U.K.'s Dounreay Nuclear Reactor Decommissioning Moves Ahead

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Researched by Industrial Info Resources (Sugar Land, Texas)--The Dounreay nuclear reactor, located in Scotland, has started the process of destroying liquid metal material. This step is considered to be the single largest hazard in decommissioning the nuclear power plant. About 5,000 liters of liquid metal material has been successfully and safely removed and treated before destruction. The process of destroying 30 batches of the liquid metal from the coolant unit of the reactor has been completed 30 days ahead of schedule. The removal and safe destruction of the highly radioactive and severely contaminated sodium-potassium (NaK) liquid metal is a very important and crucial stage in the reactor decommissioning project, supervised by the Nuclear Decommissioning Authority (London, United Kingdom). The safe disposal of the NaK coolant, the second most hazardous element in the reactor complex, in a way that meets environmental standards is a milestone for the project team at Dounreay.

Liquid metals are used in fast reactors to transmit heat to a steam-generating system, where power is produced from the reactor core. The coolant moves through circuits that are connected by heat-exchange units. The Dounreay reactor used a combination of two metallic alkalis--potassium and sodium. This alkali combination, which generates minimal waste during the reactor operations, has to be treated and handled very carefully during the decommissioning process, as it combusts very easily when in contact with water or air. This coolant is stored in a flame resistant atmosphere of nitrogen.

The redundant Dounreay nuclear reactor site, located in the Scottish Highlands, consists of two fast breeder reactors and one thermal test reactor. The complex also houses chemical plants that were used to recycle fuel from the reactors using the process of chemical separation. The recycled plutonium and uranium from the process were used as new fuel. The nuclear fuel input for the chemical plants came from Dounreay as well as other reactors in the U.K. and other countries. On April 1, 2005, the Nuclear Decommissioning Authority took over the Dounreay nuclear site, with plans to decommission the plant. The entire decommissioning operation is expected to cost about $5.8 billion.

In 1977, the NaK liquid metal coolant in the primary unit of the nuclear reactor became contaminated. Between 1979 and 1981, about 110 tons of NaK in the second unit, which was considerably less radioactive than the coolant in the primary unit, was cleaned. The process involved spray burning, followed by hydroxide conversion. After treating the hydroxides chemically, they were discharged into the sea. After destroying about 100 kilograms of liquid metal, the process in the primary unit was stalled because of concerns regarding the sea-discharge of highly radioactive cesium-contaminated effluents. Research and development efforts began to devise a more environmentally sustainable and safer method to destroy the NaK in the primary unit.

The new, safer technique transfers NaK in batches to an enclosed and protected treatment plant through a 9-kilometer pipeline from the primary unit. In an atmosphere of nitrogen at the treatment plant, which is located near the primary unit, NaK reacts with water to form hydroxide. Neutralization of the hydroxide is then carried out with the help of nitric acid. Highly radioactive saltwater, which is also produced during this reaction, is passed through ion-exchange columns, which absorb cesium. The water, which now has low levels of radioactivity, can be discharged into the sea. The level of radioactivity in the water is within the range specified for sea discharge by the Scottish Environmental Protection Agency (Stirling, United Kingdom). The cesium-absorbed columns are stored as intermediate waste.

NaK is one of the most widely used liquid metal coolants in the nuclear power sector. NaK, which is non-corrosive, is compatible with most nuclear fuels. But the combination is highly volatile and ignites immediately on contact with air or moisture. One of the first metal coolants used in nuclear reactors was mercury, which was available in liquid form at room temperature. This metal, which had very low heat conduction properties, was also found to be heavily toxic, generating poisonous fumes. Lead, which has a very high boiling point, was also one of the liquid metal coolant options. While the high boiling point of lead was favorable, as it could cool the reactor at even higher temperature, it caused the refueling and service process to be very risky. As an alternative, bismuth was used along with lead.

According to the scheduled plan published on the Dounreay website, the next step in the Dounreay reactor decommissioning process will be the dismantling of the materials test reactor. This is scheduled for 2010 and will be followed by destroying bulk liquid metal in 2013 and commencing the low-level waste disposal process in 2014. The breeder reactor removal in 2015 will be followed by dismantling tanks containing active liquor in 2018. The process of removing the fast reactor reprocessing unit is expected to begin in 2021, while the cleaning and clearing of redundant facilities is estimated to start in 2025. The land is expected to be available for re-use by 2297. The decommissioning of the Dounreay fast breeder reactor is expected to receive funding of about $255.4 million this fiscal year. The government has also allocated respective budgets of $252.3 million and $299.4 million for the 2010-11 and 2011-12 fiscal year.

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