High-temperature gas-cooled reactors (HTRs) are able to accommodate a wide variety of mixtures of fissile and fertile materials without any significant modification of the core design. This flexibility is due to an uncoupling between the parameters of cooling geometry and the parameters that characterize neutronic optimization (moderation ratio or heavy nuclide concentration and distribution).

Among other advantageous features, an HTR core has a better neutron economy than a light water reactor (LWR) because there is much less parasitic capture in the moderator (capture cross section of graphite is 100 times less than the one of water), in internal structures, and in fission products (because of a harder spectrum).

Moreover, thanks to the high strength of the coated particles, HTR fuels are able to reach very high burnups, far beyond the possibilities offered by other fuels (except the special case of molten salt reactors).

These features make HTRs potentially interesting for closing the nuclear fuel cycle and stabilizing the plutonium inventory.

A large number of fuel cycle studies are already available today on three main categories of fuel cycles involving HTRs: (a) high-enriched uranium cycle, based on thorium utilization as a fertile material and high-enriched uranium as a fissile material; (b) low-enriched uranium cycle (LEU), where only LEU is used (from 5 to 15%); (c) plutonium cycle based on the utilization of plutonium only as a fissile material, with (or without) fertile materials.

Plutonium consumption at high burnups in HTRs has already been tested with encouraging results under the DRAGON project and at Peach Bottom. To maximize plutonium consumption, recent core studies have also been performed on plutonium HTR cores, with special emphasis on weapon-grade plutonium consumption. We complete the picture by a core study for an HTR burning reactor-grade plutonium. Limits in burnup due to core neutronics are investigated for this type of core.

With these limits in mind, we study in some detail the Pu cycle in the special case of a reactor fleet made of a mixture of LWRs and HTRs. It is reasonable to assume that if HTRs are to be deployed on an industrial scale, they will coexist during a long period of time with already existing LWRs. The present paper investigates the symbiotic behavior of LWRs producing plutonium and of HTRs burning it.