A research and development program was outlined in June 1998 by the CEA’s Rhône Valley Research Center (Valrhô) to model the initial alteration rate r0 between 100°C and 50°C, and the long-term alteration rate r(t) over one year at 50°C, throughout the composition range for the UOX1 “light water” borosilicate containment glass as specified for the R7 and T7 vitrification facilities in the COGEMA reprocessing plant at La Hague.
An experimentation plan methodology was set up for this purpose. Seven groups of glass components were designated to assess the influence of composition variations: SiO2, B2O3, Na2O, Al2O3, additive oxides (Fe2O3+NiO+Cr2O3), insoluble fine particle suspensions, and fission product and actinide oxides. The other glass constituents, accounting for 9.5 oxide wt% of the composition, were maintained constant. An empirical synergistic mixing model was postulated, based on first-degree terms and enriched with second-degree terms corresponding to the major interactions liable to exist between some constituents. The NEMROD calculation code was used to select the 22 glass compositions necessary to construct the model (i.e. to determine the values of the 15 coefficients) and the 3 control compositions.
The initial rates for all 25 glass compositions were measured in pure water using a Soxhlet test device at 100°C, and in static mode at 50°C. The alteration rates at an advanced degree of reaction progress were determined in static mode at 50°C with a glass-surface-area-to-solution-volume (S/V) ratio of about 60 cm−1. The rates after one year are now available, but the tests are still in progress in order to confirm the stability over time of the steady-state conditions observed.
The test results show that the initial alteration rates are clustered around the value obtained for the SON 68 reference glass composition (about 2.4 g·m−2d−1 at 100°C and 0.031 g·m−2d−1 at 50°C), with minor variations between 1.2 and 3.4 g·m−2d−1 at 100°C and between 0.012 and 0.058 g·m−2d−1 at 50°C. These slight rate variations are of considerable significance in view of the wide range of glass compositions investigated. The one-year alteration rates for the test glasses varied by a factor of about 11 around a mean value of 1 × 10−4 g·m−2d−1 at 50°C.
The rate variations were modeled as a function of the glass composition variations using the NEMROD program. Other parameter values determined during static testing at the high S/V ratio were also modeled to improve our understanding of the phenomena involved: notably the altered glass thickness after one year.
The order of importance of the constituents on the initial rate is not the same as for the final rate, suggesting that the alteration kinetics are controlled by different mechanisms. The initial rate was influenced primarily by the following constituents (by order of importance): the fission product oxides and SiO2 (increasing durability at increasing concentrations) and B2O3 (diminishing durability at increasing concentration). Conversely, the one-year alteration rates were found statistically to depend much more on the variability of the Na2O, Al2O3 and FP oxide concentrations than on the SiO2 concentration. Several hypotheses are proposed to account for these effects.