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THE ROLE OF METAL ION SOLUBILITY IN LEACHING OF NUCLEAR WASTE GLASSES*

B. GRA•BOW Pacific Northwest Laboratory, P. 0. Box 999, Richland, Washington 99352, USA and Hahn-Meitner-Institut fuirKernforschung Berlin GmbH, 1000 Berlin 39 Glienicker Str. 100, Berlin, Germany

INTRODUCTION The leachability of solid nuclear waste forms has been studied by many investigators to evaluate the short-term kinetics of elemental release or to determine the effects of leachant composition or other system parameters. general observations from these studies have included: formation of reaction layers that contain rare-earth,

incongruent leaching; alkaline-earth,

tion metal elements; and apparent saturation of some elements in others continue to be leached.

1

'

2

'

3

Some

or transi-

solution while

Before these observations can be used to

predict long-term performance of waste glasses under repository conditions,

the

mechanisms controlling the release of elements from the solid must be understood. Solubility-related phenomena have become increasingly important in interpreting many aspects of leaching behavior. example,

Several previous studies have,

for

qualitatively considered solubility limits to be important with respect

to reaction layer formation and the prediction of long-term behavior under accident conditions in a repository.4,5 in

Rai6 showed that concentrations of Np

solutions contacting crushed Np-doped PNL 76-68 glass were controlled by a Np

solid phase that is

similar to that controlling the solubility of pure,

crystal-

line NpO2 . Since multicomponent waste glasses are unstable with respect to an assemblage of alteration products, thermodynamic equilibrium between the glass phase and the leaching solution can never be achieved.

This paper shows that the altera-

tion phases resulting from the leaching of the waste glass ultimately regulate the solution concentrations as if

equilibria with metal hydroxides or car-

bonates are established.

EXPERIMENTAL METHODS The composition of PNL 76-68 glass that was used in this study has been 3 reported by Strachan. The results to be discussed come from the MCC-lP Static Leach Test Method;3'3'8 from a solubility test with glass powder;9 or from * This work was supported by the U.S. Department of Energy under Contract DE-AC06-76RLO 1830.

94 experiments

using the MCC-5S Soxhlet Leach Test Method.I0

are summarized in Table 1.

The test conditions

Details can be found in the references indicated.

TABLE 1 EXPERIMENTAL CONDITIONS OF LEACH TESTS CONSIDERED IN THIS PAPER

Test Method

Leachant

MCC 1 (Static)

HNO 3 , NaOH or DI-H 0 DI-H 2 0 2

"

"Silicate water "EDTA (0.1m)

Temp. 0 C

SA/V m-1

!28

90

10

1 to 12

8

365 :28

90 90

10 10

8.8 to 9.6 9.5

3 3 b

:28

90

10

8.3 & 9.0

Time d

Final pH

Ref.

Solubility (Static)

H20

79

95

1200, 5000

8.8 9.6

9 9

Soxhleta (Dynamic)

H 0

n28

90

(0.01)

6.8

b

SA/V = sample surface area

a Flow rate: 1 ml/min; pH measured in leach cup.

divided by total volume of water that had con