Material Unaccounted For (MUF) refers to the difference between the book inventory of material and the physical inventory of material for an MBA over a material balance period. The following equation is used to calculate MUF:

MUF = (PB + X - Y) - PE

Where, PB is the beginning physical inventory, X is the sum of increases to inventory, Y is the sum of decreases from inventory, and PE is the ending physical inventory.

The book inventory is the algebraic sum of PB, X and Y, which implies that:

MUF = BI - PE

For item MBAs, MUF should be zero, and
a non-zero MUF is an indication of
a problem (e.g. accounting mistakes). For bulk handling MBAs, a non-zero MUF is expected for
three main reasons: 1) the measurement uncertainty; 2) the nature
of processing; and 3) operator's measurement uncertainties
associated with PB, X, Y, and
PE. These quantities are
combined to determine the uncertainty of the material balance σ_{MUF}.

The cumulative MUF is simply the algebraic sum of the MUFs for an MBA over time:

We expect that the Nuclear Material accountancy systems will be able to verify that the MUF is less than three times the standard deviation in the MUF measurements:

|MUF| < 3σMUF

and that the MUF_{c} is
less than 1 Significant Quantity (SQ) for the
time specified in the IAEA timeliness goal for
the facility (usually corresponds to the material balance
period).

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