The GUM Uncertainty Approach for Safeguards Measurements

The ability to compare measurements, such as those used to reconcile stockpile accountancy or shipper-receiver differences, requires an associated statement of uncertainty. An uncertainty statement is particularly important when comparing measurement results among different laboratories or among different techniques. An assessment of different results can only be made on the basis of two values agreeing (or not agreeing) within the uncertainty of their measurements. An uncertainty provides confidence in the value of the measurement, judgment on significance of differences between measurement results, information regarding the capability of the measurement procedure, and quality assurance. Because there is no common method for estimating uncertainty within the safeguards community, the ability to compare results is impaired. This problem can be remedied with the use of a universally adopted method. The Guide to the Expression of Uncertainty in Measurement (GUM), published by the International Organization for Standardization (ISO) and adopted by the American National Standards Institute as an American National Standard, provides the current international consensus method for estimating measurement uncertainty. It establishes general rules for evaluating and expressing uncertainty in measurements, is applicable to the wide range of methods employed in the safeguards community, and forms the basis for accreditation requirements relating to measurement uncertainty estimation. Consequently, GUM has been adopted by most of the metrology institutes in the world.