The physics of advanced reactors

Abstract

This lecture is an attempt to characterize the various promising types of solid fuel power reactors in terms of the neutron-physics characteristics of their moderators, coolants, and fuel cycles. The neutron-physics design objectives which have the greatest effect on power cost are: (i) use of a fertile isotope (uranium 238 or thorium 232) as a major source of energy; (ii) internal conversion ratio high enough to allow long fuel element life; (iii) safety characteristics favourable enough to avoid remote location and high containment cost. These objectives are not restrictive enough to define the best reactor type, but they are the major factors in determining the composition and geometric arrangement of specific reactor designs once the type has been chosen. The thermal reactors which are at present being designed or constructed utilize uranium 235 as the initial fissile isotope, and there is a strong incentive to utilize ratios of fissile to fertile material which depart as little as possible from the natural value. With this additional design restriction, the lattice characteristics appropriate to the various moderator-coolant combinations are still more sharply defined. Examples are drawn from the current group of reactor designs to illustrate these characteristics. The possible consequences of a greater latitude in the choice of fissile-fertile ratio are discussed, and the relative characteristics of the possible fuel cycles are considered. The characteristics of fast reactors fit into a quite different pattern. The possible conversion ratio is high on either fuel cycle, and the reactivity changes associated with fuel burn-up are relatively low simply because of the very large fissile-material content of the reactor. The requirement that the average neutron energy be kept high is a rather restrictive one. It appears now that the range of variation in fast reactor design will be less than that in the thermal reactor field. The characteristics of typical fast reactors are presented, their special problems are discussed, and some recent experimental results are quoted.