Appropos only of your request, and not quite 'on-topic' otherwise, according to DOE Inspector General Report 0813 dated February 18, 2009 (DOE-IG-0813), which is an Audit Report on the Department's Management of Nuclear Material Provided to Domestic Licensees, the findings determined that DOE domestic licensees had failed to accurately account for the locations and quantities of certain nuclear materials; and/or that the department had "agreed to write-off large quantities without fully understanding the final disposition of these materials". Among the findings:
"...Waste processing facilities were unable to determine whether some 6,711 grams of special nuclear material (SNM) and 35,269 kilograms of uranium were still under their control or had been treated and disposed of as waste"; and
"...DOE agreed to write-off 20,000 grams of SNM and 194,000 kilograms of... uranium" (page 2);
There are certainly steps in standard nuclear materials processing where reductions in amounts of nuclear material accounted for may be encountered. For example, in the 1990s at Rocky Flats it was determined that a percentage of material unaccounted for had leached into piping out of the acid solution flowing through the pipes.
Some may also find this disconcerting:
A story first reported by Military Times and subsequently by Global Research
about missing nukes. Interesting read.