Timing and Methods
When a nuclear reactor is to be decommissioned,
disposal of radioactive materials need to be carefully
considered.
The half lives of and the concentration of radionuclides
present in materials within a facility can vary,
and so these materials need to be classified and
disposed of accordingly.
To minimize doses to personnel, a suitable decay
period is required before activated materials
are handled for decommissioning or waste management
purposes.
Two main options:
One method still being researched:
Considerations
- Safety and environmental issues - The
radiological impact of the decommissioning method
on the public, workers and the environment is
of high concern and needs to be carefully assessed.
- Requirement to reuse plant and/or site
– Some sections of the plant may be decommissioned
earlier for new purposes. This therefore can
influence the time available for decommissioning
of the nuclear facility.
- Availability of waste disposal facilities
- In a few cases, no permanent disposal site
is available for storage of radioactive wastes
produced during the life of the reactor. This
can result in the delay of final dismantling
until suitable disposal sites are available.
- Availability of funding - Decommissioning
costs are estimated to demonstrate that sufficient
funding is or will be in place for completion
of the decommissioning project. The aim is to
optimize the dismantling sequence and timing
to minimize the total decommissioning cost.
- Stakeholder views - Different stakeholders
will have different views on the factors above.
The choice of the decommissioning method is
left entirely to the licensee. Licensers may however,
influence licensees to re-evaluate its decision
if the choice:
- Could not be completed as described
- Could not be completed within 60 years of
the permanent cessation of plant operations
- Included activities that would endanger the
health and safety of the public by being outside
of the licenser's health and safety regulations
- Would result in a significant impact to the
environment
Nuclear Waste in Decommissioning
Exempt waste (EW):
Contains negligible amounts of radioactivity and
materials in this category are excluded from regulatory
maintenance.
Low Level Waste (LLW):
LLW does not contain enough radioactivity to require
shielding when being handled or transported. It
does, however have enough radioactivity to remain
separate from exempt waste and to be stored separately
(usually shallow land burial). It contains mainly
small amounts of short lived radioactive materials.
The bulk of the waste from the nuclear facility
is LLW, which makes up 90% of the volume of nuclear
waste but only contains about 1% of the total
radioactivity.
Intermediate Level Waste (ILW):
This category of waste requires shielding, using
materials such as concrete, lead or water and
is usually stored underground.
If the ILW contains more than 4000 Bq/g of alpha
emitting radionuclides with a half life greater
than 30 years it is categorized as long-lived
waste and is dealt with more sophisticated handling
and disposal methods than short lived ILW materials.
High Level Waste (HLW):
Contains fission products and transuranic elements
from spent fuel.
HLW contains highly radioactive, long-lived alpha
emitters which also requires cooling as well as
shielding, as this waste can generate heat of
over 2kW/m3. 95% of total
radioactivity from overall nuclear waste is contained
in this category. HLW is removed during the regular
defueling of the reactor.
Early Site Release
Early Site Release is the decommissioning process
in which all components, structures and portions
of the facility are removed from the site or decontaminated.
The property is then able to be released for unrestricted
use. This method contains more ILW, which is either
shipped away to be disposed of or contained temporarily
on site. Early Site Release is often favored when
suitable disposal sites are available.
| Advantages |
Disadvantages |
| Prompt method and so the facility
or site becomes available for earlier reuse. |
More ILW involved due to less
time available for radioactive materials to
decay. |
| Need for long term security,
surveillance and maintenance is eliminated.
|
Higher worker and public doses
during transport of waste materials. |
| Greater availability of knowledgeable
facility workforce who are able to aid in
the decommissioning of the plant. |
Complications may arise if the
spent fuel must remain on site. |
| Greater assurance in the availability
of radioactive waste disposal sites. |
Larger initial commitment of
money. |
| Generally lower estimated
costs compared to the Safestore option. Main
reason for this is due to non-escalation and
the lesser risk of uncertainties. |
Potentially larger
amounts of radioactive materials are present
than the Safestore option, resulting in a
larger commitment of disposal site space. |
Safestore
The facility is prepared then sealed up and maintained
for safe storage. Radioactive materials are given
time to decay to safer levels for final dismantling
and clearing of the site.
Depending on the half-lives and the concentration
of particular radionuclides involved, the facility
can be left in storage for up to 60 years for
NRC licensed reactors.
Once the radioactivity of materials have decayed
to a satisfactory level, the facility is taken
apart similar to early site release.
| Advantages |
Disadvantages |
| Substantial reduction
in the levels of radioactivity within the
facility. |
Site is unavailable
for reuse during the storage period. |
| Reduced worker dose than Early
site release option. |
Maintenance, security, and surveillance
of the site must be regulated. |
| Potential reduction in the amount
of waste volume than Early site release. Therefore
less disposal space required. |
Shortage of personnel familiar
with the facility to support final dismantlement
and decontamination. |
Reduction in waste shipments
hence exposure to public is reduced.
Lower initial funding commitments. |
Increase in costs for uncertainties
regarding the availability and the future
costs of LLW sites at the time of final dismantlement. |
| Storage period allows
time for developments for permanent waste
disposal sites, particularly for HLW repositories. |
Due to price escalations,
final dismantlement of the facility may result
in higher total costs compared to Early site
release. |
Entombment
The facility is cleaned out and encased in a
long lasting material, such as concrete. The entombed
structure is then maintained and surveillanced
until the radioactive material decays to levels
permitting release of the property for unrestricted
use in a reasonable time period, generally in
the order of 100 years.
This is a cheaper alternative to Safestore, due
to greater reduction in radioactive waste materials.
It generally has the same advantages and disadvantages
to Safestore.
A few radioactive isotopes found in nuclear reactors
have concentrations exceeding the limits of safe
levels of radioactivity even after 100 years (usually
the case for larger reactors), and hence may not
be seen as a feasible option under current regulations.
Three small demonstration reactors have been
entombed; Piqua (Piqua, Ohio), Hallam (Hallam,
Nebraska) and Bonus (Punta Higuera, Puerto Rico),
and the results of this method are still pending.
Currently, no licensees have proposed the Entomb
option for any of the power reactors undergoing
decommissioning.
Examples of decommissioning strategies taken
for different reactors can be found on the NRC
website.
References:
http://www.world-nuclear.org/wgs/decom/intro.htm
http://www.arpansa.gov.au/pubs/rrrp/op/SAR/ch19.pdf
http://www.nei.org/doc.asp?catnum=3&catid=278
http://www.eu-decom.be/faqs/faqs2.htm
http://www.wct.doe.gov/documents/sites/oh/piqua/fact_sheet/piqua.pdf
http://journals.iranscience.net:800/Default/www.asme.org/pro_dev/akerandtaboas.pdf
http://www.orau.gov/ddsc/Resources/guidance/nrcreg1628.pdf
http://www.world-nuclear.org/wgs/decom/database/php/qry1_results_allcountries.php
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