FAILURE MECHANISMS OF FUEL PARTICLE COATING FOR HIGH-TEMPERATURE GAS-COOLED REACTORS DURING THE COATING PROCESSES

To reduce the defective coating fraction of TRISO-coated UO2 particles, failure mechanisms of fuel particle coating during the coating processes have been studied. Examinations of the coated fuel particles at every coating stage revealed two kinds of silicon carbide (SiC)-defective particles. The SiC-defective particles with partly carbonized kernels were formed by chemical reactions during SiC deposition when the coating layer of inner dense pyrolytic carbon was defective. The SiC-defective particles with nonreacted kernels were formed by mechanical shocks during unloading of SiC-coated particles from the coater. The coating processes were improved by controlling particle fluidization modes in the coater and by adopting a coating process without unloading and loading of the particles at intermediate coating stages.