Quality Control and Uncertainty Evaluation for Accountancy Analysis at PFDC-JAEA

摘要

Accurate and precise measurement of plutonium and uranium are critically important for safeguards in addition to process control and is the premise of safeguards inspection by the IAEA at nuclear fuel handling facilities. The Quality Control Section of the Plutonium Fuel Development Center of Japan Atomic Energy Agency (hereafter “PFDC-lab”) has been analyzing the plutonium and uranium isotopic composition of materials by mass spectrometry as well as elemental content by isotope dilution mass spectrometry for MOX fuel pellets or their source materials for accountancy purposes. To maintain and improve analysis quality and to have international accreditation for analysis reliability, PFDC-lab has obtained accreditation to ISO/IEC 17025: 2005 (International Standard for the General Requirements for the Competence of Testing and Calibration Laboratories). In addition to establishing a quality management system, ISO/IEC 17025 requires labs to evaluate measurement uncertainty in accordance with “The Guide to the Expression of Uncertainty in Measurement (GUM)”, which is issued by Joint Committee for Guides in Metrology: JCGM 100:2008 (also known as ISO/IEC Guide 98-3). Thus, PFDC-lab evaluates measurement uncertainty of all instruments involved in accredited measurements, such as balances, alpha spectrometers and mass spectrometers, and combines all contributions from them to estimate total measurement uncertainty. These uncertainties from each instrument are re-evaluated on an annual basis and those measurement uncertainties are compared to the International Target Values (ITV), and consistently show agreement with them. Also, in order to minimize human error in data handling and calculation, we have established the uncertainty evaluation system by integrating Excel software and “GUM-Workbench” software. Once sample measurement results as well as any standard reference material information used for isotope dilution are entered into the database, this system calculates the measurement uncertainty automatically. In addition to uncertainty evaluation of the measurement in accordance with GUM, traditional QC methods such as X-bar control charts are also applied as part of our quality control system. This paper presents details of our quality control system and measurement uncertainty evaluation systems.