Proceedings of the International Topical Workshop on Fukushima Decommissioning Research
Online ISSN : 2759-047X
2024
セッションID: 1066
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GENERATION MECHANISM STUDY OF FUKUSHIMA’S CESIUM-BEARING MICROPARTICLES
Hugo LaffolleyChristophe JourneauBernd GrambowAnne-Laure FauréManon Cornaton
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The accident of the Fukushima Daiichi Nuclear Power Station has caused radioactive releases over Japanese lands and into the Pacific Ocean. In the days following the initiating events, a new form of radioactive Cs pollution has been collected and characterized. These particles, referred to as Cs-bearing microparticles (CsMP), are made of an amorphous silica matrix, in which many elements are imbedded, including radioactive Cs. These particles are a significant source of Cs pollution resulting from the accident and a fraction of them, called type-A CsMP has traveled over several hundreds of kilometers due to their micrometric size.

The generation mechanism of these particles has remained uncertain since their discovery. The inclusion of a significant concentration of radioactive Cs suggests a damaged core related interaction, and the silica matrix implies an Si-rich source involved at the origin of the particle generation. Therefore, among other possibilities, it has been suggested that CsMP could result from molten core concrete interaction (MCCI) after molten core relocation on the concrete pedestal.

To investigate this proposed mechanism, a small-scale experimental set-up has been designed to collect and study the aerosols generated during MCCI. Prototypic corium, containing depleted uranium oxide and stable fission product elements has been reacted with basaltic concrete at about 2000°C through induction heating, and the resulting aerosol have been collected. The first phase of the experiments, carried out in pure nitrogen atmosphere has produced micrometric spherical particles made of amorphous silica, highly similar morphologically to type-A CsMP. The chemical composition shows significant similarities, such as the matrix composition and the Si-relative concentration of Na, K, Rb, Al, Sn; but also some discrepancies like the low concentration of Fe and Cs and the high concentration of Ca. For the second phase, the protocol has been modified to inject a 20% steam - 80% nitrogen gas mixture in the test section and try to reach a more representative containment vessel atmosphere condition. These experiments have proven more challenging to carry out due to the reactive atmosphere, but micrometric particles have been also observed. While the overall composition was somehow similar, the Fe and Cs concentration appeared higher and Ca concentration lower.

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© 2024 The Japan Society of Mechanical Engineers
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