Thermal Desorption Studies on Plutonium Oxides with Thermal Gravimetric Analysis Coupled with Simultaneous FTIR and Mass Spectroscopy
L. Morales and U. Gallegos Los Alamos National Laboratory S. Lemarchand, E. Post, and A. Schranner Netzsch Gerätebau GmbH K. Imrich and A. Jurgensen Savannah River Technology Center Y. Mazza and M. Brugh Rocky Flats Environmental Technology Site Scott Barney Flour Daniels, Hanford Site
A Netzsch 409 PC TGA system was interfaced to a Fourier Transform Infrared (FTIR) detector and a mass spectrometer (MS) that were employed to study the thermal de-sorption of gases from 3 gram samples of pure and impure plutonium dioxides. Compared to other methods, this system provided the most comprehensive understanding of the chemistry occurring at the solid-gas interface. The focus of the work was on the quantification of water vapor, however other vapor species were identified and studied. The simultaneous interface was made possible by a heated "Y" adaptor installed in the flow line. These auxiliary moisture detection systems are needed due to the difficulties in interpreting the TGA curves alone when multiple volatile species were removed over the same temperature range. The purpose of these experiments was to demonstrate that 1) the detector response was linear with the amount of water added; 2) the integrated peak areas obtained from a given amount of added water were independent of the peak shape and bed thickness; and 3) that one was able to accurately measure water removed from different temperature regimes. Other gases detected were CO2, NO2, HCl, SO2, and HF.
This study consisted of several parts designed to validate the TGA-based moisture measurement. Portions of this study were conducted in parallel, such as the non-radioactive surrogate materials study and the work done on pure plutonium dioxides. The linearity of the calibration curves shows that both detector systems are well behaved. The mass spectrometer provides better sensitivity or a lower detection limit for water than that provided by the FTIR instruments. These studies have also demonstrated that water can be removed at different temperature regimes. A careful selection of the IR frequency ranges will eliminate or minimize interferences from other volatized species.
The data show that bed effects are minuscule and should not affect the performance of this method to provide a moisture measurement. For the alumina and ceria surrogates and the pure plutonia, kinetic effects do not hinder water removal from room temperature to 550 oC from these materials. In addition, there do not seem to be any significant chemical differences between the alumina, ceria, or plutonia surfaces that would contribute to the non-linearity of the calibration curves.
The results of this study show that the TGA-FTIR and TGA-MS systems are more than adequate for determining the moisture content and providing a pass/fail decision at the 0.5 weight percent level with 95% confidence. The detection limits reported clearly show that these methods can detect and quantify small amounts of water removed from the samples. The kinetics of the water evolution are rapid and complete by 1000oC for the samples studied. Long hold times at 1000oC for moisture measurements are probably not necessary.
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