Windscale gets down to CORE VALUES

Pull up a piece of clip-art or go to a newspaper photo library for an image that symbolises nuclear power and what do you get? Almost certainly, at least one of the images will be of the `golf ball' reactor building that has dominated the Cumbrian landscape at Windscale for the past 40 years. But that image of 20th century technology has long outlived what is inside the dome itself, which last month entered its active decommissioning stage.

Operated by the UKAEA, the Windscale advanced gas-cooled reactor (WAGR) began generating power in 1963 and was the forerunner of the modern generation of AGRs built to eventually take over from the ageing Magnox reactors. Its decommissioning actually started in 1981 when the reactor was shut down for the final time. The highly radioactive fuel was discharged in 1983, since when `Project WAGR' has become the UK's demonstration exercise for power reactor decommissioning, aimed at showing how this can be done safely, cost-effectively and with minimum environmental risk.

Somewhat confusingly, BNFL Magnox Generation, the prime contractor for the dismantling operations, refers to last month's latest stage in this decommissioning process as `active commissioning', by which it means the commissioning of the various items of dismantling and removal equipment that have been assembled in the `active' core area of the reactor.

At the heart of all this necessarily remote-controlled equipment is an £8million `remote dismantling machine' (RDM), complete with a manipulator arm that can access all areas inside the reactor's pressure vessel. Although based on a proprietary robotics design, this arm can be fitted with a host of tools such as oxy-propane torches, shears and grinders to painstakingly dismantle the core and ancillary equipment piece by piece, and pass them out from the reactor into radiologically-shielded assay and packaging areas.

These areas are part of a `waste route' established inside the dome by making use of the existing shielding that housed the reactor's four steam-generating heat exchangers. These 20m x 3.4m diameter, 190 tonne giants were lifted out of the dome in 1995 and removed to the Nirex low-level waste storage facility at nearby Drigg.

Drigg is also the destination for all the other low-level waste that will be extracted from the reactor by the RDM. Intermediate-level waste - as determined by sensors in the assay area - now has its own purpose-built shielded storage area on site at Windscale, pending the long-awaited decision on where Britain's long-term waste repository is to be sited.

While the assay area identifies the level of radioactivity of the waste components, all the waste removed from the reactor is initially treated in the same way. Individual pieces are remotely transferred into precast concrete boxes about 2x2x2m in size. When the boxes are filled, with either low or intermediate waste, the remaining voids are filled with `grout', a cement-based, quick-setting concrete that is mixed up on site. A concrete lid is then moulded in situ on to the 50tonne boxes, prior to them being transported either to Windscale's new intermediate-level storage building or to Drigg. The small amounts of liquid effluent arising from the concrete plant and the filling operations are collected and tankered across site to BNFL's own treatment plant on the adjacent Sellafield works

The first box of `active' elements removed from the reactor last month included over a hundred neutron filler plugs extracted from the fuel channels in the core. Core dismantling will continue in a top-down operation that will consist of a series of 18 stages, each one removing specific elements such as operational waste left after defuelling, hotbox, loop tubes, neutron shield, graphite core, pressure vessel and insulation.

According to UKAEA's head of site for WAGR, Barry Hickey, `the fact that we are now in the phase of active commissioning demonstrates the real progress being made on the project, and we are looking forward to further significant achievements.'

The project will, however, continue to be a long job. With an estimated 800 tonnes of intermediate waste and 400 tonnes of low level waste to be removed, core dismantling is expected to run through to at least 2006.

After that, work can start on removing the rest of the reactor structure, largely consisting of the concrete biological shielding around the reactor's 13m x 6.5m diameter pressure vessel. But this stage, says Terry Benest, UKAEA project manager, is unlikely to be completed much before 2020 because of the lack of adequate storage facilities.

As yet, no decision has been made regarding the ultimate fate of the dome itself. An on-going options study is considering whether to apply for `listed building' status - in which case, a long-term care and maintenance programme would be put into place - or to investigate techniques for returning the site to `green field' status.

And here the timescale becomes almost millennial. The WAGR diary suggests a period running through to 2130 for either option, by which time UKAEA estimates that `workers can enter the plant and, using straightforward technology and techniques, dismantle the remaining parts of the plant.'

In the shorter term, having demonstrated its decommissioning expertise, BNFL Magnox hopes to capture a good slice of the worldwide market for this highly specialised work, estimated by the company's project manager Jim Craik to be worth around £40billion over the coming years. PE