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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 system up to the high temperature required. An approach that
Fig.
3.NaK sample being transferred in Nitrogen padded container.
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.
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. Te 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. Tis 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. Te 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. Te optioneering phase to identify the 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. Tese will be installed in 2014 for treatment of the PFR to begin in 2016. ●
Jason Casper is Reactors Project Director at Dounreay, Caithness, Scotland.
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