Dealing with the coolant when decommissioning liquid metal cooled reactors, is a demanding and hazardous challenge. Jason Casper looks at how this may be addressed to complete the timely decommissioning of the UK's Dounreay reactors.
Standfirst: Last year saw the disposal of 57 tonnes of alkali liquid metal successfully completed at Dounreay, representing the destruction of one of the most hazardous legacies of Britain’s earliest atomic research. Now, parent body organisation Babcock Dounreay Partnership (responsible, for the decommissioning, demolition and clean-up of the Dounreay nuclear site), working with the Dounreay Site Restoration Limited team, face a further, significant challenge; to tackle the destruction of the hazardous alkali metal remnants inside the reactors vessel, which could not be extracted for disposal in the purpose-built chemical processing plant.
To address the challenges, the project will apply innovative approaches, while drawing on proven techniques and experience, and lessons learned, from other sites worldwide. Equally, the approach taken and knowledge gained from the Dounreay programme will be able to contribute valuable experience and expertise to future alkali metal breeder reactor decommissioning.
Dealing with the nature of the coolant – commonly sodium or the sodium-potassium alloy NaK – is a major consideration in decommissioning liquid metal cooled reactors. Sodium metal reacts vigorously when exposed to water, releasing hydrogen and large quantities of heat – thereby 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 coming into contact with skin, as well as being hazardous to ingest or inhale. Methods of destruction or disposal 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.
The challenges involved in treating the sodium or NaK in such decommissioning programmes are considerable and 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 sodium, which is 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 hundred tonnes of metallic radioactive product into a stable form, while the primary and secondary vessels (when drained of the primary and secondary coolants) will 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 cold traps (which clean the alkali metal coolant of impurities) or caesium traps. [Page Break]
First stage success
At Dounreay the two reactors are both the fast breeder type using alkali metal coolant; the Dounreay Fast Reactor (DFR) using approximately 130 tonnes of NaK, while the larger Prototype Fast Reactor (PFR) used around 900 tonnes of sodium. The first stage in dealing with these was to remove the hazardous inventory of radioactive contaminated alkali metal in a safe, environmentally responsible and cost-effective manner, leaving the reactor primary circuits and vessels in a safe state ready for the next phase.
The PFR was the first to be addressed (sodium being slightly easier to work with than NaK). A dedicated sodium disposal plant (SDP) was constructed in the former PFR reactor turbine hall, and operated from 2004 to 2008 to process over 1500 tonnes of sodium metal and a small quantity of NaK from the PFR. The SDP reacts small quantities of sodium with large quantities of aqueous sodium hydroxide which, following neutralisation with hydrochloric acid, produces salt water. The salt water passes through an ion exchange process to remove caesium radionuclides before it is discharged to sea, in accordance with the site’s waste disposal authorisation.
To treat the NaK coolant from the DFR, a dedicated NaK disposal plant was constructed in the DFR sphere. This began operating in 2008 and completed its role to destroy 57 tonnes of primary (radioactive) NaK in April 2012. An estimated 1000 trillion becquerels of caesium-137 was removed from the coolant during the chemical process, which again turned the liquid metal into 20,000 tonnes of salty water. Liquid metal was lifted in small batches, the alkalinity neutralised with acid, and the caesium extracted via ion exchange. (Designers thought the plant would decontaminate the effluent by a factor of 1000, but decontamination rates of up to 4 million were achieved 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.
The challenge ahead
With the bulk volumes of sodium and NaK now removed from the reactors and dealt with safely, in a readily controlled manner, attention now turns to the remaining element. It is estimated that around 3.5 tonnes of residual NaK remains inside the pipes and vessel of the DFR, with a further 9 tonnes of sodium still estimated to be in the PFR reactor vessel, which needs to be cleansed and/or destroyed. In both cases this is extremely difficult to access, and the destruction therefore more complex than the sodium and NaK destruction projects to date.
The project to deal with this residual sodium/NaK presents numerous challenges. Those associated with dealing with alkali metals include the potential for violent reaction, particularly in high humidity, and hydrogen production with its potential for ignition if oxygen is present (which is avoided with the use of inert purge gases), in addition to radiological challenges associated with the high dose involved (up to 400 sieverts in the PFR core, and around 240 in the DFR).
Among the 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. This minimises the hazards and risks associated with cutting into the reactors to remove the affected components and, importantly, although not used before in the UK has been proven at other sites around the world, in projects such as the Experimental Breeder Reactor II in Idaho, USA, of which the team has direct experience and knowledge of the processes and the risks and safety issues to be considered.
Detailed development of the project methodology is now underway and will involve a number of techniques. The first is to inject superheated steam (which is above water’s boiling point to avoid any condensation) into an inert gas system at 340°C that contains alkali metals. 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 very controllable and safe. Application on a larger scale, however, can be more complicated, due to the difficulty of getting the entire system up to the high temperature required.
An approach that does not require the very high temperatures is to use low concentration wet vapour nitrogen, which again reacts with the sodium in a series of ‘bubble and pops’ to prevent the build up of significant sodium hydroxide layers. Any hydroxides and salts are then flushed out of the system. In this case the complexities include getting exactly the right balance to control the process, without allowing a sufficient build-up of hydroxide crust (which builds on the sodium at lower temperatures) to cause shutdown of the reaction, or potential break-through of the hydroxide layer resulting in violent sodium and water reactions.[Page Break]
A third approach, which 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, resulting in small controllable excursions and removal of the hydroxide layers. The acid promotes the reaction and reacts with the hydroxide preventing the 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. Additionally, it is 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 (for example smaller liquid metal reactors in Germany have been entirely dismantled and treated externally), the process has been found to be laborious, time-consuming and carry greater risk compared to the in-situ methods, so will only be used at Dounreay to a limited extent.
The optioneering phase to identify the exact methodology to be implemented for the PFR was completed in 2012, with designs sufficiently complete to enable skid systems for the selected treatment to be ordered. These will be installed in 2014 for treatment of the PFR to begin in 2016.
For the DFR there is a requirement to remove 1000 or so breeder elements from the vessel, a number of which are swollen, split or stuck, before commencing treatment. Some of these will need to be cut out of the reactor grid using tools on a deployment arm while others can simply be pulled out, and 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. Meanwhile, the optioneering phase for the residual Nak removal will be completed over the next two years, including characterisation and system design, with system installation scheduled for 2016 (subject to removal of the fuel) and the NaK treatment to start in 2017. The treatment of both the PFR and DFR will be followed by reactor and vessel sizing and ultimately building demolition in 2022 and 2023.
In short, the next phase of the metal reactor decommissioning at Dounreay is complex, but it is achievable, and while the proposed approach is innovative, the methods have been proven to be safe and successful at other sites internationally. The experience of the Dounreay team brings world class expertise together to address the challenge. Moreover, as two significant liquid metal fast breeder reactors, the experience at Dounreay will also be invaluable as a contribution of highly specialist expertise, technique development and demonstrable experience, to other metal reactor decommissioning and sodium or NaK disposal projects around the world.
Jason Casper is Reactors Project Director at Dounreay, Caithness, Scotland. www.dounreay.com