15.38. The use of refractory-coated fuel microspheres is a key feature of the MHTGR as a result of their ability to retain fission products under severe conditions. There are two types of microspheres: fissile, containing a uranium oxycarbide (UCO) kernel with the uranium enriched to 20 percent, and fertile, containing a kernel of Th02. The fertile material enhances the conversion ratio.

15.39. Each fissile kernel, of about 350 (xm in diameter, is first coated with a porous graphite buffer, followed by three successive layers of pyr­olytic carbon, silicon carbide, and pyrolytic carbon, with an outer diameter of about 800 |xm achieved. The fertile particles, are similarly coated, but slightly larger. In earlier reactors, only the fissile particles, designated as TRISO particles, contained a silicon carbide layer, to permit fuel recycling, which is not a current option. Both types of coated particles are mixed with an organic binder and graphite filler, then fired to form fuel rods, which are inserted into holes in hexagonal graphite core blocks (§15.41).

15.40. The inner layer of silicon carbide prevents the escape of most fission products, while the outer layer of dense pyrolytic carbon provides mechanical support and acts as backup containment. Extensive testing has confirmed the ability of the coated particles to maintain their integrity and retain fission products up to sustained fuel temperatures of 1760°C. There­fore the safety design philosophy of the MHTGR is that control of radio­nuclide releases can be accomplished by their retention within the fuel particles rather than by active features or operator actions [5].