5.1 Certified reference materials (CRMs) prepared from nuclear materials are generally well characterized and sufficiently homogenous and stable for their intended use. Usually they are certified using the most unbiased and precise measurement methods available, often with more than one laboratory being used on a national or international level, and they are at the top of the metrological hierarchy of reference materials. A graphical representation of a typical national nuclear measurement system is shown in Fig. 3.

FIG. 3 Typical National Nuclear Measurement System

5.2 Working reference materials (WRMs) need to have quality characteristics that are similar to CRMs, although the rigor used to achieve those characteristics is not usually as stringent as for CRMs. Similarly, producers of WRMs should comply with applicable requirements of ISO 17034, which are less stringent for WRMs than the requirements for producers of CRMs. Where possible, CRMs are typically used to calibrate the methods used for establishing reference values assigned to WRMs, thus providing traceability to CRMs as required by ISO/IEC 17025. A WRM is normally prepared for a specific application.

5.3 Because of the importance of having highly reliable measurement data from nuclear material analysis, particularly for material control and accountability purposes, CRMs are used for calibration when available. However, CRMs prepared from nuclear materials are not always available for specific applications. Thus, there may be a need for a laboratory to prepare nuclear material WRMs to meet specific needs. Also, CRMs are often too expensive, or their supply is too limited for use in the quantities needed for long-term, routine use. When properly prepared, WRMs will serve equally well as CRMs for most applications, and using WRMs will help preserve supplies of CRMs.

5.4 Difficulties may be encountered in the preparation of RMs from nuclear materials because of the chemical and physical properties of the materials. Chemical instabilities, problems in ensuring stoichiometry, homogeneity, and radioactivity are among the factors to be considered, with all three factors being involved with some materials. Those preparing WRMs from nuclear materials need to be aware of how these factors may affect preparation, as well as being aware of the other criteria governing the preparation of reliable WRMs.