EXPERIMENTAL INVESTIGATION OF AQUEOUS CORROSION OF R7T7 NUCLEAR GLASSAT 90-DEGREES-C IN THE PRESENCE OF ORGANIC-SPECIES

The initial dissolution kinetics of the French ''R7T7'' inactive LWR r eference glass was investigated in organic media at 90-degrees-C by st atic experiments at imposed pH values. Tests were conducted with vario us organic acids or acid salts (formic acid, citric acid, humic acids, sodium citrate and sodium formate) at different concentrations and at pH values ranging from 3 to 9. The leachates were submitted to ICP-AE S analysis, and the alteration films on the glass specimens were exami ned by SEM (equipped with an energy-dispersive chemical analysis syste m) and XRD. Results are discussed for the following areas. Effect of p H: different mechanisms are involved at pH 2.5 and pH 9, but in both c ases, the R7T7 glass dissolution kinetics are always controlled by a s urface reaction mechanism. The most significant effects of the organic acids on the glass dissolution kinetics were observed at pH 3, which was not unexpected. Nature of the acid (or acid salt): this is an esse ntial parameter. Formic acid (monoacid) and sodium formate had no effe ct on the initial glass dissolution rate nor on the nature of the alte ration film; conversely. citric acid (triacid) and sodium citrate sign ificantly affected glass dissolution. The role of citric acid in the s urface reaction mechanism has been demonstrated experimentally at pH 2 .5; it is also known to favor that extraction of Si (directly or indir ectly) and Zr and probably of Fe. The humic acids are strong catalysts of the R7T7 glass dissolution reaction. The conclusions drawn from th ese observations corroborate other published findings, suggesting that the catalytic effect is appreciable only for species with at least tw o carboxylic acid functions. Effect of concentration: the effect of th e concentration was observed at basic pH values in the presence of sod ium citrate and humic acids. This parameter may thus be quantified and incorporated in the glass dissolution kinetic law.