Radionuclide Retardation During Transport Through Fractured Granite
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RADIONUCLIDE RETARDATION DURING TRANSPORT THROUGH FRACTURED GRANITE
Ian G.McKinley, Julia M.West Environmental Protection Unit, Institute of Geological Scinces, AERE Harwell, Oxfordshire. OXiI ORA, United Kingdom.
Building 151,
INTRODUCTION In several countries low permeability crystalline rocks
(e.g. granites) are
under consideration as potential hosts for radioactive waste repositories. such formations groundwater flow occurs predominantly in
In
specific fractures
rather than being a general porous flow through the entire rock matrix.
By
considering fractures to be simple parallel plates various authorsI'2,3 have demonstrated the potential importance of diffusion into dead-end pores and the rock matrix itself
('matrix diffusion')
migrating radionuclides.
as a mechanism for the retention of
Complementing these theoretical studies,
situ migration experiments are planned in in Sweden,
the U.K.
single fissure is,
and the U.S.A. however,
The
single fissures in 'parallel plate'
several in-
crysalline rocks
approximation to a
acknowledged to be a gross simplification of any
real case where "flowing" fractures are expected to be either filled or coated with secondary minerals, In
formed by hydrothermal alteration of fracture surfaces
the evaluation of net radio-nuclide retardation,
therefore,
the effect of
sorption onto such secondary minerals must be carefully considered. In
this paper the results of laboratory sorption studies of particular
radionuclides
(isotopes of Cs,
Sr, Co and Ce) onto naturally weathered granite
fracture infill
are reported.
Complementary mechanistic studies investigating
the effect of reaction direction, temperature and competing ions on sorption isotherms are also briefly summarised.
By use of simple computer models of
groundwater flow, the consequences of observed 'isotherm non-linearity'
on
resultant nuclide migration are illustrated and compared with retardation calculated assuming 'matrix-diffusion'
effects.
MATERIALS AND METHODS Sorption studies are reported on weathered granite fracture infill proposed fissure migration site in a near surface mine at Camborne, The fracture infill
from a Cornwall.
4
consisted of granite residuum with a fine grained
component including:- quartz,
smectite,
mica, chlorite and kaolinite,
weathered rock assemblage sampling and pre-treatment were designed to
For this
812 minimise pertubation
(e.g. loss of natural moisture).
It
must be emphasised
that mineralogical analysis of such material gives an incomplete picture of composition as many poorly ordered or amorphous intermediates in
the process of
weathering from component granite minerals to thermodynamically stable clays are present.
Poorly formed surfaces present will be very complex chemically
but are likely to be important in
the sorption of trace concentration species.
A standard batch reaction technique was utilised in which rock samples are equilibrated with groundwater spiked with y-emitting radionuclides under conditions of controlled tem
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