Electrolyte diffusion in compacted montmorillonite engineered barriers
The bentonite-based engineered barrier or packing is a proposed component of several designs conceived to dispose of high-level nuclear waste in geologic repositories. Once radionuclides escape the waste package, they must first diffuse through the highly impermeable clay-rich barrier before they reach the host repository. To determine the effectiveness of the packing as a sorption barrier in the transient release period and as a mass-transfer barrier in the steady release period over the geologic time scales involved in nuclear waste disposal, a fundamental understanding of the diffusion of electrolytes in compacted clays is required. We present, and compare with laboratory data, a model quantifying the diffusion rates of cationic cesium and uncharged tritium in compacted montmorillonite clay. Neutral tritium characterizes the geometry (i.e., tortuosity) of the particulate gel. After accounting for cation exchange, we find that surface diffusion is the dominant mechanism of cation transport, with an approximate surface diffusion coefficient of 2 x 10 W cmS/s for cesium. This value increases slightly with increasing background ionic strength. The implications of this work for the packing as a migration barrier are twofold. During the transient release period, K/sub d/ values are of little importance in retarding ion migration. This is because sorption also gives rise to a surface diffusion path, and it is surface diffusion which controls the diffusion rate of highly sorbing cations in compacted montmorillonite. During the steady release period, the presence of surface diffusion leads to a flux through the packing which is greatly enhanced. In either case, if surface diffusion is neglected, the appropriate diffusion coefficient of ions in compacted packing will be in considerable error relative to current design recommendations. 11 refs., 4 figs., 1 tab.
- Research Organization:
- Lawrence Berkeley Lab., CA (USA)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 6315009
- Report Number(s):
- LBL-20406; CONF-850981-6; ON: TI86005461
- Country of Publication:
- United States
- Language:
- English
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54 ENVIRONMENTAL SCIENCES
ALKALI METALS
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
CATIONS
CESIUM
CHARGED PARTICLES
CLAYS
DATA
DIFFUSION
ELECTROLYTES
ELEMENTS
ENGINEERED SAFETY SYSTEMS
ENVIRONMENTAL TRANSPORT
EXPERIMENTAL DATA
HIGH-LEVEL RADIOACTIVE WASTES
HYDROGEN ISOTOPES
INFORMATION
INORGANIC ION EXCHANGERS
ION EXCHANGE MATERIALS
IONS
ISOTOPES
LIGHT NUCLEI
MANAGEMENT
MASS TRANSFER
MATERIALS
METALS
MONTMORILLONITE
NUCLEI
NUMERICAL DATA
ODD-EVEN NUCLEI
PACKAGING
RADIOACTIVE MATERIALS
RADIOACTIVE WASTE DISPOSAL
RADIOACTIVE WASTES
RADIOISOTOPES
RADIONUCLIDE MIGRATION
TRITIUM
UNDERGROUND DISPOSAL
WASTE DISPOSAL
WASTE MANAGEMENT
WASTES
YEARS LIVING RADIOISOTOPES