Technical Basis for Proposed Fissile Exemption Criteria for Transport Packages

The U.S. experience in applying the fissile exceptions from the 1996 Edition of ST-1 has 1) identified concerns regarding the potential for inadequate criticality safety in certain shipments of excepted quantities of fissile material (beryllium oxide containing low-concentration of high-enriched uranium), 2) identified multiple examples where fissile-excepted consignments were accumulated on a conveyance in quantities which exceeded the excepted fissile material limits (demonstrating that the ad hoc control provided by a consignment limit was insufficient), 3) demonstrated continued confusion among licensees regarding what constitutes a \"lattice\" or \"essentially homogeneous\" material as required for 672(b), and 4) identified additional special moderating material (reactor-grade graphite) that the IAEA regulations do not address, thus permitting a potential for inadequate safety margin for exempt packages.