Parameters of Iodine Migration in Soil
Hirofumi TSUKADA, Mutsumi YAMAGAMI, Akira TAKEDA,
Yuichi TAKAKU, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
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Iodine-129 (half-life, 1.6 × 10 y) is one of the important radionuclides discharged from the firstcommercial nuclear fuel reprocessing plant in Rokkasho, Japan that must be considered for the assessmentof radiation dose to the public. A part of the 129I discharged to the atmosphere from the plant is depositedon the land surface and retained in surface soil. Downward migration of 129I in soil is important for theprediction of its concentration in both ground water and surface soil during and after long-term operationof the reprocessing plant. The aims of this study are to evaluate the rate of downward migration of 129I in
soil around the reprocessing plant and to clarify physico-chemical and biological factors affecting themigration rate. In FY 2012, we studied: 1) the downward migration rate of 125I- and 125IO3- in core samples
of surface soil; 2) the effect of soil temperature on the chemical form of stable I in soil solution; and 3) theeffect of rice plant root activity on the chemical form of stable I in cultivation solution.
Undisturbed soil core samples were collected from a grassland area in Rokkasho to study thedownward migration rate of I. The distribution coefficient (Kd) values of 125I- and 125IO3-, as well as 85Srand 137Cs, in the soil samples fractionated from different depths (0 – 50 cm) of the soil core samples weremeasured by the batch sorption method. Downward migration rate of the nuclides in the soil wasestimated by using a retardation factor that was obtained from the measured Kd value. The downward
migration rate of 125I in the two forms examined ranged from 0.3 to 2.3 mm y-1, which was lower than therate for 85Sr and higher than that for 137Cs. Undistributed soil columns were prepared from the coresamples, and the solution of 125I (125I or 125IO3-), 85Sr or 137Cs was added onto the surface of each column.
The vertical distribution of the nuclides in the column was investigated after passing the artificial
rainwater through it. Since the fraction of 125IO3- at a depth to 20 cm was larger than that of 125I-, it wassuggested that the downward migration rate of 125IO3- was higher than that of 125I-.
Surface soil samples collected from three sites in Rokkasho were used in the soil incubation experiment
under different temperatures: 4, 15, 30 and 45°C. Soil solution samples were obtained by high-speed
centrifugation after the incubation within 18 d, and concentrations of I-, IO3- and total I in the samples
were analyzed. The IO3- concentrations in all soil solution samples were under the detection limit. Whilethe concentrations of total I and I- did not change in the samples incubated at 4, 15 and 30°C, the total I
concentration for the 45°C incubation was significantly higher than that in the lower temperatures.
Dissolved organic iodine was the main form of iodine solubilized under the highest temperature, though
the I- concentration in the soil solutions was also increased.
To investigate the effect of plant root activity on iodine speciation, rice plant cultivars were cultivatedby sand-hydroponics using the cultivation solution with added I- and IO3-. The concentrations of both Iand IO3- in the solution decreased in the cultivation period. The I- concentration depended on the rice
plant cultivars, while the IO3- concentration did not.