Paul A. Demkowicz

TRISO-Coated Particle Fuel Performance

Tristructural isotropic (TRISO)-coated particle fuel is used in all current and planned high-temperature gas-cooled reactors (HTGRs). The robustness of this fuel, coupled with the high heat capacity of graphite, has led to the development of modular HTGRs with a high degree of passive safety. In this chapter, the irradiation and accident performance of modern TRISO-coated particle fuel around the world are reviewed. For all HTGRs, TRISO-coated particle fuel forms the heart of the concept. Such fuels have been studied extensively over the past four decades around the world, for example, in countries including the United Kingdom, Germany, Japan, the United States, Russia, China, and more recently, South Africa.

Ceramographic Examinations of Irradiated AGR-1 Fuel Compacts

The AGR 1 experiment involved irradiating 72 cylindrical fuel compacts containing tri-structural isotropic (TRISO)-coated particles to a peak burnup of 19.5% fissions per initial metal atom with no in-pile failures observed out of almost 300,000 particles. Five irradiated AGR 1 fuel compacts were selected for microscopy that span a range of irradiation conditions (temperature, burnup, and fast fluence). These five compacts also included all four TRISO coating variations irradiated in the AGR experiment. The five compacts were cross-sectioned both transversely and longitudinally, mounted, ground, and polished after development of careful techniques for preserving particle structures against preparation damage. Approximately 40 to 80 particles within each cross section were exposed near enough to mid-plane for optical microscopy of kernel, buffer, and coating behavior. The microstructural analysis focused on kernel swelling and porosity, buffer densification and fracture, debonding between the buffer and inner pyrolytic carbon (IPyC) layers, and fractures in the IPyC and SiC layers. Three basic particle morphologies were established according to the extent of bonding between the buffer and IPyC layers: complete debonding along the interface (Type A), no debonding along the interface (Type B), and partial debonding (Type AB). These basic morphologies were subdivided according to whether the buffer stayed intact or fractured. The resulting six characteristic morphologies were used to classify particles within each cross section, but no spatial patterns were clearly observed in any of the cross-sectional morphology maps. Although positions of particle types appeared random within compacts, examining a total of 830 classified particles allowed other relationships among morphological types to be established.