Breeding Potential of Thermal Reactors

The breeding potential of thermal reactors is re-evaluated on the basis of present nuclear data. It is concluded that positive breeding on the U²³³, thorium cycle is possible with a variety of moderators including heavy water, graphite, beryllium, and ordinary water. Current measurements indicate that the accepted thermal value of η₂₃ = 2.28 ± 0.02 is somewhat conservative. Neutron spectrum considerations show that η₂₃ decreases gradually with increasing resonance absorption to a minimum of 2.14 ± 0.04. When neutron losses to the moderator are considered, maximum breeding gains of 0.26, 0.22, 0.21, and 0.19, respectively, are obtained for D₂O, graphite, Be, and H₂O moderated reactors. The breeding gain in reactors partially or completely moderated by beryllium can be considerably increased if use is made of the fast effect, presently estimated at 1.075 ± 0.02 for pure beryllium. Probable breeding gains in proposed full-scale fluid fuel breeders are estimated at 0.09 for the Aqueous Homogenous Reactor and 0.05 for the graphite moderated Liquid Metal Fuel Reactor and Molten Salt Reactor. Breeding in predominantly thermal, solid fuel reactors also appears within reach if neutron losses are minimized. The possibility of positive breeding in near thermal, plutonium fuelled reactors is unsettled although this goal can be approached by maximum use of the fast effect in U²³⁸. Estimates of breeding ratios in plutonium fuelled reactors depends on the variation of η₄₉ with neutron temperature which is still inadequately known.