UKAEA ^{A World Leader in Decommissioning & Waste Management.}

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Project background

The Windscale Piles were among Britain’s earliest nuclear reactors, constructed shortly after the Second World War. Operational between 1950 and 1957, they were primarily used to produce plutonium for the UK nuclear defence programme.

Following a core fire in Pile 1 in October 1957, both piles were permanently shut down. Pile 2 was subsequently completely defuelled and all except 15 tonnes of damaged fuel was removed from the Pile 1 core. The reactors were then kept in safe care and maintenance until the early 1990s, with regular inspections to ensure that no radioactivity was being discharged. Decommissioning work then began in 1993.

The unknown conditions within the Pile 1 reactor and the remaining inventory in Pile 1 make the Piles project one of the world’s most challenging nuclear decommissioning tasks.

Key challenges

The objective of the first stage of the decommissioning project was to improve the safety of the facilities and to provide data for future phases.

This phase of decommissioning involved the installation of active ventilation and core condition monitoring systems, sealing the biological shields of the reactors and removing debris from the air and water ducts. In addition surveys were completed to assess the scale of damage to the Pile 1 core and to verify conditions within Pile 2.

Our approach

This project was one of the most complex civil and mechanical engineering operations we’d faced to date, involving over 2,000 hours of remotely-operated vehicle and manipulator activity.

Specialist staff and contractors with experience of working in hazardous environments were employed to carry out the various decommissioning tasks. Wherever possible, we used existing equipment and technologies that we modified and adapted to suit the unique environment.

Clearing the water ducts was particularly challenging. The work involved removing fuel debris from the fire and accumulated sludge, which was submerged under 750m3 of water in each duct. We developed innovative techniques, adapted from North Sea oil technology, including the use of remotely- operated vehicles. The vehicles were equipped with tools to handle the solids and sludge, and were deployed on existing rails in the ducts.

The results

We successfully completed the first phase of decommissioning in 1999 – isolating the Piles from the fuel storage pond and the chimneys, and providing appropriate levels of ventilation and monitoring. The air and water ducts have been completely cleared of fuel and isotope debris, and the water ducts have been fully drained. We now have a clearer picture of the conditions in the fire-affected region of the Pile 1 core and confirmation of conditions within the Pile 2 core, thanks to a series of surveys and analysis of graphite samples retrieved from the reactors.