Yoichiro Ohmomo

Tracer experiments on transfer of radio-iodine in the soil — rice plant system

In order to study the transfer of 129I from soil into rice, tracer experiments have been carried out in the soil-rice plant system. The soil-plant transfer factors, which are defined as ‘concentrations of the nuclide in a plant organ’ divided by ‘concentrations of the nuclide in dry soil’, were 6.0 × 10−3 for brown rice, 3.2 for the first leaf blade (flag leaf), 5.1 for the second leaf blade, 2.6 for stem, 1.4 for rachis and 9.0 for root on a dry weight basis at the time of harvest. The ratio of radio-iodine in brown rice to polished rice was about 1:0.3. The transfer factor of radio-iodine from soil to polished rice was estimated to be about 2 × 10−3. It was observed that the concentration of radio-iodine in the soil solution increased with the plant growth. This was attributed to the reduced condition in the soil caused by the effects of root and/or micro-organisms. Release of I from rice plants into the air in an organic form was found. The chemical form of radio-iodine in the soil solution appeared to be iodide, and the radio-iodine in the plant tissue was found in a low molecular weight fraction by gel-filtration. The vertical distribution of radioiodine in the soil was also studied.

Iodine separation procedure for the determination of129I and127I in soil by neutron activation analysis

A radiochemical neutron activation analytical method for the determination of129I and127I in soil samples was studied. Iodine was separated from the sample prior to the irradiation by volatilization, i.e. by combustion of the sample and trapping of the iodine in an alkaline solution together with a reducing agent. This method enables one to digest samples containing up to 100 g dry matter. The chemical yield was mostly more than 90%. After irradiation the iodine fraction was further purified by solvent extraction. The detection limit of the129I/127I ratio was 1×10−9.

Neutron activation analysis of iodine-129 and iodine-127 in environmental samples

An analytical method for129I and127I in various environmental samples is described. The method consists of sample digestion by alkali fusion, iodine separation by solvent extraction, neutron irradiation, radiochemical purification of iodine and gamma-spectrometry. The detection limit of129I and the129I/127I ratio are 1·10−3 pCi and 1·10−9, respectively. The range of the129I/127I ratios obtained in the environmental samples collected from the Tokaimura area in Japan was between 1·10−9 and 7.9·10−6. The highest ratio was observed in pine needles followed by rain water, soil, swamp water and algae.

Iodine-131 and other radionuclides in environmental samples collected from Ibaraki/Japan after the chernobyl accident

Measurements were made of 131I, 137Cs, 134Cs and 103Ru in environmental samples collected from Ibaraki-Prefecture, Japan, after the Chernobyl accident. The highest values of 131I obtained in May 1986 were 98 Bq l-1 (2.7 nCi l-1) for rain water (collected at the beginning of the rainfall), 400 Bq kg-1 (11 nCi kg-1) for pine needles (wet), 160 Bq kg-1 (4.3 nCi kg-1) for vegetables (wet) and 52 Bq kg-1 (1.4 nCi kg-1) for seaweed (wet). Most of the radioiodine in rain water was present as IO3- and I-. The proportion of IO3- in fresh rain water was higher than that of I-, but the IO3- was converted to I- during storage. About 10% of 131I was removed from leaf vegetables by washing, and about 70% by boiling.

Determination of iodine-129 and iodine-127 in environmental samples collected in Japan

Analytical method for the determination of129I and127I in environmental samples has been developed by using radiochemical neutron activation analysis. The129I levels in the samples such as soil (0.9–41 mBq/kg), precipitation (0.002–0.11 mBq/kg), pine needles (1.2–32 mBq/kg) and seaweed (<0.1–17 mBq/kg) collected near the nuclear facilities in Tokaimura were higher than those from the other areas in Japan. The highest129I concentration was found in surface soil (0–5 cm), and the highest129I/127I ratios were found in pine needles and precipitation. The129I/127I ratio was higher in rice paddy soil than those in wheat field soil collected around Tokaimura, while the concentration of129I somewhat higher in wheat field soil.

Determination of I-129 and I-127 in soil and tracer experiments on the adsorption of iodine on soil

Neutron activation analysis of129I and127I in soil has been studied. The limit of detection for129I in soil was about 0.05 mBq/kg or 1×10−9 as129I/127I atom ratio. The range of129I concentration in surface soils collected around Tokaimura (Ibaraki Prefecture) was 0.9–41 mBq/kg.Tracer experiments on the adsorption of iodine were also carried out, in order to obtain information on the behaviour of iodine in soil-water systems. Different adsorption patterns of iodide and iodate on soil were found. It was supposed that iodide was adsorbed by the soil fraction which became unstable at about 200° C and iodate by the fraction which was relatively stable to heating.

Tracer experiments for the determination of chemical forms of radioiodine in water samples

Two simple methods, (1) isotope exchange method and (2) anion exchanger column method, are developed for the determination of chemical forms of radioiodine (iodide and iodate) in water samples. Using these methods, transformations of chemical forms of iodine in various water samples were studied. It was observed that iodate in rain water (unfiltered) and milk tended to change iodide form, whereas iodide was converted to iodate form in seawater and tap water. After the Chernobyl accident both chemical forms of131I (iodide and iodate) were found in rain water samples collected in Japan.

Deposition velocity of gaseous organic iodine from the atmosphere to rice plants

To obtain parameter values for the assessment of 129I transfer from the atmosphere to rice, deposition of CH3I to rice plants has been studied. The mass normalized deposition velocity (VD) of CH3I for rough (unhulled) rice was 0.00048 cm3 g-1 s-1, which is about 1/300 of that of I2. Translocation of iodine, deposited as CH3I on leaves and stems, to rice grain was negligibly small. Distribution of iodine between hull and inner part of the grain was found to depend also on the chemical forms of atmospheric iodine to be deposited. The ratio of the iodine distribution in a grain exposed to CH3I was as follows: rough rice:brown rice (hulled rice):polished rice = 1.0:0.49:0.38. The distribution ratio in polished grains for CH3I exposed rice was about 20 times higher than that for I2.

Biological half-life of gaseous elemental iodine deposited onto rice grains.

In order to obtain the biological half-life (Tb) of iodine deposited on rough rice grains, rice plants of four different growing stages--heading, milky, dough, and yellow ripe--were exposed to elemental gaseous iodine. After the exposure, the rough rice samples were collected at different intervals and analyzed for iodine to estimate the value of Tb. The average value of Tb obtained by the experiments at the dough and yellow ripe stages was about 200 d. This value is considerably larger than those for pasture grass and leafy vegetables.

Transfer of radionuclides to crop plants through roots - Radioiodine.

長半減期核種の一つである129I (半減期: 1.57×107年, 放出放射線: β-線, X線およびγ線〔39.6keV〕) の経根吸収経路による農作物への移行を研究するため, 131I (半減期: 8.04日, 放出放射線: β-線およびγ線〔364keVほか〕) をトレーサとして水耕実験を行った。ポット栽培試験を実施するに当たっての問題点について検討し, 131Iの作物体内分布を求めた。さらに, 農作物の可食部への移行速度を求め, その作物間差から, 129Iの被曝評価上, クリティカルと考えられる農作物の選定を試みた。