Determination of Defective Particle Fraction in HTGR Fuels

Book excerpt: The fuel for the High-Temperature Gas-Cooled Reactor (HTGR) consists of uranium and thorium species in the form of microspheres individually encapsulated in layers of pyrolytic carbon and silicon carbide. In the characterization of these particles, an important consideration is the fraction in a particular sample that may have defective coatings which would allow the release of gaseous and metallic fission products. A method for this determination, which has been in use for almost a decade, is the chlorine leach in which the gas is passed over the particles at 1000°C, and the fuel that is exposed by coating defects is volatilized and collected as the heavy metal chlorides. However, when the particles have been bonded into a fuel rod, the removal of exposed fuel from particles in the intact rod requires long exposure times (16-50 hours). A much more rapid procedure (2-3 hours) is described in which the chlorination of fuel rods is done at 1500°C. The method is also applicable to loose particles. Also described is an adaptation of the chlorine leach at 1000°C for the treatment of irradiated fuel rods in a hot cell.