<p>Very-high-temperature gas-cooled reactor (VHTR) has been identified as one of the six advanced nuclear reactors by Generation IV International Forum (GIF), and its prototype is high-temperature gas-cooled reactor (HTGR). HTGR is characterized by inherent safety, superior proliferation resistance, and high efficiency, with a promising prospect on not only electric power product but also high-temperature heat, hydrogen production, etc. for industrial markets. The enhanced safety mostly relies on the good retention of tristructural-isotropic (TRISO) coated particles for fission products. Most radioactive nuclides are retained in the fuel element dispersed with a large amount of TRISO coated particles during normal and off-normal transients. However, silver can pass through the intact TRISO coated particle and this has been reported by researchers worldwide. Numerous studies have focused on the transport of silver and tried to interpret its unique behavior. Additionally, several theories have been proposed and abnormal occurrences have been ascribed to excess silicon, chemical degradation of SiC, grain boundaries, long columnar grains, nanopores, nanocracks, re-wetting process, and lattice diffusions. However, this problem has not been completely solved so far. This paper introduces the general diffusion theory for fission products in the spherical particle, reviews recent theoretical models and mechanisms on silver transport, summarizes the diffusion coefficients of silver in SiC from various experiments and theoretical calculations, and discusses the diffusion features of silver under different mechanisms. Based on the above investigations, several key questions have been posed and should be well explained if a new mechanism for the abnormal diffusion characteristics of silver will be proposed, which can lay a good foundation for further studies on transport behavior of silver.</p>

Transport Behavior of Silver in High-Temperature Gas-Cooled Reactors

Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE28/volume/85246

2021 28th International Conference on Nuclear Engineering (ICONE28)

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