福島第一原子力発電所事故により1号機から放出された放射性粒子の放射光マイクロビームX線分析を用いる化学性状の解明  [in Japanese] Investigation of the Chemical Characteristics of Individual Radioactive Microparticles Emitted from Reactor 1 by the Fukushima Daiichi Nuclear Power Plant Accident by Using Multiple Synchrotron Radiation X-ray Analyses  [in Japanese]

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Abstract

2011年3月の福島第一原発事故により,1号機由来の放射性物質が飛来したと考えられる原発北西地域の土壌から,強放射性の粒子を7点分離した.分離された粒子は100 μm前後の大きさで<ruby>歪<rt>いびつ</rt></ruby>な形状のものが多く,2号機から放出されたとされる直径数μmの球形粒子(Csボール)とは明らかに異なる物理性状を有していた.これらの粒子に対して,大型放射光施設SPring-8において放射光マイクロビームX線を用いた蛍光X線分析,X線吸収端近傍構造分析,X線回折分析を非破壊で適用し,詳細な化学性状を解明した.1号機由来の粒子はCsボールに比べて含有する重金属の種類に富み,特にSrやBaといった還元雰囲気で揮発性が高くなる元素が特徴的に検出され,粒子内で明確な元素分布の不均一性が見られた.粒子本体はCsボールと同様にケイ酸塩ガラスであったが,Feなど一部の金属元素が濃集した数μm程度の結晶性物質を含有していた.これらの粒子は3月12~13日に大気中に放出されたものであると考えられ,核燃料と格納容器との<ruby>熔<rt>よう</rt></ruby>融がかなり早い段階で進行していたことが示唆された.さらに放出源の推定において,放射性物質自体の化学組成情報が放射能比に代わる新たな指標となることが実証された.

Seven radioactive particles were separated from a soil sample collected at the Northwest region of the Fukushima Daiichi Nuclear Power Plant (FDNPP). It has been pointed out that the soil is contaminated by radioactive materials emitted from reactor 1 of the FDNPP by the accident that occurred in March, 2011. The physical characteristics of these radioactive particles with –100 μm in diameter and non-uniform shape are clearly different from those of spherical microparticles, known as Cesium-balls, thought to be emitted from the FDNPP reactor 2. Three kinds of synchrotron radiation-based X-ray analyses (X-ray fluorescence analysis, X-ray absorption near edge structure analysis and X-ray diffraction analysis) were nondestructively applied to radioactive particles using a micro-focused X-ray beam at the SPring-8 to investigate their detailed chemical properties. Various elements related to fission products of nuclear fuel and components of the reactor were detected from the particles emitted from the FDNPP reactor 1 with an obvious heterogeneous elemental distribution. In particular, the chemical compositional feature of these particles was characterized by several elements (Sr, Ba <i>etc.</i>), which were easily volatilized in a reducing atmosphere. Although a main component of the particles was identified as silicate glass similar to the Cesium-balls, some crystalline materials were also found in microscopic regions containing Fe and other metallic elements. We concluded that these radioactive particles were emitted from reactor 1 to the atmosphere during 12<sup>th</sup> to 13<sup>th</sup> March, 2011. Our results suggest the fact that the nuclear fuel and the reactor vessels around the fuel were melted together at a very early stage of the accident. In addition, it was demonstrated that chemical compositional information of individual radioactive materials can be a new indicator as an alternative to the radioactive ratio to estimate the source of emissions.

Journal

  • BUNSEKI KAGAKU

    BUNSEKI KAGAKU 66(4), 251-261, 2017

    The Japan Society for Analytical Chemistry

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