Understanding the Behaviour of the Actinides under Disposal Conditions: A Comparison between Calculated and Experimental Solubilities

Abstract

Actinides /Solubility /Repository /ModellingAbstract The solubilities of plutonium, americium and neptunium measured in simulated near-field waters have been compared with those predicted using the simple thermodynamic model NearSol. The dependence of solubility on pH and redox potential is examined in an effort to understand the behaviour of actinides in disposal.The agreement was variable. Differences could be appreciable, in particular for neptunium under oxidizing conditions; conversly, the model successfully predicted the behaviour of neptunium under reducing conditions. Such comparisons pinpointed défi-ciences in the thermodynamic data base and showed the sensitivity of solubilities to certain experimental parameters such as Eh and the concentration of carbonate ions.A comparison between NearSol and the reaction pathway program PHREEQE gave generally good agreement. NearSol was quicker and easier to use, requiring only limited preselection of participating species; however it did not account for the behaviour of bulk inactive species in solution; this feature will be built into an updated version.

Actinide (III) solubility in WIPP Brine: data summary and recommendations

“…Am(III) is not easily oxidized in aqueous solution (Hobart et al 1982);however, Am(V) Pryke and Rees (1986) and Felmy et al (1990) …”

Appendix SOTERM-2009 Actinide Chemistry Source Term

“…In the expected WIPP environment, however, where anoxic and reducing conditions with microbial activity and reduced iron are expected to be present, Np(IV) is expected to be the predominant oxidation state (Rai and Ryan 1985, Rai, Strickert, and McVay 1982a, Pryke and Rees 1986. This is based on studies of the solubility of NpO 2 OH in 1 M and 5 M NaCl solutions at pH 6.5 where the reduction of Np(V) to Np(IV) was observed (Kim et al 1985, Neck, Kim, andKanellakopulos 1992).…”