Management of Commercially Generated Radioactive Waste - U.S. ...

4.3.2.2 Glass Waste Forms
4.27
Vitrification (conversion to glass) of high-level liquid wastes is being developed in
Germany, France, India, Russia, Great Britain, Belgium, Japan, Canada and the United States.
A facility for vitrification of the HLW from the Marcoule reprocessing plant has been oper-
ating in France since the summer of 1978 (Bonniand et al. 1978). The various HLW vitrifica-
tion processes and properties of the glasses made by them have been well described in recent
reports and symposia proceedings (McCarthy 1979, Chikalla and Mendel 1979).
Low-Melting Glasses
Low-melting glasses are glasses that can be processed at temperatures below about
1200°C. The most well developed vitrification processes throughout the world all pro-
duce low-melting glasses of a borosilicate formulaion, although a small amount of develop-
ment continues on phosphate glass formulations (Kelley 1975, Wiley and LeRoy 1979, Gombert
et al. 1979, Kupfer 1979 and Mendel 1978). The product of these borosilicate glass pro-
cesses is a glass casting in a metal canister. The castings vary in size depending on the
process and the amount of radioactivity, but are generally cylinders from 0.3 to 0.6 m in
diameter and 1 to 3 m long.
Borosilicate waste glasses can contain one-third or more (by weight) HLW oxides; the
remainder is inert glass-forming material added during vitrification processing. The
glasses can tolerate wide variations in HLW composition without sacrificing their prop-
erties. The glass castings contain some fractures caused by thermal stresses induced as the
large monoliths cool. Waste glasses are metastable materials and they must be cooled fairly
rapidly (a cooling rate of at least 10OC/hr between 900°C and 600 0 C is satisfactory for most
formulations) to prevent excessive devitrification from occurring. At lower temperatures,
e.g., those encountered in geologic disposal, the rates of thermal devitrification are too
slow to be a factor. Extensive studies have shown that the only significant effect of devi-
trification, if it does occur, is a small increase in leach rate. The increase is usually
less than a factor of three even in fully devitrified glasses but in some formulations may
be as high as 10. The glass phase exhibits excellent stability in radiation fields as shown
by tests simulating over 500,000 years of alpha radiation.
Borosilicate waste glasses also exhibit good chemical durability; however, there is a
finite reaction rate with water. The reaction rate is dependent on many factors but for
typical waste glasses is usually in the range 10 -7 to 10 -5 g glass/cm 2 -day after a few weeks
of leaching at 25°C. The rate increases with temperature, rising a factor of 10 to 100 for
a 100*C increase in temperature.
High-Temperature Glasses
In the context of this discussion, these are glasses that melt above 1200 0 C. They con-
tain more silica or alumina than the low-temperature glasses. An early example of a high-
temperature waste glass is the nepheline syenite waste glass made in Canada from 1958 to
1960. Blocks of this glass, without canisters, were buried below the water table at Chalk
River in 1960. The leaching behavior of these glass blocks has been monitored by means of