Analysis of uranium and plutonium materials by radiometric methods

Abstract

The radiometric methods used in the IAEA Safeguards Analytical Laboratory for the analysis of uranium-and plutonium-containing materials sampled in nuclear facilities under IAEA Safeguards are presented. Uranium-235 concentrations are determined in solutions by intensity measurements of the 185.7-keV γ rays with a scintillation detector. The factors affecting the accuracy of results, e.g. dead time and self-absorption corrections, background subtraction, and calibration procedure are discussed. When all these factors are properly taken into account the standard deviation of measurement approaches the value resulting from counting statistics. The 238Pu content in freshly separated plutonium solutions is measured by α-particle spectrometry in combination with mass spectrometry. The alpha sources are prepared by evaporation of analyte solution with tetraethyleneglycol on glazed porcelain discs and subsequent ignition at 850°C. The alpha-spectrometric measurement is performed using the spectrometer with a surface-barrier detector. The alpha-activity ratio of 238Pu to 239 Pu+ 240 Pu is calculated taking into account the tailing correction for 238Pu under the 239 Pu+ 240 Pu peaks. The two methods of tailing correction are described: (i) exponential fitting, and (ii) geometric progression. The atomic ratio of 238Pu/ 239Pu peaks is derived from the alpha activity, and finally, the 238Pu abundance is calculated using the values for 239Pu and 240Pu contents from mass-spectrometric measurement. Plutonium isotopic composition, elemental concentration of Pu, 241Am, 237Np and 235U/Pu ratio are assayed using a γ-ray spectrometer with a high resolution planar germanium detector. A set of computer programs is used for spectral evaluation and calculation of respective concentrations. The results obtained on a number of samples using this method are compared with those of other methods.