Certified Reference Material 116-A: A New High-enriched 4 Uranium Metal Standard

Certified Reference Material (CRM) 116-A is a uranium amount content (assay) and isotope amount ratio (isotopic composition) standard for nuclear material safeguards/nonproliferation analyses. CRM 116-A was specifically created as a replacement for the CRM 116 U metal standard. Each unit is comprised of a 1.1- g metal pellet of 93 percent 235U enriched uranium. The certified U amount content value of the CRM is (0.999450.00014) g U g metal-1 with the quoted uncertainty at the 95 percent confidence level. The amount content value was determined by High Precision Titrimetry and is directly traceable to the national metrology database for the United States. To verify the certified value and to assess the potential for sample-to-sample heterogeneity, the amount content for CRM 116-A was also measured using the Davies and Gray titrimetry and isotope dilution mass spectrometry techniques. Data from the verification analyses are consistent with the certified value but indicate that, at the limits of measurement resolution, there may be sample-to-sample variability. Accordingly, the GUM-compliant uncertainty estimate for the amount content includes a factor to account for potential sample-to-sample variability. This factor was determined by performing a second set of High Precision Titrimetry analyses on six 1.1-g CRM units. A discussion of the certified U isotope amount ratio determinations is provided in a separate publication.