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Proof-of-principle work using CeO2 as an analogue to understand surface species and interactions between adsorbed chloride and water on stored PuO2


EMSL Project ID
48811

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 objective of the proposed work is to test this hypothesis through a study of Cl- adsorption behavior on the PuO2 analogue CeO2 to determine combined physical (particle size/morphology/porosity) and chemical effects. EMSL resources including the dual Focused Ion Beam (FIB) - scanning electron microscope (SEM), transmission electron microscopy (TEM)-energy dispersive x-ray (EDX) spectroscopy and surface sensitive x-ray photoelectron spectroscopy (XPS) in the Radiochemistry Annex are required to conduct a proof-of-concept study for the PuO2 investigation (EMSL Science Theme Proposal 48801). Work described in this rapid access proposal will provide valuable evidence to support a strategy for dealing with chlorine-contaminated PuO2.

Project Details

Project type
Limited Scope
Start Date
2015-04-01
End Date
2015-06-01
Status
Closed

Team

Principal Investigator

Carolyn Pearce
Institution
Pacific Northwest National Laboratory

Co-Investigator(s)

Robin Taylor
Institution
University of Manchester

Team Members

Mark Engelhard
Institution
Environmental Molecular Sciences Laboratory

Eugene Ilton
Institution
Pacific Northwest National Laboratory

Kevin Rosso
Institution
Pacific Northwest National Laboratory

Related Publications

Sutherland-Harper S., R. Taylor, J. Hobbs, S. Pimblott, R. Pattrick, M. Sarsfield, and M.A. Denecke, et al. 2018. "Surface speciation and interactions between adsorbed chloride and water on cerium dioxide." Journal of Solid State Chemistry 262. PNNL-SA-128945. doi:10.1016/j.jssc.2018.02.018