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A good amount of experience is available in Germany from previous work on
decommissioning of nuclear facilities. This applies e. g. to
decontamination and dismantling techniques, the radiation protection and also to waste management,
including the release of materials.
Disassembly of a steam generator in the Greifswald nuclear power
plant (KGR) for the transport into the neighbouring North
Storage Facility (Zwischenlager Nord-ZLN, cf. photo “interim or
decay storage”). Source: EWN GmbH
Radioactive substances on surfaces can be removed by chemical
and/or mechanical means (decontamination), in order to reduce the activity
and the dose rate of plant components.
After the final shut-down of a nuclear facility, entire systems are
usually decontaminated (e. g. rinsing of the primary circle of a
reactor), in order to reduce occupational exposure during
the subsequent dismantling steps.
Dismantled components, in particular metallic ones, are systematically decontaminated, so that they can possibly be
released if decontamination is successful.
For decontamination, chemical (e. g. with acids, bases or complexing
agents), mechanical (e. g. through scrubbing, brushing, grinding or
polishing), and electric (e. g. electro-polishing) techniques are
applied according to requirements. If necessary, techniques are
combined. In decommissioning reliable decontamination techniques are available for all relevant cases of application.
Decontamination techniques can be used also, if building structures are
contaminated directly. The surface is removed - usually by means of
milling techniques or needle guns. Cracks and dowels are chiselled out
to remove potential contamination. The measurements for release of
regulatory control are usually done in-situ. After being released, the building can be reused for further purpose or be demolished
conventionally.
When dismantling nuclear facilities, the existing components must be
dismantled and cut into manageable pieces, taking into account
that the wall thickness of the components may vary between a few
millimetres (e. g. in the case of pipelines) and several ten centimetres
(e. g. in the case of the reactor pressure vessel).
Comprehensive experience with the dismantling of components has been
gained in previous decommissioning projects in Germany. In
decommissioning reliable dismantling techniques are available for all
relevant cases of application.
Typically, thermal (e. g. welding or plasma burning) or mechanical (e. g.
cutting, sawing, or water jetting) cutting techniques are applied.
Blasting techniques can also be used to demolish steel and concrete
structures.
In order to guarantee the necessary radiation shielding remote-controlled or under water techniques are required in
many cases.
Remote-controlled manipulator carrier system for
dismantling activities in the Wiederaufbereitungsanlage Karlsruhe (WAK) reprocessing
plant which is under decommissioning. The robot arms
are equipped with the respective tool required for work steps (e.
g. gripper, scissors, saw). Source: Karlsruhe Research Centre
Basically, it has to be decided, wether existing techniques are to be used for
the dismatling of components, after they have been adapted to the
special needs of nuclear engineering, or whether purpose-built new techniques, which are e. g. supported by the Ministry of Research, are to be applied. In practice, both roads are
followed, depending on the task.
The dismantling of the reactor pressure vessel of the Kahl experimental nuclear
power plant Versuchsatomkraftwerk (VAK) can be mentioned as an example for the enhancement of existing technologies
and the introduction of new techniques. At VAK good experience
was made with the water abrasive jetting technique. This technique was
used for the first time to cut through wall thicknesses of 130 mm without any problems.
Despite decontamination, areas with high dose rates can exist in the
decommissioning of nuclear facilities. Therefore radiation protection
must be included in work planning. Experience with previous
decommissioning projects has shown that for German reactors the mean
annual occupational collective dose is lower during decommissioning
than during power operation (incl. revision doses).
For adequate radiation protection appropriate measures are
taken (e. g. shielding measures, housing, directed pressure difference or inhalation protection), which ensure that occupational
exposure - both due to direct radiation and
the release radionuclides - remains below the permissible limits laid
down in the Radiation Protection Ordinance (StrlSchV).
The discharge of radionuclides via exhaust air and waste water during
the decommissioning process is limited through technical measures in
such a way that the protection of the general public is guaranteed
according to the Radiation Protection Ordinance (StrlSchV). By
monitoring the emissions, fulfilment of the legal provisions is
proven. In practice the protection goals of the Radiation Protection
Ordinance (StrlSchV) are fulfilled due to the approved
limits for exhaust air and waste water.
Only a small part of all the matters resulting from the
decommissioning of nuclear facilities must be disposed of as
radioactive waste. The major part is either not or weakly radioactive and can be
released by permission of the
regulatory body from nuclear regulatory control, if certain conditions are fulfilled. The released substance is
then subject to the Kreislaufwirtschafts- und Abfallgesetz (law
on cycle economy and waste). This allows the reuse of materials.
Measuring facility for batch-wise clearance
measurements of materials from the decommissioning of the Greifswald (KGR) nuclear power
plant. The material to be measured is packed in boxes which are lifted
with a forklift onto a conveyor system. Then they are moved into the measurement
chamber with integrated scales. After the activity measurement has been carried out and
evaluated with a computer a decision can be made on the release of the material. Source: EWN GmbH
The Radiation Protection Ordinance (StrlSchV) includes in § 29 and
Annex III detailed radionuclide-specific information on unrestricted
release and on the release of solid substances and liquids of buildings
for demolishing and of scrap metal for recycling. Release of
materials is granted in writing by the regulatory body (release
notification), if the effective dose to an individual is limited to
about 10 Microsievert per calendar year. Compliance with the requirements of the
Radiation Protection Ordinance (StrlSchV) represents a particular
challenge due to the very low activity values. At the end of a number
of measures, a decision measurement is made to decide whether the
material can be released.
Materials that cannot be released from nuclear regulatory control must be disposed of as
radioactive waste. The decommissioning waste must be stored until a
repository will be available. On-side storage facility is typically applied.
Storage of dismantled steam generators
of the nuclear Greifwald (KGR) power plant in the North Interim
Storage Facility (ZLN). During the storage period of about 35 years
a part of the radionuclides decays. The steam generators are to be cut down
in the ZLN and a decision on the release or the disposal of the materials will be made. Source: EWN GmbH
Before the radioactive waste is delivered to a storage or disposal
facility, it must be properly conditioned. Radioactive waste arises in solid or liquid form. Solid
waste is, for example, compacted, melted or simply packaged in
containers, in dependency of its type. Liquid waste is converted into solid
waste forms, e. g. by drying / evaporating or cementing. The waste
is packaged e. g. into drums, steel containers,
concrete containers or cast-iron containers and stored.
For the disposal of the waste additional packaging (e.
g. packaging of drums into containers) may be required.
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