Decommissioning strategies

The main objectives of decommissioning are to place nuclear facilities that have reached the end of their useful life in such a condition that they pose no unacceptable risks to workers, to the public and to the environment, and to reuse materials, facilities and sites for new purposes.
In this perspective, simply abandoning a facility after a permanent shutdown is not considered an acceptable alternative to decommissioning because, if not decommissioned, it could degrade and potentially present an environmental hazard in the future. Furthermore, the cost of not taking prompt action is normally also high.
However, different decommissioning options could be identified and, case by case, the optimum strategy could be selected in accordance with national policies and taking into account technical as well as non-technical needs, priorities and constraints.
Policy for decommissioning of nuclear facilities consists in a set of established goals or requirements for the safe, effective and efficient decommissioning of such facilities. The national policy usually includes a specification of national roles and responsibilities, and is mainly established by the national government. Furthermore, the national policy should define safety and security objectives fostering the safety culture, identify the main approaches for the decommissioning of nuclear facilities, make provisions for the management of the generated radioactive waste and for public information and participation.
The strategy reflects and elaborates the goals and requirements set out in the policy statement. Strategy is the means for achieving the goals and requirements set out in the national policy for the decommissioning of nuclear facilities. It is normally established by the relevant facility owner or operator.
Decommissioning can be a complex activity, whose implementation can require a series of stages, according to the graded approach, lasting several years. To achieve decommissioning goals, some basic actions are normally required but the specific approach to decommissioning could be different from country to country. Some countries have chosen to decommission their nuclear facilities as soon as they cease to generate nuclear energy or, in general, their operation. This option is named immediate dismantling and it normally starts within a few years from the shutdown of the

facility, giving time for transition from operational status to decommissioning status and allowing for removal of spent fuel, in the case of a reactor, and of residual radioactive waste from the facility to another new or existing licensed facility (temporary or final disposal facility). The equipment, structures and parts of a facility containing radioactive contaminants are removed or decontaminated to a level that permits the facility to be released for unrestricted use, or with restrictions imposed by the regulatory body. This strategy implies prompt completion of the decommissioning project by external experienced staff, taking advantage of the knowledge of the existing workforce.
Other countries delay the process for a number of years. In the deferred dismantling (also called safe storage or safe enclosure), after removal of spent fuel and some peripheral items, the facility is kept in a state of safe enclosure, for a period of 30 to 100 years before dismantling. This period of time enables the radionuclides mainly responsible for the radiation dose to workers to decay (usually after some half-lives, 5-6 times) and thus an easier access to structures. This involves the need, and thus additional costs, for a control of the facility throughout the “safe enclosure” period to ensure the necessary level of safety. However, the waiting time enables the proper training of the facility workers, even if some knowledge about facility operational lifetime could be lost. Parts of a facility containing radioactive contaminants are either processed or placed in such a condition that they can be safely stored and maintained until they can subsequently be decontaminated and/or dismantled to levels that permit the facility to be released for unrestricted use or with restrictions imposed by the regulatory body.
Other countries convert their facilities into a form of waste disposal, after ensuring that they are safe. This strategy is defined as entombment and it is the case for encapsulating the facility on site and keeping it isolated until radioactivity decays to a level permitting the unrestricted release of the facility, or release with restrictions imposed by the regulatory body.
Figure 1 shows a view of the old sarcophagus of the 4th block of the Chernobyl Nuclear Power Plant in 2007: it is a massive steel and concrete structure covering the nuclear reactor building, built to limit radioactive contamination of the environment following the 1986 Chernobyl disaster

by encasing the most dangerous area and protecting it from weathering.

Figure 1 - Chernobyl Nuclear Power Plant showing the sarcophagus in 2007. Credit: Vadim Mouchkin/IAEA - CC BY-SA 2.0

The operating organization is normally responsible for defining the decommissioning strategy on which the planning for decommissioning will be based. The strategy should be consistent with national decommissioning and waste management policies. However, often the approach adopted by a country lies somewhere between these categories: for example the chosen strategy could be the partial dismantling followed by a period of safe enclosure for the remaining parts. Indeed, the choice of the approach depends on many factors, such as:

• policy, regulatory and socio-economic factors, including national policy, regulatory arrangements (for ensuring public and occupational health and safety, environmental protection and safe waste management), end-states for decommissioned facilities (site reuse for a new nuclear facility, redevelopment with restrictions on future site use or brown field, site release without restrictions or the so-called green field), costs, availability of spent fuel and radioactive waste management systems, knowledge management and availability of qualified staff and social and community aspects;
• technological and operational factors, including radiological aspects, availability of technology for decommissioning and physical and radiological state of facilities;
• long-term uncertainties, including evolution of regulatory standards, costs and fund management, evolution of facility ownership and availability of qualified staff, availability of radioactive waste disposal facilities, evolution of policy on future of nuclear power.