COATING PARAMETERS THAT INFLUENCE THE DEFECT RATIO OF TRISO COATED FUEL PARTICLES

Coater parameters such as deposition temperature, volume percent of methyltrichlorosilane, and total gas flow were varied to study the effect on the ratio of defective TRISO nuclear fuel particles. The burn-leach test and other leach tests were performed to determine the defect ratio on samples of particles representing these variations. In the narrow ranges that were used, none of these parameters showed any correlation with the burn-leach result. However, a reduction in the density of the directly underlying carbon layer showed a marked increase in the defect ratio of particles. No trend could be observed when the density of the carbon layer was varied in the range of 1.8 to 2 g/cm(3), specified for TRISO particles. But, when the density was reduced to 1.7 and 1.6 g/cm(3), it was seldom possible to produce a batch that did not leach uranium, in spite of having a good quality SiC layer. This indicates that the integrity of the SiC layer is influenced by the quality of the underlying carbon layer. Mechanical damage is proposed as a mechanism responsible for the defective particles that are detected with the leach methods. This mechanism could be the reason for the variations in the leach results. Calculations and some examples show that all defects are not detected with the leach methods, probably because of the limited duration of these tests.