Stepwise dismantling of nuclear facilities
The example of a nuclear power plant is used to show how nuclear facilities are dismantled immediately.
Before the dismantling work starts, the plant is largely in the same technical condition as in post-operation. Fuel and operational waste have, as far as possible, been removed.
In preparation for decommissioning, a detailed overview of the plant’s radioactive inventory is produced. For this purpose, further measurements are taken in all areas of the facility and samples are collected and analysed.
On this basis, the final plan for the dismantling can be prepared. The radiological characterisation is continuously added to and updated in order to support dismantling.
Dismantling in stages
The following figures explain the dismantling by means of a simplified section through a nuclear power plant. In each dismantling step, the systems, structures and components highlighted in red are removed.
Dismantling in stages
One possible strategy is to start the dismantling of components in areas with low contamination and then to continue in areas with higher contamination; dismantling is carried out “from outside to inside”. In the vacated areas, e. g. parts of the turbine building, equipment needed to segregate, decontaminate and reprocess the waste and residues can then be installed.
Disassembly of contaminated systems and components
Almost all removed parts are disassembled into easily manageable pieces and decontaminated if necessary. Decontamination can take place, depending on the circumstances, before or after disassembly. Each piece is examined for radioactivity. This process is reviewed by independent experts. Based on this assessment, a decision is taken on whether the piece has to be conditioned and handed over as radioactive waste, or whether it can be further treated and cleared, which means that it can be fed back into general substance cycles as non-radioactive material.
Computerised systems ensure continuous tracking of the pieces from the place of their dismantling and subsequent treatment steps until their removal from the plant. The logistics needed for residue and waste management are substantial and represent a significant cost factor.
Key installations, such as ventilation, power and media supply, are kept in operation or ready to use through all steps of dismantling. These systems are available from the operating phase and their continued operation is essential. Depending on the requirements, they will be retrofitted or replaced to meet the new specifications.
Disassembly of activated components
The activated components include the reactor pressure vessel (RPV), the near-core area (biological shield) and the adjacent systems and components within the containment vessel.
These dismantling steps are largely carried out by remote control in order to avoid any human presence in areas of high radiation. Much of the work is carried out under water, since water is an effective radiation shield. In some cases, major components are removed from the plant for decay storage and subsequent segmentation. The materials arising from this dismantling step are, for the most part, activated. They account for a significant proportion of the radioactive waste produced during decommissioning.
After complete vacation of the building, only residual contamination remains on the surfaces of the building structures. In the last step, they are decontaminated and re-examined for residual contamination. This step is repeated until the contamination is lower than the limits specified in the Radiation Protection Ordinance.
After successful clearance, the building can be released from regulatory control and can then be conventionally used or demolished.