Dealing with the 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, which stopped generating power in 2003. The buildings are now in a period of asset management until all fuel is removed from the reactors. This has led to a number of issues which have needed action to ensure that the building’s integrity is maintained, ensuring the assets are safe and available for defueling.
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. As soon as the power generation stopped, the subsequent loss of heat meant the asbestos quickly started deteriorating.
This was particularly concerning in external areas where there was the added problem of weather damage and corrosion of outer protector layers which was causing sections to fall away creating health and safety hazards.
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 and covered Calder Hall’s 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.
Hertel has been providing maintenance services at Sellafield for around 20 years. The company’s specialist asbestos removal division was appointed to handle the project which would eventually take four and a half years and more than 1 million man hours to complete.
To complete the work Hertel formed four teams, totalling about 100 workers who 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; all the pipework and bridges; and surveillance and follow-up maintenance.
Team members were rotated across the different teams to minimise concerns of personal radiation dose uptake. This was important because there was an increased risk of exposure to radiation as the asbestos was stripped away to reveal steelwork. Stringent health physics checks confirmed there was no need for pressurised suits as they faced a nuclear radiation, not contamination, risk.
All the areas where the asbestos was being removed, which ranged from scaffolding on the heat exchangers to the most confined areas of pipework, were contained with tenting to ensure that asbestos fibres were not released into the atmosphere.
All the asbestos had to be dampened, prior to removal, with an injection of a pressurised fluid. This was to reduce the amount of airborne particles produced making it safer and easier to remove, all which had to be done by hand.
Work on the external heat exchangers was on the programme critical path and so it was important to try and avoid any weather delays which could impact on the whole project. Access was by a specially designed 36m high scaffold which was encapsulated with polythene sheeting to protect the inner asbestos tents and the workforce from the weather.
Wind loading would only allow one half (top or bottom) to be encapsulated at any one time. The inner asbestos tents provide the sealed containment for removing asbestos and prevent the release of any airborne particles. The asbestos had to be removed in a pattern which would not put any undue stress on the heat exchanger structure and ensure that a uniform loading was maintained.
Special attention had to be paid to the complex network of pipes, valves, pumps and tanks at Calder Hall which were often sited in confined spaces. Before work could start, Hertel had to plan how work could be carried out safely in these areas while ensuring that the required tenting was accessible and secure.
In all areas it is necessary to install an air duct, which was connected to a negative pressure unit, to allow for adequate air flow for the workers while minimising airborne fibre release.
Once the asbestos was removed it was important to ensure all surfaces were clean and the complexity of the project presented challenges as conventional methods were often not feasible as it could lead workers to be exposed to radiation.
The solution was to use grit blasting, with a Quill Falcon Kwikblast unit for those difficult to reach areas or where they needed to minimise time at the workface to keep dose uptake to a minimum.
In total more than 2,300 tonnes of asbestos was stripped and removed from site for disposal from Calder Hall. The waste was bagged and transferred to a buffer store at ground level, where the contents were monitored to ensure there was no nuclear contamination before being transported for disposal at a designated site.
Throughout the project the primary concern was safety. Weekly safety audits were carried out by HSEQ teams and rescue procedures, which provided valuable knowledge in such a complex project, were practiced in partnership with the Sellafield site emergency services.
Within the asbestos tents CCTV cameras were also installed in addition to the normal viewing windows. This was particularly useful where teams were working at height and could be monitored from ground level. It also allowed the HSE inspectors to carry out regular inspections without having to enter the asbestos containment tent.
As part of the asset management process 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 identified as most in need of immediate attention and it was decided to remove the top duct for safety reasons.
Before the top duct could be removed from the building, a full method statement was prepared by Hertel to outline the work needed prior and post removal.
Scaffolding was erected around the top of the unit to provide access for cutting and lifting preparations. A 3d model was used to allow a detailed designed cutting plan to assist in dismantling the steelwork surrounding the top duct. Whilst doing this it was important to ensure the structural integrity of the unit at all times.
Around the top duct was a steel framework bridge used to give foot 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 steel work together. To achieve this it was necessary to fix the springs in place otherwise the steel work would have buckled under the energy in the springs and its own weight. 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.
Before the start of the cutting work, it was important that all the active liquor in the local filter system was drained, collected and transported away from site. All contaminated waste was also packaged and transferred to the Sellafield site waste disposal team for removal. At the same time any free-release materials were segregated and sent for recycling.
When the top duct was ready for lifting Hertel engaged a heavy lift contractor who brought in a mobile crane capable of lifting 1,200 tonnes. The weight to be lifted had to be assessed from historic plant drawings. It also required ground preparations to be made using load bearing concrete plinths and spreader mats.
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. This was encased within a 40 mm thick split metal casing. It was necessary to remove the asbestos prior to sending the downsized equipment to the waste stream.
The 10Te bellows unit, at the end of each top duct, was located inside an asbestos containment enclosure which housed an overhead lifting frame and hoist. It was intended to raise the top outer casing from the bellows unit using the overhead crane.
Initially it was hoped that it would be possible to remove the holding bolts on the bellows to simply separate the inners from the outer casings. Having been operational for more than 50 years, however, 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. When it was built 60 years ago, there was little understanding of the metallurgical reaction to the heat and other impacts 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 30t jacks connected to a 110v power pack through the bolt access holes around the outer casing along the unit. By operating them in tandem, it would ‘break’ the welded affect at the spigot and allow the top casing to be lifted off allowing access to the convoluted duct.
A team of three asbestos strippers worked on the unit once the lagging materials were accessible. This asbestos would be stripped using a wet spray technique and conventional methods of hand and manual scrapers.
Due to potential radiation exposure the team were limited, by the day and by the week, in the amount of time they could work. In addition to the asbestos team, Hertel’s mechanical team were working on dismantling other parts of the top duct. As a minimum all team members were also given full asbestos training.
This was the first time that a top duct had been removed at Calder Hall. Whilst the removal of the bellows and the asbestos should have been a straightforward process, there was no way the problems which were encountered could have been foreseen. To respond to those issues required the pooling of experience and innovation. The knowledge that has been gained will give a better understand of what can be expected in the programme of the removal Calder Hall’s more than 128 bellows units.
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