Sodium Coolant
Fifteen hundred tonnes of sodium liquid metal was used in the PFR to transfer heat from the nuclear reaction in the core to the steam-generating plant where electricity was produced. This sodium coolant flowed between the core and the steam-generating plant through circuits linked by heat exchangers.
The use of sodium produced little waste during operation but the volatile compound needs unique and complex treatment during its decommissioning. The sodium combusts in contact with air or moisture and is normally kept safe in a fire-suppressant atmosphere such as nitrogen.
The original reactor design gave no consideration for subsequent decommissioning, resulting in access and engineering difficulties associated with removing the sodium after plant shutdown. This gave rise to many technological challenges to be overcome during the design and construction phase of the plant required to remove and dispose of the sodium.
The Sodium Disposal Plant (SDP) was constructed in a building adjacent to the reactor, on the footprint of the dismantled steam turbine and is the largest of its kind in the world. Its design is based on the successful “NOAH” process that was used to dispose of the sodium coolant from the Rapsodie reactor in France.
Behind thick concrete walls designed to protect workers from the toxic hazard, the alkali metal was converted to salt water by mixing small batches with large quantities of aqueous sodium hydroxide and then neutralising it with hydrochloric acid.
Radioactive caesium was extracted by passing the liquid through an ion exchange column, leaving a cleaned up salt water that was discharged safely to the sea. In total, 1533 tonnes was destroyed this way. The previous record for liquid metal destruction was just over 1000 tonnes set during the decommissioning of a fast reactor in the USA.
“The whole process generated 10 tonnes of clean effluent for every tonne of sodium metal received,” explained project manager Neil McLean.
“The level of caesium in the metal when it came from the reactor was 60 becquerels per millilitre. The level in the salt water that left the plant was below the limit of detection of 0.3 Bq per millilitre. In between, we used 22 ion exchange columns to extract the caesium and these will now be stored as intermediate-level waste.”
Checking the sodium inventory disposal plant at PFR
The sodium inventory disposal plant at PFR
Schematic of the NOAH process in use at PFR
Sodium Reaction Vessel
Sodium Disposal Plant
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