On the Minimum Overall Size of a Reflected Pebble-Bed Reactor

The minimum overall size of a reflected pebble-bed reactor is, in general, considerably smaller than that of the corresponding bare-critical assembly. For a reactor fueled with low-enriched uranium fuel elements at average burnup, the minimum outer radius for the reflected system was found to be up to 20% smaller than the bare-critical radius. The fact that the graphite reflector can effectively be so much“more reactive” than core material in the outer regions of such a reactor is shown to be largely a consequence of the relatively high degree of voidage (∼40%) inherent in pebble-bed cores.