Evaluation of Hydrogeo‐chemical Conditions for Development of Nuclear Criticality in Low‐Level Waste Disposal Facilities

The under ground disposal of fissile isotope-bearing wastes poses some unique issues. Specifically, radionuclides such as 235U disposed in low-level waste facilities, if present in the correct concentration and geometry, can create a nuclear criticality event that releases radioactivity to ground water. This paper reports the results of a study of the potential for 235U to be transported by ground water within low-level waste facilities and to concentrate to form a critical mass. Coupled hydrologic and geochemical modeling was used to investigate two possible mechanisms for concentrating mobile uranium: sorption on high capacity sites and precipitation in a reducing zone. The hydrogeochemical modeling showed that (1) it is difficult to mobilize uranium from sorption sites then re-deposit it; (2) if uranium is already in solution it can accumulate in zones of high sorption, and (3) reducing zones can accumulate sufficient uranium in the presence of oxygenated ground water. Site-specific disposal practices, such as the commingling of large quantities of depleted (nonfissile) uranium and the difficulty of bringing sufficient mass together in the correct geometry, limit the potential for criticality safety concerns. In order to determine appropriate disposal practices, hydrogeochemical modeling can be used to study the future mobility and accumulation of the waste.