Among the most demanding and potentially hazardous aspects of decommissioning liquid metal cooled reactors is dealing with the coolant.
This substance is commonly sodium or the sodium-potassium alloy NaK, which can react with water to release hydrogen and large quantities of heat, providing not only an explosive gas mixture but also a source of ignition. The products of combustion are also toxic and can cause severe caustic and thermal burns on skin contact, as well as being hazardous to ingest or inhale.
The considerable challenges involved in treating this type of coolant in reactor decommissioning programmes will be encountered at various stages. At the defuelling phase, for example, the assembly will be covered by a residual film of sodium that has to be removed before storing the elements in the pond. Every component extracted from the reactor will also be covered by a film of sodium and can sometimes retain larger amounts of the metal that are best removed before dismantling the components.
Additionally, the metallic coolant from the primary and secondary circuits has to be chemically treated to transform what could be several tonnes or several hundreds of tonnes of metallic radioactive product into a stable form. The primary and secondary vessels, when drained of their coolants, will also have some residual liquid metal stuck to the surface, or retained inside the structures as non-drainable retentions. Several secondary wastes may also contain sodium, such as the cold traps, which clean the alkali metal coolant of impurities, or caesium traps.
Methods of destruction or disposal of sodium or NaK can range from a purpose-built sodium disposal plant, which essentially treats and neutralises the sodium to produce salt water, to high temperature incineration, among others. Indeed the successful disposal last year of 57 tonnes of alkali liquid metal at Dounreay represented the destruction of one of the most hazardous legacies of Britain’s earliest atomic research.
Now, the parent body organization, Babcock Dounreay Partnership, which is responsible for the decommissioning, demolition and clean-up of the Dounreay nuclear site, is working with Dounreay Site Restoration Limited, and faces a further significant task.
As the metal reactor decommissioning at the site moves into the next, highly demanding phase, the challenge now is to tackle the destruction of the remnants of hazardous alkali metal still inside the reactor vessel, which could not be extracted for disposal in the purpose-built chemical processing plant. It’s a challenge that will be addressed with the application of innovative approaches while drawing on proven techniques and the experience and lessons learned from other sites and projects worldwide.
Similarly the approach taken and the knowledge gained from the Dounreay programme will, in turn, be able to contribute valuable experience and expertise to future decommissioning of alkali metal breeder reactors around the world.
|Inside the DFR sphere, the chemical plant was installed above the reactor to neutralise and destroy batches of liquid metal lifted from its primary circuit.|
The two reactors at Dounreay in Scotland are both the fast breeder type, and use alkali metal coolant – some 130 tonnes of NaK in the case of the Dounreay Fast Reactor (DFR) – and around 900 tonnes of sodium at the larger Prototype Fast Reactor (PFR). Removal of the hazardous inventory of radioactive contaminated alkali metal in a safe, environmentally responsible and cost-effective manner was the first stage of the work, leaving the reactor primary circuits and vessels in a safe state ready for the next phase.
First to be addressed was the PFR, because sodium is slightly easier to work with than NaK, and a dedicated sodium disposal plant (SDP) was constructed in the former PFR reactor turbine hall for this purpose. The SDP reacts small quantities of sodium with large quantities of aqueous sodium hydroxide. Following neutralisation with hydrochloric acid, this produces salt water. The salt water passes through an ion exchange process to remove caesium radionuclides before it is discharged into the sea in accordance with the site’s waste disposal authorisation. The SDP operated from 2004 to 2008, successfully processing over 1,500 tonnes of sodium metal and a small quantity of NaK from the PFR.
|The sodium disposal plant constructed in the former PFR reactor turbine hall|
To treat the NaK coolant from the DFR, a dedicated NaK disposal plant was constructed in the DFR sphere. This operated over four years from 2008, and completed its role to destroy 57 tonnes of primary radioactive NaK in April 2012.
During the chemical process, which again turned the liquid metal into salty water – some 20,000 tonnes of it – an estimated 1,000 trillion becquerels of caesium-137 were removed from the coolant. Liquid metal was lifted in small batches, the alkalinity neutralised with acid and the caesium extracted via ion exchange. Designers thought that the plant would decontaminate the effluent by a factor of 1,000, but decontamination rates were up to 4 million during the operation, reducing levels of radioactivity in the effluent to below the limit of detection. The resin columns used to trap the caesium will now be cemented up and managed as higher-activity waste.
Having removed the bulk volumes of sodium and NaK from the reactors, and having dealt with these safely in a readily controlled manner, attention now turns to the element that remains within the reactors, that cannot easily be removed.
It is estimated that around 3.5 tonnes of residual NaK remain inside the pipes and vessel of the DFR, and a further 9 tonnes of sodium are still estimated to be in the PFR reactor vessel. This needs to be cleansed or destroyed – or both – but the destruction is more complex than the sodium and NaK destruction projects to date, given that in both cases it is difficult to access.
Among the numerous challenges presented by the need to deal with this residual sodium or NaK are those associated with the alkali metals themselves, including the potential for violent reaction, particularly in high humidity, and hydrogen production and its potential for ignition if oxygen is present – although this is avoided with the use of inert purge gases.
There are additional radiological challenges associated with the high dose involved. This is up to 400 sieverts in the PFR core, and around 240 sieverts in the DFR.
While there is a variety of techniques that could be used to address these issues, with varying degrees of success and risk, a proven innovative approach is being taken to treat as much of the sodium and NaK as possible in situ – an approach that will minimise the hazards and risks associated with cutting into the reactors to remove the affected components. Importantly, although it has not been used before in the UK, this has been proven at other sites around the world in projects such as the Experimental Breeder Reactor II in Idaho, USA. Our team has direct experience and knowledge of the processes, risks and safety issues of that project.
Detailed development of the Dounreay project methodology is now underway. A number of techniques will be involved, one of which is the injection of superheated steam into an inert gas system – at 340°C – that contains alkali metals.
Using steam at a temperature above water’s boiling point avoids any condensation. The alkali metal is converted into hydroxides and, at these temperatures, the hydroxide remains molten, and sinks through the molten sodium, always leaving a fresh layer of sodium to react. Once the alkali metal has been converted to hydroxide the system is flushed with an acid solution to remove any residual salts.
This approach has been tried and tested a number of times for single or groups of components and has proven to be controllable and safe. However, application on a larger scale can be more complicated because of the difficulty of getting the entire system up to the high temperature required.
An approach that does not require high temperatures is to use low concentration wet vapour nitrogen. This again reacts with the sodium in a series of so-called bubbles and pops to prevent the build up of significant sodium hydroxide layers. Any hydroxides and salts are then flushed out of the system.
The complexities in this scenario include reaching exactly the right balance to control the process without allowing a sufficient build up of the hydroxide crust that accumulates on the sodium at lower temperatures. This could cause shutdown of the reaction or potential breakthrough of the hydroxide layer, resulting in violent sodium and water reactions.
|The control panel and parts of the pipework installed in the reactor hall|
A third approach that has been proven in the decommissioning of the Idaho Experimental Breeder Reactor II is to use water jets to spray a low volume acidified liquid solution directly into the reactor on sodium layers. This results in small controllable excursions and removal of the hydroxide layers. The acid promotes the reaction and reacts with the hydroxide, preventing crusting.
Once the sodium has reacted, the vessel is then filled with liquid and flushed. This has been shown to be extremely effective, although it again involves challenges – in particular the need for careful control of liquid dosing and monitoring to avoid large violent reactions.
The approach for the Dounreay reactors is likely to involve elements of all three of these methods. It is also likely that some portion of the alkali metal will be extracted and then treated in specific pressure vessels using either the superheated steam or wet vapour nitrogen technique, given that a suitable and safe removal method can be identified. While this approach does have a precedent – smaller liquid metal reactors in Germany, for example, have been entirely dismantled and treated externally – the process has been found to be laborious and time consuming, and to carry a greater risk compared with the in-situ methods, so it will only be used at Dounreay to a limited extent.
For the PFR, the optioneering phase to identify the exact methodology to be implemented ended in 2012. Designs were sufficiently complete to enable the ordering of skid systems for the selected treatment. These will be installed in 2014 for the treatment of the PFR to begin in 2016.
Meanwhile, for the DFR there is a requirement to remove 1,000 or so breeder elements from the vessel before starting treatment. A number of these are swollen, split or stuck, and some will need to be cut out of the reactor grid using tools on a deployment arm. Others can simply be pulled out. However, if many are stuck, a contingency measure will be to deploy a further long-arm manipulator. The items will then be removed through an inert flask and then through an inert cell, where they will be washed to remove the NaK before being packed in a flask for shipment. In the meantime the optioneering phase for the residual NaK removal will be completed over the next two years, including characterisation and system design. System installation is scheduled for 2016, subject to removal of the fuel, and the NaK treatment is to start in 2017. The treatment of the PFR and DFR will be followed by reactor and vessel sizing, and ultimately building demolition in 2022 and 2023.
Importantly, while the proposed approach to the next phase of metal reactor decommissioning at Dounreay is innovative, the methods have been proven to be safe and successful at other sites internationally. The project ahead is complex but it is achievable, and the experience of the Dounreay team brings world-class expertise together to address the challenge.
Equally, there will be opportunities for the Dounreay project to contribute to other international projects, since sodium or NaK disposal as part of the decommissioning of liquid metal reactors and related development projects and research facilities is ongoing, or will be required in many world regions. In France, for example, there are significant quantities of sodium metal to process, while the fast reactor communities in Kazakhstan and the USA also have an alkali metal disposal requirement.
Expertise and experience gained in projects successfully undertaken can in turn be applied to further projects in these regions, and potentially in future in areas of expanding fast reactor development, such as Russia, China and India.
As there are two significant liquid metal fast breeder reactors at Dounreay, other metal reactor decommissioning and sodium or NaK disposal projects around the world will find the Scottish experience invaluable as a contribution of highly specialist expertise and technique development, and the experience will be demonstrable.
Jason Casper is reactors project director at Babcock Dounreay Partnership
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