16. Site Characterization and Data Requirements

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Lecture Notes: Site characterization and data requirements for
implementing the BDC (30 minutes)
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 provides a brief description of the purpose of site characterization and the
activities that it would entail when applied in the context of the BDC. The lecture also
describes the data that would be generated by site characterization and explains how
these data would be used.
Where a number is placed at the start of a paragraph this indicates the corresponding slide
number. See also the complete set of slides at the end of this note.
SCOPE/STRUCTURE
2 The lecture has the following contents list which also defines its scope.
Step by step development and implementation of the BDC
The nature of site characterization
Site characterisation techniques
Uses of site characterization data
These are described in more detail in the next section
CONCEPTS
Step by step development of the BDC
4-6 This is a short section that introduces the idea of step-by-step development for the
BDC [SSR5] and shows how site characterization fits into the overall process. The
section also describes the permissions and iterations to the safety case that might allow a
BDC development to proceed. This includes the idea that a preliminary safety case would
use the established inventory and any information that was already known about the
proposed site(s) in order to obtain approval-in-principle and permission to investigate the
site in detail. This would be followed by a site-specific safety case that would underpin a
decision to characterize the site and construct a disposal borehole.
1
The nature of site characterization
8 This section describes the nature of site characterization explaining the need to use
specialist contractors and the multi-disciplinary aspects. The main disciplines are outlined
(see Table below) and the need for coordination and control that arise from this aspect.
9 The use of safety assessment as a primary driver for site characterization is also
explained.
DISCIPLINE
TOPIC
MAIN RELEVANCE
Geology
Geology
Geotechnical properties
Hydrogeology, chemistry, seismic
Borehole construction/ stability
Geochemistry
Chemistry of host rocks
Chemistry of groundwater
Sorption properties, groundwater chem.
Corrosion of containers
Origin/ age of groundwater
Hydrogeology
Hydraulic properties/ flow
regime
Transport of radionuclides in groundwater
Seismology
Earthquake magnitude and
probability
Impact of earthquakes on engineered
barriers
Surface
properties
Erosion/ hydrology
Topography
Reduction of cover / loss of isolation
Inadvertent human intrusion
Ecology
Plant and animal life/
farming
Conversion of radionuclide flux to annual
dose to humans
Resources
Scope for resource
exploration & exploitation
Inadvertent human intrusion
Site characterisation techniques
11 This section describes the techniques that are widely used in site characterization but,
first, emphasizes the need to interrogate existing sources of information such as national
institutes and, to the extent possible, involve them in the work.
An site characterization programme will be defined in a scope document that will define
the activities to be undertaken. This will cover issues such as the number of investigation
boreholes to be drilled and their depth; the type and amount of sampling and testing that
is to be performed and so on.
12 The scope of the required investigations will, in turn, depend on

the geological and hydrogeological environment to be investigated eg their
complexity;
2



the inventory of wastes for disposal – especially the longevity of the radionuclides
to be disposed of which may require closer investigation of the flow and transport
properties;
the national regulations and approaches to authorisation;
the local availability of resources
13 The site investigation strategy should take maximum advantage of existing
information and use techniques that are best suited to obtain the required information and
are available locally at reasonable cost. Such investigations will normally be performed
by specialist contractors and this entails a need for technical specifications.
14 IAEA has prepared tech specs for the investigatory and disposal boreholes. Obviously,
these are generic (non-site specific) but they do indicated the general requirements for
undertaking the different investigation activities. They place the responsibility for
providing more detailed information on how activities are undertaken with the
implementing organisation. This is so that the detailed requirements can take account of:



Local regulations and approaches to undertaking site investigations;
Available local resources; and
Local language
15 The types of work that are covered by the generic technical specifications include:





Surface-based geophysical surveys;
Drilling and down-hole testing of exploratory boreholes:
Interpretation of results;
Ecological survey
Investigation management (if above activities are undertaken by multiple
contractors)
16 Some important considerations are

Investigations are designed to gather the information needed for the project - it is
important to avoid getting side-tracked;

The Investigations will be multidisciplinary and you may require there to be
collaboration between a range of contractors – this requires active coordination;
and

Contractors may need to wait whilst other activities are being undertaken, e.g.
drilling may be halted whilst testing is being carried out. The contractor needs to
know this from the outset because it will affect his costs. It follows that
investigatory drilling is not like drilling a water well
17 Surface-based geophysical surveys are used to provide data and information on the
subsurface environment, in particular, lithological contrasts, geological structures such as
faults and the depth to groundwater. The most commonly used techniques are seismic
surveys and ground penetrating radar. These surveys may be used to identify prospective
locations for siting the investigation boreholes and/or the disposal borehole(s).
18 At least one exploratory borehole will be needed. These will be smaller diameter (eg
50mm) than the disposal borehole (>260mm) and they are used to:
3





Define the geological succession. Here one might use diamond drilling to extract
rock core for subsequent examination. If percussion drilling is used, cuttings
should be collected at metre intervals for examination.
Define the characteristics of the geological materials, e.g. geo-mechanical,
geochemical and hydrogeological properties.
Define the groundwater chemistry;
Establish groundwater pressures/ flow directions; and
Provide information to calibrate the surface-based geophysical surveys.
19 Diamond drill bits for extracting 50mm core and a schema showing how such drill bits
are used.
20 Many down-hole techniques are available. Geophysical wireline logging is performed
using a range of tools lowered into the hole on wire. This provides a continuous record of
rock properties with depth. Typical properties measured are electrical resistivity, porosity,
permeability, natural gamma radioactivity all of which may then be correlated with the
lithology of the extracted core.
Hydrogeological Testing applies when the borehole contains water (ie saturated
conditions). It is done by isolating a length of borehole using inflatable “packers” and
then measuring the response when water is pumped into or extracted from the isolated
portion. In simple terms one wishes to know how easily water can flow in and out of the
isolated portion – it is a measure of permeability and, thus, of the ease with which
contaminants might migrate away from a disposal. This may need to be done over the full
length of the borehole. Alternatively, sections of interest may be identified from
examination of the extracted core or from the geophysical data.
Groundwater sampling and testing is important for two main reasons. First, because of its
effect on the corrosion of the waste packages. We especially need to know the oxidation
potential and the concentrations of ions that could cause accelerated corrosion or
degradation of concrete (eg chloride and sulphate). That makes it clear that we should
take steps to avoid contamination of the extracted water. Oxidation potential may be
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difficult to measure and because the mineralogy of the host rock will have an influence
on the groundwater chemistry, it may be possible to infer the oxidation potential from the
mineral assemblage. The second reason is to provide for an understanding of the origin of
the water and its age. If it transpires that the extracted water has been in position for
hundreds of thousands of years, for example, we can have some confidence that
groundwater movement is very slow. In this way groundwater chemistry can be sued to
provide information about the origin of the water and its rate of flow.
21 Example of a wireline log that includes data (bottom to top) for borehole diameter
(calliper log), density and resistivity (Wikipedia)
22 A site characterization programme will yield an enormous quantity of information and
interpreting the results is a specialist job. A technical specification will be needed to
explain to the contractor what has to be done. The regulator will expect the licensee to
have a good understanding of the characteristics of the site so the contractor will provide
a description of, for example, the geological formations found at the site and the way
these have evolved. Any associated uncertainties will need to be described. In relation to
the disposal borehole you will be looking for information that might influence its location,
its construction (principally rock competence – strength and in-situ stress) and the
geological formation in which the disposal zone is to be located.
Site characterization data and their uses
24 This section describes the information generated by site characterization and its
potential uses in the context of the BDC. This is divided into five sub-sections:





Site selection
Borehole design optimization
Borehole construction
Post-closure safety
Understanding of the geological evolution of the site
Site selection
25 Site location may be determined by site properties but is more often a political
decision.
Optimization of the borehole design
26 An important task is to optimise the design to make best use of the site. The number of
disposal packages (and hence the length of the disposal zone) derives from the number of
sources (the inventory) and the package-to-package spacing but where, within the site,
the disposal borehole is to be located and where, within the borehole, the disposal zone is
to be placed will depend on the site properties. The aim will be to choose a suitable host
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geology and a depth of disposal that respects the minimum 30m depth and avoids the
phreatic surface and any loss of isolation of the waste due to long-term erosion.
Borehole construction
27 Construction of the disposal borehole will be facilitated by the presence of competent
rock ie rock that does not disintegrate or fracture heavily when the borehole is drilled.
Such effects can be brought about by the presence of low strength rocks caused, perhaps,
by extensive weathering (often near the surface) or by high rock stresses at depth that
may cause borehole breakout. Warning of such possibilities will be provided by the
results of site characterization.
Post-closure safety
28 The BDC is close to a one-size-fits-all design that, using a combination of stainless
steel and concrete, relies heavily on long-term physical containment
Important inputs for post-closure safety are parameters that might reduce the integrity of
this stainless steel and concrete system. These include the presence, at disposal depth, of
groundwater with high oxidation potential and high concentrations of ions that could
degrade either the stainless steel (eg chloride) or the concrete (eg sulphate). It follows
that geochemical characterization of groundwater is a key activity.
If the containment system were to fail, radionuclides would escape into the surrounding
environment and dissolve in the groundwater or be present as colloidal material.
Movement of this groundwater could transport these radionuclides into the human
environment. It follows that the site characterization programme needs to provide
information on the flow of groundwater through the geosphere and the corresponding
transport of radionuclides.
General understanding of the evolution of the site
29 The licensee will be expected to have a good overall understanding of the geological
evolution of the site because it is this that informs him about its likely future evolution
Hence key inputs could relate to issues such as past climate conditions at the site,
seismicity and geomorphology – the last as an aid to understanding the future rate of
erosion at the site which could affect the minimum depth of disposal.
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.
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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
30 The lesson has covered

Site characterization in the context of the step by step development and
implementation of the BDC and the stages in the development of the safety case
used to underpin the decision making process

The nature of site characterization, its multi-disciplinary nature and the
requirements that this imposes in terms of overall control and coordination

Site characterization techniques, emphasizing the need to collect and use existing
information. Site characterization techniques are then broadly divided into
surface-based geophysics and down-hole exploration.

Five potential uses of site characterization data are described for (1) site selection,
(2) borehole design optimization, (3) borehole construction, (4) post-closure
safety, (5) understanding the geological evolution of the site.
QUESTIONS
1. Which of the following may be said to be a purpose of site characterization in
relation to the BDC?
a) To provide data to support the safety case
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b) To provide data that will help with borehole construction
c) To demonstrate that disposal will meet the regulatory requirements
d) To gain an understanding of the geological evolution of the site
Answer a), b) and d). Item c) is a purpose of the safety case not site characterization.
2. What are the main design parameters that will be determined by the information
derived from site characterization for the BDC?
a) Number of packages to be disposed
b) Host geology
c) Maximum depth of disposal
d) Waste package material
Answer b) and c). a) is derived from the inventory; d) is fixed
3. Which of the following types of work are likely to be used in site
characterization?
a) Desk studies
b) Surface-based geophysics
c) Exploratory boreholes
d) Interpretation
e) Monitoring for environmental radioactivity
Answer: a) to d). Item e) is incorrect: environmental monitoring is not part of
site characterization
4. Why is it important to characterize groundwater chemistry for the BDC?
a) Because groundwater could have adverse reactions with the concrete
backfill
b) Because groundwater chemistry will determine the rate of package
corrosion
c) Because the groundwater could be slightly radioactive or contain toxic
elements
d) Because knowing the origin of the groundwater will provide information
about its age and flow rate
Answer a), b) and d). c) is of minor importance
5. What are the main uses of down-hole wireline logging?
a) To provide information about groundwater flow;
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b) To provide a detailed characterization of the lithology and properties
of the intersected rocks;
c) To make best use of available information
Answer b) is correct.
6. What are the main purposes of down-hole hydrogeological testing?
a) To locate the phreatic surface (water table)
b) To determine the water flow characteristics of the surrounding rocks;
c) To provide information about the chemistry of the groundwater.
Answer b) is correct.
7. Geophysical techniques that are commonly used for site characterization are:
a) Seismic surveys
b) Resistivity surveys
c) ground penetrating radar
Answers a) and c) are correct. Resistivity surveying is a geophysical technique
but it is seldom used in site characterization because it yields results about the
very near surface
8. Surface-based geophysical surveys provide information about:
d)
e)
f)
g)
lithological contrasts
geological structures such as faults
depth to groundwater
water flow in the subsurface environment
Answers a) to c) are correct.
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