Nuclear decommissioning project passes milestone

AV Technology's work on the decommissioning of the Trawsfynydd nuclear power station involves monitoring stresses in the reactor walls during boiler height reduction work.

AV Technology (AVT) has completed the second of four phases of a critical structural monitoring project during the decommissioning of the Trawsfynydd nuclear power station in the heart of Snowdonia National Park.

The work is being carried out for main decommissioning contractor Doosan Babcock in conjunction with British Nuclear Group (BNG) and involves monitoring stresses in the reactor walls during boiler height reduction work.

The station ceased electricity production in 1991 and the decommissioning of the Trawsfynydd site reached a critical stage in 2005, with the commencement of major works to reduce the height of the two 52m high reactor buildings and reconfigure their appearance so that they blend more aesthetically with the surroundings.

Each of the reactor buildings houses six boilers arranged three either side of the reactor.

Each boiler, approximately 40m tall and 6m in diameter, weighs 1000 tonnes and these have to be reduced in height before work can start on the reactor buildings themselves.

Trawsfynydd site manager Simon Parsons puts the task into perspective: "This is a historic milestone for everyone involved and the culmination of four years of effort and preparation".

"In this first stage we are reducing the height of the old boilers by about one third by cutting them into sections".

"Reducing the height of the boilers is a huge challenge and this is the first time such an operation has been carried out anywhere in the UK".

"The boilers are constructed of solid steel up to 180mm thick and we have had to develop new technologies and techniques to facilitate their safe removal".

The cut sections, weighing up to 105 tonnes each, are being lowered into suitable storage spaces inside the building using a specially developed mobile boiler lifting rig mounted across the reactor walls.

The project has been divided into four stages, each involving work on three boilers at a time.

After each stage, which lasts approximately 12 months, the lifting rig is moved and a new monitoring system is set up.

From the outset it was clear that measures had to be taken to ensure the reactor structure could safely withstand the additional loads introduced by the lifting process and as Doosan Babcock's Project Manager Terry McAleer explains: "The first boiler deplanting at Trawsfynydd was carried out in 2005".

"In conjunction with BNG and AVT, a wall monitoring system was installed which allowed us to demonstrate effectively that the structure of the building was not being loaded beyond the planned limits during these complex lifting operations".

"In 2006 AVT won the contract to install the second set of wall monitoring equipment which was safely and successfully installed to programme".

AVT's 36-channel wall monitoring system is designed to measure the stresses in the concrete reactor walls and boiler box walls.

It records data from vibrating wire gauges (VWGs), strain gauges, crack displacement sensors and temperature sensors strategically placed at different heights and locations on the walls and selected steel work.

The VWGs are mounted directly to the concrete walls and are suitable for such applications, being insensitive to moisture, long cable runs and low signal levels.

Their operating principle is very simple: the units comprise a taut steel wire tensioned between two steel mounting blocks which are in turn attached to the surface of the structure.

An electromagnetic coil is used to 'excite' the wire at its natural resonant frequency and to measure the changes in vibration frequency as the strain in the wire changes.

The data from the gauges and sensors is recorded every 20 seconds on the Campbell Scientific CRX data logger.

The proprietary logging software then downloads data every 60 seconds to the PC.

Both the data logger and the PC are mounted at the 36m level.

Real-time data monitoring (RTDM) software is used to display data on the screen in two separate formats.

One shows the data in numeric format, displaying the last data value collected for each sensor location.

The second shows the data for the last 60 minutes displayed in graphical format.

The modem built into the data logger allows AVT personnel to access data remotely at their offices in Stockport.

In the event of the outputs from any of the gauges exceeding predetermined threshold levels, the system triggers an audible and visual alarm mounted at the 40 metre level to warn the crane driver and other site personnel, allowing time for appropriate action to be taken.

AVT has considerable experience in such complex data collection operations and as their Technical Director Neil Parkinson explains: "The key to successful structural monitoring is being able to collect, analyse and interpret large amounts of good quality information and then to present this information clearly and concisely for others to make informed decisions relating to the structure with high levels of confidence".

"All round good communication and the ability to work closely with other interested parties are also vital ingredients".

"This project has presented a number of technical challenges, including working in a highly controlled nuclear environment".

"In addition the complexities of securing and wiring gauges at height posed its own problems, requiring a strict health and safety regime".

"In the second phase roped access was used during the fitting of some sensors and this technique removed the need for time consuming scaffolding erection".

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