Nuclear Materials Balance figures (NMB, also known more widely in the industry as Inventory Difference or ID) reflect differences between the inventory of nuclear material (plutonium, uranium and thorium) recorded in the nuclear materials accounts for a facility, and the inventory which is measured physically.
Inventory differences can arise from a number of factors, the largest of which is uncertainties inherent in measurement systems. 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 they cannot be mathematically precise. 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 a number of 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.
ID is part of the terminology used in nuclear materials accountancy and works in a manner analogous to financial book-keeping. At the start of an accountancy period, the books are in agreement with the stock or inventory. As new items are brought on to the inventory the book account is increased, and for items leaving the inventory it is decreased. At the end of the accountancy period, the stock is compared with the book account. This is no different in principle to what is done in shops, banks, warehouses and factories.
ID = (Physical Inventory) - (Book Inventory)
Book Inventory = (Opening Inventory) + (Receipts) - (Issues)
Whenever nuclear material is measured, there is an uncertainty associated with the measurement. Some forms of material, e.g. finished product such as Plutonium Oxide, can be more accurately weighed and the uncertainty will be smaller. For other forms, e.g. material held up in process and material in bulk processes, measurements may be more difficult and the associated uncertainties higher. Such measurement uncertainties are a major cause of ID figures, 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. This is particularly the case in bulk processes such as reprocessing, where large volumes of material (hundreds of Tonnes per annum) pass through the plant, often in liquid solution form.
Increasingly the material being reprocessed comprises the oldest fuel from the UK’s Magnox fleet of nuclear reactors and a higher proportion of corroded fuel from long term storage. The physical state of the fuel, arising from the end of a reactor’s generating life, is such that representative sampling and modelling of input streams is more challenging than for earlier reprocessing campaigns. Moreover, historical models for calculation of the composition of spent nuclear fuel have higher uncertainties associated with the end of reactor life, which create additional imprecision in the nuclear material accountancy processes. This may all, in turn, lead to an increase in some IDs associated with current reprocessing operations.
The Office for Nuclear Regulation (ONR), the UK’s independent nuclear regulator, is satisfied with Sellafield Ltd’s activities to verify that the discrepancies are not the result of leaks, theft or diversion of material. The current ID reported at Sellafield is the subject of ongoing investigations supported by Euratom and ONR. However, it should be noted that the findings may not be sufficiently quantitative to allow for/justify correction of the ID, so similar anomalies may persist for subsequent ID figures.
Decommissioning of nuclear facilities also has an effect on Inventory Differences (e.g. the processing or re-packaging and re-measurement of historic residues using modern equipment and/or recovery and accurate measurement of material during plant clean out). Clearly, such IDs will depend on the quality of assumptions on plant hold-up (a phrase describing material deposited in machinery and pipework over the course of its operation) and the content of wastes/residues made in the original operator’s accounts. Decommissioning introduces two further factors: firstly material is liberated from the fabric of the building and other areas for which the operator had previously made no provision for hold-up; secondly some decommissioning arisings may be recorded as waste, and not balanced against the previously recorded IDs.
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 Division of ONR and the measures taken exceed international recommendations in this area.