The base of the HTMR's unique design is the spherical fuel elements called "pebbles". These tennis ball-sized pebbles are made of
pyrolytic graphite (which acts as the moderator), and they contain thousands of micro fuel particles called TRISO particles. The TRISO fuel particles are fundamentally responsible for HTMR’s high temperature operation and its inherent safety. The TRISO fuel particles consist of a fissile material (such as U235) surrounded by a coated ceramic layer of SiC for structural integrity and fission product containment.
The fuel kernel, at the centre of the TRISO fuel kernel, is the primary power source for the HTMR reactors and produces almost all of the fission products. The fuel kernel also serves as a significant barrier to radionuclide release by immobilizing many of the fission products as stable oxide compounds and delaying the diffusive release of others, allowing them to decay into more stable isotopes. These processes substantially reduce fission product release from particles.
The buffer layer bonds to the fuel kernel and is a low-density, porous carbonaceous layer. The buffer layer provides void volume for the accumulation of gaseous fission products released from the fuel kernel, accommodates fuel kernel swelling, and serves as a sacrificial layer for fission fragments.
The inner pyrocarbon (iPyC) layer, between the buffer and SiC layer, is a gas-tight coating that protects the kernel from hot gaseous chlorine compounds during SiC decomposition and provides a smooth substrate for SiC deposition. The iPyC completely retains fission gases xenon, krypton, and iodine and also serves as a diffusion barrier to metallic fission products. During irradiation this layer shrinks and the contraction helps to reduce tensile stresses on the SiC.
The third layer is a near-theoretical density SiC layer which serves as the pressure bearing component of the particle and the primary metallic fission product diffusion barrier as well as retains all gaseous fission products. The SiC layer is the primary load bearing layer of the particle.
The fourth and outer layer is another high-density, isotropic layer, called theouter pyrocarbon (oPyC). This layer serves as a further diffusion barrier for gaseous and metallic fission products, and like the iPyC layer, it too contracts during irradiation helping to reduce tensile stress on the SiC. The oPyC also protects the SiC during particle handling and sphere/compact formation and provides a bonding surface to the carbon matrix in the Fuel Element.
Depending upon customer’s long-term fuel supply strategy and fuel resource availability, a customized fuel manufacturing and technology-transfer package can be provided upon the request. A complete manufacturing of pebble fuel, TRISO coated particle fuel and fuel kernels can be established on a commercial basis at location identified by HTMR customers, providing the customers with secured supply of fuel for sustainable operation of their HTMR plants. The fuel to be manufactured include both low-enriched uranium fuel option (enrichment not included) and thorium fuel option.The latter provides the customers with a complete assurance of independent and long-term fuel supply.