Fuel element, design graphite coated

Fuel The fuel and fuel element design is derived from that of the MHGR, and illustrated in Figure 3. The TRISO fuel is protected from the lead by the graphite fuel element structure. However, there is no chemical reaction between molten lead and the silica-carbide coating of the TRISO fuel particles, and the solubility of silica-carbide in lead is negligible. 

An HTGR fuel element is a composite structure. Coated microspheres of enriched uranium are bonded together with fertile microspheres in a carbonaceous matrix to form rods. These rods are then inserted into hexagonal graphite blocks to form the finished clement. Both the rods and elements contain fissile material in dilute form and present no special criticality problems. Safety limits can be met by maintaining adequate spacing between units and the fabrication equipment requires no special design features to achieve the desired production rates. 

Among the reactor accidents which have occurred up to the present, Windscale-1 and Chernobyl-4 (see Section 7.4.3.) represent by far the most serious ones with respect to the thyroid burdens to the public as well as to the field contamination of the surrounding areas. In the days and weeks after the Windscale-1 accident, it was possible to measure the released radionuclides in low concentrations even in Norway and in Germany. As was pointed out before, accidents of this type cannot happen in light-water reactors on the other hand, such release values are not to be expected from gas-cooled reactors of modern design, since the fission products in their fuels are confined in graphite-coated fuel elements which are stable even under very high temperatures. 

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