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Incident in the THORP reprocessing plant at Sellafield

ThorpA substantial leakage of radioactive liquid was discovered on 21 April 2005 in the feed clarification cell of the THORP plant at Sellafield in Great Britain. The THORP plant – operated by the British Nuclear Group Sellafield Limited (BNGSL) – is designed to reprocess irradiated fuels produced by advanced gas-cooled reactors (AGR) and light water reactors. Such uranium oxide fuels are enriched in uranium-235 by up to 5%. Its reprocessing design capacity is estimated at 800 tonnes per year, with the plant having already reprocessed around 5,700 tonnes since its commissioning in 1994. Plant operations consist in cutting the fuel into sections of a few centimetres long, before dissolving them in boiling nitric acid and centrifuging the resulting fluids to remove fine solids. A liquid-liquid extraction process using the selective solvent tributyl phosphate (TBP) separates the uranium and plutonium from the fission products. The finished purified products are then stored in the form of uranyl nitrate and plutonium oxide awaiting recycling.

On 21 April 2004, about 83 m3 of clarified radioactive fluid leaked into one of the recovery pans and was discovered during a camera inspection of the main feed clarification cell. This cell is closed off to personnel at all times and its walls guarantee the radiological protection of the adjacent premises. The toxic fluid present in the recovery pan contained uranium and plutonium that were yet to be separated from the fission products and estimated to represent about 20 tonnes and 200 kg respectively. The plant was shut down as soon as the incident was discovered.

The main cause of the leakage was said to come from a fractured pipe running between two accountancy tanks. The origin of this fracture is not yet known (welding error, mechanical stress or corrosion, etc.) and no further information is available on why this leakage was not detected at an early stage.

The plant’s reprocessing units are all equipped with three containment barriers (the reprocessing equipment, the cell walls, the building and related ventilation filters) that are designed to isolate the radioactive material from the environment. Only the first barrier consisting of the transfer pipe was at fault during this incident. The static and dynamic integrity of the two remaining containment barriers remained intact. The operator emphasized that the leak posed no danger to the workers or the environment. In particular, no abnormal activity around the plant’s stack has been detected. The operator also underlined the absence of any risk of criticality*, which was corroborated by the British Safety Regulator.

The operator’s corrective actions involve recovering the liquids to be stored in existing buffer storage tanks before being able to clean the cell and repair the pipes. These operations – currently under investigation – are likely to take a long time and require special procedures to avoid all related radioactive risks (radioactive dispersion, criticality, etc.).

In similar facilities belonging to COGEMA at La Hague (cutting & dissolving workshops in the UP3 and UP2-800 reprocessing plants), a range of measures exist, designed to detect major leaks enabling a quicker response from operators. Despite these rather positive aspects, such facilities will be thoroughly re-examined once more ample information becomes available.

* Risk of uncontrolled chain reactions.

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