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

Dependency of radioiodine root uptake by crops on soil characteristics

The aim of this study was to quantify the parameters of root uptake of radioiodine by agricultural crops under steady state conditions depending on the main soil characteristics. For this purpose, a long-term (483-days) pot experiment was conducted under natural conditions in the Chornobyl Exclusion Zone to grow radish in soils of four different types with added isotope ¹²⁵I. The experiment demonstrated an increase in root uptake of radioiodine by radish roots in the following sequence of soil types: clay soil < loam soil ≪ sandy soil (Chernozem ≈ Phaeozem < Greyzem ≪ Podzoluvisol). The obtained results have been analyzed in conjunction with the results of our previous studies to identify the factors determining the parameters of root uptake of radioiodine by the studied crop species. The ¹²⁵I concentration ratios (CRs) in edible parts of crop species (radish roots: from 0.003 for Chernozem to 0.02 for Podzoluvisol; lettuce leaves: 0.004-0.04; bean pods: 0.0003-0.004; wheat straw: 0.01-0.1; and wheat seeds: 0.0001-0.001) anticorrelated with the characteristics of the soils studied: the distribution coefficients Kd of ¹²⁵I (from 112 L kg^-1 for clay soils to 19 L kg^-1 for sandy soil, R² = 0.56-0.97) and Kd' of stable iodine (93-19 L kg^-1, R² = 0.43-0.74), stable iodine concentration in soil (6.2-0.5 mg kg^-1, R² = 0.71-0.88), and humus content (4.0-0.8%, R² = 0.44-0.78). The obtained steady-state CR values and their dependence on the soil characteristics can be used to model the root uptake of ¹²⁹I, a long-lived radiological contaminant, and to predict its accumulation in human food and animal feed.

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.

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.

Temporal variation of iodine-129 concentrations in kelps (Saccharina) from coastal waters off northern Japan

Concentrations of ¹²⁹I and ¹²⁷I in kelps (Saccharina) collected from coastal waters off northern Japan were monitored from 2007 to 2019. During the 2007-2008 test operation of the Rokkasho nuclear fuel reprocessing plant, ¹²⁹I discharge from the plant increased, and the ¹²⁹I concentration and ¹²⁹I/¹²⁷I atom ratio in the kelps reached maxima of 42 μBq/g-dry and 264 × 10^-11, respectively. By 2009, both had decreased by one order of magnitude. After the Fukushima Dai-ichi Nuclear Power Plant accident in 2011, the ¹²⁹I concentration and ¹²⁹I/¹²⁷I atom ratio in the kelps increased to 2.24 μBq/g-dry and 11.6 × 10^-11, respectively. After 2012, the ratio in kelps decreased to (2.1-8.9) × 10^-11, which is almost the same as the seawater value off Aomori Prefecture before the test operation. The ¹²⁹I/¹²⁷I atom ratio in kelps thus represents the ambient seawater ratio during the growth period of the kelps.

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.