The uncertainty was related to the fuel of the Annular Core Research Reactor (ACRR), maintained by Sandia. The reactor uses fuel with 35% enriched uranium. The fuel, which contains 21.5 weight-percent UO2 (and 78.5 weight-percent BeO), apparently can be categorized differently - either as Category IC or as Category IIID - under different criteria. According to Sandia, the fuel is Category III material because of the low uranium weight contents. However, the currently existing guidelines (Nuclear Material Control and Accountability, DOE M 470.4-6), if applied strictly, would require categorizing the material as Category IC.
Categorization of HEU fuel at Sandia
Background Removal of Categories I and II Special Nuclear Material from Sandia National Laboratories-New Mexico, Inspection Report DOE/IG-0833, 15 January 2010, issues by the DoE Inspector General outlines some uncertainty about categorization of the HEU material that remains at Sandia after it completed the removal of Category I and II materials in February 2008.