The first visual inspection of the damaged areas of the Windscale 1 reactor core has been carried out, over 50 years since a fire ruined the military unit. The extra information should help workers dismantle it more quickly.
|Preparatory work at the pile face
(Image: Sellafield Ltd)
The Windscale reactors were plutonium production units built in the pioneering days of nuclear energy. Their graphite cores were found to suffer from build-ups of Wigner energy and although a process was developed to release this energy, it was not fully understood. On 10 October 1957 a runaway Wigner release led to a fire that damaged unit 1 beyond repair.
Unit 2 was closed after the accident, which was at the time the world's worst. That reactor was defuelled after the accident and its chimney stack was been taken down some years ago. Its core - a 'pile' of graphite bricks - remains.
Clean-up efforts at pile 1 have been made more difficult by a lack of information on the true state of the reactor core. In the absence of hard information, planners have had to make the most conservative assumptions on conditions and potential accident scenarios. Fuel channels unaffected by the fire have been cleared, but those in the fire-affected zone (FAZ) still contain nuclear fuel elements - some of them badly damaged.
However, a major advance was made when a computer modelling exercise showed that under no circumstances could the reactor go critical again. Given this information, engineers were able to gain approval from safety regulators to look inside the FAZ for the first time. The FAZ covers about one third of the core, and only about one third of the FAZ is actually damaged.
Using an endoscope, the team was able to find enough information to be sure that their proposed Fuel Channel Removal Tool would be robust enough for the job. They will now undertake more studies towards applying for permission to remove the remaining nuclear fuel and isotopes cartridges before storing and disposing of these. Information gathered during the studies should increase certainty when planning these steps.
The overall fuel removal plan involves cutting through the top of the reactor's reinforced concrete cap and shield to deploy tools and manipulators on both sides of the core to remove the damaged fuel and isotope cartridges. After that, shutdown and control rods will be removed from both piles and reactor internal materials will be removed remotely. The graphite blocks that form the piles will be removed in bulk where possible. After the reactor cores are removed, the remaining internal surfaces will be removed or decontaminated prior to final demolition.