Hybrid Fusion Fission Molten Salt Reactor

A single HMSR (Hybrid Fusion Fission Molten Salt Reactor) is designed to produce 5 GW thermal power at 700 C, thermal conversion systems should be able to generate 2.1 GW gross electricity. For comparison, the largest PWR’s now under construction will convert the same thermal power to 1.6 GW of electricity, i.e., 500 MW less. One could consider using a portion of the extra 500 MW of electrical power resulting from the greater thermal conversion efficiency to operate the HMSR’s driven DT fusion source of energetic neutrons.

Because simulations showed that less than 1% of the MSR power is adequate for the HMSR, no more than 50 MW of average DT fusion power is needed for a 5 GWth HMSR if all fusion neutrons are absorbed in the molten salt.

A hybrid molten salt reactor includes a source of energetic neutrons, the energetic neutrons having a typical energy per neutron of 14 MeV or greater, a critical molten salt reactor, and a molten salt comprising a dissolved mixture of fissile actinides and fertile actinides. The molten salt circulates in a loop through the reactor vessel and around the source of energetic neutrons. The fissile actinides and fertile actinides sustain an exothermic nuclear reaction in which the actinides are irradiated by the energetic neutrons, the energetic neutrons inducing subcritical nuclear fission, and undergo critical nuclear fission when circulating through the critical molten salt reactor. A portion of the daughter neutrons generated by nuclear reactions are captured by the fertile actinides in the molten salt and induce transmutation of the fertile actinides into fissile actinides and sustain critical fission chain reactions in the molten salt reactor.

The proposed Fission-Fusion Hybrid Molten Salt Reactor (FFHMSR), combining two subsystems, a deuterium + tritium (DT) fusion reactor surrounded by a neutron-absorbing Fusion Blanket (FB) and a critical Molten Salt fission Reactor (MSR). The molten salt, which contains dissolved actinides, circulates at a high rate between them.

As envisioned the MSR exhibits the large Conversion Ratio of graphite moderated reactors having small fissile and large fertile inventories. DT fusion neutrons irradiating actinides in the molten salt release additional neutrons which increase isotope conversion and fission. Actinide fuel is continually added while fission products are continually removed so the system’s operation never requires refueling interruptions. The choice of molten salt as a eutectic mixture of the fluorides of lithium, sodium, and actinide fuel is explained by eliminating other options.

Molten Salt Reactors (MSRs) form a class of novel nuclear fission reactors which in the past have been operated experimentally. The MSR class of designs has been adopted internationally as one of the Generation IV reactor design families to be developed for possible future use, and there is substantial technical research interest in MSR’s within the international nuclear engineering community.

Ever since fission breeder design difficulties, costs and constraints were recognized in the 1950s, there have been efforts to find alternative approaches to harvesting fission energy from the more abundant non-fissile but fissionable actinides.

Two pathways for using fissionable actinide isotopes
1. Provide an external source of sufficiently energetic neutrons to induce the fissions without any chain reaction.
2. Transmute the fissionable isotopes into fissile isotopes.

Unlike accelerator driven systems, net energetics need not be an issue in a FFH since fusion releases its own nuclear energy. Proposed FFH schemes can be classified according to whether their fusion fuel feeds are DD or DT. If the proposed fusion system uses a feedstock of deuterium only, then half of its resulting DD fusion reactions would produce 2.45 MeV neutrons. These do not carry enough energy for pathway one but are adequate for pathway two. The other half of the DD reactions would produce 1.01 MeV tritons which, if confined in the plasma, would
fuse with deuterons to yield 14.1 MeV neutrons adequate for pathway one. In this scheme the fusion system is self-supporting since it only exports neutrons and is fueled by natural deuterium. However, the neutron yield is weak compared with the DT case.

If instead the fusion uses a 50/50 DT feedstock of deuterium and tritium, then almost all neutrons produced will be 14.1 MeV neutrons adequate for pathway one.

Ralph Moir has been instrumental in the development of FFH design concepts.

The HMSR can completely consume all supplied actinides using uranium, SNF or thorium fuels, an the steady fission to energetic neutron power ratio is sufficiently large for the neutron source to be less than 1% of total plant power.

HMSR has novel synergistic design features are combined to reduce device size and to simplify maintenance, thus limiting costs. After further development of this conceptual energetic neutron source design, it may be feasible to deploy fusion-fission HMSR systems in the near-term using fusion energy gain factors that have already been demonstrated, without waiting for additional pure fusion research progress.

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