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The importance of Evaporation

The inventive engineering and technological trickery of hundreds of Sellafield Ltd and supply chain employees has ensured our reprocessing work and hazard reduction can continue into the future. Read on to hear this tale of triumph over adversity, and crucially, why it matters.

As far as nuclear plants go – the Highly Active Liquor Evaporation and Storage Plant (HALES) is one of the more simple to explain, because it does exactly what it says on the tin – evaporates and stores highly active liquor from our reprocessing plants, before it is sent for vitrification.As far as nuclear plants go – the Highly Active Liquor Evaporation and Storage Plant (HALES) is one of the more simple to explain, because it does exactly what it says on the tin – evaporates and stores highly active liquor from our reprocessing plants, before it is sent for vitrification.

Whilst the process might be simple, the complexity of keeping three ageing evaporators running cannot be understated, and nor can the importance of the plant to operations at Sellafield. In one of the largest construction projects in our recent history, we are currently building a new evaporator, to increase our evaporation capacity, to cover the end of reprocessing operations and to support the post operational clean out of these facilities. The new evaporator is due to be operational during 2017, but until that point, we need to ensure that we continue to operate our current three evaporators, known as A, B and C.

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Evaporation allows us to reduce the volume of our most hazardous waste product, highly active liquid waste, to a level as low as reasonably practicable – this can be a reduction of a factor of fifty times (for oxide wastes) or 100-150 times (for Magnox wastes). There are clear safety and financial reasons for reducing the amount of this highly active waste we store, prior to eventual disposal. Our initial series of six evaporators quickly became obsolete many years ago, as they were not designed for reprocessing. This led to the development of evaporator A (which started operations in 1970), evaporator B (in 1984) and evaporator C (in 1990).

Each of these three evaporators was designed to increase our evaporative capacity, with a specific purpose in mind. As the liquids (known as raffinate) we evaporate have different properties – looking and acting differently depending on the source – they cannot all be evaporated in the same way. This means that we have only one evaporator, C, that can handle raffinate from Thorp reprocessing. The simplest explanation of an evaporator is that it acts like a giant jacketed kettle for liquid waste, with heat provided through steam coils and jacket. The pressure in these is controlled, along with the pressure in the evaporator, so as to reduce corrosion of the coils.

However, this description downplays the complexities involved when the liquid is highly active. As a result of the nature of this liquid, over time corrosion does occur, and this means that each evaporator has a limited life. So for the less technologically minded, this is likely to be the most inhospitable working environment imaginable – due to both the temperature, and the acidic, corrosive and highly radioactive nature of the liquid.

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Evaporator B – delivering the seemingly impossible

In fact, it was corrosion of a different kind that led to evaporator B being taken out of service in December 2009 – from an acid leak to the support steelwork. This corrosion caused serious damage, and at the time it looked highly unlikely that the evaporator would be used again. As you might imagine, it’s very difficult to make repairs to a large, specialist piece of dated equipment, on a congested industrial site. It’s even more challenging when the kit in question handles highly radioactive liquid, is shielded behind concrete and other barriers, and the plant in question is Europe’s most complex industrial site. Human access is impossible, and this means that any access is via the originally installed kit or remotely operated equipment.

However, the need for ongoing evaporative capacity meant we needed to look again at whether evaporator B could be brought back into operation. This was a significant decision which weighed the costs of the remediation work required against the benefits the additional capacity would bring. As a consequence, a complex and challenging project to return this evaporator to service was instigated. Preparatory work included CCTV investigations of the evaporator and the creation of a full scale replica of the damaged area. This allowed us to develop the design, and trial deployment techniques away from the plant.

Thanks to work like this, and the dedication of the project team – which included both Sellafield Ltd employees and our partners from James Fisher Nuclear and Shepley Engineers – the once-thought-impossible was achieved, and evaporator B was brought back in to service in April 2014. It is now used as the duty evaporator for Magnox reprocessing waste, with evaporator A remaining on stand-by. Tom Gardner, the head of HALES explains: “Evaporator B is a 45 year old facility, operating safely well beyond its original design life. This is a reflection of the hard work of the engineering and maintenance teams that have made this happen, who have adapted so professionally to the challenges that the evaporator has posed to us.”

Evaporator C – life expectancy on the rise

As our most modern evaporator, with the largest capacity, evaporator C is important to reprocessing and hazard reduction. As our only evaporator that can handle Thorp liquids, it is a single point of failure – that is, if we lose evaporator C, we lose the ability to reprocess waste from Thorp. Despite its relative youth, it’s still a dated facility, and one that has been pushed hard over the years. The oxide waste that comes from Thorp is far more corrosive than that from Magnox reprocessing. This means that the lifespan of the facility reduces much more quickly than the other evaporators.

We assess the remnant life of an evaporator based on the level of corrosion that the metalwork (namely the coils and jacket) has received. Once the corrosion in one of the coils reaches an agreed limit, this coil is taken out of action. Once the corrosion in the jacket reaches its limit, we have to stop using the evaporator. Assessing the corrosion is a straightforward idea, but the challenge again comes from the shielding and protective barriers that encase the evaporator. These mean that it’s incredibly difficult to ascertain and evidence the levels of corrosion. In fact, we don’t think that there is a similar system anywhere else in the world – nuclear or otherwise. Based on the worst corrosion we’d found to the coils, our conservative predictions for corrosion to the jacket and base were that evaporator C might be reaching the end of its usable life. As the prediction was conservative, we believed the evaporator actually had a far longer remaining life, but this had to be proven. To address this, we worked to better understand the corrosion levels in the evaporator. Advances in technology have allowed us to work with our partners at the National Nuclear Laboratory to develop a new base inspection device which has measured thickness, as well as a coil inspection device and we have developed numerous test rigs to challenge and enhance our corrosion knowledge.

On top of this, we have changed the way we use the evaporator to minimise future corrosion. The changes we have made have resulted in a lengthening of the evaporator’s life, which have enhanced confidence we can bridge the gap until evaporator D is online. This was made possible, in part, thanks to a new approach to risk management developed to break down the barriers to progress at Sellafield. This ‘G6’ thinking is so called because it is a collaborative way of working developed by the six organisations with responsibility for the safe and effective clean-up of the site (the Department of Energy and Climate Change, Shareholder Executive, Office for Nuclear Regulation, Environment Agency, the Nuclear Decommissioning Authority and Sellafield Ltd) – all of whom have a common goal to speed up decommissioning and hazard reduction.

Andy Lindley, director of the Office for Nuclear Regulation’s Sellafield programme said: “We have been working closely with Sellafield Ltd to better understand the corrosion in Evaporator C and the risks posed by this; ensuring that we have confidence in its continued operation.”

Evaporator D – making high hazard reduction possible

The advanced solutions delivered above have lengthened the life of our current evaporators, but they haven’t removed the need for new, future-proof evaporative capability. This is where evaporator D comes in. This new evaporator is in the final stages of construction and will be ready for operations by 2017. Its development has been complex and challenging, and not without its setbacks.

However, once complete, the new facility will provide us with the evaporative capability we need to both complete reprocessing fuel in Thorp and Magnox, and clean out these plants, the old evaporators and the storage tanks. The evaporator has been designed to handle these clean out wastes, which contain greater concentrations of suspended solids than the other evaporators. Thanks to changes made early in the design an additional, planned evaporator, will no longer be required.

Evaporator D, which will have an operational life of 25 years, is a building of over 23,000 tonnes of concrete with 22,000m of pipework. It is built on an extraordinarily compact site adjacent to critical buildings. The evaporator was built in separate modules, which were delivered individually by sea, and were connected when on site. Even by Sellafield’s standards, it has been a feat of engineering and of logistics, requiring the removal of bridges and street furniture from the site – such is its size. It’s a project which has built on the lessons we learned in all of our previous evaporators – so it’ll be easier to do the corrosion inspections, will have more modern safety systems installed from day one, and will operate with the fundamentally safer segregated cooling water system – which none of its predecessors had.

A dedicated team from the HALES facility is now working to ensure we are ready for the final installation of the evaporator, which will include both inactive and active commissioning. Once this is complete, our most advanced evaporator will be switched on, and the future of the crucial task of high hazard removal through the evaporation of highly active liquor, will be secured.


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Fit for the future

As we’ve outlined, evaporation is crucial to Sellafield Ltd’s reprocessing work. It’s also important to the future of the site, when reprocessing concludes and we move into post operational clean out (or POCO). When we move to POCO the liquid wastes (or raffinates) that are transferred to the HALES plant for evaporation and storage will be different to those from reprocessing – in density and chemical makeup. This means they are likely to move and act differently to those we are used to. However, we’ll still be using the same network of pipes to make these transfers.

This is where a team from our supply chain partner, the National Nuclear Laboratory (NNL), come in. A group of chemical engineers is currently researching how this new raffinate will act. At their Workington laboratory and rig hall facility, they have created a full size mock-up of the pipelines that transport it. As the pipelines are gravity fed, they’ve also recreated the slight downward gradients found in the pipelines on the Sellafield site.

By recreating this they can understand how the solid-carrying liquid will move through the pipework, whether it will settle within it, and what clean-up is required.

The NNL lead chemical engineer on the project, Dr Donna McKendrick, explains the importance: “The change in materials coming through the pipes will influence the clean-up regime required. By fully understanding this now, we can put in place the very best process, minimising the amount of fluid required. This will ultimately lead to savings on the storage and disposal costs for this and could potentially negate the need for a new storage facility in the future.”