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A decommissioning success story

Published: 10 February 2012 - Joe Bush

Radars and switches from VEGA Controls are playing a key role in the decommissioning of Dounreay Power Station in Scotland

The process of decommissioning the UK’s nuclear power stations can create a range of different types of waste - from conventional industrial wastes to hazardous materials such as asbestos and radioactive materials.

Waste which cannot be safely recycled is segregated and processed at site ready for transportation and further processing, storage or disposal. About 50 facilities on the site at Dounreay have a history that involved the presence of radioactive materials, and special controls are in place around these to contain radiation. Careful planning and risk assessment are key factors in the decommissioning process - keeping the exposure of workers to radiological and other hazards to a minimum.

Some items need packaging and shipping in approved transport flasks. The loading of these flasks involves radioactive materials being placed inside and then filled with water, which protects the contents during transport. This loading area is very hazardous and requires remote, robotically operated handling and carefully designed process control systems. The water filling requires precision as the flask needs to be filled right to the top, but must not over spill.

Radars

An EMPHASIS approved non-contact (or through air) VEGAPULS radar is being used for the continuous level measurement. VEGASWING has also been supplied for the high level switch (a fail safe vibrating fork), also approved for use as it is classified as a ‘simple device’ (no microprocessors). They have been specially engineered for the application by VEGA engineers working with the customer.

The extensions to the VEGAPULS waveguide and antenna, as well as the VEGASWING tuning fork, also have bends to enable them to reach as well as accommodate manoeuvring inside the filling area to carry out their tasks. Both devices were specially designed and fabricated for the application. They are mounted on a pivoting rig, which can be swung over prior to filling for level monitoring, and swung back when the flask is lifted in and out.

The radar electronics housing, which incorporates the microwave generator, is mounted well away from the container and hazardous area. The only radar parts inside the area are a stainless steel tube wave guide and a cone antenna, which means electronics exposure to radiation is avoided. The microwaves, which carry out the measurement, are guided along the tube then transmitted and received back by the cone antenna at the end.

VEGASWING offers a level switch point which uses a tuning fork-based level switch, consisting of a small piezo drive screwed to the back of the forks to produce the vibration needed to detect liquid. A cable runs up inside the tube to the electronics module in the housing, where the vibrating element is monitored and the switch output connected. The VEGASWING also has a bent tube design engineered to work with the filling arm and mount alongside the radar. The tuning forks are also mounted in a protection cage in case of accidental damage during manoeuvring of the filling rig and flask.

Technology explained

VEGAPULS Radar uses microwave pulses which transmit to, and reflect back from, the product surface. In this instance they travel initially inside a tube several metres long before being transmitted through air. Many non-contact systems would pick up too much noise and signal interference from the ‘tube’ to offer the reliability and accuracy needed. As VEGA uses pulsed radar with advanced ECHOFOX signal processing, all echoes are discrete in time and the software can be easily set-up to only measure, in this case, over a specific 180mm measuring range beyond the end of the cone antenna. Radars can measure far beyond the antenna - still with good accuracy and repeatability.

The VEGAPULS radar level transmitter, originally assessed by Exida for SIL applications, was also independently assessed by Sellafield as part of the EMPHASIS project to satisfy the Nuclear Installations Inspectorate (NII). It is now deemed acceptable for this sensor to be deployed in suitable SIL1 applications for the UK nuclear industry.

VEGASWING is a universal liquid level switch. As it has no microprocessors, this allows it to be classed as a ‘simple device’ thus making it suitable for many applications in the nuclear industry. By using an ‘active element’, it has an inherently fail safe design, where power loss, damage, build-up or corrosion will cause the switch to fail safe. When it touches liquid, the vibrating tuning fork element is damped and it gives a switch output. This micro-dynamic ‘solid state’ mechanical technology enables it to detect almost any type of liquid, without needing floats or linkages. It doesn’t require the ‘tuning with product level’ for set-up associated with solid state electrical level probes. It conforms to IEC 61508/11: SIL 2 and there is also a remote self test option for checking operational readiness.

VEGAPULS EMPHASIS approved radars look set to play a big part in the nuclear industry, both on decommissioning and future new builds - helping to meet the needs of some key applications as well as the security of design, cost and engineering criteria for more ‘routine’ applications. VEGASWING also has the potential to provide reliable, fail safe level detection on many liquids with its capability for ‘universal’ level detection and classification as a ‘simple device’.

VEGA Controls

www.vegacontrols.co.uk

T: 01444 870 055

Industry Connections: VEGA Controls Ltd


 
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