Criticality safety evaluation for disassembly basin sand filter

As a result of the Reactor Division's disassembly basin cleanup program, it has been determined that fissile-isotopes are present in the sludge that has accumulated at the bottom of the disassembly basins. Good criticality safety practices require that the potential for obtaining a critical configuration with this fissile material be evaluated. As part of this process, the disassembly basin sand filter system has been identified as a potential area of concern. Because disassembly basin water flows through the sand filter, it is conceivable that fissile material, from the basin, could accumulate in the sand filter. Previous calculations have indicated that the mass of some fissile isotopes in the basin sludge exceeds subcritical mass limits. This report documents the criticality safety evaluation that was performed to address the possibility of forming a critical configuration within the sand filter. This evaluation is applicable to K and L Areas, since the fissile masses listed in Table 1 are bounding for both areas. Applicability to P Area will be examined following the completion of sludge sample analyses for that Area. Although it is conceivable that fissile material could accumulate in the sand filter, because of the required fissile mass and necessary critical geometries it is highly unlikely that a critical configuration could be assembled. The mass of fissile material required for criticality, for present and anticipated sand filter geometries and operational characteristics, is much greater than that available in the sludge, as indicated by sludge sample analyses. In short, there is no identified mechanism by which a critical configuration could be assembled in the disassembly basin sand filter