Nuclear Power
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.
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. Te 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. Te 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. Te SDP reacts small quantities of sodium with large quantities of aqueous sodium hydroxide which, following neutralisation with hydrochloric acid, produces salt water. Te 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. Tis 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). Te 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. Dealing with this residual sodium/NaK presents
numerous challenges. Tose 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. Tis 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. Te 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. Te 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. Tis approach has been tried and tested a number of times for single or groups
Fig. 2. Operator taking NaK sample for transfer to Labs.
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