Abstract
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 the235U 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.
Highlights
As world reliance on nuclear energy increases, concerns about proliferation of materials that could be used for weapons increase
(1) Observed (2) Best estimate (3) Naive estimate (b) one might expect to require the calibration protocol to count for sufficiently long time that the impact of errors in predictors is negligible during calibration. It would be desirable if estimation error in β1 and β2 was essentially all due to the limited number of standards used in the calibration, Figure 2 and our analyses indicate that some estimation error in β1 and β2 will be due to error in X1 and X2, even for reasonable count times such as 300 seconds per standard
This paper described challenges in Uncertainty quantification (UQ) for Nondestructive assay (NDA), some of which are addressable using the guideline for expressing uncertainty in measurements (GUM)’s concept of a measurement equation
Summary
As world reliance on nuclear energy increases, concerns about proliferation of materials that could be used for weapons increase. Enrichment can be measured using the 185.7 keV gamma-rays emitted from 235U by applying the enrichment meter principle (EMP), which we consider here as our case study [2]. These challenges include item-specific biases, calibration with errors in predictors, Journal of Sensors and model error, especially when the model is a key step in the assay. A case study is presented using low-resolution NaI spectra and applying the enrichment meter principle to estimate the 235U mass in an item. The case study illustrates how to update the current international standard test method (ASTM) for application of the enrichment meter principle using gamma spectra from a NaI detector.
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