【国外标准】 Standard Test Method for Determination of Uranium Isotopic Composition by Modified Total Evaporation (MTE) Method Using Thermal Ionization Mass Spectrometer

5.1 Uranium material is used as a fuel in certain types of nuclear reactors. To be suitable for use as nuclear fuel, the starting material shall meet certain specifications such as those described in Specifications C753, C776, C787, C833, C967, C996, and C1008, or as specified by the purchaser. The isotope amount ratios of uranium material can be measured by mass spectrometry following this test method to ensure that they meet the specification.5.2 The MTE method can be used for a wide range of sample sizes even in samples containing as low as 20 µg of uranium. If the uranium sample is in the form of uranium hexafluoride, it has to be converted into a uranium nitrate solution for measurement by the MTE method. The concentration of the loading solution for MTE has to be in the range of 1 mg/g to 6 mg/g to allow a sample loading of 2 µg to 6 µg of uranium. A minimum loading of 3 µg uranium per filament is strongly recommended. This is needed to have a sufficient and stable ion signal especially for the two minor isotopes (234U and 236U) thus enabling the internal calibration of SEM versus the Faraday cups using the 234U ion beam signal during the measurement.5.3 Until now, the instrument capabilities for the MTE method have only been implemented on the TRITON™ TIMS instrument.5 Therefore, all recommendations for measurement parameters in this test method are specified for the TRITON instrument. The manufacturers of other TIMS instruments (for example, IsotopX and Nu Instruments) have indicated plans to implement the modifications needed in their instruments to use the MTE method.5.4 The MTE method described here can also be extended to measurement of elements other than uranium. Note that the MTE method has already been implemented for plutonium and calcium.1.1 This test method describes the determination of the isotope amount ratios of uranium material as nitrate solutions by the modified total evaporation (MTE) method using a thermal ionization mass spectrometer (TIMS) instrument.1.2 The analytical performance in the determination of the 235U/238U major isotope amount ratio by MTE is similar to the (“classical”) total evaporation (TE) method as described in C1672. However, in the MTE method, the evaporation process is interrupted on a regular basis to allow measurements and subsequent corrections for background from peak tailing, perform internal calibration of a secondary electron multiplier (SEM) detector versus the Faraday cups, peak centering, and ion source refocusing. Performing these calibrations and corrections on a regular basis during the measurement, improves precision, and significantly reduces uncertainties for the minor isotope amount ratios 234U/238U and 236U/238U as compared to the TE method.1.3 In principle, the MTE method may yield major isotope amount ratios without the need for mass fractionation correction. However, depending on the measurement conditions, small variations are observed between sample turrets. Therefore, a small correction based on measurements of a certified reference material is recommended to improve consistency. The uncertainty around the mass fractionation correction factor usually includes unity.1.4 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.