Siltbuster – The Demolition of Sellafield’s nuclear chimney; alkaline water neutralisation.
Background
Director of the Atomic Energy Research Establishment (AERE), John Douglas Cockcroft famously insisted the chimney stacks of the Windscale (now Sellafield) plutonium production reactors be fitted, at great expense, with high-performance filters. Because it was decided to fit them after the stacks had been designed, the filters became pronounced lumps at the top of the chimneys.
The reactors had been designed to remain clean and uncorroded during use, so it was thought there would be no particulate for the filters to catch. The filters therefore became known as “Cockcroft’s Folly”. However, when the core of one of the two reactors ignited the Windscale fire of 1957, the filters prevented a far worse release of radioactive material. It remains as Britain’s worst nuclear accident.
More recently, nuclear clean-up teams have removed the huge diffuser at the top of the Windscale Pile Chimney on the Sellafield site.
Overall Objective
The chimney’s diffuser had to be cut up in-situ, more than 100 metres above ground. Parts were still very radioactive and any dangerous particles had to be prevented from escaping to the environment.
Sections were transported separately down to the ground requiring around 200 moves to clear the entire block.
Work is now taking place to remove a metal frame platform and reduce the access shaft. Once that work is complete, a key delivery milestone will be met.
Following this, work will start on demolishing the chimney barrel.
Key facts
- Demolition work started in December 2018
- The first block was removed from the diffuser in January 2019
- A total of 226 blocks have been removed from the top of the chimney stack
- Each block weighed between 2 and 9 tonnes
- The total weight of all 226 blocks is 1232 tonnes
- This has reduced the height of the stack by 15.42 metres
Workers using a specially-built 500ft (152m) crane cut out concrete slabs with diamond wire saws.
The saw coolant water became highly alkaline and also potentially contained radioactive particles.
Solution
Coolant water from the diamond wire saw fell into the base of the structure and collected in subsurface chambers. This is where Siltbuster’s diffuser plates and pH probe continually measured the pH value and automatically released carbon dioxide gas until neutralisation of alkaline conditions had occurred, the water could then be sent for further treatment in the Sellafield site.
The main pHD unit was placed on the outside of the chimney and away from the radioactive ‘hot zone’, this remote dosing and separate process control is an almost unique attribute of the Siltbuster pHD unit and helped keep operators safe.
When Area Sales Manager for Siltbuster, James Baylis asked how the pHD unit was performing, Project Engineering Manager – Demolition for Sellafield Ltd, George Frost said:
“Other then gas changes and the odd function check, it has been a case of install and leave be, it just does its thing. I am pleased to say there has not been a single instance where pH level has prevented us from obtaining the necessary transfer authorisation. [for the treated water]”
