Neutronic characteristics are investigated of critical configurations consisting of U and moist rock, which may be formed if large enough quantities of highly enriched U (HEU) are released, transported, and deposited in the rock below the repository. A companion study investigated neutronic characteristics of similar critical configurations consisting of weapons-grade Pu (wPu) and moist rock.

Two modes of U deposition are considered: uniform homogeneous and heterogeneous. The latter is assumed to be of the form of thin planar deposition layers separated by relatively thick slabs of rock, making a constant pitch lattice. Three neutronic characteristics are examined: the critical U loading, and the variation, with the change of operating conditions, of the multiplication factor k; the time-eigenvalue ; and the effective neutron generation time . They are compared against the characteristics of the corresponding wPu-rock systems. The study is done parametrically by solving time-independent transport equations. The effect of various mechanisms (including water removal, U and rock temperature increase, homogenization of fissile and rock materials, and the system expansion) on the neutronic characteristics is studied independently.

A surprising finding is that heterogeneous depositions of HEU in moist rock can have a positive reactivity feedback due to spectrum hardening. Moreover, the magnitude and temperature dependence of heterogeneous U-rock systems can be comparable to those of Pu-rock systems despite the fact that they are due to entirely different mechanisms: reduction in the spatial self-shielding in the HEU systems compared with an increase in the effective fission-to-absorption cross-section ratio in the wPu systems.

It is concluded that heterogeneous HEU deposits in moist rock can be just about as autocatalytic as critical wPu deposits but require approximately twice the critical mass. Three mechanisms were found to have a potential for a large positive reactivity insertion: water removal, rock temperature increase, and homogenization. The magnitude of each of the three positive reactivity feedbacks in the HEU systems is similar to the magnitude in the corresponding wPu systems.