Year
2005
Abstract
Plutonium dioxide (PuO2) standards are often used both as heat standards and isotopic standards for calorimetric assay. Calorimetric assay is the combination of the power in watts measured in a calorimeter with the effective specific power (Peff) in watts/g Pu, determined either by nondestructive gamma-ray assay or by destructive mass spectrometry, to yield the total elemental plutonium mass in the sample. To use a PuO2 sample as a heat standard for calorimetry, one must determine both the plutonium mass and Peff with very small uncertainties and then calculate the sample watts from the known plutonium mass, specific powers, and isotopic composition. Well-characterized PuO2 standards have plutonium mass values determined by analytical chemistry with a precision and accuracy on the order of 0.1%–0.2 % relative to the total mass of the sample. Mass spectrometry, typically used to determine the isotopic fractions of plutonium standards, is very accurate and precise for the major isotopes but is somewhat less precise for low-abundance isotopes. The characterization of the 241Am/Pu ratio in the standard is also of great importance because 241Am can contribute significantly to Peff and to the heat output of the standard. The determination of the 241Am/Pu ratio in a plutoniumbearing sample is a process that is less standardized than mass spectrometry. There are no certified reference materials (CRMs) traceable to the national measurement system for 241Am in plutonium, and routine analytical 241Am/Pu ratio measurements often exhibit uncertainties of several percent relative to the total plutonium or greater. The usefulness of a PuO2 standard for calorimetric assay is seriously degraded if the 241Am concentration has a large uncertainty. The discussion in this paper quantifies the effect of the 241Am characterization uncertainty on the total uncertainty of Peff and suggests target values for 241Am uncertainty required to characterize PuO2 materials used for calorimetric assay standards.