Decontamination and decommissioning

Dissolution solution

13 November 2009

Ten years on, the Dungeness A Magnox dissolution plant is proving to be a UK decommissioning success story. By reducing the amount of waste, the plant is expected to save GBP35 million over its lifetime at the shut down gas-cooled, graphite-moderated reactor. Other UK sites are now considering building similar facilities.

Dungeness’ Magnox dissolution (MXD) plant drastically reduces the amount of waste on site by dissolving metal fuel element debris into magnesium hydroxide and carbon dioxide, to produce magnesium bicarbonate that is discharged to sea. More than 200m3 of debris has been reduced to 3m3 of intermediate level waste.

By reducing the volume of ILW, the long-term storage costs associated with this type of waste are lowered. The MXD plant is expected to save in the region of GBP50 million over its lifetime. Savings are calculated by estimating the disposal, storage and packaging costs of the original volume of waste before the dissolution process (around GBP34.26m) and subtracting the estimated packaging and disposal costs of treated waste (GBP172,000).

MXD plant manager, Ian Cuthbert, said: “We are now in our tenth year of successfully and safely operating the MXD plant. “During this time we have processed a total of around 80 tonnes of intermediate level waste which has seen a big reduction in the amount of storage we need on site.

“The MXD team has built up a wealth of knowledge which we are sharing across the Magnox estate and we are expecting to process all the remaining fuel element debris at Dungeness by the end of 2011.” By then, all the fuel is scheduled to have been removed. Dungeness A was shut down at the end of 2006.

The MXD plant is currently used to process the waste in storage vaults containing fuel element debris (FED), predominantly in the form of solid magnesium-alloy lugs. Lugs are the fin-shaped parts on the outside of the Magnox fuel element casing. They were shaved off to allow the rest of the casing to be tightly packed into flasks for transport and reprocessing at the UK’s Sellafield nuclear facility.

The two vaults, next to the fuel cooling ponds on reactor 1 and reactor 2 respectively, originally contained about 30 tonnes of lugs; so far about 15 tonnes have been dissolved. The reactor 1 lug vault was emptied in April 2009.

An overhead crane with grab lifts lugs from the lug vaults on to a tray. There, workers behind a shielded window sift through the debris with hand tools to find the cobalt-steel springs originally mounted in a clip at the end of the rod. These are now very reactive, and loaded into their own shielded container. The lugs are then tipped into a drum, and transported to the MXD plant, where a tonne of debris takes five to six weeks to dissolve.

The MXD process was originally developed by the Environmental Services department of British Nuclear Group, at the time part of British Nuclear Fuels Ltd, and now owned by VT Nuclear Services. The MXD plant was built in 1986 by Davy McKee, although it was little used until the late 1990s when the decommissioning of the station began to loom large. The MXD plant was refurbished and returned to service in 1999. It was originally designed to process the corroded sludge and debris accumulated in the splitter storage vaults. Splitters, originally used to direct the flow of gas in the reactor, are a different type of FED and with a different metallic make-up. This waste, which represented about 60 tonnes of magnesium, was successfully processed between 2000-2004. The original splitter vaults are now empty. The UK’s Nuclear Decommissioning Agency owns the Dungeness site, whose decommissioning is being carried out by contractor Magnox South, a subsidiary of EnergySolutions.

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