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Green, green grass returns to Welsh hotspot

Laing O’Rourke and Magnox North are the key players involved in the decommissioning of a nuclear power plant in north Wales

Project Intermediate level waste store
ClientNuclear Decommissioning Authority
Value £25 million
Tier 1 contractorMagnox North
Tier 2 contractorLaing O’Rourke
Slate wall contractorJames Cyf

For large image click here

The nuclear power station at Trawsfynydd in north Wales is one of a kind. There are other magnesium non-oxidising or Magnox reactor power stations scattered around the country but this is the only one built inland. It is also the only one built slap bang in the middle of a National Park.

Over the years it has been both derided by tourists and environmentalists who for various reasons wish it had never been built and supported by locals who see it as the only major local source of decent wages.

Despite ceasing active power production in the early 1990’s the plant is still the area’s largest employer and actually boasts more staff now than it did during its working heyday.

Most of these employees are working on its decommissioning, a long-term process that will eventually, albeit not for the rest of the 21st century, see all of the site returned to its former glory.

Part of the decommissioning process will see the site team reduce the height of the two 70 m high reactor buildings by almost 30 m. But another vital cog in the decommissioning wheel has seen site operator Magnox North open up a vital storage centre for some of the radioactive waste produced by the power station during its active days.

The £25 million intermediate level waste store will see material from the main site stacked into containers and held in the storage vaults until it can be treated or dealt with as part of a national radioactive waste strategy.

“The facility will not be used for the storage or treatment of material from any other site,” Dave Soar, Magnox North’s site engineer on the waste store project is quick to point out, “It will only be Trawsfynydd produced waste.”

That material will be shielded from the public by a double skin building designed to be able to withstand severe earthquakes and bomb blasts as well as the harsh weather this part of north Wales can throw at it.

Kalzip roofing system

Built by main contractor Laing O’Rourke its double skin features a 200 mm thick reinforced concrete outer wall with 190 mm thick Welsh slate cladding tied into it. Above 6 m this concrete wall is replaced by a steel frame that supports a metal wall cladding system which ties into a steel Kalzip roofing system.

But this outer skin is not much more than the acceptable face of the inner, business end, of the building. “Its really just to pretty up the outside,” says Mr Soar, “Its not doing any major structural work.”

The inner core is though. Featuring 13,500 cu m of lightweight LC45 concrete mixed at Hanson’s Minffordd Quarry eight miles away its huge 600 mm thick reinforced concrete walls and roof slab contain 500 tonnes of steel reinforcement at various diameters. The reinforced concrete floor slab houses some 1,000 tonnes of rebar and rests directly onto the underlying granite. There is nothing subtle about the building. It is the clunking fist of the decommissioning process.

Divided into four separate areas the building oozes raw engineering power and precision. It has to - if anything goes wrong here then there could be serious consequences.

The new intermediate level waste store has been pared right back to its most fundamental performance requirements.

It contains no heating system in it, no water, no sewerage, no air conditioning and what little lighting and cabling exists has been installed at a low level to help ease maintenance. Here there is little more than a great grey concrete box built to house radioactive waste.

But although the building is relatively simple it is built to incredible precision.

One side of the facility will store solid waste entombed in cement grout, wrapped in a sealed steel liner and eventually encased in a thick 3 m x 3 m reinforced concrete overpack. These overpacks are sited on stacking nodules that rise up out of the floor.

The other storage area is dedicated to housing drums of resins used to clean out parts of the reactor.

The resins are mixed up with a polymer to become a solid material and then placed in six or eight drum stillages - large steel crates - before being stacked, 10 high, on top of one another.

Floor and stacking pod foundation tolerances in the two storage bays are set at plus or minus 1 mm and the two huge 45 tonne gantry cranes that crank 19 m above the floor in the 94 m x 34 m storage areas will return each load they pick up to exactly the same place each time it is lifted. These stacking pods are levelled across a four corner grid ensuring that the four sections the overpack sits on are level with each other.

Any intolerance in this storage area can be ironed out by the lead pads that sit on top of the reinforced concrete nodules, although in practice this has never been required.

In the drum storage area specialist flooring subcontractor Surface Solutions used concrete dabs placed at exactly the right level to act as guides for its installation team. Self-levelling flooring compound was then poured in around these dabs up to fill level.

“The tolerances have to be that high. If you imagine that the bottom stillage is even just say 3 mm out, by the time that intolerance transfers to the top stillage it could be so far out that the crane is unable to pick it up,” Mr Soar says.

A reception hall at the front of the building will receive waste from the reactors which will be placed on a transfer bogie before being drawn into the building.

Here the waste will be picked up and lifted into position by the overhead gantry cranes which serve the storage areas. Should there be any problems with any of the overpacks or stillages operators are able to lift the waste back from the storage areas and into a specially designed inspection chamber.

Its 1,100 mm thick reinforced concrete wall, 6 tonne shield door and two 1,000 mm thick glass windows will allow operators to lift the lid from the waste packages and inspect the contents without any fear of exposure to radiation.

Rather than lower the huge gantry cranes in through the roof the project team decided to bring them in through the main reception hall door.

Crane hirer Ainscough Heavy Lift supplied mobile machines to lever the gantry cranes into the storage halls where they were shunted into position using some of its specialist hydraulic jacks. This simple measure helped shave weeks from the works programme, according to Mr Soar.

Proud of the job

“It meant that we could get on with pouring the walls and roof slab. We could use the 13 tonne, 6 m x 6 m steel concrete shuttering system more efficiently,” he says.

The inner section of the store and outer steel frame has a design life of 150 years although the cladding and roof system is less at just 40 years. This is to reflect the long-term solution for dealing with radioactive waste and is expected to be a national waste repository.

But despite knowing that the building is likely to become redundant well within its design life Mr Soar is proud of the job Laing O’Rourke and the rest of the project team has done.

“There were 70 to 80 guys working on this project at any one time,” he says, smiling “A quarter of a million man hours without incident is a fantastic safety record for any site.”

For large image click here

The mother of all cleaning jobs

Building the new Intermediate waste store is only one part of the whole decommissioning of Trawsfynydd’s two reactor buildings.

Current strategy will see the buildings converted into safe storage areas for the reactors themselves, the large boilers and plant such as pipes and ducting.

This process will involve placing a reinforced concrete slab capping roof across the entire building footprint at the new height reduced level.

Almost 30 m will be chopped from the existing height with the capping roof placed directly above the charge face – the business end of the power station where the nuclear fuel rods were lowered into the reactor.

The new waterproofed reinforced concrete roof must be placed before any of the height reduction work is carried out and one of the essential jobs is to carry out a complete de-plant or strip out of the top floors of the building.

“The object is to get each room in the reactor building ‘cleaned’,” says John Marron, de-planting team leader for Magnox North. “Each room has been surveyed to list exactly what is in there and each piece of plant will be ticked off as it is removed.”

That survey will be carefully stored to provide an exact record of the material removed from each room for future reference.

Once stripped out most of the material can be sent for recycling but at the moment the de-planting team is concentrating on getting the work spaces clear so that Costain, the contractor awarded the three year, £11 million project to place the capping roof across the two buildings, can get on with the project.

“Once the capping roof is in place we will be responsible for everything below it,” says Magnox North site engineer Gareth Jones. “When we have finished with the work spaces all that will be left are the bare walls and a basic electrical system.”

Use of GGBS in Nuclear Decommissioning Jobs

Mike Connell, technical and environmental manager at UK Ground Granulated Blastfurnace Slag producer Civil and Marine, explains how the benefits in its use has helped the decommissioning of Britain’s nuclear power plants:

Sellafield Limited first tested the suitability of Ground Granulated Blastfurnace Slag for the encapsulation of medium-level radioactive waste in the 1980’s. But since its £3.2 billion decommissioning programme gathered pace with the encapsulation of waste streams from the Thermal oxide reprocessing plant at the Sellafield plant in Cumbria it now uses up to 2,000 tonnes of GGBS a year.

Sellafield acts as contractor to the Nuclear Decommissioning Authority to ensure the civil nuclear sites it owns are decommissioned and cleaned up safely.

By using a high ratio 90 per cent GGBS to 10 per cent CEM I Portland cement grout mix in its encapsulation process the heat generated within the grout is reduced, minimising the risk of thermal cracking and the diffusion of aggressive chemicals.

This Sellafield grade GGBS grout has a precisely controlled particle size distribution that gives it the high flow, extended working life and low bleed, without the use of chemical admixtures – all vital for the process of encapsulating waste within the concrete grout.

Grout made with a high proportion of GGBS gains strength much more slowly than that made with a high proportion of Portland Cement, generating less heat resulting in a lower thermal differential in the encapsulation container. This reduces the risk of cracking and desiccation of the core and the deterioration of the hydrate structure is minimised. Because of the alkaline reducing nature of the pore water in high GGBS grouts they are able to stop some of the radioactive particles dissolving, keeping them in an insoluble state and helping immobilise them.

Commenting on the project, Sellafield contracts specialist Alan Watkin said: “It is paramount that the grout we use in disposing of waste from nuclear power stations provides long-term durability, stability and peace of mind.

“Our tests showed that a grout with 90 per cent GGBS, with its high flow, extended working life and effective immobilisation properties makes it a suitable grout for this demanding task.”

Although its use in Sellafield’s decommissioning programme is relatively recent, GGBS has actually been used in concrete production for over 100 years. Today, over half of all ready-mixed concrete deliveries in the UK contain the product.

With a worldwide production of 3 billion tonnes a year, the manufacture of Portland Cement is considered responsible for five per cent of global carbon dioxide emissions.

But GGBS production requires less than one fifth of the energy, produces one tenth of the carbon dioxide emissions and does not require the quarrying of new materials.

When used as a partial replacement for cement, the ratio of GGBS to PC is usually less than 60:40.

But it can be used in percentages of up to 90 per cent to obtain specific properties - secant pilling applications use 90 per cent GGBS to give the low early strength for cutting the secant while retaining the later high durability and impermeability.

As well as low heat concrete it is also be used in concrete requiring high durability performance in aggressive ground conditions. GGBS combinations are suitable in all designated chemical classes, generally at lower cement contents than other cement types.

For example, in certain conditions, such as those where there is a risk of thaumasite attack, a 70 per cent GGBS mix is preferred; while in chloride conditions the low permeability and high binding capacity of GGBS concrete is an advantage. Other concrete durability problems such as alkali silica reaction, are also comfortably resolved by the use of GGBS.