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Radiation shielding - Massive high density concrete chambers are nearing completion beneath a new hospital project in Leeds.Dave Parker investigates.

Radiotherapy is a proven technique for treating many types of cancer - but there is a growing problem which is beginning to limit its effectiveness. Patients are becoming ever more obese, so the linear accelerators (Linacs) which produce the tightly focused beams of x-rays that target tumours have to develop greater power than ever.

'Until recently 8MV was thought to be enough. Now we're having to install Linacs with up to 25MV, ' explains Faber Maunsell project director Steve Gunning.

'This takes us into new regions when it comes to dealing with the radiation produced.' So, when FaberMaunsell took on design responsibility for the new £220M ($381M) oncology wing at the world famous St James Hospital in Leeds, UK, on behalf of design and construct contractor Bovis Lend Lease, the design team knew the 12 treatment rooms in the basement would be one of the toughest challenges. 'The main structure is a fairly conventional insitu concrete frame in three sections, using flat slabs throughout, ' Gunning explains.

'Insitu concrete was chosen for its low vibration characteristics and inherent fire resistance. Insitu concrete was also the obvious choice for the treatment chambers - but it would have to be something special.' Shielding everyone outside the chambers from the intense X-rays inside would involve much more than simply constructing massive 'bunkers', Gunning adds. There was the question of practicality. The hospital required each chamber to be capable of treating a patient every 10 minutes, giving the facility the ability to deal with a maximum of 864 patients in a single 12 hour day. Easy access was essential.

'Heavy doors weighing up to 4t are the most space efficient way of sealing off entrances, but neither patients nor radiologists like them very much, ' says Gunning. 'The preferred solution is a 'maze', an access passageway with overlapping walls blocking the radiation.

'But these take up a lot more room than doors, so the thickness of the walls becomes critical.' Faber Maunsell's preliminary calculations showed that to block the radiation produced, walls made of normal concrete with a high quality whinstone aggregate would have to be up to 2.7m thick. Replacing the whinstone with barites (barium sulphate) from the Lake District would increase concrete density from 2,350kg/m 3 to 3,200kg/m 3 and slash wall thickness to a maximum of 2.0m.

Magnetite (black iron ore) promised even greater reductions. Concrete made with magnetite could reach densities of no less than 3,800kg/m 3, which would bring wall thickness down a whole metre, to 'only' 1.7m.

'Magnetite concrete has some attractive properties, but there were drawbacks, ' explains Gunning. 'It's four times more expensive than normal concrete, and truckmixers can only run half full, so delivery is complicated.

'The real problem, however, is availability. Magnetite comes from Sweden, and there's never more than 5,000t in the UK, enough for only 1,250m 3 of concrete, and we would need more than 5,000m3.' Luckily, not all the walls and roofs would have to be the same thickness. The degree of shielding required varied according to the distance of the concrete from the source of radiation, and its orientation.

Gunning says the decision was taken to use magnetite concrete only where absolutely necessary, and use conventional concrete wherever possible.

Also to be considered were the structural functions of the chamber complex. All the loads from the 10 storeys of hospital wing above the chambers have to be transferred down to a 1.2m thick concrete raft below, which sits on sandstone bedrock.

This is some 12m below the deepest point of excavation on the sloping site. A contiguous piled wall on the high side of the excavation will be permanently propped off the chamber complex.

'We decided to use staggered 50mm steel plates stacked up to 350mm thick encased in normal density concrete for the roof, ' Gunning reports. 'This makes it possible to do larger pours with fewer health and safety issues.' Specialist radiation protection advisors had to be called in to finalise the details of the chambers, not least because the radiation levels involved would be well outside those covered by current guidance. Pouring the 6m high walls in a single lift was an obvious precaution, although achieving this with the magnetite concrete placed some special demands on the shuttering, which was supplied by Ischebeck Titan.

Maximum practical pour size was 70m 3. All concrete comes ready mixed from Tarmac in nearby Yeadon (see box). Every possible precaution was taken to minimise the heat produced as the cementitous content of the mix hydrated; even so, thermocouples were cast into the pours to monitor temperatures closely.

'We were originally worried about the effects of any cracking on the shielding potential of the concrete, ' Gunning recalls. 'But the specialist advisors were quite relaxed. Typical shrinkage cracks apparently let through no more radiation than the solid wall.' Nevertheless, Faber Maunsell specified a minimum concrete tensile strength of 2.5N/mm 2 - as well as minimum density. With such thick walls reinforcement was there mainly to control drying shrinkage.

Magnetite concrete pours began in April last year and were effectively complete by November. A total of 1,600m 3 was supplied by Tarmac, along with 3,500m 3 of normal density mix. Pours went well, Gunning reports.

He adds: 'Obviously such massive walls will be releasing water vapour into the chambers for a very long time. There has to be a completely dry environment for the Linacs to work, so the air conditioning system is 'oversize' to cope with the moisture from the concrete.' Concreting of equipment supports is taking place at the moment. Once the basic mechanical and electrical fit out is complete sometime in the autumn, the tricky installation of the 3t Linacs can begin. Final commissioning takes around three months per chamber; it will be 12 months before all 12 are in action.

When the new oncology wing is fully opened early in 2007 it will add 66,500m 2 of wards, diagnostic and treatment areas and research facilities to St James, and house up to 1,600 staff. Its radiotherapy centre will be one of the largest in Europe. Almost as valuable, at least in some people's eyes, will be the 1,200 space multi-storey car park that Bovis will build on the same site.

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