Failure Mechanisms of Fuel Particle Coating for High-Temperature Gas-Cooled Reactors during the Coating Processes

Abstract

The high-temperature engineering test reactor (HTTR), a 30-MW(thermal) high-temperature gas-cooled reactor, is being constructed by the Japan Atomic Energy Research Institute. The HTTR uses TRISO-coated fuel particles: UO{sub 2} microspheres coated with layers of porous pyrolytic carbon (PyC), inner dense PyC (IPyC), silicon carbide (SiC), and outer dense PyC (OPyC). To reduce the defective coating fraction of TRISO-coated UO{sub 2} 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.