Nuclear Materials Balance figures also known as Inventory Difference reflect differences between the inventory of nuclear material (plutonium, uranium, and thorium) recorded in the nuclear materials accounts for a facility, and the inventory that is measured physically.
Inventory differences vary from one accounting period to another showing apparent gains or losses each year. Though procedures for nuclear materials accountancy are well developed, measurement results are subject to uncertainty due to errors that are inherent in all measurement systems. The presence of positive inventory differences does not mean that material not in existence has somehow been found just as a negative figure does not imply a real loss of material. They are instead one amongst several indicators used in the control of nuclear materials. There are, in addition to such nuclear materials accountancy indicators, a variety of distinct security measures (e.g. preventing/restricting access to plant areas and personnel monitors to detect nuclear material at the exit to plants) to detect the unauthorised removal, including theft, of nuclear material.
The IAEA monitors how civil nuclear materials are handled and accounted for to confirm that nuclear material is not misused or diverted. The monitoring involves a series of assessments and inspections carried out by multi-national teams of inspectors. This verification provides an overview of the systems that keep track of civil nuclear material and the records of the quantities involved.
As part of international agreements, the UK has committed to publish information each year on inventories of civil plutonium and uranium in the UK.
The figures are published below and by the IAEA as part of its Information Circular number 549.