The behaviour of coated fuel particles at very high temperatures

Coated particles of different kernel and coating design (U, Th)O2-Biso (methane), ThO2-Biso (propene), ThO2-Biso (mixgas), UC2-Triso (propene), UO2-Triso (propene) have been treated at very high temperatures (2,300-2,5000C). After this treatment the samples have been analysed microradiographicaly and by metallographic cross sections. After the 24 h heat treatment at 2,5000C the methane-derived pyrocarbon (HTI) particles of THTR-design [(U, Th) O2-Biso] do not show any change on the heavy metal distribution. Propene-derived pyrocarbon is under these conditons quasi-permeable to CO, so that the oxide kernel can be reduced to carbide, which then loses heavy metal through the coating. The SiC-interlayer of the Triso coatings decomposes very fast under those conditions to carbon and silicon and therefore the retention of this interlayer is destroyed. The SiC-layer can also be destroyed by the interaction with the diffused heavy metal, if the kernel containes a carbide like the fissile particle design of General Atomic. These results are in a good agreement with thermodynamic considerations made for the various observations. (orig.)