Material unaccounted for (MUF), in the context of nuclear material, refers to any discrepancy between a nuclear-weapons state's physical inventory of nuclear material, and the book inventory. The difference can be either a positive discrepancy (an apparent gain of material) or a negative discrepancy (an apparent loss of material). Nuclear accounting discrepancies are commonplace and inevitable due to the problem of accurately measuring nuclear materials. This problem of inaccurate measurement provides a potential loophole for diversion of nuclear materials for weapons production. In a large plant, even a tiny percentage of the annual through-put of nuclear material will suffice to build one or more nuclear weapons.