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WASTE GLASS/REPOSITORY INTERACTIONS D.E. CLARK AND C.A. MAURER Department of Materials Science and Engineering, College of Engineering University of Florida, Gainesville, Florida 32611, USA. ABSTRACT The effects of repository material and a tailored backfill or overpack on the leaching behavior of glass have been studied. were used in

alkali oxide-67 mol% SiO2 , without simulated wastes. placed in

Two types of glasses

this investigation: 1) Model glasses comprised of 33 mol% and 2)

alkali borosilicate glasses with and

Several types of repository material were

the same containers as the glasses to determine if

presence would alter the extent of leaching.

their

A backfill material consist-

ing of phosphate slime/sand mixtures was evaluated with the same procedure. The results indicate that the leaching behavior of the glass may be influenced by the presence of some materials.

Preliminary results of a

161 month burial experiment in Florida are also discussed.

INTRODUCTION Water chemistry is glass leaching.

probably the single most important variable in

Orders of magnitude variations in both the 'Leach rate and

extent of degradation can be achieved by modifying the composition and pH of the water in contact with the glass. the presence of certain species in

Investigations have shown that

solution,

such as Al, Zn,

Ca and Mg

are effective in improving the leach resistance of both alkali silicate and alkali borosilicate glasses under certain conditions 1-4 . In a repository the water chemistry will inevitably be altered and possibly controlled by the materials in overpack materials).

close proximity to the waste form (ie, Additionally,

repository and

the chemistry of the water in

with the waste form will be complex and probably vary with time, ture,

flow rate, geology,

tempera-

radiation field and the ratio of surface area

of material to volume of water in the repository. expected to provide the first important to understand its

contact

Since the waste form is

barrier to release of radionuclides it

is

leaching behavior over the widest possible

range of potential repository conditions. Other barriers such as tailored overpacks and backfills may provide additional protection against radionuclide release from the repository. Some of these barriers, including bentonite,

clay/charcoal mixtures and

72 metals,

5-6 . have been discussed by other investigators at this conference

These tailored materials are designed specifically to perform one or all of the following functions: minimize water flow throught the repository adsorb hazardous species released from the first waste form)

barrier

(ie,

the

reduce the rate and/or extent of leaching from the waste form by controlling the water chemistry. The last function can be effective in

immobilizing the radionuclides

by producing protective surface films on the waste form and by controlling saturation and buffering capacity of the repository water.

In the

present investigation'we examined the effects of several types of repository and over