At 10 years after shutdown, the total activity will be dominated by the 2.7 year half-life, X-ray emitting isotope iron-55 arising from activation of reactor in-core steel components. This will decay and within a few years the total activity, which is important to waste disposal, will be due to carbon-14 in the graphite cores of gas-cooled reactors and nickel-63, half-life 92 years, present as a trace element in carbon steel and a necessary alloy in stainless steels, especially in PWR pressure vessels and internal core structures. Cobalt-60, half-life 5.7 years, also a trace element in carbon and stainless steel, gives rise to a gamma dose rate far too high to permit manual access for disman­tling the reactor structures. Early Stage 3 therefore, requires the use of remote methods to cut, handle and package active wastes. The option to defer Stage 3 for up to 100 years offers advantages because it is expected that dose rates will decay sufficiently to allow access to set up and maintain automatic equip­ment (see Fig 4.14). This would simplify dismantling, speed-up work and reduce costs, but account must also be taken of the long surveillance and maintenance of the residual buildings. Because of other very long — lived isotopes such as nicel-59 and nicobium-94, no further benefit will accrue from longer delay.

The CEGB will remain responsible under the terms of the site licence, for all aspects of safety until Stage 3 decommissioning is complete and the Nil has de­clared the site to be free from harmful radiation. Thus CEGB is bound to exercise strict management control over the whole decommissioning project, including the handling and transport of all waste to designated dis­posal sites.

Radioactive wastes arising from Stages 2 and 3 will be low or intermediate level. The intention is to limit the requirements for cutting material during disman­tling operations or subsequent conditioning to the minimum necessary to reduce potential radioactive doses and costs. Large reinforced-concrete containers or boxes will be used, therefore, to carry such wastes to suitable disposal or storage facilities.

Regulatory authorities now require more attention to be given to station design to identify the features needed for operation and other features specifically introduced that could assist decommissioning when that time comes. This includes:

• Layout and outline plans for decommissioning.

• Selection and control of primary circuit materials.

• Maintenance of lifetime records.

• Surface treatment and decontamination.

• Recovery from design basis accidents.

Proposals for decommissioning magnox nuclear power stations have been prepared over the last five years and are up-dated as required. These provide a basis on which detailed engineering plans can be built-up when the time is nearer to hand. Basic information on the physical layout of the reactor structure, materials and operational history has been used to calculate a radio­active inventory of activated and contaminated ma­terial. This identifies the mass and isotopic contents including important data on long level trace elements which have been checked by measurement of samples

when the opportunity is presented. From this, pre­dictions may be made of the radioactivity decay and requirement for engineering plant, shielding, waste management and transport.

The major task at Stage 2 of dismantling and re­moving the low level contaminated boilers has been studied. It has shown that suitable lifting methods with capability of well over 1000 t could be used to transfer these boilers as single items to a suitable multi-wheeled vehicle and thence onto a barge for transport to a designated land or sea disposal facility. Alternatively the boilers may cut down into pieces and moved by road/rail in packages weighing up to 100 t or so. These methods have been proved by similar tasks undertaken in the offshore oil industry.

The expected approach to Stage 3 is that access to the reactor will be gained from a shielded, ventilated temporary containment through the pile cap. Up to some 80 years after shutdown, remote methods will be essential to cut and remove material and to effect routine plant modifications, maintenance and recovery. This will be controlled from outside the containment using TV, sound and other navigational control sys­tems. Wastes will be lifted out in skips and taken through an airlock to the waste assayence packaging facility using the existing irradiated final route.

It is proposed that the 2000 t of graphite per re­actor will be lifted out using a multi-fingered grab mounted on a small commercially available tracked vehicle. Removal of the 2500 t or so of steel will be divided between abrasive disc cutting of the core sup­port and restraint structure, and hot cutting of the pressure vessel itself. Methods for removing the inner activated layer of the biological shield include impact hammers, concrete saws, explosives and high pressure water jets. All these methods, including designs for ventilation system and filters, are being developed and will be demonstrated initially in the complete dis­mantling of the Windscale AGR.

Decommissioning of PWRs has been reviewed, mak­ing use of extensive studies and development work already being undertaken in USA and Western Europe.

Costs have been assessed and for operating reactors financial provision is made each year to build ade­quate funds to meet their costs.

Appendix A