TRANSPORT BEHAVIOR OF SILVER IN HIGH-TEMPERATURE GAS-COOLED REACTORS

High-temperature gas-cooled reactor (HTGR) is characterized by inherent safety, superior proliferation resistance, and high efficiency, with a promising prospect on electric power product and high-temperature heat and hydrogen production. The enhanced safety mostly relies on the good retention of tristructural-isotropic (TRISO) coated fuel particles for fission products. Most radioactive nuclides are retained in the fuel element dispersed with a large amount of TRISO coated fuel particles during normal and off-normal transients. However, silver can pass through the intact TRISO coated fuel particle and this has been reported by researchers worldwide. Numerous studies have focused on the migration 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, nano-pores, nano-cracks, re-wetting process, lattice diffusions, and so on. 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 migration and discusses the diffusion features of silver under different mechanisms. Based on the current investigations, several suggestions have been posed to end debate on silver migration in SiC. These suggestions can lay a good foundation for further studies on migration behavior of silver.