A template for lecture notes is provided below:
Lecture Notes for Section X, Borehole Sealing and Monitoring
1. Introduction
The purpose of this lesson is to explain how a borehole disposal facility is closed and
progresses into post-closure monitoring. The objectives of the lecture are to define
closure terms and describe the sequence of events from end-of-operations through sealing
the borehole to institutional controls that include monitoring.
2. Scope/Structure
The scope of this lesson is the sealing of a borehole, the establishing of institutional
controls (initially active than passive) and the monitoring needed as the facility
transitions from operations into the post-closure mode.
The structure of the lesson is below:
•
•
•
•
•
•
•
Presentation Resources
Regulatory Framework
Definitions
Borehole Sealing
• What is sealing?
• How is it done?
Institutional Controls (IC’s)
• Needed?
• Active vs. Passive Controls
Post-closure Monitoring
• Why monitor?
• Generic requirements for post-closure monitoring,
• What to monitor? And where?
Summary
3. Concepts
The key concepts in this lesson are: sealing the borehole, institutional controls and
monitoring of the facility during active institutional controls.
Scheme of a Borehole Disposal Facility:
Closure starts with the removal of the upper sections, at least the top 30m, of casing. If
HDPE casing was used, it might be possible to unscrew the casings. If steel casing was
installed, down hole cutting may be needed. The force needed to withdraw the casing is
highly dependent on the geology of the area and how much material is lodged against the
casing. If the geology indicates a lot of material may lodge against the casing, steel
casing should be used as it is significantly stronger than plastic pipe.
After the casing has been removed, enough grout is placed in the borehole to cover the
top of the disposal zone casing. An anti-intrusion plate, made of steel, should be fitted
above the disposal zone casing to redirect any intruding drill off into the surrounding rock.
The size of the plate is related to the inside diameter “D” of the borehole. The plate is
rectangular with length “D” and a breadth of 0.71D. With these dimensions, the plate can
be placed at a 45° inside the borehole. Plate thickness should only need to be 10mm.
After placing the plate, it should be surrounded with additional concrete using a tremie
pipe or similar.
Final borehole sealing is accomplished by filling the hole with concrete until a few
meters (e.g., 2m) below ground surface. The remaining few meters can be backfilled
with soil. In this manner the exact location of the borehole is not easily determined,
except by disposal site records.
After facility closure, the regulator may require institutional controls at the site.
Institutional controls are optional. One of the main goals of geological disposal of
radioactive waste, including DSRS, is the system be passively safe, without human
intrusion to maintain safety. So why are active institutional controls needed.
Institutional controls are the prerogative of the regulator. Active controls typically run for
at least five years. If for no other reason, the environmental sampling and analysis
performed will work to help validate the models used to predict post-closure behavior of
the disposed radionuclides. It is vital that pre-disposal baseline levels of radionuclides be
obtained so post-closure results can be compared to pre-operational values. Other active
institutional controls include maintaining the fence around the facility and perhaps a full
time guard force. Active measures can be very expensive and not absolutely needed.
Passive institutional controls include requiring government land ownership, the use of
warning markers in the upper portion of the vadose zone, lodging facility information in
numerous archives.
Often during active institutional controls, the environmental monitoring program is
continued. In this manner, historical information is available about the level of
radioactivity in the air, soil, vegetation on site, surface water (if available), and
groundwater. Post-closure monitoring is necessary to ensure the integrity of disposal.
Plausible exposure pathways are:
• Leaching from packages and transport to groundwater.
• Airborne release of gaseous radionuclides such as radon.
The main objectives of post-closure surveillance and monitoring:
• Show compliance with reference levels established by RB for public
health and environmental protection,
• To confirm, as far as possible, relevant assumptions made in the safety
assessment,
• To provide indications of any malfunctioning of the containment leading
to unpredicted releases of radionuclides, and
• To provide reassurance to concerned persons living in the vicinity of the
waste disposal facility, thus building public confidence.
Initially in the active institutional control period, the sampling frequency should stay the
same as during the operational phase. Depending on the length of the active phase, the
regulator can relax the frequency based upon the result trends. If the analysis reveals no
leakage from the boreholes, frequency can be reduced (e.g., monthly to quarterly). At
some disposal sites, some analytes are only sampled once a year. In this case, sampling
could be done every three or five quarters, to set up a rotation, to evaluate seasonal events
at the site. Sampling terminates at the end of the active institutional control phase. By its
nature, passive controls don’t involve human intrusion or activities.
In order to compile a list of nuclides to monitor, the site operator & regulator need some
physical properties of the site, such as distance and travel time to groundwater for each
isotope and groundwater travel time to the point of compliance. While these parameters
will not impact long-lived mobile isotopes, a rapid travel time to groundwater and the
point of compliance will cause many short-lived isotopes to be included in the monitoring
program.
The radionuclides to be sampled need to be part of the inventory of the facility. Typical
DSRS that may be at the facility include: Pu-238, Pu-239, Am-241, Cs-137, Sr-90, Co60, and Ra-226/228 and Radon.
In Washington State, our shallow land burial site is about 90 meters above the
groundwater which is stagnant. The site started operations in 1965, but the facility’s
environmental monitoring program did not start until 1988. Before the site program was
instituted, nearby Department of Energy wells were used to evaluate any leakage from
the site. The facility’s monitoring program database now has 27 years of data. We’ve
seen an occasional non-reproducible isotopic find (above background). The only
consistent isotope in groundwater is tritium from past Dept. of Energy work to the west
of our facility.
In summary, when the facility places its last disposal package into the borehole, the
closure work becomes the primary focus of site operations. The facility must seal (all)
the boreholes to prevent intrusion and discovery. Institutional controls may be used to
ensure the site is performing as modeled in the safety case. Initial controls may involve
active human involvement at the site, including environmental monitoring for any signs
of leakage from the packages entombed in the boreholes. When the site transitions from
active controls to passive controls, intrusive human activities cease.
4. Definitions
Sealing – the removal of the upper 30 m of casing, covering the disposal zone casing
with concrete, placement of an anti-intrusion plate, the addition of more concrete and
bringing the hole up to grade with several meters of top soils.
Monitoring – obtaining and analyzing samples for radionuclides that may impact human
health and/or damage the environment and other physical facility aspects (e.g., fence)
needed during the active institutional control period.
DSRS – disused sealed radioactive source.
5. Additional information
Scheme of a Borehole Disposal Facility:
Any additional information should be detailed in this section. Additional information may
include examples or case studies from existing situations in the world, which supports
any of the concepts highlighted in the lecture, may include photographs or additional
figures which depict concepts learned, etc.
6. References
•
•
•
International Atomic Energy Agency, BOSS: Borehole Disposal of Disused
Sealed Sources – A Technical Manual, IAEA-TECDOC-1644, IAEA Vienna
(2011)
International Atomic Energy Agency, Borehole Disposal Facilities for
Radioactive Waste, Specific Safety Guide No. SSG-1, IAEA Vienna (2009).
International Atomic Energy Agency, Disposal of Radioactive Waste, Safety
Standard Series No. SSR-5, IAEA Vienna (2011).
7. Summary of the Lesson
A borehole disposal facility may operate for 10’s of years. All disposal facilities must be
closed at some point. This lesson examines those end-of-operations activities specific to
a borehole disposal facility.
•
•
•
Presentation Resources
Regulatory Framework
Definitions
•
•
•
•
Borehole Sealing
• What is sealing?
• How is it done?
Institutional Controls (IC’s)
• Needed?
• Active & passive controls.
Post-closure Monitoring
• Why monitor?
• Generic requirements for post-closure monitoring,
• What to monitor for?
• Where to monitor.
Summary
8. Questions and Answers
A list of 5-10 questions, which will be used to assess the effectiveness of the lecture in
relaying the intended information/concepts to the participants. Questions should not focus
on direct regurgitation of the presented material, but should force the participants to apply
the concepts learned and think critically. The questions may take any form: fill in the
blank, matching, multiple choice, problem solving, narrative, etc. The answers to the
questions should also be provided.
Question #1: What are the major steps in sealing a borehole?
Answer: The major steps in sealing a borehole are:
 Removing the upper (e.g., 30m) casing,
 Covering the top of the disposal zone casing with concrete,
 Installing an anti-intrusion plate at about 45 degrees,
 Adding more concrete to about 2-3m below grade,
 Filling the last few meters with site soil to hide the location of the borehole.
Question #2: What are the types of institutional controls? Describe each.
Answer: The two types of institutional controls are active and passive. In essence
active controls imply a human presence still at the facility undertaking maintenance
(e.g., fence repair) and monitoring activities (e.g., obtaining a ground water sample
for radionuclide analysis). Passive controls imply no human activities at the site.
These controls include the placement of warning markers, land owner restrictions
(e.g., government maintains custody) and the filing of information with various
archives, both public and governmental.
Question #3: What are the main media types sampled at a disposal facility?
Answer: The main media types are air, soil, vegetation and water (surface, if
available and groundwater).
Question #4: Describe the anti-intrusion plate? What is its purpose?
Answer: The anti-intrusion plate is a rectangular steel plate with dimensions
related to the inner diameter “D” of the borehole. The dimensions are “D” x 0.71D
x 10mm (nominal) thick. Its purpose is to deflect any drill into the surrounding
rock and prevent intrusion into a closed borehole containing radioactive waste.
Question #5: List five isotopes that may be sampled for during the “active” institutional
control period at a borehole disposal facility. What factors influence this list?
Answer: Isotopes that may be sampled during active control period include:
Pu-238, Pu-239
Am-241
Cs-137
Sr-90
Co-60
Ra-226/(228)
Radon
Others from inventory as appropriate.
The primary factor influencing the isotopes is whether the isotope was among the
radionuclides in the facility inventory. Other factors include the isotope’s half-live,
travel time to groundwater, and groundwater’s travel time to the point-ofcompliance (e.g., well).