Decommissioning in the nuclear industry

The nuclear industry lays on the operation of different kind of facilities, that have the same complexity of some industrial plants but also some peculiar features, such as the presence of radioactive materials. Exploitation of nuclear technologies has left a legacy that needs to be addressed. Indeed, at the end of their service life, these facilities have to be managed in such a way that any radioactive material must be disposed of without becoming a danger to the public or the environment.
Thinking to the nuclear fuel cycle (see Figure 1), each step results in a facility that needs to be decommissioned. Decommissioning refers to the administrative and technical actions taken to remove all or some of the regulatory controls from an authorized facility, so that the facility and its site can be reused. It includes actions to reduce the levels of the residual radioactivity, ensuring the protection of the public and the environment.

Figure 1 - Nuclear Fuel cycle. Credit: Nuclear Regulatory Commission, Infographic from 2018-2019 Information Digest, NUREG 1350, Volume 30. Published in August 2018 - CC BY 2.0

Decommissioning of a conventional power station is a challenge because the plant consists of large and complex buildings, containing reinforced concrete, steel or chemically hazardous materials. In a nuclear facility, the additional presence of radioactive material prevents from using traditional dismantling and demolition methods, and implies adaptation in the operating procedures and adoption of actions for workers safety specifically designed. It is therefore necessary to preliminarily decontaminate the material to reduce the radioactivity to levels where the workers/operators can work or, if this is not possible, to work remotely by robotics. It is of paramount importance to identify and quantify all the risks related to the decommissioning activities, to propose mitigation actions able to reduce the risks to acceptable levels: decommissioning has to be conducted using the Defense in Depth¹ principle (DiD) for safety, appropriate to the degree of hazard. The operator (licensee) is responsible for safety (radiological and nonradiological). Consequently, the licensee is required to perform a detailed safety assessment and take protective measures, in accordance with a graded approach². The ALARA process is used to this purpose. ALARA is an acronym for ‘As Low As Reasonably Achievable’ and it is a decision-making tool with the goal to maximize the total benefits of the radiological protection provisions relating to an activity that is likely to expose members of the public to ionizing radiation. This occurs when both the costs of radiological protection and detriment are minimal.
During the whole decommissioning process the licensee has to establish a safety culture within the organization, that is an assembly of attitudes in which safety is an overriding priority and not a blind compliance with rules and regulations

imposed by a regulatory body.
The decommissioning starts with a proper characterization of the facility in order to obtain a detailed overview of the actual condition of the plant and a detailed inventory of all the radioactive and non-radioactive materials present.
Then, the proper decommissioning strategy is selected. Indeed, three different strategies can be adopted: immediate dismantling, deferred dismantling and entombment. The option “no actions” is not acceptable.
Immediate dismantling starts shortly after the shutdown, this is the fastest option, but it is more difficult to perform due to the radioactivity levels involved. Otherwise, it can be decided to wait some time, so that the radioactivity naturally decreases. This option is the deferred dismantling: the facility is placed in a safe storage condition for a period of time up to several decades, then the other steps of decommissioning are performed. Entombment is the strategy in which the remaining radioactive material is permanently encapsulated on a site, but the site will not be available for other purposes. Decommissioning follows a series of steps called decommissioning plan (see Figure 2).

Figure 2 - Flow chart of typical decommissioning project.

The overall decommissioning strategy to be adopted should be identified as early as possible in the planning process. This can be updated on the basis of national policy, socio-economic factors, technological advancement, national regulations, availability of radioactive waste disposal, condition of the plant and so on.
The decommissioning plan requires the definition of the surveillance and maintenance program, the systems necessary for the facility maintenance, the system used for the dismantling procedures, the staff required and their qualifications, the financial resources available, the waste management, and so on. This plan has to be approved from the regulatory body in order to obtain a decommissioning license. This approval is a complex process that needs long time: specific licenses have to be requested and obtained for the different components of the facility according to a step-by-step approach. Furthermore, the decommissioning license is continuously subjected to the authority control that, by focused actions, verifies the fulfillment of the requirements.
The decommissioning involves the decontamination of the structures and components of the plant: for example, in this stage chemicals are often introduced to remove or

reduce residual radioactivity. Then, dismantling can be performed: this includes the removal and/or recycling of the radioactive materials and the isolation and shut down of the unrequired safety-related systems. Dismantling could start from the outer parts of the reactor (the non-radioactive turbines etc.) and working inwards towards the reactor pressure vessel and core. Otherwise, it could be carried out from inside out. This step requires proper equipment and may also require further decontamination.
Finally, after all the plant equipment is removed, the building itself is demolished (see Figure 3 ans 4). At this point, little or no radioactive material remains and demolition is completed using normal civil engineering practices.
A final radiation survey and a final report are prepared in order to complete the decommissioning procedure.

Figure 3, 4 - Decommissioning of the Merlin Research Reactor in Germany, IAEA Imagebank. Credit: Forschungszentrum Juelich GmbH - CC BY-SA 2.0

In general, decommissioning results in large volumes of radioactive waste. Such waste is classified according to the type and quantity of radioactivity they contain. Fortunately, most of the waste coming from nuclear decommissioning activities belongs to the low activity level waste category, such as scrap metal, paper and plastics. Each country defines a proper national waste classification by interpreting waste classification proposed by IAEA, and in each decommissioning project disposal routes have to be defined accordingly to safely manage such nuclear waste.
Decommissioning involves very specific tasks and requires several competencies to meet the requirements given by authorities and inspectors. Radiochemists are fundamental to support decommissioning activities in order to optimize the process and proceed safely.