Positive reactivity effect in hollow critical assemblies with moderated neutrons

Computational results obtained using the neutron-gas-dynamic codes for fiss ion bursts in supercritical hollow assemblies of fissioning substances with a moderated neutron spectrum, specifically, in assemblies containing a neu tron absorber, with a core consisting of a mixture of uranium , enriched to 90% with U-235, and polyethylene are presented. Sharp manifestations of po sitive reactivity in the assemblies and a difference of the effect from tha t in fast systems are found. It is shown that in assemblies with moderated neutrons, as compared with fast systems, 1) the scale of the increase ill t he energy release in a burst is substantially reduced by inserting boron in to the cavity; 2) in many cases the effect has the classical form with real growth of k(eff) mid ii in time; 3) the effect is essentially independent of the properties of the moderator in the cavity; and 4) the effect is less dangerous with respect to nuclear safety 2 figures, 3 tables, 12 reference s.