Decommissioning is the term used to describe the whole process which follows reactor final shutdown. It includes defuelling, dismantling plant and build­ings, transport of waste materials to authorised dis­posal sites and site clearance.

6.3Decommissioning stages

Three decommissioning stages may be identified:

• Stage I decommissioning relates to the period im­mediately following final shutdown. It is usually assumed that this is a planned operation rather than precipitated by a breakdown or accident, so that some small benefit may be realised by op­timising the management of the last fuel change. Gas-cooled reactor defuelling is a major task that could extend over several years to make best use of flasks, transport and reprocessing services. Dur­ing this time other tasks such as removal of opera­tional solid wastes, sludges and resins and removal of thermal insulation will be carried out using exist­ing plant and facilities by operating or decommis­sioning staff.

• Stage 2 has the objective of dismantling all plant and buildings external to the reactor biological shield or prestressed concrete pressure vessel. The major part of this will be inactive and removed by con­tractors under the strict control of the CHGB. The radioactive, mostly contaminated waste material, will be taken off-site in large packages which com­ply with IAEA Regulation for the Safe Transport of Radioactive Waste for disposal or storage. The residual reactor structure, which contains over 90% of the total radioactive inventory after defuelling, is sealed off until Stage 3 commences

For a magnox station, the radioactive plant re­moved includes the contaminated boilers, main ducts, radwaste and fuelling equipment, fuel ponds, laundries, workshops, etc. This plant will be dismantled using built-in facilities and contract engi­neering plant and equipment under controlled condi­tions, to ensure that safety of the workforce and public is maintained to the standards required by NIL The waste arising, though significant in terms of mass, will not generally require elaborate or remote techniques.

• Stage 3 is concerned with the removal of the reactors themselves and final clearance of the site rendering it safe for future use. This is a very dif­ficult task because the activated reactor structure, which for the early magnox design comprises the graphite core, steel restraint structure and diagrid support, steel pressure vessel and inner 1 m layer of reinforced concrete biological shield, requires the use of specialised remote methods (see Fig 4.13). Suitable techniques are being developed but the detailed design, planning and proving the complete system will be a substantial project. For the late magnox and AGR stations the boilers are within the prestressed concrete vessel and will be removed during Stage 3.

• The time required for each stage of decommission­ing varies for each reactor system and individual site but is likely to take 5 to 7 years.

It is intended that Stage 2 should follow on directly from Stage 1. There is an option between proceeding to Stage 3 immediately, so that decommissioning is completed as a single project in about 15-17 years from shutdown, or to defer Stage 3 up to 100 years.