X-ray Computed Tomography of Tristructural Isotropic (TRISO) Fuel from the AGR-5/6/7 Irradiation Tests

The development of graphite-moderated high-temperature gas-cooled reactors (HTGRs) is currently underway. Typical HTGRs use helium as a coolant and are expected to have a core outlet temperature greater than 650°. Prismatic-type HTGRs use fuel in the form of cylindrical compacts that contain thousands of tristructural isotropic (TRISO)-coated particles embedded in a graphitic matrix. In pebble-bed HTGRs, TRISO particles are contained within spherical fuel pebbles. TRISO coated particles consist of three dense coating layers encapsulating the low-density carbon buffer layer and the central fuel kernel. In the U.S. TRISO fuel program, the fuel kernels are a heterogenous mixture of uranium oxide and uranium carbide. All the features of the TRISO particle work together to retain radioactive fission products during normal as well as abnormal conditions [1]. Figs. 1 and 2 show the structure of TRISO fuel compact and an individual TRISO particle. As shown in Fig. 1, this particular fuel compact is loaded with roughly 2200 TRISO particles to give a nominal 25% volume packing fraction. Figure 2 shows the structure of a TRISO particle, where a fuel kernel consists of a surrogate ZrO2 encapsulated in a porous buffer layer of carbon. It is further surrounded by layers of inner pyrolytic carbon, silicon carbide, and outer pyrolytic carbon, respectively.