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Dounreay DFR decommissioning
Dounreay, near Thurso, Caithness, Scotland, UK
associated engineer
Dounreay Site Restoration Ltd
date  2008 - 2025 (proposed date)
era  Modern  |  category  Power Generation  |  reference  NC985670
ICE reference number  HEW 1708
The Dounreay Fast Reactor — the UK’s first fast breeder nuclear reactor to generate electricity for public consumption — is now being decommissioned. It ceased operating in 1977 and is one of three nuclear reactors at Dounreay. The whole site closed in 1998.
The Dunreay Fast Reactor (DFR) went critical on 14th November 1959 and first exported power to the UK's National Grid on 14th October 1962. Dounreay power station lead fast breeder reactor research until 1994.
In 1998, it was announced that the whole site would close. Dounreahy Site Restoration Ltd, which is a subsidiary of the UK Atomic Energy Authority, has been responsible for the site's closure programme since 1st April 2008. They have a Lifetime Plan for the site, which outlines the schedule of works. The decommissioning of the DFR is a top priority for the government's Department of Energy & Climate Change, but only part of the overall plan.
The DFR’s reactor is housed in a vault covered by a 41.1m diameter mild steel sphere, painted green and weighing 1,500 tonnes. Air pressure inside the sphere is below atmospheric pressure, and entrance into it is through a small airlock. It was designed to implode and contain contamination in the event of an accident.
The DFR converted unusable uranium 238 into plutonium 239, an alternative fuel for nuclear reactors that is 30,000 times more active than uranium 235 (the more usual fuel). A by-product of the chain reaction is caesium, also radioactive. Unlike most later reactors, the DFR used NaK (an alloy of sodium and potassium) as the coolant. It can cause fires and reacts with water.
After the DFR ceased operating in 1977, there was one fuel assembly and 977 fuel breeder rods stuck in the reactor’s core. There were almost 1,700 tonnes of NaK in the secondary cooling circuit, which were removed in the earlier stages of decommissioning. However, the primary cooling circuit — 9km of pipework closer to the nuclear material inside the core — still contains 57 tonnes of NaK. This coolant residue has become contaminated with caesium.
The residue is a mixture of three metals with similar chemical properties, which proved difficult to break down to remove the caesium. A resin was developed in Finland that traps the larger caesium ions while the NaK ions pass through, and it has a decontamination factor of around four million — four thousand times more effective than expected.
The contaminated NaK is removed in 160kg (200 litre) batches through the entry original point and kept warm to prevent crystallisation, in shielded stainless steel pipework. Each batch is added to water, giving a metal hydroxide mixture. Nitric acid is added to produce metal nitrates. The 2,000 litres of effluent is twice the strength of standard household drain cleaner. Then the ion exchanger recovers the radioactive caesium.
Batches take 16-18 hours to process and one batch is treated every 2-3 days. The total quantity of radioactive caesium recovered is likely to be about 8g for the entire Dounreay site — a tiny amount. By February 2010, some 12 tonnes of contaminated NaK had been destroyed and the decontamination should be completed by 2013. The spent resin has been sealed inside a drum and stored as intermediate-level waste on site.
Once this has been done, the nuclear fuel and breeder elements can be removed by a decommissioning robot developed at Dounreay. It will then dismantle the reactor, which is likely to take until 2016. During these operations the sphere — which is also radioactive — will be contained within a new structure so that the airlock can be removed.
The vexed question of what to do with the iconic sphere has yet to be resolved. It would be expensive to maintain — painting the outside costs £500,00.
Other works to complete the decommissioning of the DFR include upgrading the electrical and ventilation systems, refurbishing the Goliath crane inside the reactor, and emptying and demolishing a fuel storage pond.
All DFR decommissioning operations should be completed by 2025, at a cost of some £240m.
Main contractor: Dounreay Site Restoration Ltd
Research: ECPK
bibliography
www.dounreay.com
www.nce.co.uk
reference sources   CEH SHI
Location

Dounreay DFR decommissioning