MASS CHANGE AND EVOLVED GAS ANALYSIS OF ACTINIDE CERAMICS

The United States Department of Energy requires thermogravimetric analysis (TGA) of plutonium-bearing materials for moisture content prior to placement in interim storage in 3013 containers per DOE-STD-3013. Following the 20 Points agreement, small (3 to 5 gram) samples are heated to 1100oC at a rate of 10oC / min. The furnace chamber cannot be evacuated prior to analysis, and the working gas must be ultra high-purity (UHP) argon. Analysis is initiated after the furnace atmosphere stabilizes to a constant partial pressure of Ar, as inferred from 36Ar peak monitoring. The H2O content of these materials is quantified using quadrupole MS of the headspace gases evolved during TGA, and must not exceed 0.32 wt. %. Quantitative H2O analyses are obtained using a calibration curve constructed using the total areas of the H2O and (OH) peaks at m/z = 18 and 17 for materials containing known amounts of structural water. Other gas species such as NO, CO2, and H2 are also monitored during thermal analysis, but are not quantitatively analyzed. Since May 2010, LANL has subjected approximately 100 PuO2 samples from a variety of process streams to TGA-MS analysis. Relatively pure Pu-bearing materials from different process streams within TA-55 display a predictable set of responses both in terms of mass change, and the type and relative amounts of gases evolved during heating. Materials oxidized using LANL’s “Direct Metal Oxidation” process, for example, have relatively low volatile contents, but also undergo a slight weight gain at high temperature. High-purity oxides from nitric acid-rich processing streams have exceptionally high NO peaks, even after high-temperature calcination. Together with other physical characteristics such as particle size distribution and specific surface area, TGA-MS results can be diagnostic of the origin and subsequent process history of Pu-bearing materials