Development of a Backfill for Containment of High-Level Nuclear Waste
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DEVELOPMENT OF A BACKFILL FOR CONTAINMENT OF HIGH-LEVEL NUCLEAR WASTE*
FLOYD N. HODGES, JOSEPH H. WESTSIK, JR., and LANE A. BRAY. Pacific Northwest Laboratory, P. 0. Box 999, Richland, Washington 99352, U.S.A.
ABSTRACT Sodium and calcium bentonites, pressed to densities between 1.9 and 2.2 13 g/cm 3, have hydraulic conductivities in the range of 10-11 to 10cm/s. Batch sorption distribution ratios (Rd) indicate that Sr, Cs, and Am are strongly sorbed on bentonites and zeolites, that Np and U are moderately sorbed on bentonites and zeolites, and that Am, Np, U, I, and Tc are strongly sorbed on charcoal. Sorption results with basalt and tuff ground waters are similar; however, iodine in tuff ground water sorbs more strongly on bentonites Thermal diffusivity measurements for dry, compacted (p - 2.1 g/cm ) sodium bentonite indicate that the thermal conductivity of a high density bentonite backfill should be roughly similar to that of silicate host rocks (basalt, granite, tuff). These results indicate that a bentonite backfill can significantly delay the first release of many radionuclides into the host rock and that by forming a diffusion barrier a bentonite backfill can significantly decrease the longterm release rate of radionuclides from the waste package. Introduction The inherent uncertainties
involved in
the predicting
of long-term be-
havior of large-scale geologic and hydrologic systems make the use of engineered
backup
isolation. waste
(waste
important functions. water
systems
A backfill form
waste
an
important
(buffer)
plus
canister
package
part
material, and
of
placed
overpack)
component
that
the waste
package;
2)
3)
complexants
containerized waste;
to the
the
for
rock
several
is
an
useful
1) control the migration of
4)
agents and
control the Eh and pH of
(1,2,3,4,5).
Initial backfill work at Pacific Northwest Laboratory materials
waste
and 5) provide mechanical isolation
and protection for the containerized waste
backfill
host
perform
retard the migration of corrosive
water reaching the containerized waste,
develop
nuclear
retard the migration of radionuclides
away from the waste form; inward
and
may
A properly designed backfill may:
through
geologic
between the containerized
non-salt
geologies
(PNL),
(i.e.,
intended to
basalt,
Work performed for the U. S. Department of Energy under Contract DEAC06-76RLO 1830. Operated for the U. S. Department of Energy by Battelle Memorial Institute.
tuff,
642
etc.)
has followed
trated
on
materials.
The choice
desirable
and
bentonite is
including
ability
its
clays
(bentonite)
of bentonite
properties,
plasticity,
for U.S.
the lead of the Swedish KBS program
montmorillonite
low
to swell.
is
reasonable
potential
because
permeability, The
(1) and has concen-
as
high
major question
backfill
of its
highly
sorption about
ratios,
the use of 0
stability at the higher temperatures
(T > 100 C) proposed
high-level nuclear waste repositories.
Backfill
development
work
at
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