Dounreay Site Restoration Ltd -Sodium Residue

Sodium Residue

PFR circuitry and other equipment, such as the reactor vessel and gas circuits, dump tanks, and other circuitry, are coated in sodium. This hazard needs to be dealt with before these items can be removed.

The removal of the bulk sodium means the decommissioning team can now “steam-clean” this coating of sodium using a Water Vapour Nitrogen (WVN) process, making it safe to begin dismantling the entire system.

The WVN process was developed at PFR, and DSRL (supported by NNC Ltd) has carried out an extensive development programme to qualify the process for treating the sodium residue. The process involves in-situ cleaning using mobile units, as well as operation of an on-site sodium inventory disposal plant and an off-site facility at the nearby Janetstown test facility. The WVN process is also to be used to remove the sodium residue from the reactor vessel.

Sodium residues from the secondary sodium circuits have already been removed using this process. These circuits consisted of over 12 km of sodium-contaminated pipe work plus nine vessels each weighing up to 50 tonnes, and three steam drums each weighing up to 80 tonnes (see pictures).

Over 900 tonnes of steel was subsequently removed from the secondary sodium cells during this strip out and size reduction. The size reduction of the major vessels was accomplished using dry diamond wire cutting. The picture opposite shows a vessel being cut using this cold cutting technique. During this size reduction the diamond wire process cut the equivalent of the DFR sphere in half a total of four times.