| AVS 57th International Symposium & Exhibition | |
| Actinides and Rare Earths Topical Conference | Tuesday Sessions |
| Session AC-TuA |
| Session: | Science and Technology of Actinides and Rare Earths |
| Presenter: | M. Borkowski, Los Alamos National Laboratory |
| Authors: | M. Borkowski, Los Alamos National Laboratory H. Khaing, Los Alamos National Laboratory J.-F. Lucchini, Los Alamos National Laboratory D.T. Reed, Los Alamos National Laboratory M.K. Richmann, Los Alamos National Laboratory J.S. Swanson, Los Alamos National Laboratory D. Ams, Los Alamos National Laboratory |
| Correspondent: | Click to Email |
The mobility and potential release of actinides into the accessible environment continues to be the key performance assessment concern of nuclear repositories. Actinide, in particular plutonium speciation under the wide range of conditions that can exist in the subsurface is complex and depends strongly on the coupled effects of redox conditions, inorganic/organic complexation, and the extent/nature of aggregation. Understanding the key factors that define the potential for actinide migration is, in this context, an essential and critical part of making and sustaining a licensing case for a nuclear repository. Herein we report on recent progress in a concurrent modeling and experimental study to determine the speciation of plutonium, uranium and americium in high ionic strength Na-Cl-Mg brines. This is being done as part of the ongoing recertification effort in the Waste Isolation Pilot Plant (WIPP).
A key feature of salt-based repositories is the relatively rapid self-sealing nature of the salt. This feature leads to geologic isolation of the waste form, and when reduced metals are present (e.g., iron containers), the system is driven anoxic by corrosion leading to strongly reducing environments. The consequence of this is that the combination of anaerobic microbial activity, reactions of reduced metals, and, when present, reactions of organics leads to the reduction of higher valent Pu(V) and Pu(VI) species to the lower valent Pu(III) and Pu(IV) species. The reduction of Pu(V/VI) species has been studied extensively. Less is known about microbial effects with halophiles although there is no question that bioreduction of higher valent plutonium occurs readily by soil bacteria under anoxic conditions. These lower valent oxidation states have lower solubilities and correspondingly lead to lower solubility and mobility of the plutonium.
The oxidation-state specific solubility of actinides were established in brine as function of pCH+, brine composition and the presence and absence of organic chelating agents and carbonate. An oxidation-state invariant analog approach using Nd3+ and Th4+ was used for An3+ and An4+ respectively. These results show that carbonate and hydrolysis predominate at pCH+ above 8. Organic complexation is more important for An3+. Carbonates are the key factor for U(VI) solubility. Modeling efforts are focused on the use of Pitzer parameters to correct for high-ionic strength effects and show that there is still some uncertainty about the predominant carbonate and hydrolytic species, particularly when longer-term timeframes are considered.