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Gently does it

One of four European nuclear reactor decommissioning pilot projects is entering the delicate main phase. Andrew Mylius went to Windscale to see work in progress

Stripping the radioactive core from the UK's experimental Windscale advanced gas reactor is little more than 'scrap business, ' says engineering manager Stephen Haslam.

But the complexity of handling wastes safely means work inside the landmark 'golf-ball' started in 1981 and has been advancing at a snail's pace.

When the project ends in 2005, the ga in, it is hoped, will b e to have proved to a nuclear-wary public that the atomic genie can be stuffed back into its bottle, or at least into the ground.

The UK Atomic Energy Authority, facility owner and project client, started building the prototype advanced gas reactor, known as WAGR, in 1957. The facility, completed in 1961, was designed as a research facility for the development of new fuels. Little thought was given to how it should be taken apart, Haslam observes.

'The reactor is incredibly well built, but taking it apart was someone else's problem, ' he says.

Construction practices have also changed in the 40 years since WAGR was built. Despite high build quality, design changes made during construction were poorly documented. In some cases drawings have vanished entirely, Haslam adds.

The ú80M ($125M) contract to strip out the reactor's contents involves standard civil engineering techniques. However, working with toxic materials in a confined space calls for meticulous planning and robust methodologies. Safety is paramount: work is vetted by industry regulator the Nuclear Installations Inspectorate and parent body the Health & Safety Executive. Haslam says of the nuclear energy industry as a whole: 'If anything is not safe, it is now recognised that's the end of business.' Lengthy periods of testing are required to make sure solutions are failsale. Meanwhile, the long lead-in times and the creeping pace of work means any miscalculations have a bad impact on the project's critical path.

WAGR's reactor core is contained within a 6.5m diameter, 13.1m high steel pressure vessel, in turn isolated inside a 1.5m thick reinforced concrete biological shield. It is here that work is now focused.

Prime contractor, BNFL Magnox, carries full financial risk. It has let 10 fixed price subcontracts through competitive tender. The first, for tooling and methodology, was awarded to Entech, a joint venture of contractors Strachan & Henshaw and French firm SGN, in 1997.

Human contact with the contents of the pressure vessel is out of the question.

A robotic arm with a 14m reach is used and can be fitted with tools including gas cutters, shears and grabs. It will be lowered on a mast as work advances. Two cranes equipped with grabs allow operators, viewing the site via video, to manhandle waste out of the pressure vessel and into the adjacent chamber, where it is packed into steel baskets. These baskets are then lowered to an assaying chamber where their radiological load is measured before being incarcerated in giant reinforced concrete boxes.

Supplied by Tarmac, the boxes measure 2.2m by 2.2m by 2.4m, but with an internal volume of just 1.4m 2. Ordinary concrete provides adequate shielding for low level waste, says Haslam. Higher levels of protection are delivered by adding magnetite, an iron oxide compound, to the mix.

Waste material is grouted into the box with a high workability mix of Ordinary Portland Cement and pulverised fuel ash before the box is sealed. Low level waste is transported to the nearby Drigg storage facility. It is calculated the entire decommissioning project will generate 137 low level boxes.

Intermediate level waste, with a half life of between 20 and 600 years, is being stored on site pending delivery of a permanent repository for hazardous nuclear waste. Some 220 boxes of waste are expected to arise. The on-site bunker has a design life of over 100 years.

An initial 'mop up' operation to remove loose material from inside the pressure vessel has already been carried out. Work now is to cut up and remove the hot gas manifold, or 'hot box', sitting above the core.

When the reactor was operational, gas was pumped up through the core, where it was super-heated. The hot box channelled the gas to four heat exchangers. It is built from 30mm thick carbon steel, 5m in diameter by 1.2m high.

During preliminary decommissioning in the early 1990s the fuel channels were randomly chopped off above the hot box. To break the structure into manageable, 1m 2plates, it is now necessary to cut the channels flush with the hot box top surface. A plasma head on the robotic arm is being used to cut each channel.

Later this year subcontractor Nuclear Services Group will start work on removing the steel restraints binding the reactor's graphite core structure.

Though it generates more particulate debris than cutting with a flame, the restraints will be ground off.

Haslam explains: 'If you subject graphite to heat over long periods, for instance in a reactor core, it retains energy. And if you heat it again suddenly, it relieves itself of that energy.' Fire is a real, but avoidable, danger.

The graphite core will also contain radiologically 'hot' Caesium 136 and Carbon 14, which are long-lived and highly soluble. It will be classed as intermediate level waste and placed in the controlled on-site environment.

But the most challenging element of the decommissioning will be removal of the pressure vessel structure.

Entech will cut the 102mm thick steel with an oxy-propane torch. However, the 200mm asbestos insulation coating the vessel reflects all but the hottest flames, and iron particles will be added to the gas mix.

Removal of all hazardous reactor structures is scheduled for completion by 2005-6, leaving just the biological shield and the outer membrane. Providing operations are glitch-free UKAEA and BNFL will have developed some leverage for assuaging public anxiety about nuclear energy.

Core data

WAGR generated 33MW of energy from enriched uranium rods inserted into 247 fuel channels running vertically through its graphite core. Fuel was installed in the core via corresponding channels in a concrete neutron shield and steel hot gas manifold above.

Core, shield and hot box are supported by a 1.5m tall steel diagrid, sitting above a conical steel tundish intended to catch debris from the core.

The assembly is contained within an insulated steel pressure vessel, in turn encapsulated by a 1.5m thick reinforced concrete biological shield.

WAGR's structural frame, supporting reactor core, refuelling machine, heat exchangers and its outer protective sphere, is built of reinforced concrete.

Fuel was removed from the structure in 1983 and in 1989 the refuelling machine was dismantled. The top of the biological shield and top dome of the pressure vessel were removed between 1990 and 1992, and a heavy duty concrete shield installed to contain radioactivity within the core structure.

Where the refuelling machine had been housed, above the reactor core, a reinforced concrete floor was constructed to support a remote robotic arm and two cranes, installed in 1994.

Modification of the reactor building's reinforced concrete structure provided chambers where waste from the core could be packed, or 'sentenced', and assessed for radiological load before safe storage.

WAGR's four heat exchangers, weighing 190t each, were lifted out and transported as low level waste to the nearby nuclear storage facility at Drigg in 1995.

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