Failure mechanisms of coated fuel particles during fabrication

In order to decrease failure fraction of as-fabricated coated fuel particles for HTGR, failure mechanisms of the coated fuel particles during fabrication were studied. More than eight thousand as-fabricated coated fuel particles from seven coating batches were observed by means of X-ray micro-radiography, ceramography, optical microscopy, etc. Two possible failure mechanisms were suggested through the observations. One of SiC failure mechanisms is that SiC will not deposit on defective inner pyrolytic carbon (IPyC) or gas-permeable IPyC because CO gas generated by carbonization of UO2 kernel interferes with SiC deposition. The other SiC failure mechanism is that flawed SiC will deposit under undesirable fluidizing condition of the particles and that the flawed SiC is apt to fail because of its low strength. It was also suggested that SiC failure lead to total coating failure easily during fuel compact fabrication. The relation between coating failure and odd shape of the particles was not clear. (author)