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2011年3月11日,日本东北部地区发生里氏9.0级地震并引发海啸,在地震和海啸的双重破坏下,位于福岛县的福岛第一核电站核电机组相继发生堆芯熔毁和氢气爆炸。事故发生后,东京电力公司为了降低核反应堆内气压而将堆内气体排放到大气环境,为了冷却核反应堆而向堆内注入大量冷却水,之后又将此冷却水排入大海。这些危机处理措施以及其他的意外与失控事件,使得福岛核反应堆内的放射性物质持续大规模外泄释放到周边环境。日本灾区民众遭受地震、海啸和核辐射污染三重打击,同时造成了日本和邻国民众对日本环境和食品安全的担忧。限于篇幅,笔者主要从放射性铯和锕系元素等方面介绍日本福岛核电站事故对环境和食品安全可能造成的影响,对如何加强安全防范和科学研究等问题做一些概要的介绍和讨论。
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核泄露后人体健康可能会因为内外辐射而受到影响。内辐射主要来源于呼吸暴露、皮肤暴露和食品摄入。核事故发生后的前两年,人体摄入的放射性物质主要来源于绿叶植被上的沉降,而不是来源于根茎类食品;另外一个主要途径则是来自肉类和奶制品的摄入,因为牲畜可能食用了被污染的饲料[2]。福岛核事故后,日本厚生劳动省根据日本原子力规制委员会、国际放射防护委员会和国际原子能机构等推荐的以每年最高摄入剂量5 mSv、甲状腺等效剂量50 mSv的食品安全摄入限值为基准,制定了食品中放射性物质暂定限制(表1、表2)。2012年4月1日福岛核事故1年后,现行的食品中放射性物质的基准值开始实施(表1)[32]。考虑到食品的摄取量和可能含放射性物质的食品比例等因素的影响,因此不能仅通过数据比较进行衡量。国际食品法典委员会、欧盟和日本对来自食品的辐射剂量上限值的设定同为1 mSv/年。随后日本厚生劳动省进行了前所未有的食品安全检测。随着放化检测仪器的增多,检测的食品样本数量逐年递增(图3)。以福岛县为例,事故发生后第1年检测的食品样本为21 549个,截至2019年2月26日共计检测了271 654个样本(图3)[33]。日本政府为减少在生活空间中受到的辐射量而实施了清除放射性物质或用土覆盖等作业。日本农林水产省、地方自治体和生产者通过饲养管理、使用钾肥等措施降低食品中放射性物质的活度[34]。基于这些有效的措施以及半衰期较短、比活度较高的134Cs和131I能够快速地衰变等原因,目前大部分食品中放射性物质的活度均低于基准值,放射性Cs的活度大幅降低,超过基准值的样品数量逐年递减。从2015年起截至2019年2月26日,超过基准值的样品占比均低于1%(图3)[33]。福岛核事故污染最严重的地区约75%被森林覆盖,这些地区很难进行除污[35-36]。但是,森林具有富集放射性物质的属性,放射性物质在森林系统的生物半衰期较长,并被不断的排入河流等其他生态系统[35-38]。此外,由于动植物的某些特殊属性导致其被污染的水平较高,比如蘑菇富集Cs,野猪、野鹿、黑熊等主要以蘑菇或其他易富集Cs的植被为食[1]。目前,在福岛地区的蘑菇、野生动物肉制品、野生蔬菜、淡水鱼和一些咸水鱼等非人工饲养的食品中,放射性Cs的活度仍然超过基准值。需要长期检测这些食品[33]。基于检测的食品中放射性活度数据和日本人的饮食结构,对人体辐射剂量进行评估,结果显示其低于最高限值(1 mSv/年)[39-42]。总体来说,福岛核事故后,日本采取的表层土壤除污、相应法规制定和实施、农产品生产规范和上市前检测等措施取得了积极效果,具有借鉴意义。
不同组织或国家 辐射剂量上限值/年(mSv) 含放射性物质食品比例的假定值(%) 国际食品法典委员会 1 10 美国 5 30 欧盟 1 10 日本暂定值 5 50 日本现在值 1 50 表 2 各国和各国际组织关于辐射剂量上限等的相关规定
Table 2. Relevant regulation on radiation dosage upper limit adopted by the United States, the European Union, the Codex Alimentarius Commission and Japan
核素 食品类型 国际食品法典委员会基准值 美国基准值 欧盟基准值 日本暂定基准值 日本现在基准值 放射性铯(Bq/kg) 饮用水 − 1200 1000 200 10 牛奶 − 1200 1000 200 50 婴幼儿食品 1000 1200 400 500 50 一般食品 1000 1200 1250 500 100 注:表中,“−”表示无此项数据。 表 1 各国和各国际组织关于食品中放射性铯相关指标的规定
Table 1. Derived intervention levels for radiocesium in foods adopted by the United States, the European Union, the Codex Alimentarius Commission and Japan
福岛核事故对日本环境和食品安全的影响
Environmental impact and food safety in Japan after the Fukushima Daiichi Nuclear Power Plant accident
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摘要: 笔者阐述了日本福岛核事故对当地环境的影响,系统介绍了同位素示踪技术在日本福岛事故发生后的发展和应用,分析并指出福岛核事故释放的放射性微粒对环境和辐射剂量评估带来的新挑战,总结了福岛核事故后日本当地食品安全规范和现状。希望能引起核工业从业人员、核应急管理人员以及公众的重视,并引以为鉴。Abstract: This review illustrated the radiation effects on the environment in Japan, the improvements and applications of source identification by using radioisotope tracer, the resulted challenge on the environment and radiation dose estimation due to the released radioactive microparticles, and the regulations and current situation of food safety in Japan, after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The FDNPP accident allowed the staffs in the nuclear industry, managers for the nuclear emergency, and the public to recognize the importance of nuclear security and nuclear emergency response again.
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表 2 各国和各国际组织关于辐射剂量上限等的相关规定
Table 2. Relevant regulation on radiation dosage upper limit adopted by the United States, the European Union, the Codex Alimentarius Commission and Japan
不同组织或国家 辐射剂量上限值/年(mSv) 含放射性物质食品比例的假定值(%) 国际食品法典委员会 1 10 美国 5 30 欧盟 1 10 日本暂定值 5 50 日本现在值 1 50 表 1 各国和各国际组织关于食品中放射性铯相关指标的规定
Table 1. Derived intervention levels for radiocesium in foods adopted by the United States, the European Union, the Codex Alimentarius Commission and Japan
核素 食品类型 国际食品法典委员会基准值 美国基准值 欧盟基准值 日本暂定基准值 日本现在基准值 放射性铯(Bq/kg) 饮用水 − 1200 1000 200 10 牛奶 − 1200 1000 200 50 婴幼儿食品 1000 1200 400 500 50 一般食品 1000 1200 1250 500 100 注:表中,“−”表示无此项数据。 -
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