In installations using nuclear materials in their processes, physical security, particularly monitoring the access and integrity of the various protective areas, ensures protection for the material in the first instance.
The first protective area is often formed by the process itself (containment glove boxes and the method of accessing the material, for example). A second protective area may be formed by the building or premises housing the process (physical barriers or access controls, for example).
However, outflow processes can not be directly reconciled with inflows for two reasons:
- In processes, materials during processing may accumulate locally or settle on equipment walls: Such material is described as “retained” . They are, in part, recovered during the regular cleaning processes, or during disassembly and decontamination of the process elements.
- Physical measurements are taken to determine the inflows and outflows of nuclear materials, as well as specific steps in the transformation process of these materials. These measurements, however, involve uncertainties related to the accuracy of measuring equipment employed and methods used.
- Over a given period, the difference between the quantity of nuclear materials being put in and initially present in the process on the one hand and on the other hand those removed and present the final process is called "discrepancy assessment".
To provide a defined probability that the "discrepancy assessment" does not involve misuse of nuclear materials, the operator calculates a "confidence interval" within which this "discrepancy assessment" must fall. This confidence interval reflects the uncertainty of measurements or estimates related to analyses of in/outflows and materials used.
1- Materials which have not been recovered during cleaning after processing, before an inventory is taken.