Last updated 17:15, Friday, 22 August 2008

WITH tonnes of melted nuclear fuel still potentially too dangerous to move, engineers at Sellafield have started the first close-up examination of the Windscale Pile One reactor.

Heart of the matter: Looking into the heart of the Windscale Pile One reactor

The reactor was the scene of the 1957 fire that sent a plume of radioactivity over north-west England.

For the first time in over 50 years the Windscale Pile One Decommissioning Project team has looked inside the heart of Pile One reactor core -at the fire affected zone.

Since the fire in Pile One, in 1957, the reactor has been maintained in a safe and secure manner.

Accessible fire damage has been cleared and safety equipment installed to prevent any release of radioactivity.

Because of the difficulty of physically examining the contents of the core, pessimistic assumptions about the consequences of accident scenarios had to be made which led to extremely complicated and costly engineering solutions being proposed for the safe dismantling of the reactor and its content.

The Sellafield in house magazine Energize reports this week that an extensive study using some of the most up-to-date computer technology to model a range of scenarios, demonstrated that the Pile One fire could not start again and the remaining fuel could not go critical.

As a result of this modelling, the team has been able to gain approval for a safety case which allowed them to look inside the reactor’s fire affected zone (FAZ) meaning that they could see inside the core for the first time in half a century.

Inspections have now been carried out inside the FAZ and the team used an endoscope to obtain pictures from within the core.

The results of the inspection will enable them to ensure that the proposed equipment used to remove the remaining fuel and isotopes is suitably robust.

The current design for the Fuel Channel Retrieval Tool is expected to be successful in removing the fuel and isotopes from the reactor and the majority of the graphite core will be removed in bulk. The more recent survey revealed that the FAZ is approximately one third of the size of the reactor and of that only one third of the FAZ is damaged. It is estimated that about 90% of the core is undamaged.

Further surveys will be carried out in the FAZ to enable the team to gain a complete understanding of the condition of this section of the core. This in turn will allow the Decommissioning Safety Case, the fuel and isotope removal design and development work and the waste storage and disposal strategy to be based on factual evidence rather than assumptions.