Understanding surface species and interactions between adsorbed chloride and water on stored PuO2
EMSL Project ID
48628
Abstract
Radiation induced degradation of chlorinated polymers, such as poly(vinyl chloride) (PVC), used historically in packaging of plutonium oxide (PuO2), has led to chloride-contaminated PuO2, which constitutes a significant proportion of the DOE Pu inventory. An option for plutonium disposal includes re-use as mixed oxide fuel but further work is needed to underpin this option and in the interim period before any disposal route is implemented, safe and secure storage of PuO2 is required. Reuse of the chloride contaminated plutonium requires treatment of the PuO2 to reduce Cl- contamination to acceptable levels for fuel manufacture; whereas storage requires stabilization coupled with a predictive understanding of the evolution of chlorinated PuO2 over time, particularly with respect to gas pressurization from radiolytic and thermal degradation. The PuO2 in storage containers is subject to a range of conditions from the center of the container, where it will be hot and dry with the potential for gas formation as steam, to the outside where it is cooler and water may be present that can undergo radiolysis to produce H2; therefore, interrogation of the behavior of chlorinated PuO2 under these conditions is required. Furthermore, one potential treatment for the chloride contaminated PuO2 is the use of pyrohydrolysis - reaction with steam at elevated temperatures. Hence, the interactions between chloride species and water/hydroxyl ions are key to understanding and managing these materials. In preliminary experimental trials, heating chlorinated PuO2 results in part Cl- removal as HCl gas in the vapour phase while a significant amount of residual Cl forms a sorbed (removable by leaching) and a chemically/physically bound (non-leachable) Cl- species. It is hypothesized that the distribution of Cl- between these three species, and thus the extent of Cl- removal from the PuO2, will depend on: (i) the temperature at which the Cl- contaminated sample is heat treated; and (ii) the physical characteristics of the PuO2, including the specific surface area (SSA, m2/g), the concentration of surface bound OH/H2O and the presence of radiation-induced defects. The objective of the proposed work is to test this hypothesis through a detailed study of Cl- adsorption/desorption behaviour on PuO2 surfaces, including interaction/competition of Cl- with surface bound OH/H2O, in parallel with analogue studies on CeO2 and UO2 to determine the combined effects of physical (particle size/morphology/porosity), chemical (e.g. formation of PuCl3), and radiation-induced phenomena. EMSL resources including X-ray diffraction (XRD), transmission electron microscopy (TEM)/scanning electron microscopy (SEM)-energy dispersive x-ray (EDX) spectroscopy, surface sensitive x-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy in the Radiochemistry Annex, combined with supporting computational chemistry computing performed using NWChem, are required to investigate the interfacial chemistry of radioactive Pu and accomplish this objective.
Project Details
Project type
Special Science
Start Date
2015-06-03
End Date
2015-12-31
Status
Closed
Released Data Link
Team
Principal Investigator
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