Detailed records are kept of all nuclear materials that are received and processed at nuclear sites. The records are the basis for nuclear materials accountancy, which works in a similar manner to financial book-keeping.
At the start of an accountancy period, the books are in agreement with the stock or inventory. As material is received and brought on to the inventory the book account is increased, and for material leaving the inventory the book is decreased. At the end of the accountancy period, the inventory of material in stock is counted or measured and compared with the book account. Any inventory difference found will be an apparent gain or loss of material, and is known internationally as an Inventory Difference (ID) or material unaccounted for.
Whenever nuclear material is measured, there is an uncertainty associated with the measurement. For some forms of material, e.g. finished product, the material can be accurately weighed and the uncertainty will be small. For other forms, e.g. material held up in process, measurements may be more difficult and the uncertainty higher. Such measurement uncertainties are a major cause of inventory differences, and their existence does not mean that material has been found or lost. The magnitude of ID due to measurement uncertainty will depend strongly on the throughput of material at the plant concerned, i.e. the amount of material processed over the accountancy period.
International safeguards inspectors from the European Commission and the IAEA regularly monitor how civil nuclear materials are handled and accounted for. This is to confirm that nuclear material is not misused or diverted. The monitoring involves a series of checks 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.
It is also important to recognise that nuclear material accountancy is only one of a number of techniques to provide safeguards assurance that nuclear material is not diverted from nuclear facilities. Other means, for example, the use of containment and surveillance devices and regular re-verification of plant design, also contribute to the conclusions drawn by the international safeguards inspectorates about the absence of material diversion.
In addition, the materials are protected at all times and levels of security at nuclear sites are very high. All sites are required to comply with a security plan approved by the Civil Nuclear Security programme and the measures taken exceed international requirements in this area.