In this fuel, uranium dioxide or uranium nitride is embedded within tristructural-isotropic (TRISO) particles, which use several layers, including a silicon carbide layer, to contain fission products .
(ii) Uranium nitride fuel (enriched to 99.5% [sup.15]N)
For uranium nitride, if natural nitrogen were used, a significant enrichment of the uranium would be required.
In the uranium nitride cases, the exit burnup for natural uranium exceeds that of natural U[O.sub.2], so the exit burnup values calculated for each case are listed.
Uranium nitride has the potential downside of natural nitrogen being a strong neutron absorber, requiring significant uranium enrichment to compensate and reducing the uranium economy of the fuel cycle.