Disposal criticality analysis for immobilized Pu: External configurations

The problem of potential external criticality is defined according to the following features and issues: The environment outside the waste package is most conveniently divided into 3 zones: (1) Near-field, in the drift, external to the waste package; (2) Near far-field, several meters into the host rock immediately adjacent to the drift; (3) Far far-field, everything beyond the near far-field. These zones have different mechanisms for accumulating a critical mass from the passing waste package outflow. Fissile material can be transported outside of the waste package thereby becoming separated from the neutron absorber. Descriptions of uranium mineral deposits imply that concentrations sufficient for criticality can be accumulated by natural processes, particularly if the uranium is highly enriched. Some configurations of fissile material in the external environment have been identified as having the potential for autocatalytic criticality behavior, but without examining the likelihood of the processes necessary to achieve such configurations. The following factors limit the possibility of external criticality: absence of geologic formations in Yucca Mountain which can cause sufficient concentration, and length of time required to form the typical geologic mineral deposit. The paper summarizes the approach used to analyze criticality and gives results and conclusions of the analysis