Uncategorized

Decommissioning Dounreay

Issue 1 and Volume 3.

By Nancy Spring, Editor

Click to Enlarge
Dounreay from the air, July 2006. All photos, courtesy Dounreay Site Restoration Ltd.

Located on the north coast of Scotland, Dounreay was the center of the U.K.’s fast breeder reactor research program from 1954 until 1994. Three reactors—two fast breeder reactors and one research reactor—a chemical reprocessing plant and various waste facilities were built there.

But the fast reactor proved to be more expensive than thought and in 1988 Britain cancelled the program. All research and development there has now ceased and each facility has to be cleaned out, the waste managed safely and the structures dismantled. With a finish date of 2025, Dounreay is one of the most complex decommissioning projects in the world.

Click to Enlarge
Manufacturing fuel elements in the D1202 fuel fabrication plant prior to its closure in 2004 and subsequent decommissioning and demolition.

Dounreay’s purpose was to research and develop more efficient ways to generate electricity from uranium and plutonium. The knowledge gained from this work made Dounreay a world leader in the development of nuclear energy for electricity production.

Various fuel types were tested in the reactors. The used fuel was recycled in chemical plants and the recovered uranium and plutonium turned into new fuel elements. By 1994, the site had built up a stock of more than 100 tons of uranium and plutonium, much of it in a variety of specialist forms.

Click to Enlarge
Decommissioning the D1204 materials test reactor fuel reprocessing plant.

The experimental fast breeder reactor at Dounreay—the DFR—led British research and development of nuclear energy during the 1950s and 1960s. Housed inside a steel sphere, it was built between 1955 and 1958 and became the first fast reactor in the world to provide electricity to a national grid in 1961. The 14 MW DFR was closed down in 1977.

Dounreay’s prototype fast reactor (PFR) was the second and last fast reactor to be built in the U.K. Construction commenced in 1968 and the PFR went critical in 1974.

Click to Enlarge
Workers used a Brokk remotely-operated machine to break up reinforced concrete plinths that had provided shielding for the active liquor slab tanks.

The PFR had the dual role of providing power to the national grid and offering unique research and development facilities. The reactor was 250 MWe—660 MW thermal output, although the original design intent was 600 MW.

The plutonium metal fuel was cooled by sodium liquid metal designed to remove heat from the reactor core. The heat was transferred via the primary and secondary sodium circuits to the steam raising plant that fed a conventional steam turbine.

Click to Enlarge
Archive image from 1985 of operations in the laboratory (pulsed column lab).

The PFR closed in 1994. It was de-fuelled and the 1,500 tons of bulk sodium that once flowed through the primary and secondary circuits removed.

In early 2009, a 7-year project to clean up the dirtiest area of Dounreay’s uranium conversion plant was successfully completed on time. A team of 12 workers managed by Dounreay Site Restoration Limited (DSRL) carried out the clean-up work in the “amber” area. DRSL, a subsidiary of the Babcock International Group, is the site licence company responsible for the closure program at Dounreay. Dounreay belongs to the Nuclear Decommissioning Authority, a non-departmental body of the U.K. government.

Click to Enlarge
Construction of the plutonium criticality laboratory in the 1950s.

When the plant was operating, the amber area housed equipment to dissolve uranium in acid to recover the reusable material and remove the waste products. Industry regulators placed a high priority on the decontamination of the amber area.

Before any clean-up work could start, the ventilation system had to be upgraded. Redundant glove boxes and solvent extraction plant were stripped out, cleaned up and size-reduced. Four-inch-thick slab tanks that once held radioactive liquid were removed and the team employed a remotely-operated Brokk to help demolish the 32 reinforced concrete plinths that separated the tanks. Approximately 130 tons of concrete rubble were designated as low level waste.

Click to Enlarge
Demolition of the plutonium criticality laboratory, February 2009

Preparations for the removal and clean-out of the third PFR dirty dump tank are underway. The massive cylinder has been isolated and dismantled from its surrounding support structures and is ready to be hoisted out.

Once an integral part of the heat transfer system, two of the four metal tanks have already been safely removed from the redundant plant. The solidified sodium resembles pink putty. After it is dug out using high powered chisels, it is ready for size reduction and disposal. Removal of all four tanks is expected to be complete by May 2010.

Decommissioning the DFR is one of the most significant challenges. A blanket of uranium and plutonium breeder material is submerged in 57 tons of liquid metal. Both need to be removed to enable the rest of the reactor to be cleaned out and dismantled. Decommissioning is expected to be complete by 2024.

 

More Nuclear Power International Issue Articles

 

 

Nuclear Power International Issue Archives

 

 

View Power Generation Articles on PennEnergy.com