Uncertainty Quantification in Application of the Enrichment Meter Principle for Nondestructive Assay of Special Nuclear Material

Nondestructive assay (NDA) of special nuclear material (SNM) is used in nonproliferation applications, including identification of SNM at border crossings, and quantifying SNM at safeguarded facilities. No assay method is complete without “error bars,” which provide one widely used way to express confidence in assay results. NDA specialists typically partition total uncertainty into “random” and “systematic” components so that, for example, an error bar can be developed for the SNM mass estimate in one item or for the total SNM mass estimate in multiple items. Uncertainty quantification (UQ) for NDA has always been important, but greater rigor is needed and achievable using modern statistical methods. To this end, we describe the extent to which the guideline for expressing uncertainty in measurements (GUM) can be used for NDA. Also, we describe possible extensions to the GUM by illustrating UQ challenges in NDA that it does not address, including calibration with errors in predictors, model error, and item-specific biases. A case study is presented using gamma spectra and applying the enrichment meter principle to estimate the 235U mass in an item. The case study illustrates how to update the ASTM international standard test method for application of the enrichment meter principle using gamma spectra.