Radiation Effects in Nuclear Waste Glasses

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RADIATION EFFECTS IN NUCLEAR WASTE GLASSES J. F. DeNATALE, D. K. McELFRESH, and D. G. HOWITT Department of Mechanical Engineering, University California 95616 USA




Abstract The radiation damage of a nuclear waste glass is shown to be associated with enhanced phase decomposition, oxygen bubble formation, and, when the glass is exposed to air saturated water, enhanced leaching. INTRODUCTION The role of radiation damage to the disruption of stability in a nuclear waste glass is a dual one involving radiation damage to the material itself and to the environment surrounding it. In the event that the surrounding environment is air saturated water, both the radiolytic products and the structural irregularities can have a significant effect upon the leach rate (1,2]. In the work described here, these radiation effects have been induced and monitored using the techniques of high voltage electron microscopy where the electron beam has served as both an imaging and irradiation source. MATERIALS AND METHODS The glasses used for this study were based upon the original Battelle 76-68 composition with the accompanying PW-8A waste approximation [1]. This glass is a low melting temperature sodium-borosilicate with an iron and uranium rich waste. The calcined waste contains some 17 component oxides but most are considerably less than 1 atomic % (Table I). The preparation of specimens for examination by electron microscopy was performed by ion-milling techniques using a 6 keV argon plasma in an Ion-Tech micro-lap thinner. The specimens were prepared for ion-milling by cutting the bulk specimens into 400 micron thick slices using a low speed diamond saw and subsequently polishing these to a thickness of 100 microns. The specimens were examined in a high voltage electron microscope and maintained in environments that corresponded to temperatures between 200 C and 850 0 C in vacuo and 200 C in an atmosphere of water saturated helium at 150 torr. Results and Discussion The radiation damage to glass itself is manifest as the formation of oxygen gas bubbles, which result from the fragmentation of oxygen bonds (fig. 1) and as a decomposition into one or more phases (fig. 2). The formation of oxygen bubbles from electron irradiation occurs within a narrow range of temperature at rates that are reproducable only in that they fall below quite well defined limits (fig. 4). The bubbles can be made to shrink by irradiating the glasses at temperatures above the high temperature limit of the "swelling" peak and the process is often accompanied by radiation induced phase separation and devitrification, the latter occurring

698 The phase preferentially at the interfaces of the shrinking bubbles. separation which occurs at these high temperatures yields two amorphous phases, is quite spontaneous in the presence of the electron beam and can be induced throughout the foil. Similar behavior can be induced at much lower temperatures in these glasses but only in isolated regions of the foil suggesting that variations i