were evaluated through two kinds of experiments: incubation using a sediment core sample for 24 h, and observation of ¹²⁷I and ¹²⁹I concentrations in sedimentary pore water. For ¹²⁷I, the dissolution flux evaluated in the incubation experiment was comparable with that obtained from the vertical gradient of ¹²⁷I concentration in pore water in the observation experiment. This suggests that degradation of organic matter in the surface sediment is an important source of dissolved ¹²⁷I found in the water. For ¹²⁹I, the dissolution flux estimated in the incubation experiment showed negative values, indicating the transfer of ¹²⁹I to the sediment from the overlying water (i.e., absorption). Moreover, the flux evaluated from the observation experiment was positive. This result suggests that degradation of organic matter in the surface sediment is scarcely important to the supply of ¹²⁹I from the sediment to the water in the studied lake. The dissolution flux of ¹²⁹I estimated in the observation experiment was smaller than the absorption flux of ¹²⁹I in the incubation experiment. This potentially indicates that the dissolution of sedimentary ¹²⁹I does not significantly change ¹²⁹I concentrations in the water and sediment of the lake. This hypothesis was consistent with previous research conducted for the studied lake.

Short-term metabolism of biologically incorporated 125I ingested by olive flounder (Paralichthys olivaceus)

Iodine-129 with a long half-time of 1.6 × 10⁷ y was discharged into the Pacific Ocean during the final safety tests of the first commercial nuclear fuel reprocessing plant in Japan, at Rokkasho, Aomori Prefecture. Olive flounder (Paralichthys olivaceus) is an important fishery along this coast. It is necessary to determine whether ¹²⁹I accumulates in this species to assess the possible public acceptance. We developed a short-term metabolism model of ¹²⁵I in the flounder using retention data for 1-6 days after the olive flounder had ingested a freshwater fish species, medaka (Oryzias latipes), that had been labeled with ¹²⁵I by keeping them in water containing ¹²⁵I for 7 days. A single compartment model constructed from whole-body retention data for ¹²⁵I in the olive flounder, excluding the gastrointestinal tract and its contents, revealed a biological half-time of 2.9 days for ¹²⁵I. When the gill and other tissues were separated to individual compartments, the biological half-time in the gill was three times longer than that in the other tissue, though the half-time in the gill is not statistically significant. The distribution of ¹²⁵I among various tissues in the flounder 6 days after the ingestion of labeled medaka once a day for 6 days differed from that of stable I, suggesting that the biological half-time is longer in certain tissues. Further study is necessary to elucidate the metabolism of radioiodine in the flounder.

Evaluation of dissolution flux of iodine from brackish lake sediments under different temperature and oxygenic conditions

Dissolution flux of iodine from aquatic sediments in a brackish lake (Lake Obuchi), facing the Pacific Ocean and adjacent to a nuclear fuel reprocessing plant in northeast Japan, was evaluated using incubation experiments on sediment core samples. The experiments were performed under three different temperatures (29, 17, and 6 °C) and oxygenic (air flow, N₂ gas flow, and untreated) conditions for 48 h. The total dissolved iodine (TDI) concentration (i.e., the sum of iodide, iodate, and dissolved organic iodine, DOI) increased under all temperatures and oxygenic conditions in the first 6 h of incubation. From 6 to 27 h, noticeable increases in TDI concentration only occurred at high temperatures. Dissolution fluxes of iodine estimated by linear regression analysis of the measured TDI concentration in the first 6 h were always higher than those estimated in the first 27 h. This result indicates that dissolution flux of iodine should be evaluated through short-term (within several hours) incubation experiments because absorption reactions which transport iodine from the overlying water back to the sediment become active in the long-term. No substantial difference in dissolution flux, estimated by TDI concentration, was observed under different oxygenic conditions in the first 6 h. However, dissolution flux increased significantly with an increase in temperature. Increases in flux and temperature were significantly and positively correlated (R² = 0.90), suggesting that temperature was the dominant factor that regulated iodine flux during the incubation. Changes in TDI concentration at all temperatures and oxygenic conditions corresponded to those in iodide concentration, indicating that iodide was the main form of iodine dissolved from the sediments. In later stages of the experiments, from 27 to 48 h, the TDI concentration in overlying water increased only at high temperature, while concentrations at medium and low temperatures remained constant or decreased. In particular, oxic experiments showed substantial decreases in iodide concentration at medium and low temperatures. This suggests that oxic conditions promote the absorption of iodine from the overlying water to the sediments. Finally, the dissolution flux of radioiodine (iodine-129) from the sediments of Lake Obuchi to the overlying water was estimated by combining these results with data from earlier studies. The results suggest that only 0.006% of the iodine-129 accumulated in the sediments is released through dissolution to the overlying water per year, suggesting that this radioactive isotope is essentially stable in the sediments.

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.

Concentrations of iodine-129 in livestock, agricultural, and fishery products around spent nuclear fuel reprocessing plant in Rokkasho, Japan, during and after its test operation

Concentrations of iodine-129 (129I) and atomic ratios of 129I/127I in livestock (grass and milk), agricultural (cabbage, Japanese radish, and rice), and fishery (flatfish and brown alga) products collected from locations around the first Japanese commercial spent nuclear fuel reprocessing plant in Rokkasho were measured from 2006 to 2016. The actual spent nuclear fuel rods were cut and processed to test the functioning of the plant that discharged controlled amounts of 129I to the atmosphere and coastal seawater during the period from 2006 to 2008 (the "cutting period"). Statistically significant increases in 129I concentration and 129I/127I ratio were observed during the cutting period in livestock products and flatfish. On the other hand, these parameters were statistically comparable during and after the cutting period in the other products. The radiation dose through the ingestion of the maximum 129I concentrations, measured in the different products, was estimated to be in the nanoSievert per year level. This value is much smaller than 1 mSv yr-1, which is the permissible authentic radiation dose for the general public. The 129I levels in the samples, especially in milk and flatfish, are discussed in context of the 129I discharge history from the plant.

Factors affecting the 7Be surface concentration and its extremely high occurrences over the Scandinavian Peninsula during autumn and winter

Relationships between the beryllium-7 activity concentrations in surface air and meteorological parameters (temperature, atmospheric pressure, and precipitation), teleconnection indices (Arctic Oscillation, North Atlantic Oscillation, and Scandinavian pattern) and number of sunspots are investigated using two multivariate statistical techniques: hierarchical cluster and factor analysis. The beryllium-7 surface measurements over 1995-2011, at four sampling sites located in the Scandinavian Peninsula, are obtained from the Radioactivity Environmental Monitoring Database. In all sites, the statistical analyses show that the beryllium-7 concentrations are strongly linked to temperature. Although the beryllium-7 surface concentration exhibits the well-characterised spring/summer maximum, our study shows that extremely high beryllium-7 concentrations, defined as the values exceeding the 90^th percentile in the data records for each site, also occur over the October-March period. Two types of autumn/winter extremes are distinguished: type-1 when the number of extremes in a given month is less than three, and type-2 when at least three extremes occur in a month. Factor analysis performed for these autumn/winter events shows a weaker effect of temperature and a stronger impact of the transport and production signal on the beryllium-7 concentrations. Further, the majority of the type-2 extremes are associated with a very high monthly Scandinavian teleconnection index. The type-2 extremes that occurred in January, February and March are also linked to sudden stratospheric warmings of the Arctic vortex. Our results indicate that the Scandinavian teleconnection index might be a good indicator of the meteorological conditions facilitating extremely high beryllium-7 surface concentrations over Scandinavia during autumn and winter.