Dealing with hidden problems

Managing the building fabric is an important part of a maintenance process once nuclear power stations reach the end of their working lives.

This is the case at Calder Hall, Sellafield, whose four 60MW Magnox gas-cooled reactors stopped generating power in 2003. The buildings are now in a period of asset management until all the fuel is removed from the reactors. The asset management programme has identified a number of issues and actions required to ensure that the building’s integrity is maintained, and that the assets are safe and available for defuelling.

At Calder Hall, an immediate effect of stopping generation was the impact on the asbestos insulation and lagging. Previous inspections had shown that the asbestos was safe. But as soon as the power generation stopped, the subsequent loss of heat meant the asbestos quickly started deteriorating. It was neither economical nor practical to repair the insulation so the decision was taken to remove it all.

The project would become one of the largest asbestos removal projects in Europe. It covered the 16 heat exchangers linked to each of the four reactors; the eight HP and eight LP pump houses; four long and four short blower houses; the 300mm diameter steam pipes, including metal cladding and valve boxes from each heat exchanger to the turbine halls; and the steam pipes and turbine casing in two turbine halls.

The specialist asbestos removal division of the industrial services group Hertel was appointed for the project, which would eventually take four and a half years and more than 1 million man hours to complete.

In total more than 2300 tonnes of asbestos was stripped and removed from the Calder Hall site for disposal. The overall cost of the project was GBP 26.25 million.

Hertel formed four teams, comprising 100 workers in total, that worked on a 14-day shift pattern. Each team was responsible for different areas of the project: the heat exchangers; the turbine hall and the ancillary buildings; the pipework and bridges; and surveillance and follow-up maintenance.

In-situ cleaning

Work on the external heat exchangers was on the programme critical path. Access to the vertical, multi-storey, external heat exchangers was by a specially designed 36m high scaffold, encapsulated with polythene sheeting to protect the inner asbestos tents and the workforce from the weather.

Wind loading would only allow one half of the scaffold (top or bottom) to be encapsulated at any one time. And the asbestos had to be removed in such a way that it would not put any undue stress on the heat exchanger structure.

Team members wore standard asbestos protection suits and were rotated across the different teams to minimise their personal radiation dose uptake.

The areas where the asbestos was being removed were contained within tents to ensure that asbestos fibres were not released into the atmosphere. These tents contained CCTV cameras, which allowed teams working at height to be monitored from ground level. In addition, air ducts (connected to negative pressure units) were installed to ensure adequate airflow for the workers.

Once the asbestos was removed, all the surfaces were cleaned. Conventional cleaning methods were not feasible due to the potential radiation exposure, so grit blasting with a Quill Falcon Kwikblast unit was used.

Weekly safety audits were carried out by health, safety, environment, & quality (HSEQ) teams and rescue procedures were practiced in partnership with the Sellafield site emergency services.

Top duct removal

After the asbestos removal, an inspection of the building found that the heat exchangers’ top ducts and the surrounding access steelwork were showing signs of heavy corrosion. They had been in place since the station was built in the early 1950s.

One heat exchanger was in need of immediate attention and it was decided to remove the top duct for safety reasons.

Scaffolding was erected around the top of the heat exchanger unit to provide access for cutting and lifting preparations.

Around the top duct was a steel framework bridge, used for pedestrian access. Heavily loaded springs mounted on top of the steelwork were connected to the top duct, which flexed to allow for expansions. It was too difficult to move the steelwork in situ so it was decided to lift the top duct and steelwork together. To achieve this without the steelwork buckling, the springs had to be fixed in place. To achieve a ‘clean’ lift bolted restraints had to be designed and fixed in place on the springs. Additional restraints were also welded to the bridge to give further strength and to stop it buckling under its own weight.

When the top duct was ready for lifting Hertel engaged a heavy lift contractor who brought in a large mobile crane. Once the lift took place, the top duct was moved to a pre-designated compound where work could start on downsizing.

Meanwhile, pressurised blanking plates made from 20mm steel were fitted to the top of the heat exchanger and the reactor face to effectively reinstate the containment to the reactor circuit, which operates at a negative pressure.

Plates were also needed on the ends of the top ducts, which had been removed to ensure that any radiological contamination contained within the unit did not escape. There was also an asbestos lining surrounding the inner convoluted ductwork and had to be removed before material could be sent to the waste stream.

Bellows

The 10 tonne bellows unit, at each end of each top duct, was located inside an asbestos containment enclosure that housed an overhead lifting frame and hoist.

The team intended to remove the holding bolts on the bellows and raise the top outer casing from the bellows unit using the overhead crane. However, after being operational for more than 50 years, constant heating and cooling had affected the close-machined tolerances within the unit.

The flanged spigot on the bellows had originally been machined to have a tolerance of approximately 0.010 inch. But when it was built 60 years ago, there was little understanding of the metallurgical reaction to the heat and other factors that would occur over time. The result was that the spigots on the internal duct had virtually welded themselves to the rebates in the outer casings.

Hertel had to devise a way of being able to part the two bolted outer casings to access the inner lining of asbestos, whilst maintaining asbestos control regulations and being mindful of any radiation residues within the top duct.

After some consideration it was decided to insert eight 30 tonne jacks connected to a 110V power pack through the bolt access holes around the outer casing along the unit. By operating them in unison, the combined force would break the welded effect at the spigot and allow the top casing to be lifted off, giving access to the convoluted duct.

Once the lagging materials were accessible, a team of three asbestos strippers worked on the unit. This asbestos was stripped using a wet spray technique, and conventional methods of hand and manual scrapers. Meanwhile, Hertel’s mechanical team worked on dismantling other parts of the top duct.

This project represented the first time that a top duct had been removed at Calder Hall. While the removal of the bellows and the asbestos should have been a straightforward process, there was no way the problems that were encountered could have been foreseen. However, the knowledge that has been gained will give a better understanding of what can be expected when the more than 128 bellows units are eventually removed from Calder Hall.