Bespoke equipment trial completed at Dounreay

25 March 2015

A trial of bespoke equipment to lift off the top plate of the Dounreay Fast Reactor (DFR) in Scotland in order to remove trapped fuel elements has been completed successfully, the Nuclear Decommissioing Authority (NDA) said on 19 March. The authority said decommissioning the 50-year-old experimental reactor is one of the most technically challenging projects in its estate.

DFR-Breeder-Top-Plate - 360 (NDA)
Breeder fuel matrix (Image: NDA)

A camera looked inside the reactor for the first time in 2012 and subsequent inspections over a three-month period enabled the DFR team to assess the internal conditions in detail, providing an accurate basis for planning future fuel removal, NDA said. The reactor top plate - a stainless steel honeycomb structure - has been submerged in hazardous radioactive alkali metal for decades.

Working with Caithness-based Enterprise Engineering Services Ltd, the DFR team has now carried out a series of trials, using specialised equipment to demonstrate that sections of the reactor top plate can be removed to release the trapped elements underneath, NDA said.

The equipment was manufactured and trialled extensively at the company's off-site facility, before installation works on the reactor top were completed successfully and on schedule. Additional work is planned during the summer to prepare for removal and repackaging of the remaining fuel.

Five tonnes of cables connected to the hub that once controlled the reactor have been ripped out. Large quantities of mineral-insulated copper conductors were used because of their resistance to heat and overall strength even when flattened.

The cables fed into equipment used for reactor control, temperature monitoring, trace heating systems and distribution boards. Their removal will allow the DFR decommissioning team to clear old distribution boards and cables from areas next to the reactor floor, in turn making space for the removal of other fixtures and fittings. The work is due to be complete by June.

Dounreay's experimental fast breeder reactor, housed inside a steel sphere, led British nuclear R&D during the 1950s and 60s. It became the world's first fast reactor to provide electricity to a national grid in 1962. Its 14 MWe output was enough to power a small town like Thurso, with a population of about 9000. It closed in 1977.

The reactor was one of only two ever built in the UK to run on liquid metal – an alloy of sodium and potassium known as NaK. Following closure, the reactor was defuelled, the liquid metal removed from the secondary circuit and some of the breeder material taken out.

Construction started in 2003 on the plant needed to remove the rest of the breeder material and destruction of the bulk liquid metal in the primary cooling circuit was completed in 2012. Once the residues of liquid metal have been cleansed from the circuits and all the nuclear material removed, work will begin to dismantle the reactor structure, including the sphere.

Researched and written
by World Nuclear News