Lecture Notes: Borehole Completion and Repository Design (30 min.)
INTRODUCTION
The borehole disposal concept (BDC) is a disposal system developed by IAEA for the
permanent disposal of small volume radioactive wastes, in particular, disused sealed
radioactive sources (DSRS). It is intended for use in countries that do not have an
extensive nuclear infrastructure. The BDC relies on a combination of stainless steel
containers and concrete grout to provide long-term physical containment and it is
intended that it should be capable of being deployed in a very wide range of geological
and geochemical environments.
This lecture begins with a description of the main elements of the BDC design and the
safety functions of the various components. This is followed by a description of the
factors that can influence a site-specific design and another section that shows how the
design might be optimized to allow for the properties of two example sites. Finally, the
steps required to construct and complete a disposal borehole are outlined.
In the text that follows, where a number is placed at the start of a paragraph this indicates
the corresponding slide number. See also the full set of slides at the end of this note.
SCOPE/STRUCTURE
2 The lecture has the following contents list which also defines its scope.
Main components of the BDC design and their safety functions
Influences on the site-specific design
Adapting the generic design to a specific site
Drilling and completing a disposal borehole
These are described in more detail in the next section
CONCEPTS
Main components of the BDC and their safety functions
5-10 The main components of the BDC are described with reference to the two diagrams
shown opposite. The safety function(s) (if there is one) of each component is then
described in a table which here appears immediately after the two diagrams.
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COMPONENT
SAFETY FUNCTION/ DESIGN RATIONALE
Disused sealed sources
None – is assumed that the radionuclides in the sealed
sources are free to migrate
Capsule and container are made from corrosionresistant stainless steel which provide absolute physical
containment for a time that is dependent on the rate of
corrosion
All these materials are cementitious. They produce high
pH conditions that reduce the rate of container and
capsule corrosion. The cement also provides surfaces
on which radionuclides can sorb thereby retarding their
migration
Facilitates operation by supporting the borehole wall,
excluding groundwater, providing a smooth surface to
prevent snagging when emplacing waste packages
It has no long-term safety function
Facilitates operation by ensuring that the borehole is
close to vertical
Provides adequate concrete to provide pH buffering and
dilution by spreading out any contaminant plume
Prevents inadvertent drilling into the disposal zone
Capsule
Container
Container backfill
Repository backfill
Borehole grout
Casing
Casing centraliser
Container to container spacing
Anti-intrusion plate
Closure zone minimum 30m
Concrete closure seal
Casing removal
(top 2m)
Geosphere
Provides isolation of the waste by placing it out of reach
of normal deep excavation activities eg roads,
foundations of buildings
Seals the borehole and provides isolation of the wastes
through its physical presence and containment of
radionuclides by protecting the waste packages from
damage and corrosion. Also retards migration of
radionuclides through sorption
A security measure that prevents unauthorized access
to the waste by hiding the borehole location from sight.
The precise location will be known to the authorities
who, in any event, would have equipment that was
capable of finding it
Provides isolation of the wastes from the human
environment and containment of radionuclides by
protecting the waste packages from damage and
corrosion. In the event of radionuclide migration from
the facility, it will also provide retardation (through
sorption) allowing more time for radionuclides to
decay. The geosphere should have low resource
potential to deter inadvertent human intrusion
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Influences on the site-specific design
12 Many of the BDC design parameters are fixed (the waste package for instance) and,
putting these aside, there are four general issues that have the capacity to influence the
site-specific design. These may be divided into economic & practical matters, legislation
& regulation, the waste inventory and the site properties. Safety assessment is used as a
means of checking that the design is adequate. This is expressed in the diagram below.
Economic and practical matters
13 A number of parameters will have an impact on the practicability and the cost of
disposal and these could vary greatly between different countries. The most important of
these is the choice of site, which may well be a governmental decision.
Other examples are:
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Remoteness of the site and the ability to transport the sources there and the
equipment needed to do the investigations and construct the borehole. Water will
be needed to make the concrete and support human wellbeing and this may have
to be transported to the site.
Drilling contractors are widely available but not necessarily contractors who
would be qualified to perform the required site investigations. The disposal
borehole has a diameter of at least 0.26m and this size is widely used for water
extraction equipment so the equipment will often be available locally
Availability of construction and emplacement materials eg casing, cement, sand,
water
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

The site properties (groundwater, weathered zone, geological complexity,
topography…) will have an impact on the difficulty (hence cost) of the site
investigations, Geological complexity could make it difficult to make an
acceptable safety case
Finally potential licensees should not underestimate the effort required to have
useful Stakeholder and Regulatory interactions. These activities can be very time
consuming
Legislation & regulation
14 Potential licensees need need to have a very good understanding of how the law and
regulations will impact on what is planned. In brief the regulations will determine the
safety standards to be met. In general we would expect these to be similar to IAEA
standards – especially if the Model Regulations are used as a basis. That said, some
countries may impose other restrictions e.g. the format of the safety case or safety
assessment, who is permitted to design the facility, the need (or not) for retrievability
If the Model Regulations are used, almost all of these issues will have been answered by
the design
Waste inventory
15 Compared to the disposal of nuclear power plant wastes, characterization of an
inventory of disused sealed sources is straightforward because the wastes (the disused
sealed sources) generally contain only one radionuclide. The key parameters to be
determined for each source are:

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Identification of radionuclide/ nuclear reaction/ emissions (hence half-life)
Activity at a specified date
Physical size of sources to be placed into each capsule (after removal of the
source from the associated equipment and shielding)
Current storage and handling arrangements / means of retrieval from store /
weight to be lifted
Means of removal of bare source from operating shield
Need for shielding during conditioning/ containerization
Of these, the parameters that are most relevant to the design are the physical size of the
sources and their activity because (assuming that different radionuclides can be mixed in
one package) these will largely determine the number of waste packages and, hence, the
required overall length of the disposal zone.
16 The inventory will also influence the extent to which long-term physical containment
will need to be relied upon in the safety case. The figure that follows indicates this. The
left hand side of the figure shows radionuclides that are commonly used for sealed
sources. The axis at the bottom shows years. The individual bars indicate the number of
years needed for a source to decay to exemption levels; the width of the bars represents
the range of radioactivity typically found in the various types of source that use this
radionuclide. Hence the figure provides an indication of the length of time that a
radioactive source would need to be contained in order for the radionuclide inside it to
decay in situ to its exemption level. In the case of Pu-239 (half life 24,000 years) the time
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can be in excess of 1 million years but, that said, Pu-239 is an unusual radionuclide to
find in a disused sealed source. More commonly encountered is Ra-226 (half life 1600
years) which, even for the largest sources would require less than 20,000 years to reach
exemption levels. Such considerations will not affect the site-specific design but they will
affect the focus of the safety case.
Site properties
17 Site properties are dealt with elsewhere (under site characterization) but, in summary,
the most important ones for the BDC are (in no particular order)

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Depth to water table
Existence of near-surface weathered rocks
Separation of near-surface and deep water bodies
Rate of surface erosion/ geomorphology
Properties of potential host geology
o Structural
o Geomechanical
o Geochemical
o Etc
Adapting the generic design to a specific site
The most important design issues needing to be decided - and their most likely method of
resolution - are listed in the following table
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ISSUE
HOW DECIDED
Which site?
Governmental or community decision
Where on the site?
Avoid large faults, nearby surface facilities
How many boreholes, what depth?
Design optimization based on the inventory and
the site properties – see below
New borehole or modify existing
characterization borehole?
Largely an economic and practical issue
20 The issue of design optimization is first addressed by determination of the required
total length of the disposal zone. This is equal to the number of packages multiplied by
the package to package spacing. The number of packages is dependent on the number,
geometry and type of disused sealed sources. The number of sources that can be placed
inside a capsule depends upon the size of the sources and the size of the capsule (two are
currently available: small and large). All conditioning to date has ensured that
radionuclides are not mixed within a capsule but, provided that is chemical compatibility,
this need not be the case. Mixing of sources and use of the larger capsule could greatly
reduce the number of waste packages and, thus the length of the disposal zone. Of course,
the larger packages would have to be consistent with handling requirements – less than 1
Ci of Co-60 in the lightly shielded conditioning facility.
The BDC reference design has a package to package spacing of 1m but, if this could be
reduced to, say, 0.5m, it would half the length of the disposal zone and have knock-on
benefits in terms of cost. The reduction in spacing could have an effect on safety in the
long term (after the packages have degraded) but this effect is probably extremely small,
Calculations would be needed to confirm this.
Having established the overall length of the disposal zone we then look to optimize the
design to make best use of the site. We do this by reference to two examples.
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21 The minimum depth of disposal is 30m but in the case shown above the water table
depth varies between (say) 25m depth in the rainy season and 32m depth in the dry
season. To avoid the fluctuating phreatic surface the disposal should be placed below the
minimum groundwater depth. In this case a decision over whether there should be one or
two boreholes may be largely an economic matter. The cost per metre of a borehole
increases with depth; on the other hand two boreholes will require two closure zone ie
much more concrete.
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22 A second hypothetical example is shown in the first of the two figures above. A
suitable host rock exists from 20 to 70m depth. Below this is an aquifer that may not
always be fully saturated ie the water does not extend to the top of the aquifer. This is a
feature to be avoided because, while the BDC can tolerate fully saturated or fully
unsaturated conditions, cycling between the two is undesirable. The disposal zone must
also be deeper than 30m. This means that the available suitable host rock has a thickness
of 40 metres. If the required disposal zone length were 70m, a single borehole would
extend to 100m (30m minimum depth plus 70m) which would intrude into the aquifer.
An acceptable way of avoiding this would be to drill two disposal boreholes (lower of the
two figures above).
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Drilling and completing a disposal borehole
24 The BDC is designed to use techniques that are widely available. The borehole
minimum diameter of 0.26m minimum is typical of boreholes used for groundwater
abstraction. A rotary air percussion drill would normally be used. This does not allow
coring but for the disposal borehole this is not strictly necessary. Samples of chippings
should be retained for every metre of borehole however. The figure above illustrates
rotary air percussion drilling
25-30 The sequence of activities for borehole construction is as follows:
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Borehole drilling to full depth
Insertion of mild steel or high density polyethylene (HDPE). The diameter of the
casing should be such that, with a 260mm borehole, the annular gap between the
casing and the borehole wall is more than 50mm. Centralisers are used to make
sure that the casing sits centrally in the borehole. The 50mm gap should provide
sufficient space to allow a tremmie pipe to be inserted for grouting.
Formation of a bottom plug by inserting a tremmie pipe into the annulus and
running it to the bottom of the borehole. Grout is then pumped through the
tremmie pipe to form a concrete plug that holds the bottom of the casing in place.
Once the plug has hardened (overnight) the annulus itself is grouted, again using
the tremmie pipe (see photograph below). After checking its as-built status, the
borehole is then ready for waste emplacement.
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DEFINITIONS AND ABBREVIATIONS
Most of the words and expression used in these notes can be found in the IAEA Safety
Glossary or an ordinary English dictionary. Those that are specific to the BDC may be
found below
BDC
Borehole disposal concept
Capsule
A small, 3mm thick stainless steel container into which disused sealed
sources are placed prior to storage.
Conditioning
The placing of a disused sealed source into a capsule followed by
seal welding and leak testing.
Container
A 6mm thick stainless steel vessel into which filled capsules are placed
prior to disposal.
Containerization
The act of placing a capsule into a BDC container followed by seal
welding and leak testing.
DSRS
Disused sealed radioactive source(s).
SOURCES OF INFORMATION/REFERENCES
INTERNATIONAL ATOMIC ENERGY AGENCY, Disposal of Radioactive Waste,
IAEA Safety Standards Series No. SSR-5, IAEA, Vienna (2011).
INTERNATIONAL ATOMIC ENERGY AGENCY, Borehole Disposal Facilities for
Radioactive Waste, IAEA Safety Standards Series No. SSG-1, IAEA, Vienna (2009).
INTERNATIONAL ATOMIC ENERGY AGENCY, BOSS: Borehole Disposal of
Disused Sealed Sources. A Technical Manual, IAEA-TECDOC-1644 Vienna (2011)
INTERNATIONAL ATOMIC ENERGY AGENCY, DRAFT SAFETY REPORT,
GENERIC POST-CLOSURE RADIOLOGICAL SAFETY ASSESSMENT (GSA),
BOREHOLE DISPOSAL OF DISUSED SEALED SOURCES
INTERNATIONAL ATOMIC ENERGY AGENCY, IAEA Safety Glossary:
Terminology Used in Nuclear Safety and Radiation Protection, Vienna (2007).
SUMMARY OF THE LESSON
The lesson has covered
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Main components of the BDC design and their safety functions
o Containers and capsule, backfill, casing etc
Influences on the site-specific design
o Practical/ regulatory/ inventory/ site properties
Adapting (optimizing) the generic design for a specific site
o Two examples given – one leads to one borehole, the other, two
Steps in the construction of a disposal borehole
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QUESTIONS
1. What are the safety functions of the borehole casing?
(a) to aid emplacement of waste packages
(b) to exclude groundwater during operations
(c) to prevent long-term radionuclide migration by excluding groundwater
(d) to produce a localised geochemical regime that is favourable for
radionuclide retention
Answer (a) and (b) are correct. (c) and (d) refer to long term safety but the casing
has no long-term safety function
2. What are the safety functions of disused sealed sources in the BDC?
(a) Physical containment provided by the seal
(b) Low mobility because of the chemical compounds used
(c) None
Answer: only (c) is correct.
3. What are the safety functions of the repository backfill?
(a) Produces high pH conditions which reduces the corrosion rate of the
stainless steel
(b) Provides physical containment
(c) Provides surfaces on which radionuclides can sorb, thus retarding their
migration
Answer (a) and (c) are correct. (b) is incorrect because the concrete could crack
4. The safety of the BDC rests partly on long-term physical retention of
radionuclides. What aspects of the design contribute to this?
(a) the combination of concrete and stainless steel
(b) choice of site
(c) choice of depth of disposal
(d) regulatory requirements
Answer (a) to (c) are correct. Regulatory requirements (d) are not part of the design
5. 30 metres is the minimum depth of disposal for the BDC. Why?
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(a) It is a depth at which inadvertent human intrusion is thought to be very
unlikely
(b) It is below the weathered zone
(c) It is above the saturated zone
(d) It is below the saturated zone
(e) Less than 30m is defined as near-surface disposal
Answer only (a) is correct.
6. 30 metres is the minimum depth of disposal for the BDC. Why?
(a) It is a depth at which inadvertent human intrusion is thought to be very
unlikely
(b) It is below the weathered zone
(c) It is above the saturated zone
(d) It is below the saturated zone
(e) Less than 30m is defined as near-surface disposal
Answer: only (a) is correct.
7. In what way will the inventory for disposal affect the site-specific design?
(a) Need for long-term containment
(b) Host geology
(c) Length of the disposal zone
(d) Minimum depth of disposal
Answer: only (c) is correct.
8. Why will a Ra-226 sealed source require more extended physical containment
than a Cs-137 sealed source?
(d) Caesium reacts with the stainless steel
(e) Ra-226 has a much longer half-life than Cs-137
(f) Radium is more mobile than caesium
(g) Minimum depth of disposal
Answer: only (b) is correct.
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