Neutronic characteristics of critical configurations, which may be formed if large enough quantities of weapons-grade plutonium that might be stored in a geologic repository are released, transported, and deposited below the repository on rock surfaces in fractures, are investigated. Three neutronic characteristics of the plutonium-rock-water systems are examined: multiplication factor k, time eigenvalue , and effective neutron generation time . A time-independent, parametric neutronic study is performed to address two questions:

1. For a given combination of design variables (including distance between fractures, fracture width, fissile material layer thickness, water contents in the rock, and concentration of 240Pu), what is the critical thickness of the plutonium deposition layer?

2. How will the neutronic characteristics vary as any one of the performance variables of this study (including water removal; fissile material and rock temperature increase; homogenization of fissile and rock materials; buildup of fission and transmutation products; and, for finite cores, core expansion) vary from their reference values?

Three processes are identified that have the potential for a large positive reactivity feedback: (a) water removal, (b) spectrum hardening, and (c) homogenization. The higher the initial water concentration, the more absorbing the medium, the more heterogeneous the plutonium deposition, and the larger the core volume, the larger the magnitude of positive reactivity feedback can be. Critical configurations were identified in which all but one (i.e., core expansion) of the reactivity feedback mechanisms are positive. Scenarios are described in which natural phenomena could "drive" slightly subcritical configurations to develop an autocatalytic prompt supercritical chain reaction.