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Material unaccounted for is the difference between a recorded closing inventory and the measured physical inventory in nuclear material accountancy. With a consistent accounting period and material definition, $\mathrm{MUF}=B+R-S-E$, where $B$ is beginning inventory, $R$ receipts, $S$ removals and $E$ ending physical inventory.^[ref]

For instance, measured amounts $100+20-5-114$ leave an apparent difference of one mass unit. Interpretation requires measurement uncertainty: if the four measurement errors are independent, their variances add in the variance of the difference; correlated errors require covariance terms. A nonzero balance difference is therefore a measurement-and-reconciliation result that must be evaluated, rather than an automatic finding of missing physical material.^[ref]

Nuclear material accountancy records and reconciles inventories and transfers of nuclear materials. It combines records of what entered and left a defined accounting area with measurements of what is physically present. A basic balance isAll four terms must describe the same material quantity and accounting period. The difference $\Delta$ is called material unaccounted for: it compares the amount inferred from the records with the closing physical inventory.^[ref]

A nonzero difference requires interpretation using measurement uncertainty, including uncertainties in the individual balance terms. It is not automatically evidence that material has disappeared. For example, equal true opening and closing amounts can yield a nonzero measured difference when the two measurements have different errors. This makes nuclear material accountancy an application of mass balance together with statistical inference and measurement control.^[ref]