Misako Sumiya

Concentrations of radiocesium and potassium in basidiomycetes collected in Japan

Concentrations of 137Cs, 134Cs and 40K in about 60 mushroom samples (fruit bodies of basidiomycetes) belonging to 25 species collected in Japan have been studied. The levels of 137Cs varied very widely, ranging from less than 3 to 1520 Bq kg-1 (dry wt), while those of 40K were relatively constant. Concentrations of 137Cs in common edible mushrooms of Japan such as Lentinus edodes, Flammulina velutipes, Pleurotus ostreatus and Pholiota nameko were low (normally less than 50 Bq kg-1, dry wt). Concentrations of 134Cs in many samples were below the limit of detection (usually less than 5 Bq kg-1, dry wt). The median concentrations of 137Cs and 40K were 41 (dry wt) and 1150 Bq kg-1 (dry wt), respectively. From the 137Cs/40K ratios it was found that cesium rather than potassium was selectively taken up from the soils by fungi such as Suillus granulatus and Lactarius hatsudake. The 134Cs/137Cs ratios in mushrooms are related to the depth of the mycelium in the soil. The effective dose equivalent due to the dietary intake of radiocesium through mushrooms was estimated to be only 1.6 x 10(-7) Sv.

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.

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.

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。