Influence of Radiolysis by-products on the Actinide Chemistry
in Brines from Geological Saline Repository*
J-F. Lucchini, A. Rafalski, M. Borkowski, J. Conca
Los Alamos National Laboratory, EES-12 Carlsbad Operations
MS A141, Carlsbad, NM 88220
505-234-5556, fax: 505-887-3051, email@example.com
Because geological salt formations are considered possible or operational sites for radioactive waste disposal, actinide chemistry in brines has been the subject of numerous investigations within the last two decades. Oxidation states, redox behavior, solubility and speciation of actinides in solution are the main chemical processes involved in the potential migration of actinides in the environment1. In the near-field chemistry of a salt repository of nuclear waste, ionizing radiations can strongly affect these processes, and thus the actinide mobility in concentrated saline solution. So the effects of radiolysis on high-saline brine under simulated repository conditions, and the actinide behavior towards radiolytic by-products are of particular importance2.
The aim of this work is to give a non-exhaustive review of the brine radiolysis effects on the actinide chemistry.
The chemistry of concentrated saline solutions under ionizing radiations is extremely complex. Radiolysis can locally modify the redox conditions, and therefore the speciation and the solubility of the actinides compounds.
Concerning water radiolysis, the same amounts of oxidizing and reducing species are generated3. The main oxidizing species are hydrogen peroxide (H2O2) and hydroxyl radical (· OH). The main reducing species are hydrogen molecules (H2), thermalized hydrated electrons (e-aq) and hydrogen atom (· H). Because oxidizing species are highly reactive and H2 is expected to escape from the solution, the local conditions may become oxidizing.
The radiolysis of brines saturated with chloride salts results in the formation of a transitory equilibrium system, made of Cl3-/Cl2/HClO/ClO-/Cl- species. These species can also modify increasingly the redox conditions, and thus may have an impact on the chemistry of the actinides compounds in solution.
The higher oxidation states of actinides are known to be more soluble in solution than the lowest ones. Most of the studies report that hypochlorite (ClO-), generated by alpha irradiation of a concentrated sodium chloride (NaCl) solution, oxidizes Am(III) to Am(V), Pu(IV) to Pu(VI), and Np(IV) to Np(V)4-8. However, in the case of an underground disposal, we must keep in mind that the chemical conditions of the local environment, and the reducing radiolytic by-products, can also play a significant role on the stability of actinide oxidation states in the aqueous phase and on the overall actinide mobility.
Unfortunately, publications concerning the influence of radiolysis by-products on the speciation of actinides are few, especially in synthetic brine and in conditions as representative as possible of geological salt repository conditions. The complex chemistry of transuranic waste in contact with brines needs to be experimentally investigated by examining the sub-effects influencing the overall pcH-Eh system, which includes radiolysis effects.
This work presents the mechanism involved in radiolysis of chloride-ion aqueous solutions, and gives some predictions of the possible reactions and rate constants in the solutions. A summary of the significant reaction processes taking place during the radiolysis of NaCl solution is discussed.
The interaction of actinides with radiolytic species can significantly affect solubility predictions. Because radiolysis is likely the major driving force in oxidizing actinides, it needs to be studied and understood in order to be considered in risk assessments of nuclear repositories in salt formations, and to evaluate more precisely and efficiently the long-term evolution of geological nuclear waste repositories.
* : This paper is also presented at the Plutonium Futures The Science Conference, held on July 6-10, 2003 at Albuquerque, NM.
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