DEVELOPMENT OF THE RUSSIAN CERTIFIED URANIUM ASSAY REFERENCE MATERIAL (CRM) OF URANIUM OXIDE (U3O8) FOR DESTRUCTIVE ASSAY METHODS

Under the framework of the Project on the development of Russian uranium and plutonium reference materials for destructive analysis methods, A.A. Bochvar VNIINM working in cooperation with U.S. Department of Energy New Brunswick (NBL) and Los Alamos National (LANL) Laboratories has developed uranium assay reference material (RM) of uranium oxide (U3O8). Bulk material for the RM was fabricated by the Urals Electrochemical Integrated Plant (Novouralsk) by pyrohydrolysis of high-purity uranium tetrafluoride. Total content of metal and gas-producing (nitrogen, carbon, fluorine, and chlorine) impurities in the material does not exceed 0.00012 mass fractions. The material was processed (blended) at the FSUE VNIINM in order to achieve maximum homogeneity; the value of the parameter of heterogeneity of the bulk-material was estimated, which was found to be equal to +/- 0.000020 mass fractions for the uranium oxide (U3O8) sample mass equal to 0.6 g. Interlaboratory comparative measurements (ICM) of uranium in uranium-containing materials were organized preliminarily in order to select among the Russian Federation Federal Agency for Atomic Energy (FAAE) facilities the best qualified laboratories for carrying out certification and verification measurements. Twenty laboratories from 11 facilities participated in the ICM. Certification measurements of the RM material were performed at the VNIINM Laboratory for Metrological Support of Analytical Control by precision gravimetry method with peroxide precipitation. Confirmation measurements were conducted at 6 laboratories of four facilities using different measurement methods: Davies-Gray titration, gravimetry with peroxide precipitation, coulometry with controlled potential and high-precision titration using the NBL method. The reference material was certified with the following characteristics: - Certified value of the RM: total uranium mass fraction - 0.84784 g U/g U3O8; - Absolute uncertainty of the RM certified value (including the uncertainty due to heterogeneity): ± 0.00016 g U/g U3O8 for P=0.95; - Mass of the least representative sample for analysis is 0.6 g; - Average uranium atomic mass - 238.044 a.m.u. The reference material was approved by the RF Gosstandard, was entered in the State Registry with registration number GSO 8363-2003, and allowed for usage in the Russian Federation.