Candidate ceramic materials were studied to determine their suitability as Gas-Cooled Fast Reactor particle fuel coatings. The following ceramics examined in this work: TiC, TiN, ZrC, ZrN, AlN, and SiC. The studies focused on (i) chemical reactivity of the ceramics with fission products palladium and rhodium, (ii) the thermomechanical stresses that develop in the fuel coatings from a variety of causes during burnup, and (iii) the radiation resiliency of the materials. The chemical reactivity of TiC, TiN, ZrC, and ZrN with Pd and Rh were all found to be much lower than that of SiC. A number of important chemical behaviors were observed at the ceramic-metal interfaces, including the formation of specific intermetallic phases and a variation in reaction rates for the different ceramics investigated. Based on the data collected in this work, the nitride ceramics (TiN and ZrN) exhibited chemical behavior that is characterized by lower reaction rates with Pd and Rh than with the carbides TiC and ZrC.The thermomechanical stresses in spherical fuel particle ceramic coatings were modeled using finite element analysis, and included contributions from differential thermal expansion, fission gas pressure, fuel kernel swelling, and thermal creep. In general the tangential stresses in the coatings during full reactor operation are tensile, with ZrC showing the lowest values among TiC, ZrC, and SiC; TiN and ZrN were excluded from the comprehensive calculations due to a lack of available materials data. The work has highlighted the fact that thermal creep plays a critical role in the development of the stress state of the coatings by relaxing many of the stresses at high temperatures. To perform ion irradiations of sample materials, an irradiation beamline and high-temperature sample irradiation stage was constructed at the University of Wisconsin's 1.7MV Tandem Accelerator Facility. This system is now capable of irradiating of materials to high dose while controlling sample temperature up to 800ºC. iv v
EXECUTIVE SUMMARY
IntroductionSeveral fuel forms are currently being considered for the Gas Fast Reactor (GFR), including coated particle fuel embedded in an inert matrix. The development of this fuel form would build on existing experience with high temperature gas-cooled reactor (HTGR) coated particle fuel, but would differ in several key aspects, including the elimination of pyrolytic carbon as a coating material and an increase in the volume ratio of the fuel kernel to satisfy GFR fissile loading requirements. Because of the severe in-core environments of the GFR (including high temperatures and high neutron fluences) the requirements for fuel materials are demanding. Based on materials property surveys, several ceramic materials have been identified as potential particle coating materials; these are TiC, TiN, ZrC, ZrN, and SiC.This report presents the results of a cooperative investigation by the INL and the University of Wisconsin into the suitability of these materials for GFR fuel particle coatings. The research...
