Nuclear challenge

11 April 2008

Contractor Doosan Babcock is preparing for what client British Nuclear Group (BNG) refers to as deferred site clearance (DSC) at the UK's Trawsfynydd power station. Typically when the decommissioning of nuclear facilities are concerned, the time spans involved are immense – far longer than those facing most demolition contractors in their daily work – and the hazards are obvious.

DSC is the current accepted strategy for decommissioning the UK's Magnox nuclear reactors (Magnox is the name of the magnesium-aluminium alloy used for fuel cladding in such facilities). It allows for full site clearance around 100 years after the station stops being used for power generation. The strategy helps to achieve a number of benefits. It is a safe option that minimises any risk to the public, workers involved on the job and the environment, while at the same time minimises the volume of radioactive waste. In addition, from a technical perspective it is the most straightforward option while at the same time makes the best financial sense.

DSC works through a number of distinct phases. All fuel is removed from the reactors and sent to the UK's reprocessing facility at Sellafield – once the fuel is removed, 99% of the radioactive hazard is also removed. In addition to the refueling work, the site is also prepared for its 100-year care and maintenance phase. This involves the progressive removal of existing buildings on the site – with the exception of the reactor buildings themselves – and the erection of temporary buildings, including those required to help with retrieval and packaging of radioactive wastes.

Meanwhile, the reactor buildings are placed into a safe storage condition and modified as necessary to ensure that they remain safe, secure and weatherproof. Some of the remaining radioactive waste will be collected and packaged. Since there is currently no central disposal facility in the UK for intermediate level wastes, facilities will be provided on each site to allow for its safe passive storage.

Trawsfynydd Decommissioning

Located in the scenic Snowdonia National Park, the Trawsfynydd power station ceased generating electricity in 1991 and the decommissioning of the plant then began. In 2005, major work began to reduce the height of the two reactor buildings. The aim of this work is to alter their appearance to allow them to blend in better with the surrounding countryside. Each reactor building houses six boilers, with three to each side of the reactor.

Each boiler, standing some 40 m tall, is 6 m in diameter and weighs 1000 tonnes, and is constructed from solid steel up to 180 mm thick. The height reduction work is aimed at removing almost one third of each boiler's height, and must be completed before work can begin on the reactor buildings themselves. This is the first time work of this nature has been undertaken in the UK

A four-stage programme has been developed for the work, with each stage working on three of the boilers at a time. Each takes almost one year to complete, and the first began in 2005.

To reduce the height of the boilers, each is cut into sections that weigh up to 105 tonnes apiece, which are then lowered into suitable storage spaces inside the building using a specially developed mobile lifting rig, costing in excess of UK£ 1 million (€ 1,4 million), that is mounted across the reactor building walls.

Once each 12 month stage is completed, the lifting rig is moved to a new position to begin work on the next phase. Two of the stages have now been completed and work on the third phase has commenced.

Given the nature of the work and the location, it was obvious from the start that a means would be necessary to monitor the structure of the reactor to ensure that it could safely withstand the additional loads that it would experience during the lifting process.

As a result, AV Technology (AVT) was brought on site for both phase 1 and 2 of the boiler reduction programme to install a wall monitoring system that allowed main contractor Doosan Babcock to demonstrate that the structure of the building was not loaded beyond the planned limits during the complex lifting operations.

AVT installed a 36 channel wall monitoring system designed to measure the stresses in the concrete reactor walls and boiler box walls. It recorded data from vibrating wire gauges, strain gauges, crack displacement sensors and temperature sensors located at strategic locations at different heights and locations on the walls and selected steelwork.

The data from the various gauges and sensors was recorded every 20 seconds on a Campbell Scientific CRX data logger. The proprietary logging software then downloads data every 60 seconds to a PC (both mounted at the 36 m level). Real time data monitoring software was used to display data on the screen in two separate formats – one showing the data in numerical format displaying the last data value collected for each location sensor and the second shows the last 60 minutes of data in graphical format.

Should any outputs from any of the individual gauges and sensors exceed established threshold levels, an audible alarm and a visual alarm mounted at the 40 m level act as a warning for the crane operator and other personnel on site to allow appropriate action to be taken.

The success of the monitoring systems over the first two phases of the work has resulted in AVT having been selected to install a similar monitoring system for phase three of the boiler height reduction work.

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