Guidance document on the variance propagation requirements in DOE Order 5633.3

DOE facilities handling special nuclear material are required to comply with DOE Order 5633.3, which addresses various aspects of nuclear material accounting. In particular, it requires that control limits be calculated for nuclear material inventory differences. These requirements and methods of meeting them are described. The four steps in this process are the creation of a variance propagation model of the nuclear material accounting system, the validation of the model, the application of the model, and the documentation of the model; the first two of these steps are discussed at length. Topics addressed include: the mathematics of variance propagation, the structure of nuclear material accounting systems, sources of uncertainty in real accounting systems, statistical models of these sources of uncertainty, methods of estimating error standard deviations, calculation of control limits from inventory difference standard deviations, and comparison of models with historical inventory difference data. A chapter is also devoted to calculational methods. The approach emphasizes constructing models that can be implemented relatively easily and improved as data is accumulated and understanding of the system is gained, rather than attempting all-inclusive, highly detailed models at the outset.