Measurement of the Pu Content and Isotope Abundances by Combined Calorimetry, Neutron Coincidence Counting and Gamma Spectrometry (CANEGA)

The prime nondestructive assay technique currently employed in EURATOM Safeguards for the plutonium assay in plutonium-bearing solid materials like PuO2 or MOX materials bases on neutron coincidence counting combined with gamma spectrometry. In this assay the plutonium isotope abundances needed for the interpretation of the neutron measurements are mainly determined from high- resolution gamma spectra, which are recorded in parallel and analysed with standard isotope analysis codes like MGA and FRAM. Suitable instrumentations for this type of measurements are provided by the combined neutron-gamma counters as used, for example, in the EURATOM On-Site Laboratories at La Hague and Sellafield. One limitation of this kind of plutonium assay arises from the fact that the abundance of the isotope 242Pu cannot be determined by gamma spectrometry because of the absence of a detectable gamma-ray signature from this isotope. This deficiency starts to limit the accuracy of the plutonium assay particularly for higher burn-up materials, where 242Pu contributes 25% or more to the measured neutron coincidence rate. In this situation recourse is therefore often made to isotope correlations in order to obtain a rough estimate for the 242Pu isotope abundance. The underlying semi-empirical correlations developed and adopted for this purpose work reasonably well only for defined categories of plutonium materials of known burnup history. However, this information is normally not available for the materials to be assayed, leaving therefore relatively large uncertainties for the estimated 242Pu abundances. The use of a combined Calorimetry-Neutron Coincidence Counting-Gamma Spectrometry (CANEGA) assay can improve considerably this unsatisfactory situation. The five directly measured quantities as obtained from the three measurement techniques, namely m240-effective from neutron coincidence counting, the thermal power P from calorimetry, and the plutonium isotope ratios 238/239, 240/239 and 241/239 from gamma spectrometry not only provide more reliable estimates for the 242Pu isotope abundance, but also allow to improve the overall accuracy of the plutonium assay. In this paper we will outline the principle of the CANEGA assay approach. A detailed parameter study has been carried out to evaluate the impact of the uncertainty of the different measurement variables on the final assay result. The new approach is demonstrated from combined calorimetry, neutron coincidence counting and HRGS measurements, which have been performed on a set of realistic Pu and MOX samples. The results obtained by CANEGA for the 242Pu abundance and the Pu content are evaluated in comparison with analysis results from parallel measurements done with destructive methods