Assessment of coated-particle failure with the IMGA device

The coating layers of modern TRISO (tri-structural isotropic) high-temperature reactor fuel provide a barrier against the release of fission products both in normal operation and off-normal conditions. Experimental simulations of D-LOFC (depressurization and loss of forced circulation) accidents up to 1600°C have demonstrated a very low failure fraction of coated particles (CPs). In most instances, not a single particle out of the up to 15,000 CPs contained in a fuel pebble suffered mechanical failure, and all observed fission product release was due to diffusion through intact coating layers. When fuel is irradiated at temperatures well above 1000°C or when accident tests are pushed to more extreme temperatures, single CPs may fail. To selectively examine the defective particles, fuel elements must be deconsolidated and the large number of CPs contained in them analysed in an automated manner. For this purpose, an irradiated-microsphere gamma analyser (IMGA) has been developed at JRC-ITU. In this device, particles are first singularized and then assessed by gamma spectroscopy. The activity ratio of selected fission product pairs with different mobility can be used to assess whether or not a particle has suffered mechanical failure and to separate defective from intact CPs.