Effects of Waste Composition and Loading on the Chemical Durability of a Borosilicate Glass
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- V VASTE MANAGACD? FORRADIOACTIVE SCISWIFIC BASIS Wrner. l-tze, editor EFFECTS OF WASTE COMPOSITION AND LOADING ON THE CHEMICAL DURABILITY OF A BOROSILICATE GLASS
D.E. CLARK, C.A. MAURER, A.R. JURGENSEN AND L. URWONGSE Department of Materials Science and Engineering, College of Engineering, University of Florida, Gainesville, Florida 32611, USA.
ABSTRACT The effects of waste composition and percent loading in a borosilicate glass designed for US defense high level wastes (HLW)
have been evaluated.
Three types of simulated wastes were investigated; high alumina, high iron and a composite representative of an average waste composition from Savannah River Plant (SRP) waste tanks. Corrosion resistance of the borosilicate glass is
significantly enhanced by the presence of any of
the three types of wastes. as the % waste loading is
Additionally,
corrosion resistance is
improved
increased in the glass. The best corrosion
performance was obtained with the high alumina waste in deionized water.
INTRODUCTION During the last several years a large number of glass compositions has been evaluated for encapsulating high level wastes (HLW) commercial nuclear power plants and defense applications. variables in
the vitrification
from both Two important
process are the composition and the
percentage of the waste in the base glass.
Both of these may affect the
processing and will alter physical and chemical properties of the waste form. The single most important property of the waste form will be its ability to minimize the release of hazardous radionuclides during the period when the radiation level is years after burial).
siginificant
(ie,
several thousand
The most likely form of release will occur by
contact of the waste form with groundwater resulting in a leaching of the radionuclides. simple (ie,
Previous investigatorsI'2 have shown that leaching of
two and three component glasses) glasses in contact with
aqueous solution can occur via two mechanisms; dissolution.
In ion exchange protons
ion exchange and network
(or hydronium ions)
exchange with mobile species from the glass resulting in of a surface film.
from the solution the development
2 Network dissolution may also occur if the solution pH is sufficiently 3 high , and this process results in the removal of the species in the same simple systems
the glass. Even in
proportions as they are present in
the solution in
additional processes may become important if
the glass contains species other than H+ and OH-
contact with
or becomes concentrated
(or saturated) with respect to any of the species from the glass. As the solution approaches saturation the leach rate due to both ion exchange and precipitation from solution 4 will occur when solubility limits are exceeded . Thus, the ratio of the and network dissolution will decrease,
surface area of the glass to volume of solution (SA/V) leaching variable.
High SA/V's will result in
is
an important
a rapid approach towards
solution saturation and equilibrium while small SA/V's will req
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