Models for Fission-Gas Release from Pyrocarbon-Coated Particles

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Abstract
Mathematical models describing idealized mechanisms of fission-gas release were used as criteria to determine the mode of release from fully enriched UC2-fueled pyrocarbon-coated particles that had slightly 235U-contaminated outer coatings. Below 1600°C the release of krypton, and probably iodine and xenon, was due to fissions which occurred in the contaminated outer coating, with the products escaping by solid-state diffusion from the coating. Above 1600°C the krypton release increased more rapidly with temperature. The krypton originated in the fuel core and traversed the outer coating either by solid-state diffusion or Knudsen flow through micropores in the outer coating. The overall increase in release rate with time was probably due to migration of the 235U initially in the outer coating, and to the over five-fold increase in 235U contamination of the outer coating during irradiation.

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