Subsurface Barriers for Underground Storage
Tanks
The U.S. Department of Energy (DOE) set up the Underground Storage
Tank Integrated
Demonstration Program (USTID) to demonstrate technologies for the retrieval and
treatment of tank waste, and closure of underground storage tanks (UST). There
are more than 250 underground storage tanks throughout the DOE complex. These
tanks
contain a wide variety
of wastes including high level, low level, transuranic, mixed and hazardous wastes. Many of the
tanks have performed beyond the designed lifetime resulting in leakage and contamination of the
local geologic media and groundwater. To mitigate this problem it has been proposed that an
interim subterranean containment barrier be placed around
the tanks.
This would minimize or prevent future contamination of soil and groundwater in the event that
further tank leakages occur
before or during remediation. Use of interim subterranean barriers can also provide sufficient
time to
evaluate and select appropriate remediation alternatives.
The objective of this task was to develop advanced subsurface barrier
materials for underground storage tanks. The candidate materials for this task were identified,
evaluated and recommended by experts at a technical workshop held in February 1992, in Tucson,
Arizona. The materials were selected based on their applicability with conventional placement
technologies, low permeability to water, resistance to aggressive chemicals, radiation resistance
and
tolerance to an elevated temperature environment. Laboratory testing and evaluation of these
materials for potential applications as barriers for USTs at the Hanford Site was completed at
BNL
(TTP CH321203) during December 1992.
Application of interim subsurface barriers can result in reduction of
exposure
by several orders of magnitude. This can also reduce cost of remediation by many millions of
dollars
compared with retrieval and treatment alternatives. Use of interim subsurface barriers can provide
waste management operations sufficient time to evaluate and select appropriate remediation
alternatives for tanks currently or potentially leaking in the immediate future. The USTs
estimated
to lose their integrity in the near future can be remediated by providing subsurface barriers until
tank
operations or tank physicochemical conditions can be ameliorated.
Testing and evaluation of laboratory scale samples of candidate polymer
systems commenced in March 1992 and was completed in FY93. Prior to initiating this work, a
National Expert-Review-Team Workshop was conducted (Bovay) in the latter part of February
1992,
in Tucson, Arizona. The function of the workshop was to identify, evaluate, and recommend the
proper subsurface barrier technology for demonstration in the Hanford Site geology. The
grouting
materials selected, based on anticipated performance requirements, were polymer concretes and
modified sulfur cement.
As a result of the initial laboratory work (TTP CH321203) the report
"Polymers for Subterranean Containment Barriers
for
Underground Storage Tanks (USTs)" was submitted to the USTID. The report finds very
favorable
results for many of the polymer/soil composites. Polymer concrete soil composites (PC) appear
to be well
suited to conditions expected to be prevalent at the Hanford UST site. Short term testing in
severe conditions
have shown soil mortars made with vinylester styrene, polyester styrene, and a high molecular
weight methacrylate
to be extremely durable. For modified sulfur cement (MSC) the report recommends against using
this material for Hanford soil composites but does not rule out MSC in replacement grouting
techniques using a non-reactive, non-expansive aggregate. Some of the conclusions include:
- Sufficient strengths
are easily achieved with
polymer-soil composites.
- PCs have surpassed
90 days water
immersion with no loss in integrity.
- PC samples
survived thermal cycling
with no loss in durability.
- The thermosetting
polymers showed
strength gains averaging 32% after 108 rads irradiation from a gamma source. The strength gains
are
attributed to additional crosslinking of the polymer.
- PCs were subjected
to wet-dry
cycling with no detrimental effects. The average weight changes were ~1.0 % . No strength loss
occurred for any PC.
- PCs withstood 60
days immersion
in 70øC concentrated nitrate brine with no detrimental effects. The brine is a surrogate for the
Hanford UST supernatant and is an aggressive media.
- Soil mortar
hydraulic conductivities on the order of 5 x 10-12 M/s are achievable.
Polymer grouts having a wide range of viscosities (300, 100, and 3
cps) have been demonstrated to have desirable qualities for a
subterranean containment barrier. The resin characteristics should allow
these materials to be used in a wide range of soil types and conditions.
