Chemical behaviour of Np, Pu and Am in saline solutions: Radiolysis, solubility, hydrolysis, chloride complexation and speciation

The assessment of the migration behaviour of actinide elements (Np, Pu, Am) in the geosphere requires detailed knowledge on basic geochemical reactions, which include not only primary reactions of actinides with groundwater components but also secondary reactions, such as radiolysis, redox reaction, colloid formation, etc. The radiolytic behaviour of Pu and Am has been investigated in solutions of high NaCl concentration, in which radiolysis reactions are induced by intensive α-radiations of actinides themselves. An increase in the redox potential (Eh) is observed in solutions, which results in the quantitative oxidation of Pu(IV) into Pu(VI) and Am(III) into Am(V). These reactions are dependent on pH, salinity and specific α-activity. The results of α-radiolysis are compared with the effect of γ-radiolysis on inactive chloride solutions. As Pu(VI) can be stabilized in concentrated NaCl solutions, indicating that chloride complexation of Pu(VI) is important, the chloride complexation is studied by spectrophotometry and the stability constants of the first two Pu(VI)-chloro complexes are determined. Solubility experiments of NpO2 and NpO2OH in concentrated saline solutions are carried out under aerobic and anaerobic atmosphere (90% Ar, 1% CO2). The speciation of soluble species is undertaken by UV-VIS-spectrophotometry. The experiment of the Am(III)-hydrolysis reaction has revealed a dependence of Am solubility on the specific α-activity. However, no alteration in the chemical state of hydrolysis products is observed in different specific α-activities. The speciation is carried out by laser induced photoacoustic spectroscopy (LPAS) because of the low Am(III) solubility at pH>7. (orig./RB)