Precise and accurate determination of plutonium by controlled-potential coulometry and its uncertainty evaluation

Precise and accurate analysis of plutonium is necessary for material control and safeguards in nuclear facilities. In this study, controlled-potential coulometry, which yields absolute results and high precision, was applied to the determination of plutonium for achieving the highest level of accuracy. The measurement cell and electrodes of our coulometry system were optimized following ISO12183:2005, and the calibration of the coulometer was performed using NIST (National Institute of Standards and Technology, U.S.) certified equipments. The influences of fluoride which was used to dissolve a Pu metal and of nitrite which arise from the decomposition of nitrate were eliminated by adding sulfuric acid and amidosulfuric acid to the sample. Also, hydrogen peroxide was added to the sample to reduce Pu(VI) to Pu(IV). A plutonium sample prepared from a Pu metal (NBL CRM126) was measured according to the procedure optimized in this study. As a result, the measured value agreed well with the certified value. The relative standard deviation was 0.05% (n=17). For spike preparation of isotope dilution mass spectrometry (IDMS), a plutonium solution was prepared from MOX powder stored in Japan Atomic Energy Agency. The plutonium content in the prepared solution was characterized by coulometry. As a result, the plutonium concentration of 15.5995 mgPu/g was obtained. The uncertainty was calculated following the ISO/BIPM Guide to the Expression of Uncertainties in Measurements. The expanded uncertainty was ±0.0036 mgPu/g with confidence interval of 95%. The results obtained by coulometry were also in good agreement with those obtained by IDMS. Thus the proposed coulometry method was successfully applied for the determination of plutonium at the highest level of precision and accuracy. (author)