Weathering half-life of radioactive cesium for winter rye (<i>Secale cereale</i> L.) and Italian ryegrass (<i>Lolium multiflorum</i> Lam.) directly contaminated by the 2011 Fukushima Daiichi Nuclear Power Station accident

Changes in radioactive cesium (radiocesium) concentrations in winter rye (<i>Secale cereale</i> L.) and Italian ryegrass (<i>Lolium multiflorum</i> Lam.) grown in fields contaminated by the accident at the Fukushima Daiichi Nuclear Power Station of the Tokyo Electric Power Company were examined to clarify the half-lives of weathering as an environmental process that removed radiocesium during the early stages after its deposition. The radiocesium concentrations in the rye plants that resulted from direct deposition of fallout in 2011 decreased greatly from 140.5 kBq kg^–1 dry weight (DW) on March 31 to 13.2 kBq kg^–1 DW on April 27, during the 27 d between the vegetative growth and heading stages. The amounts of rye radiocesium per unit area decreased simultaneously, from 69.8 kBq m^–2 to 12.6 kBq m^–2. The radiocesium concentrations in rye were half and a quarter of those in Italian ryegrass during the vegetative and heading stages, respectively, although the radiocesium concentrations in Italian ryegrass also decreased with plant growth. The values of the weathering half-life of 8.0 and 11.0 d for the radiocesium concentration per unit DW and amount per unit area in rye were approximately half those of Italian ryegrass, at 14.1 and 23.1 d, respectively. Therefore, less radiocesium-contaminated material seemed to be retained on the plant surface of rye than on Italian ryegrass. The radiocesium concentrations of rye harvested in the same experimental fields in the year following the accident were below the value defined in the February 2012 provisional regulation for cattle roughage (100 Bq kg^–1 at a water content of 0.8 kg kg^–1).为阐明沉降早期环境风化过程对放射性铯（radiocesium）的清除半衰期，本研究针对东京电力公司（Tokyo Electric Power Company）福岛第一核电站（Fukushima Daiichi Nuclear Power Station）事故污染农田中种植的冬黑麦（Secale cereale L.）与多花黑麦草（Lolium multiflorum Lam.）体内放射性铯浓度的变化展开了分析。2011年沉降物直接沉降导致的黑麦植株体内放射性铯浓度，在营养生长期至抽穗期的27天内，从3月31日的140.5 kBq·kg⁻¹干重（dry weight, DW）大幅降至4月27日的13.2 kBq·kg⁻¹干重；单位面积黑麦体内的放射性铯总量也同步下降，从69.8 kBq·m⁻²降至12.6 kBq·m⁻²。尽管多花黑麦草体内的放射性铯浓度同样随植株生长有所下降，但在营养生长期与抽穗期，黑麦的放射性铯浓度分别仅为多花黑麦草的1/2与1/4。黑麦体内单位干重放射性铯浓度与单位面积放射性铯总量的风化半衰期分别为8.0天与11.0天，约为多花黑麦草（对应半衰期分别为14.1天与23.1天）的一半。由此可见，黑麦植株表面滞留的放射性铯污染物质似乎少于多花黑麦草。事故次年在同一试验田收获的黑麦，其放射性铯浓度低于2012年2月发布的牛用粗饲料临时监管限值（含水率0.8 kg·kg⁻¹时为100 Bq·kg⁻¹）。