Preparation and Certification of Highly Enriched Uranium Nitrate Solutions IRMM-3000 Series

Abstract

This report describes the certification of the IRMM-3000 series of uranium nitrate solution reference materials, certified for the uranium isotopic composition. The certified values and their uncertainties were assigned following ISO 17034 [1], ISO Guide 35 [2] and the Guide to the Expression of Uncertainty in Measurement [3]. This certification project was a collaboration between the International Atomic Energy Agency (IAEA-SGAS, Austria), the Oak Ridge National Laboratory (ORNL, U.S. DOE), Lawrence Livermore National Laboratory (LLNL, U.S.DOE) and the Joint Research Centre (JRC-Geel, Unit G.2 in Geel/Belgium and Unit G.II.8 in Karlsruhe/Germany). The IRMM-3000-series consists of a set of five highly enriched uranium nitrate solution reference materials named IRMM-3020, IRMM-3035, IRMM-3050, IRMM-3075 and IRMM-3090 with the isotopic ratio n(235U)/n(238U) varying from about 0.25 to 6.6, and the 235U enrichment varying from 20 % to 90 %. The solutions were prepared by dissolving uranium oxide (U3O8) materials, which were prepared by gravimetrically mixing oxides which were highly enriched in 235U and 238U, in a similar way as performed for the preparation of the IRMM-074, IRMM-075 and IRMM-3636a,b series of isotope reference materials [4,5,6]. The isotopically enriched starting materials were purified using identical methods involving separation on anion and cation columns followed by a precipitation as peroxide. Both starting materials were treated in separate clean glove-boxes to eliminate cross-contamination. The oxides were calcinated in parallel to convert them into U3O8, in an oven installed in a glove-box that provided a controlled low-humidity environment. For each of the mixtures, the oxides of 235U and 238U were weighed and dissolved to form a solution with a mass fraction of about 5 mg /g. The final solutions were dispensed into individual quartz ampoules containing 0.5 mL, 1 mL or 5 mL of a 1 mol/L nitric acid solution and subsequently flame-sealed. The n(235U)/n(238U) isotope ratios of the IRMM-3000-series were calculated based on the weighings of the two starting materials enriched in 235U and 238U and the measured impurities of both. The main contributions to the final uncertainties of the isotopic ratios are originating from the weighing uncertainties, the uncertainties of the measured isotope ratios and impurities in the two starting materials. The n(234U)/n(238U) and n(236U)/n(238U) isotope ratios of the IRMM-3000-series were certified by TIMS/MTE at JRC-Geel. In this report, the methods for the preparation via gravimetry and certification of the IRMM-3000 series will be described, including verification measurements for the isotope ratios by several methods of mass spectrometry, such as TIMS/MTE, TIMS/DS and MC-ICPMS. As explained in detail in the report, in the case of IRMM-3035, the isotope ratios were only certified using mass spectrometry by TIMS/MTE and TIMS/DS at JRC-Geel, with subsequent verification using TIMS/MTE and MC-ICPMS by the IAEA and ORNL. The isotopic reference materials IRMM-3020 (20% 235U), IRMM-3035 (35% 235U), IRMM-3050 (50% 235U), IRMM-3075 (75% 235U) and IRMM-3090 (90% 235U) are part of a systematic program of JRC-G.2 to supply isotope reference materials to the safeguards community. The materials are intended for the calibration of instruments and methods, quality control purposes. As with any certified reference material, they can also be used for validation studies.