A chemical plant built 20 years ago to process fuel from Germany’s high temperature reactor programme is the latest piece of Dounreay to be scrapped and turned into radioactive waste.
A special housing was built around the process line to contain any radioactive contamination released during its dismantling.
Workers entered the area wearing airline suits to protect them from any residual radioactivity.
They are now carrying out a radiation survey of the empty floorspace and walls where the process line once stood – the final step before its decommissioning can be declared complete.
The process line was installed inside Dounreay’s uranium recovery plant in 1990 when the site won a contract to recycle 70 tonnes of unirradiated fuel designed for the Thorium High Temperature Reactor, or THTR, at Julich, Germany.
The fuel was in the form of more than 360,000 graphite balls, each of which contained micro-spheres made from enriched uranium, thorium, silicon carbide and graphite.
Each sphere was mechanically broken down to allow the micro-spheres to be separated from the bulk graphite, before the micro-spheres were crushed and dissolved so the heavy metal could be chemically separated and recovered for re-use.
Processing of the fuel took place between 1992 and 1996, when the line was mothballed. The rest of the uranium recovery works shut down in 1998 and the entire plant is now being decommissioned.
The final piece of the thorium line to be removed was an evaporator.
Its successful removal means approximately one-third of the whole uranium plant has been safely dismantled so far and consigned as radioactive waste. Dismantling the thorium line generated almost nine tonnes of low-level waste and 315kg of intermediate-level waste.
The entire building is on course for demolition in 2019.
David Manson, senior project manager at Dounreay Site Restoration Ltd, described removal of the thorium evaporator as a milestone in the decommissioning of the uranium recovery works.
“For six weeks, the team entered the containment built around the redundant equipment in airline suits and used a variety of standard size-reduction techniques, such as metal saws and the like, to clear out the plant,” he said.
“We are now surveying the inside of the empty containment area to make to make sure radiation levels are low and contamination has been removed before we open up this area of the building again.
“When that’s done, we’ll dismantle the containment, move it into position around the next part of the plant scheduled for decommissioning and begin the process again.”
The uranium recovery works is one of the oldest industrial facilities at Dounreay.
Construction started in 1956 and the first nuclear material entered its system in October 1957. Its prime purpose was to provide uranium metal billets for use in the fabrication of fuel elements for fast reactors and material test reactors. The plant took the uranyl nitrate product from two nearby irradiated fuel reprocessing plants, as well as being capable of recovering uranium from fuel fabrication scraps.