There are a number of decisions to be made for each building that is to be decommissioned. This section outlines a set of criteria that can be used to select a preferred decommissioning approach.15 These criteria reflect the issues dis­cussed earlier. A range of decommissioning approaches that may be assessed by this approach are discussed16 18 and include issues of prompt decommissioning versus deferred decommissioning as well as selection of an end state.

The criteria are shown in Figure 5. The criteria form a hierarchy which, at the top level, has the three pillars of sustainability. These objectives are expanded into criteria and sub-criteria. Environmental impact is divided into radiological impact on man and the environment, resource usage, non-radiological dis­charges, local intrusion (which includes such factors as noise and visual pol­lution) and hazard potential (a measure developed as part of the NDA prioritisation process).2,19

These criteria provide a complete set of issues for the assessment of decommissioning projects, however, for any given assessment it may be useful to omit criteria which are not significantly different between all options under consideration, and it may also be convenient to subdivide other criteria to make best use of more readily available metrics.

It is important that the criteria are assessed over the whole lifecycle of each proposed decommissioning approach and that the end points are the same in each case in order to obtain the preferred solution.

If this approach is followed then optimum approaches to decommissioning will be obtained. Considering a complex site with many facilities or the UK as a whole, deferred decommissioning may be required in order to make the pro­gramme of work fit within the available annual budget. Such issues can be investigated using the same criteria and the impact in terms of cost, environ­ment and social factors assessed for leaving a plant in a state of surveillance and maintenance or care and maintenance.

By these methods programmes of work delivering maximum environmental benefit as quickly as possible can be derived.

Table 3 Environmental impacts of buildings between decommissioning stages.

Requirement for services Potential migration of

Potential for release in with associated discharges species from contaminated

maloperation and secondary waste land

Following completion of operations

Large quantities of mobile Water, steam, electricity species may be released in maloperation conditions

POCO

may produce liquid effluents. In each case suppression of the effluent produc­tion may result in formation of secondary wastes, such as filters, which then require ultimate disposal. Natural resources are also required for waste
packaging and construction of decommissioning equipment, some of which may not be able to be reused.

In between decommissioning stages, environmental impacts include the need for provision of services such as water and air and the treatment of the associated discharges; clean up and secondary waste generation; potential discharges in the event of a major building failure, perhaps as a result of a natural disaster or malicious action; and migration of species in the ground under the building from previous spillages.

8 Conclusions

Decommissioning is an important phase in the lifecyle of any nuclear facility, covering the transition from an operating facility to its planned end state. The United Kingdom has had a significant civil nuclear operation for many years, and as such has a significant decommissioning challenge associated with both fuel cycle plant and reactors. The decommissioning of legacy plant involves significant financial liabilities and involves large volumes of waste.

Whilst the process of decommissioning a facility can be described generically as a series of stages, the selection of a decommissioning strategy is typically plant, site or region specific. Decommissioning can be driven by many factors, ranging from a desire to reduce the hazard associated with an ageing facility through to a need to release the site for re-use. This paper has highlighted a series of criteria that should be considered when selecting a decommissioning strategy, of which environment and safety factors are of fundamental importance.