Underground supercriticality from plutonium and other fissile material

C. D. Bowman, F. Venneri, "Underground supercriticality from plutonium and other fissile material," Science & Global Security, 5, no. 3, (1996): 279-302.
Several widely endorsed solutions to the long-term disposition of weapons plutonium and other waste fissile nuclear material involve placement of batches of the material underground in subcritical concentrations. It is pointed out here that such concentrated subcritical fissile material underground might reach criticality that is autocatalytic or self-enhancing. This criticality could come about upon dispersion into the surrounding medium by either natural or unnatural processes, or by the fissile material being carried to other sites where it can collect into different autocatalytic critical configurations. Underground, where the material is confined and there is an abundance of moderating medium around it, the results of such supercritical excursions could range from modest energy releases to the several hundred gigajoules range from a single event. Without water, 50-100 kg of fissile material is required to reach autocatalytic criticality. Amounts as small as 2 kg can reach autocatalytic criticality with water present. In varying degrees, all categories of waste containing fissile actinide appear to be susceptible to these criticality excursions, including vitrified weapons plutonium, research reactor and DOE spent fuel, commercial, and MOX spent fuel.

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