The behaviour of spherical HTR fuel elements under accident conditions

Abstract Irradiated spherical fuel elements containing a large number of coated particles (20000–40000) with burnups between 6 and 16% FIMA were heated at temperatures between 1400 and 2500°C. HTI-pyrocarbon coating retains the gaseous fission products (e.g. Kr) very well up to about 2400°C if the burnup does not exceed the specified value for THTR (11.5%). Cs diffuses through the pyrocarbon significantly faster than Kr and the diffusion is enhanced at higher fuel burnups because of irradiation induced kernel microstructure changes. Below about 1800°C the Cs release rate is controlled by diffusion in the fuel kernel; above this temperature the diffusion in the pyrocarbon coating is the controlling parameter. An additional SiC coating interlayer (TRISO) ensures Cs retention up to 1600°C. However, the release obtained in the examined fuel elements was only by a factor of three lower than through the HTI pyrocarbon. Solid fission products added to UO2-TRISO particles to simulate high burnup behave in various ways and migrate to attack the SiC coating. Pd migrates fastest and changes the SiC microstructure making it permeable.