Statistical Analysis of Nuclear Material Accountancy Rationale, Benefits and Experience

Nuclear Material Accountancy is based on measured or estimated data, therefore is subject to a certain degree of uncertainty. For this reason, the Material Balance of a bulk handling facility never closes to zero: the unbalance is significantly called “Material Unaccounted For (MUF)” in the environment of International Safeguards, or “Inventory Difference (ID)” in other environments. A MUF/lD in the Material Balance may unfortunately be generated also by other events, such as: loss of material, clerical errors in collecting or recording data, insufficient control of material stocks, theft, etc. Although there are no accredited methods which allow the operator to discriminate whether a certain MUF/lD has been produced by measurement errors or by other reasons, there exist notwithstanding good methods to predict the range in which the MUF/lD generated by a certain measurement system is expected to be. These methods, based on the statistical analysis of the MUF/lD components, can be used for defining criteria for the acceptance of MUF/lD, but can also be used for judging the “quality” of the measurement procedures and optimizing them, also with a view to improving process management. Statistical analysis of MUF components has been used since longtime by the EURATOM Safeguards and by selected European Operators, with satisfactory results. Software for its practical implementation has been developed by the JRC. This paper describes the principles of the statistical analysis, its application to Operators’ and Inspectors’ problems and reports some experience obtained in the past years.