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

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

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 as well as radiation protection.

Decontamination techniques

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. Further information on release can be found in the article Waste Management.

Dismantling techniques

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.

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.

Radiation protection

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.