Commissioning is the sequence of actions to put a new reactor into service.
Decommissioning is the sequence of actions to remove all the radioactive material and other plant and equipment with the intention of releasing the site for unrestricted use. For every reactor, the first action is to remove all the fuel from the reactor core. This removes 99 per cent of the radioactivity and eliminates the possibility of a reactor accident that could cause a release of radioactivity.
There are then two options:
1. Immediate dismantling
The advantages are personnel with site knowledge are available, there's no requirement for ongoing maintenance and surveillance, and the site can be reused immediately.
The disadvantages are the plant is more radioactive, the dismantling may require expensive remote-handling equipment and there is the potential for higher radiation doses to workers.
The policy in France is immediate dismantling. France built a series (34) of 900 MWe reactors in the 1970s and 1980s, the first two of these, at Fessenheim, were shut down in 2020 and are expected to be decommissioned in five years. France has central facilities for the disposal of radioactive waste.
2. Deferred decommissioning ("Safstor")
The advantages are the reduced radioactivity of materials, lower radiation doses to workers, less demands for remote handling equipment, waste is easier to handle, and newer technologies for dismantling may have been developed. The disadvantages are that the owner is liable for the safety of the site for an extended time, ongoing maintenance is required, personnel with a detailed knowledge of the facility may have left and the site cannot be reused for some time.
This is the usual option for multi-reactor sites where the first reactor to be shut down is put into "Safstor" and decommissioning of the whole site starts when the last reactor is shutdown.
Many large power reactors have been decommissioned in the United States. An example is the Maine Yankee 900 MWe PWR (Pressurised Water Reactor) which operated from 1972-97 and decommissioning was completed in 2005 (eight years after shutdown) at a cost of $500 million.
Reactor operators pay into a fund during the operating life of the plant so that the money is available to cover the cost of decommissioning.
There is a good example of decommissioning in Australia: The Moata research reactor operated at Lucas Heights from 1961-95. In 2009-10 the internal components were removed and the concrete shielding cut up with a diamond saw and stored on site.
The building has been reused. Decommissioning Moata cost A$4.15 million.
Decommissioning of a reactor after a major accident (Chernobyl, Fukushima) is of course a lot more difficult process.
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