Year
2002
Abstract
Calorimetric assay is an NDA technique that typically can provide very precise and accurate measurement results, very often yielding total measurement errors of 1% or better. Proper interpretation of these calorimetric assay results for plutonium bearing materials requires an isotopic measurement that conforms to several underlying physical assumptions. This is especially true for materials with high americium concentrations where a majority of the emitted heat is due to the americium content. Recently with facility closures and shutdowns, sites are faced with consolidating wastes and residues for shipment and disposal. This has led to the mixing and blending of different isotopic source materials with varying americium contributions. Subsequent Pu isotopic measurements by gamma-ray spectrometry techniques of these mixtures can lead to indeterminate errors and hence to large and unquantified biases on the plutonium and americium content determined from calorimetry. The NDA isotopic measurement technique is detailed in the ASTM Standard Test Method C1030. This paper will explore the underlying physical assumptions inherent in C1030 and present guidance on which measurement scenarios are allowed and which are prohibited. Data from over 500 of these mixed isotopic measurements will be presented along with an interpretational model to bound and quantify the total measurement error for these items. Total measurement uncertainty estimation is important and required for waste characterization, disposal, and nuclear safety reasons. Analysis of this data yields total measurement uncertainties in the 10% to 30% range, much larger than the typical 1% measurements.