Using the Disposal Systems Evaluation Framework to Evaluate Design Tradeoffs Page: 4 of 10
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The primary purpose of DSEF is to help the user
tackle a multi-dimensional set of alternatives and
options to arrive at workable high-level radioactive
waste repository concepts.
For each combination of surface storage time,
fuel cycle, and geologic medium, there are multiple
Engineered Barrier System (EBS) design concept
options for (1) Waste package spacing; (2) Waste
package capacity; (3) Drift / borehole spacing; and
(4) EBS components, radii, and material properties.
There are many options to deal with and much
input data for all of the analysis cases. DSEF helps
the user take advantage of previous analyses to define
potential new configurations and analysis cases that
can meet the design and operating constraints.
Open and Enclosed Repository Design Concepts
The FY11 disposal concepts report, Generic
Repository Design Concepts and Thermal Analysis
(FY11) [6], recognized open and enclosed
emplacement modes and recommended further work
to evaluate one or more open modes. Enclosed
modes were defined to include disposal concepts that
call for waste packages to be in direct contact with
any surrounding solid medium such as buffer
material, backfill, or host geology. For enclosed
modes, the direct contact begins immediately at
emplacement or shortly thereafter, with that contact
influencing peak near-field temperature. Open modes
maintain unsaturated, air-filled open spaces around
the waste packages for some time prior to permanent
closure, and even after closure for some concepts.
Open mode concepts were evaluated in [5] and
[7], and are discussed further in this conference in
[2]. Reference [3] evaluated a set of base case open
mode repository design concepts and developed a
number of parametric sensitivity studies for the open
mode repository concepts.
Base Cases and Parametric Studies Evaluated
The base case analyses in Reference [3] included:
1. Commercial LWR UOX spent nuclear fuel, with
burn-up values of 40 GWd/MT and 60 GWd/MT
2. Waste package sizes of 4, 12, 21, and 32 PWR
(4P, 12P, 21P and 32P) assemblies
3. Surface storage times of 50 and 100 years4. Ventilation system operating times of 250 years
for SNF with 50 years of surface storage, and 200
years for 100 years of surface storage time (i.e.,
300 years between removal from the reactor and
start of closure operations)
5. A constant ventilation thermal efficiency of 75%
6. Backfill installation completed 10 years after
termination of the ventilation system operation,
with a mixture of 30% quartz sand and 70%
bentonite
Parametric sensitivity studies were performed in
Reference [3] as one-off studies from the base cases,
including:
1. Ventilation efficiencies of 50, 60, 70, 80, and
90%, in addition to the base case of 75%
2. Ventilation system operating times of 50, 100,
150, and 200 years, in addition to the base case
of 250 years (in combination with 50 years of
surface storage)
3. Drift/borehole spacing variations of 40, 50, 60,
and 70 m, in addition to the base case of 30 m
4. An assumed generic rock type with host rock
thermal conductivities of 1, 2, 3, 4, and 5 W/m-
K, and associated thermal diffusivities assuming
a constant volumetric heat capacity typical of
clay
5. An assumed generic engineered backfill with
thermal conductivity values of 1, 2, 3, 4, and 5
W/m-K. The higher values in this range are
achievable using a mix of bentonite, sand, and
graphite [8] and [9].
6. An uncertainty analysis for clay and alluvium
designs, assuming the mean values of volumetric
heat capacity, and with thermal conductivity plus
or minus one or two standard deviations.
The Reference [3] base case thermal response for
drift/borehole rock wall and waste package surface
temperatures in a clay/shale environment are shown
in Figure 1. Peak temperatures increase linearly with
waste package capacity. The influence of burnup is
shown by the separation between the pairs of curves,
while the minor influence of surface storage time is
shown by the close spacing of the curves.
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Greenberg, H. R.; Blink, J. A. & Sharma, M. Using the Disposal Systems Evaluation Framework to Evaluate Design Tradeoffs, article, January 28, 2013; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc838349/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.