NEUTRONIC ANALYSIS OF CRITICAL CONFIGURATIONS IN GEOLOGIC REPOSITORIES - II - HIGHLY ENRICHED URANIUM

Neutronic characteristics are investigated of critical configurations consisting of U and moist rock, which may be formed if large enough qu antities of highly enriched U (HEU) are released, transported, and dep osited in the rock below the repository. A companion study investigate d neutronic characteristics of similar critical configurations consist ing of weapons-grade Pu (wPu) and moist rack. Two modes of U depositio n 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. Th ree neutronic characteristics are examined: the critical U loading, an d the variation, with the change of operating conditions, of the multi plication factor k; the time-eigenvalue alpha; and the effective neutr on generation time Lambda. They are compared against the characteristi cs of the corresponding wPu-rock systems. The study is done parametric ally by solving time-independent transport equations. The effect of va rious mechanisms (including water removal, U and rock temperature incr ease, homogenization of fissile and rock materials, and the system exp ansion) on the neutronic characteristics is studied independently. A s urprising finding is that heterogeneous depositions of HEU in moist ro ck can have a positive reactivity feedback due to spectrum hardening. Moreover the magnitude and temperature dependence of heterogeneous U-r ock 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 increas e in the effective fission-to-absorption cross-section ratio in the wP u systems. It is concluded that heterogeneous HEU deposits in moist ro ck can be just about as autocatalytic as critical wPu deposits but req uire approximately twice the critical mass. Three mechanisms were foun d 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.