Extension of the IAEA’s GSA for the
Borehole Disposal Concept to
include High Activity Sources
Task 4: Post-closure Safety Assessment
Calculations
R H Little
QRS-1668A-TN4
Version 1.0 (draft)
November 2014
Document History
Title:
Extension of the IAEA’s GSA for the Borehole Disposal Concept
to include High Activity Sources
Subtitle:
Task 4: Post-closure Safety Assessment Calculations
Client:
IAEA
Document Number:
QRS-1668A-TN4
Version Number:
Version 1.0 (draft)
Notes:
Draft TN for review by IAEA staff
Prepared by:
Richard Little
Reviewed by:
Richard Metcalfe
Approved by:
Richard Little
Quintessa Limited
The Hub, 14 Station Road
Henley-on-Thames
Oxfordshire RG9 1AY
United Kingdom
Date: November 2014
Tel: +44 (0) 1491 636246
Fax: +44 (0) 1491 636247
info@quintessa.org
www.quintessa.org
www.quintessa-online.com
QRS-1668A-TN4, Version 1.0 (draft)
Contents
1
Introduction
1
2
Modifications to the BDC Screening Tool
3
3
Modified Input Data for Post-closure Safety Assessment Calculations
6
4
Results
8
4.1 Reference Case Calculations
8
4.2 Bounding Case Calculations
8
References
10
Appendix A – Testing the BDC Screening Tool v1.1 (Long Capsule)
11
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QRS-1668A-TN4, Version 1.0 (draft)
1
Introduction
The draft IAEA publication “Generic Post-closure Safety Assessment for the Borehole
Disposal of Disused Sealed Sources” (the GSA) (IAEA, 2013) considers the post-closure
impacts of the disposal of disused sealed radioactive sources using the Borehole
Disposal Concept (BDC). Table 17 of the GSA notes that there is no explicit
consideration in the GSA of radiolysis, criticality or thermal effects, since such effects
are considered to be insignificant for the typical inventories to be disposed. However,
there are certain high activity sources, such as the Category 1 and 2 sources, for which
it might be necessary to consider such effects. In addition, the calculations undertaken
in the GSA are performed for a reference design with fixed dimensions for the capsule
and disposal container (Table 5 of the GSA). It is recognised that capsules and disposal
containers with alternative dimensions might be used in order to accommodate
Category 1 and 2 sources. Changing the dimensions of the capsule/disposal container
will impact on steel and concrete thicknesses which in turn will impact failure times.
Therefore, it is also necessary to assess the impacts of alternative dimensions.
The present project addresses the potential impacts of the disposal of high activity
radioactive sources (specifically Co-60 and Cs-137 sources from irradiators used for
sterilisation/food preservation – see Little and Thatcher, 2014) on the post-closure
safety of the BDC. The results of this project will be used to determine the need, if any,
for additional work. The project relates only to post-closure impacts; operational safety
issues associated with the disposal of high activity radioactive sources in the BDC and
the use of alternative dimensions for capsules and disposal containers is beyond the
scope of the present project.
This draft technical note documents work undertaken under the fourth task in the
project (Perform Design and Defect Scenario Calculations) and takes account of the
findings of the three previous tasks which are documented in Little and Thatcher
(2014), Thatcher et al. (2014) and Little (2014), respectively. The scope of the current
work is to specify, undertake, check and document the calculations for the limiting
disposal system identified in the GSA (i.e., the saturated high flow rate system),
including the Design (i.e., normal evolution) and Defect (i.e., alternative) Scenarios,
using the alternative set of dimensions for the high activity capsule and disposal
container described in Little and Thatcher (2014) and summarised in Table 1-1 and
Figure 1.1. Section 2 discusses the necessary modifications to be the BDC Screening
Tool, Section 3 provides the modified input data for the post-closure safety assessment
calculations and Section 4 provides the associated results.
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QRS-1668A-TN4, Version 1.0 (draft)
Table 1-1: Capsule, Containment Barrier and Disposal Container Dimensions for the
Disposal of Irradiator Sources used in Sterilisation/Food Preservation
Waste Package
Component
Capsule
Containment
Barrier
Disposal
Container
Length (mm)
Inside
Diameter (mm)
Outside
Diameter (mm)
Thickness
(mm)
478
47
52
2.5
526
55
103
24
603
103
115
6
Figure 1.1: The Capsule, Containment Barrier and Disposal Container Suitable for
the Disposal of Irradiator Sources used for Sterilisation/Food Preservation
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QRS-1668A-TN4, Version 1.0 (draft)
2
Modifications to the BDC Screening Tool
The current task has required the modification of the hard wired dimensions of the
capsule/disposal container/cement barrier dimensions in the IAEA’s BDC Screening
Tool (Robinson et al., 2013) in order to allow stainless steel corrosion times and cement
degradation times to be calculated and used as input to the AMBER model for the high
activity capsule and disposal container. The parameters that define the alternative
geometry are given in Table 2-1. All other parameters relating to the geometry remain
the same as used in the GSA and summarised in Robinson et al. (2013).
Table 2-1: Geometry Parameters in the Original and Revised Version of the BDC
Screening Tool
Parameter
Original Value
Revised Value
Capsule Length
110 mm
478 mm
Capsule Outer Diameter
21 mm
52 mm
Capsule Thickness
3 mm
2.5 mm
Containment Barrier Length
186 mm
526 mm
Disposal Container Length
250 mm
603 mm
In addition to the changes to geometry, as discussed in Little and Thatcher (2014), the
capsule material has been changed from the Type 304 considered in the GSA to 316L
stainless steel. This is the same material as used for the disposal container, with the
relevant parameters summarised in the Robinson et al. (2013).
Previous versions of the BDC Screening tool assumed that the capsule fails once 80% of
the thickness has been corroded.
For the updated version, consistent with the
discussions with experts from the Nuclear Energy Corporation of South Africa, it has
been assumed that the capsule fails when the corrosion depth reaches the thickness of
the weld, which is 1.7 mm. The model for the failure of the disposal container remains
the same as before, i.e., it fails once 80% of the thickness has been corroded.
These modifications have been made to the tool and a modified version (“BDC
Screening Tool v1.1 (Long Capsule)”) has been produced. The testing of this version of
the BDC Screening Tool is described in Appendix A.
The modified BDC Screening Tool has been used to calculate failure times for the high
activity capsule and disposal container, and the associated cement degradation times
for the containment barrier and backfill in the disposal zone. Calculations have been
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QRS-1668A-TN4, Version 1.0 (draft)
undertaken for the saturated high flow rate system using GSA groundwater #5 (i.e. the
limiting disposal system identified in the GSA). The resulting times are given in Table
2-2, in which the equivalent times are also given for the GSA reference capsule and
disposal container.
Table 2-2: Capsule/Disposal Container Failure Times and Cement Degradation
Times for the Saturated High Flow Rate System
Parameter
Times for High
Times for
Activity
Reference
Capsule and
Capsule and
Container (y)
Container (y)
Disposal Container
4.8E+5
4.8E+5
Capsule
6.5E+5(1)
7.2E+5
Length of time in Stage 1
513
513
Length of time in Stage 2
4620
4620
Length of time in Stage 3
37
37
Length of time in Stage 1
188(2)
242
Length of time in Stage 2
1695(2)
2180
Length of time in Stage 3
14(2)
17
Failure Times
Cement Degradation
Times: Backfill
Cement Degradation
Times: Containment
Barrier
Notes:
1.
More rapid corrosion due to thinner capsule thickness (2.5 mm vs. 3 mm) and modified
corrosion model (see text).
2.
Faster cement degradation due to smaller thickness of the containment barrier (24 mm
vs. 41 mm).
Little (2014) noted that, whilst the temperature increase and radiolysis effects
associated with the high activity sources considered in the current study might cause
some increase the rate of general corrosion and cement degradation during the initial
50 to 200 year period following disposal of the sources, the primary impact is likely to
be on the potential for localised corrosion.
Although the initial generation of
temperatures in excess of the local boiling point calculated in Section 3.2 of Thatcher et
al. (2014) will promote the initial drying of the cement surrounding the disposal
container and so limit corrosion, the subsequent cooling will result in water
condensation and the potential for localised corrosion due to high concentrations of
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QRS-1668A-TN4, Version 1.0 (draft)
electrolytes on the surface of the capsule and disposal container. However, as the
temperature falls further, the concentration of electrolytes on the outside of the
disposal container is likely to reduce due to the diluting effect of groundwater and so
the rate of localised corrosion will decrease. In addition, the potential for localised
corrosion will be further limited by the high pH conditions in the borehole.
To model the behaviour of the system described above would require the use of a
complex THMC (thermal, hydraulic, mechanical and chemical) coupled model that
also took into account impact of radiolysis. The development of such a model could not
be justified especially in the absence of site-specific and engineered barrier-specific
information (e.g., exact porewater composition, groundwater flow rate and cement
grout composition). Therefore, it is beyond the scope of the current project to modify
the corrosion and degradation models used in the GSA to account for these processes.
So the same models as used in IAEA (2013) have been used, although account is taken
for the differing near-field dimensions (as discussed above). In order to account for the
potential detrimental impacts of increased temperature and radiolysis on the corrosion
and degradation rates, results for bounding calculations are provided in Section 4 (as
well as for the reference cases, i.e., the cases based on the corrosion and degradation
models described in IAEA (2013)).
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3
Modified Input Data for Post-closure
Safety Assessment Calculations
The use of the high activity capsule and disposal container described in Little and
Thatcher (2014) requires the modification of the following input data used in the postclosure safety assessment calculations performed in AMBER for the BDC:
▲
failure times for the capsule and disposal container (given in Table 3-1 for the
references cases (i.e., the cases based on the corrosion and degradation models
described in IAEA (2013));
▲
degradation times for the containment barrier and backfill (given in Table 3-1 for
the references cases);
▲
dimensions for flow and transport in the borehole (Table 3-2); and
▲
the radius of the seal source which is taken to be 5.5 mm (IAEA (2007) notes that
irradiator sources are mostly 11 mm in diameter).
Table 3-1: Times for the Failure of the Performance of the Near-field Components
for the Design and Defect Scenarios for the Associated Reference Cases
Component
Start of Failure
(y)
Totally Failed
(y)
5133
5170
0
480,001
Intact stainless steel disposal containers
480,000
480,001
Containment barrier in failed disposal container
(Defect Scenario only)
18,800
19,000
Containment barrier in intact disposal containers
481,884
481,897
0
170,000
650,000
650,001
Backfill cement grout
Failed stainless steel disposal container (Defect
Scenario only)
Failed stainless steel capsule in failed disposal
container (Defect Scenario only)
Stainless steel capsule in intact disposal container
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Table 3-2: Dimensions for Flow and Transport in the Borehole
Component
Capsule (containing
source container)
Containment Barrier
Disposal Zone
(horizontally adjacent
to capsule)
Disturbed Zone
(Backfill)
Length in
Direction of
Flow
(m)
2.35E-2
Area
Perpendicular to
Flow
(m2)
1.24E+0
Diffusion
Length
(m)
Area for
Diffusion
(m2)
2.58E-2
3.90E+0
2.40E-2
2.71E+0
1.90E-2
9.50E+0
1.25E-2
7.00E+0
3.13E-2
2.51E+1
5.00E-2
1.30E+1
2.49E-1
4.08E+1
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QRS-1668A-TN4, Version 1.0 (draft)
4
Results
4.1 Reference Case Calculations
It can be seen from Table 3-1 that, for the Design Scenario’s reference case (i.e. the cases
based on the corrosion and degradation models described in IAEA (2013)), the capsules
do not fail until 650,000 years. By this time, the Co-60 (half-life around 5 years) and
Cs-137 (half-life 30 years) inventories will have decayed. So there will be no release of
Co-60 or Cs-137 for the Design Scenario’s reference case.
The limiting Defect Scenario for Co-60 and Cs-137 disposal is Defect Scenario D4, i.e.,
all welds are satisfactory except for the closure weld in one waste container and one
capsule, with the faulty capsule being in the faulty disposal container. All other nearfield barriers perform as in the Design Scenario. Calculations show that there is no
effective limit on the activity of Co-60 that can be disposed from a post-closure safety
perspective. For the longer lived Cs-137, the limiting activity in a capsule is 5,700 TBq
which is more than an order of magnitude higher than the 470 TBq that results from
the disposal of ten Cs-137 sources in one capsule.
4.2 Bounding Case Calculations
As noted in Section 2, in order to account for the potential detrimental impacts of
increased temperature and radiolysis on the corrosion and degradation rates,
calculations for a bounding case have been undertaken. They conservatively assume
that both the capsule and disposal container fail after 50 years due to localised
corrosion and that cement degradation is an order of magnitude more rapid than for
the reference case (Table 4-1).
Table 4-1: Times for the Failure of the Performance of the Near-field Components
for the Bounding Case
Component
Start of Failure
(y)
Totally Failed
(y)
Backfill cement grout
513
517
Stainless steel disposal container
50
480,001
1,880
1,900
50
170,000
Containment barrier in disposal container
Stainless steel capsule in disposal container
These conservative calculations show that there is no effective limit on the activity of
Co-60 that can be disposed from a post-closure safety perspective. For the longer lived
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QRS-1668A-TN4, Version 1.0 (draft)
Cs-137, the limiting activity in a capsule is about 490 TBq which is marginally higher
than the 470 TBq that results from the disposal of ten Cs-137 sources in one capsule.
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QRS-1668A-TN4, Version 1.0 (draft)
References
IAEA (2007). Identification of Radioactive Sources and Devices. International Atomic
Energy Agency Nuclear Security Series No. 5, IAEA, Vienna, Austria.
IAEA (2013). Generic Post-closure Safety Assessment for Borehole Disposal of Disused
Sealed Sources. International Atomic Energy Agency Draft TECDOC, Vienna, Austria.
Little, R.H. (2014). Extension of the IAEA’s GSA for the Borehole Disposal Concept to
Include High Activity Sources. Task 3: Screening of GSA FEPs. Quintessa Report to
IAEA QRS-1668A-TN3, Version 1.0 (draft).
Little, R.H., and Thatcher, K.E. (2014). Extension of the IAEA’s GSA for the Borehole
Disposal Concept to Include High Activity Sources. Task 1: Collation and
Documentation of Relevant Data. Quintessa Report to IAEA QRS-1668A-TN1, Version
1.0.
Robinson, P.C., Watson, C.E. and Little, R.H. (2013). The Borehole Disposal Concept
Screening Tool v1.0: User Guide. Quintessa Report to IAEA QRS-3038A-2 Version 1.0.
Thatcher, K.E., Metcalfe, M. and Emery, P. (2014). Extension of the IAEA’s GSA for the
Borehole Disposal Concept to Include High Activity Sources. Task 2: Thermal and
Other Calculations. Quintessa Report to IAEA QRS-1668A-TN2, Version 1.0 (draft).
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QRS-1668A-TN4, Version 1.0 (draft)
Appendix A – Testing the BDC Screening Tool
v1.1 (Long Capsule)
Because the changes to the source code are localised and trivial, a full testing
programme has not been carried out.
To ensure that the changes were made as
intended the same case was run in the original and branched versions of the software
and the results compared. The test file SatLF(11Jun13).bdc was used for this purpose.
Figure A.1 shows the output from the tool that shows what values for the geometry
were used in the calculation. The values shown for the “long” capsule case (the bottom
figure) have been compared against those given in Table 2-1 and are correct.
Figure A.2 shows the detailed output from the tool giving the failure and degradation
times for each component. It can be seen that the failure time for the disposal container
is the same in both cases, which is expected because there were no changes to the
dimensions of this component, or to those of the surrounding backfill.
Similarly the
backfill degradation times are the same in both cases.
The barrier degradation times are shorter for the “long” capsule case, with each stage
taking approximately 78% of the time of the original case.
A quick calculation
determines that this reduction is also as expected, given that the time per flush is
proportional to the ratio of the barrier volume to the surface area. The time per flush is
defined as (Robinson et al., 2013):
𝜏=
𝜃𝑉
𝑞𝜒
where 𝜃 is the barrier porosity, 𝑉 is the volume, 𝑞 is the flow rate and 𝜒 is the surface
area. Given that the porosity and flow rate are the same for both cases, the ratio of the
times is simply the ratio of volumes to surface areas:
𝜏 𝐿 𝑉 𝐿 /𝜒 𝐿
=
𝜏 0 𝑉 0 /𝜒 0
where the superscript 𝐿 denotes the value for the “long” capsule and 0 denotes the
value for the original capsule.
Substituting in the formulae for the volumes and
surface areas and cancelling out common factors gives the expression
0
𝐿
𝐿 )2 )𝐿0
)2 − (𝑑𝐶𝐴𝑃
𝜏 𝐿 𝐿𝐿𝐶𝐴𝑃 ((𝑑𝐶𝑂𝑁
𝐶𝐴𝑃 𝑑𝐶𝑂𝑁
=
0
0 )2 )𝐿𝐿
𝐿
𝜏 0 𝐿0𝐶𝐴𝑃 ((𝑑𝐶𝑂𝑁
)2 − (𝑑𝐶𝐴𝑃
𝐶𝐴𝑃 𝑑𝐶𝑂𝑁
where 𝐿𝐶𝐴𝑃 and 𝑑𝐶𝐴𝑃 are the length and diameter of the capsule and 𝑑𝐶𝑂𝑁 is the
diameter of the containment barrier. Substituting in the values from Table 2-1 gives a
value of 0.78, agreeing with the degradation times reported by the tool.
Figure A.2 also shows that the capsule failure time is shorter for the “long” capsule
case (86,615 years) than for the original case (94,339 years); this is partly due to the
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cement barrier being thinner and, as discussed above, degrading more quickly.
However it is also compounded by the assumption that the capsule fails when the
corrosion depth reaches the weld thickness (1.7 mm), rather than 80% of the wall
thickness (0.8*3 = 2.4 mm).
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QRS-1668A-TN4, Version 1.0 (draft)
Figure A.1: The Input Data tab for the original (top) and large capsule (bottom)
versions of the tool, showing the alternative geometry parameters used.
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Figure A.2: The Detailed Information tab for the original (top) and large capsule
(bottom) versions of the tool, showing the failure and degradation times for each
component.
14