Radar and level switch in nuclear decommissioning project

14 February 2012

Dounreay in north of Scotland is one of a number of nuclear sites in the UK currently in the process of being decomissioned. Non-contact radar technology is being employed here to help ensure the safe packaging of material which is being transferred for further processing.

The VEGASWING level switch with protective cage and VEGAPULS radar mounted behind the larger filling pipe to measure and detect the water level in the flask as it is filled
The VEGASWING level switch with protective cage and VEGAPULS radar mounted behind the larger filling pipe to measure and detect the water level in the flask as it is filled

Decommissioning generates different types of waste, from conventional industrial wastes to hazardous materials, such as asbestos and radioactive materials. Waste which cannot be safely recycled has to be segregated and processed on-site, ready for transportation and further processing, storage or disposal.

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 to protect 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 process requires precision – the flask needs to be filled to the top, but must not over-spill, as a water clean-up in the filling area is highly undesirable.

An EMPHASIS approved non-contact (or through air) VEGAPULS radar is being used for the continuous level measurement in this application. 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’. 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 has 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 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, to avoid electronics exposure to radaiation. 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 is mm repeatable. It uses tuning fork based level switch, this consists 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. It also had 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.

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 use 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 over many metres beyond the antenna, still with very 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 NII. It is now deemed acceptable for this sensor to be deployed in suitable SIL1 applications for the UK Nuclear industry. More information, search on your browser for articles on ‘VEGA EMPHASIS approved radars’.

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