日本水稻核放射性污染现状及去污措施综述

2011年3月日本福岛核电站发生核泄漏事故,释放出大量放射性物质,使周边农田受到直接污染。其中对水稻影响最为严重的放射性核素为放射性铯。分析表明:水稻对铯的吸收是存在差异的,钾肥能有效降低水稻中铯含量,而氮肥会增加其铯含量;距离福岛核电站距离越远其受污染程度越低;土壤中铯的浓度、含钾量、固定铯的能力和耕作表土的厚度都影响着铯的转移;此外,水稻本身的品种也影响着对铯的吸收。水稻去污染的方法包括:利用蓝藻除去表土中放射性铯、对土壤进行深耕及防止水田二次污染等方式来降低土壤本身铯含量;另一方面通过改变土壤粒度分布,向土壤中加入施威特曼石、沸石和蛭石及控制肥料比例等方式来抑制土壤中铯的转移。以上除污染方法的综合应用效果十分显著,福岛县水稻含铯量于2014年已全部低于25 Bq/kg。该研究为中国构建农作物放射性污染特征数据库及相关应急管理提供参考。

Review on rice contamination and its remediation methods by radioactive nuclides in Japan

The accident occurring at the Fukushima Daiichi nuclear power plant on March 11, 2011 resulted in leakage of a large amount of radionuclides, contaminating farmlands, including rice field, in Fukushima and its neighboring prefectures. Potential contamination of the rice growing these areas by the radionuclides has thus attracted public attention. The objective of this paper is to analyze rice contamination in this region and its remediation. Literature review showed that the radionuclide that was most potential to contaminate the rice is the radioactive cesium. Among all available methods to detect radioactive cesium in rice grain, belt conveyor testers is quicker in measurement but is less accurate, while germanium semiconductor detectors is more accurate but is tedious. Rice contamination by cesium accumulation is modulated by a multitude of factors, one of which is fertilization. For example, potassium fertilizer (potassium chloride is more effective than potassium metasilicate) can reduce the absorption of the radioactive cesium in rice, while nitrogen fertilizer works in the opposite. The movement and transfer of radioactive cesium in soil was affected by its concentration, potassium content, immobilization of the cesium by soil, and the thickness of the topsoil. Apart from these, rice cultivars also affect cesium accumulation. Different methods for ameliorating rice contamination by radionuclides had been developed and tested. In paddy fields, rice contamination by cesium can be reduced by amending the soil or planting. The radioactive cesium is sportive to soil and cyanobacteria can be used to facilitate the exfoliation of top soil. Also, rice is a shallow-rooted plant and deep ploughing could bury the contaminated topsoil beyond the reach of roots so as to reduce its uptake by rice. Secondary contamination can be avoided by controlling fertilization and irrigation, and the transformation of the radioactive cesium from soil to rice can be inhibited by amending the soil with schwertmannite, zeolite and vermiculite, or by controlling fertilizer application rate and the secondary contamination. Implementation of above mitigating methods over the past few years reduced the radioactive cesium content in rice in Fukushima Prefecture to 25 Bq/kg in 2014 (the radiation level set by the Food Sanitation Act of Japan in 2012 was 100 Bq/kg). This paper has implications for constructing a database for mitigating and remediating crop contamination by radionuclides which is also relevant to food production in China.