Observation by Sem/Edxs of Leached Layers on SRL-165 Glass from 5-Year MIIT Test
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OBSERVATION BY SEM/EDXS OF LEACHED LAYERS ON SRL-165 GLASS FROM 5-YEAR MIIT TEST PEDRO B. MACEDO AND ANDREW C. BUECHELE Vitreous State Laboratory, The Catholic University of America, Washington, D.C., 20064 ABSTRACT Long-term tests on the durability of nuclear waste glasses, both under laboratory conditions and in actual burial environments, have shown that after periods of several months or years the rates of glass corrosion can show unexpected increases. In burial tests in brine environments the growth of the thickness of the surface layer during years 3 to 5 as revealed by SIMS and SEM measurements was found to be faster than expected on the basis of findings during the first 2 years. In this paper we report on our SEM-EDXS observations of a 5 year burial sample of SRL-165 glass from the MIIT test, and compare our findings to those of other studies, especially SIMS measurements. Our average thickness measurement for the leached layer at the glass-metal interface is 3.70 1tm, but wide variation is observed, as well as evidence of partial loss of the leached layer in some areas. A thicker, better retained layer is observed on the glass-brine interface near the core heater averaging 8 to 12 Itm in thickness with occasional local attack producing leached layers of greater depth, in one case nearly 40 14m. These findings show that it is important to extend durability tests to long periods in order to establish a reliable basis for predictive modeling of long-term glass durability. INTRODUCTION Projecting leaching behavior of nuclear waste glasses from short term test results is one of the most difficult problems in predicting the long-term behavior of nuclear waste glasses. Early tests on simulated nuclear waste glasses, usually carried out under conditions of a small surface/volume ratio and short duration (e.g., the MCC-1 test) have shown glass dissolution rates to decrease progressively with time. This would suggest the use of rates obtained at the late stages of such tests as bounding values in developing predictions and extrapolations of long-term glass behavior. Such extrapolations tacitly assume that future dissolution rates will always remain no higher than they were at the end of the laboratory tests on which they are based. Recently, the results of tests on waste glasses conducted under more realistic conditions and for longer durations have produced, in certain instances, indications that the leach behavior of glasses may be more complex than believed previously and cannot always be described by a constant or steadily decreasing leach rate. BACKGROUND One of the more comprehensive and realistic long term tests conducted recently was the Materials Interface Interaction Test (MIIT) carried out in the salt beds of the Waste Isolation Pilot Plant (WIPP) site near Carslsbad, New Mexico between 22 July 1986 and July 1991 under the direction of Martin Molecke of Sandia National Laboratories [1]. The MIIT glass studies were coordinated by George Wicks of Savannah River Laboratories (SRL) and Mat. Res. Soc. Symp. P
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