Release behavior of fission products from coated fuel particles during post-irradiation heating at abnormally high temperatures

The present report describes experimental results on release behavior of metal fission-products during isochronal and isothermal heating of TRISO-and BISO-coated fuel particles at temperatures from 1400 to 2200degC. The particles were irradiated in the eighth OGL-1 fuel assembly at 1300 - 1400degC for 53.8 days up to burnups around 1% fima. Fuel particles without failed or defective coating layer in X-ray radiography were heated up one by one in a graphite electrode, whose temperature was measured by a pyrometer. The fractional releases of 137Cs from the TRISO-coated fuel particles were only around one percent, even after isothermal heating at temperatures from 1600 to 2000degC for several hundred hours. Effective retention of 137Cs by the silicon-carbide layer was observed below 2000degC through comparison of its fractional releases from the TRISO-and BISO-coated fuel particles. Above 2050degC, however, fractional releases of metal fission-products from the TRISO-coated particles increased rapidly, due to thermal degradation of the SiC layer. (author)