Study of Radionuclide Migration from Deep-Lying Repository Sites with Overlying Sedimentary Layers
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STUDY OF RADIONUCLIDE MIGRATION FROM DEEP-LYING REPOSITORY SITES WITH OVERLYING SEDIMENTARY LAYERS +
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JOERG HADERMANN , FRITZ ROESEL , and CHARLES MCCOMBIE + Swiss Federal Institute for Reactor Research, CH-5303 Wfrenlingen; ++ National Cooperative for the Storage of Radioactive Waste, CH-5401 Baden, Switzerland
INTRODUCTION In Switzerland the host rock formations currently considered for disposal of high-level wastes lie in the northern part of the country. They form the crystalline basement and are covered by substantial layers of sedimentary rocks. First predictions of regional groundwater flow
fields for this hydrologic
system have become available recently. These define preferential migration paths to the biosphere and groundwater velocities to be used as input to radionuclide transport model calculations. We consider a one-dimensional transport along these preferential paths from a source described by a leaching model. The source is
located at notional repository depths of 1500 and 2400 meters,
In each case the depth
in granitic rock is
we have chosen the 245Cm chain because it appears to be crucial in
respectively.
1000 meters. As a source inventory
includes the nuclide 237Np which
long-term safety analyses. As far as possible field
and laboratory data are used in the calculations. However, these data have very large uncertainties,
at present most of
so that we consider the influence of
their variation on the resulting nuclide concentration at the outlet of the geosphere.
MODELS The leaching model RNRMI)
is
based on a mass balance equation for the radio-
nuclide inventory of a waste matrix. rates for the
It
takes into account time-dependent leach
individual nuclides and a constant corrosion rate for the glass
matrix. Various representations can be chosen for the matrix geometry.
The re-
sulting radionuclide injection rates are transformed to a concentration boundary 2) at the geosphere inlet with the aid of the code CONZRA . This will allow for a check whether solubility limits are surpassed at the source. We realize that representing the source term for the far-field migration simply by a leach
690
model constitutes a gross simplification. Radionuclide transport through the geosphere is code3)".
calculated by the RANCH
This models one-dimensional transport through layered media with an
arbitrarily time-dependent concentration boundary taking into account dispersion, advection and linear sorption equilibrium characteristics.
PARAMETERS The parameters used in the present calculations are discussed in detail elsewhere4) . We restrict ourselves to a few remarks. Hydraulic parameters,
i.
e. migration distances from repository to the bio-
sphere and water velocities,
are taken from ref.
5 ( tables 1 and 2 ) in which
a two-dimensional porous flow model was employed with broad estimates for various geological data which are not yet available. hydraulic conductivities.
This is
This is
especially true for
not currently of great importance since our
principal objective w
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