In the United Kingdom, the Nuclear Decommissioning Authority (NDA) is a non-departmental government body founded in 2005 to manage the UK civil nuclear wastes. The NDA defined a number of stages of the decommissioning process as illustrated in Figure 1.3

Whilst each plant and site may have their own characteristics, this series of generic stages provide a good overview of decommissioning activities from the end of operations through to site closure.

The first stage after normal plant operations cease is known as Post­Operational Clean Out (POCO). In the case of a reactor site this is removal of the fuel from the reactor; for other facilities it typically requires existing equipment, with only minor modifications, to be used by plant operators to move most of the radioactivity out of the plant. POCO will typically use only chemicals (and equipment) that were used during plant operation, and utilise existing waste and effluent treatment routes.

The next stage, Initial Decommissioning, removes or fixes loose radioactive material within pipework and vessels to reduce dose rates and ease access to facilitate further decommissioning tasks. This may use special cleaning che­micals and so require additional effluent treatment equipment. The transition from POCO to initial decommissioning may involve changes in staff and controlling procedures and so is potentially problematic; particular advice is available for making this transition.4

The Surveillance and Maintenance stage applies only to facilities that are not in a passively safe state following Initial Decommissioning and which require a period of Surveillance and Maintenance prior to Interim Decommissioning. In these cases certain plant systems would remain operational (e. g. services,
radiological monitoring and ventilation systems), maintenance regimes would remain in place and some plant enhancement may be necessary to maintain building structural integrity.

The Interim Decommissioning stage is when the work required to convert a facility to a passively safe state is carried out. Typically this would involve removal of residual radioactive inventory from the plant, dismantling and removal of plant and equipment, removal of non-radioactive facilities and, where possible, reduction of the building footprint. At the end of this stage, the plant will be in a passively safe state with systems and processes de-energised, deactivated and drained.

The Care and Maintenance stage allows limited monitoring and observation of a facility prior to final decommissioning. It can be distinguished from sur­veillance and maintenance because few resources are required. Typically care and maintenance might be used to allow levels of radioactivity to decay, but it is also possible that the facility has been made sufficiently safe that resources are most effectively used on other facilities at a given time. Effort to maintain the plant in this state would be minimal, confined to routine monitoring and sur­veillance of the facility and the building fabric with very few, if any, operators dedicated to the plant on a full-time basis. It is important the facility is ade­quately enclosed during this period and some guidance for this is provided.5

Final Decommissioning will bring a plant or facility to its agreed end-point, including final site clearance but excluding any contaminated land or groundwater remediation. This includes final dismantling of installed plant and equipment, strip-out of any remaining facilities within the building and demolition of cells, internal structures and the building envelope. All wastes generated will be disposed of or stored awaiting disposal. The end-point reached at the completion of this phase will be such that any danger or hazard that may remain to workers, the general public or the environment is at a minimum level consistent with the principles of ALARP (As Low As Rea­sonably Practicable).

Subsequent work would consider both Groundwater Remediation and Contaminated Land Remediation prior to site close-out.