Tailoring the Breeding-Fission Ratio in a Hybrid Fusion Reactor Blanket by Neutron Moderation with Iron

The hybrid fusion reactor is becoming an interesting and promising model. In the present Note, a method for controlling the breeding-fission ratio is investigated. Since ²³⁸U fission occurs mainly with fast neutrons and breeding occurs with intermediate and slow neutrons, an optimal ratio can be obtained by partial slowing down of the original 14.9-MeV neutrons. This is done using iron as the moderator. Uranium samples were irradiated with 14.9-MeV neutrons from a deuterium-tritium reaction with iron layers of various thicknesses between the samples and the neutron source. It was found that with a relatively thin layer of iron (12 cm), any breeding-fission ratio can be obtained within a range of two decades. The breeding rate changes by only 50% when the iron-slab thickness changes from 0 to 12 cm, while the fission rate follows (more or less) the 14-MeV neutron flux and drops by more than two decades. Good agreement was obtained between the measurement and the calculated results.