Positive reactivity effect in hollow critical assemblies with moderated neutrons

Computational results obtained using the neutron-gas-dynamic codes for fission bursts in supercritical hollow assemblies of fissioning substances with a moderated neutron spectrum, specifically, in assemblies containing a neutron absorber, with a core consisting of a mixture of uranium, enriched to 90% with^sup 235^U, and polyethylene are presented. Sharp manifestations of positive reactivity in the assemblies and a difference of the effect from that 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 in the energy release in a burst is substantially reduced by inserting boron into the cavity; 2) in many cases the effect has the classical form with real growth of k^sub eff^ and λ 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 references.[PUBLICATION ABSTRACT]