PLCs take charge of nuclear disposal

Mitsubishi PLCs are at the heart of a disposal system hekping to decommission the UK Atomic Energy Authority installation at Dounreay.

Even though not producing of nuclear powered energy anymore, activity at the United Kingdom Atomic Energy Authority (UKAEA) installation at Dounreay is far from over.

The contract to design, construct and commission a system for the disposal of the bulk liquid metals (sodium and sodium/potassium alloy) at the Prototype Fast Reactor (PFR) was recently awarded, with the main contractor choosing to use Mitsubishi Electric's Automation products to control the entire application.

The aim of the overall GBP 5 billion project is to decommission the former research facility and return it back to a natural landscape by the year 2060.

PLCs used on decommissioning projects at Dounreay have been Mitsubishi Electric's with IEC61131-3 programming software.

Cornerstone of the contract, the Sodium Disposal Plant (SDP), has just completed five days continuous operation, effectively bringing to a successful close the inactive commissioning of the plant.

The next (active) stage of the work will proceed when consent is given by the NII (Nuclear Installations Inspectorate).

Commissioning of the SDP and the associated equipment provided to supply the sodium to the SDP started late in 1999.

The combined commissioning team, comprising UKAEA, NNC (the main contractor) and RWE Nukem (who will provide the operations team for the plant) had a single goal of delivering a plant that is 'fit for purpose'.

Although the inevitable 'teething' troubles have arisen the plant and equipment has generally behaved well.

Where there have been problems, the UKAEA, NNC, and their subcontractors (including many businesses local to Dounreay) have worked closely together to identify the causes and resolve them to the satisfaction of all concerned.

The SDP is unique in its size and as decommissioning of other liquid metal cooled reactors progresses around the world, there is significant international interest in its operation.

Although the chemistry of sodium disposal is easily understood, finding the right process and engineering solutions required more than just thought.

As much as possible the design teams from NNC and its subcontractors undertook to apply proven technologies to avoid the risk of delay in development.

The central processing plant and the fluid systems have been integrated through the application of a Mitsubishi Q4AR 'hot standby' process system connected to a central SCADA based control system.

Although backed up by hardwired safety circuits, this automatic system is key to maintaining the reaction conditions.

Commissioning experience so far has demonstrated that the control system has easily met the design requirements set for it.

Extraction of the sodium from its various locations at the PFR and its transfer to the SDP presented a number of other challenges.

The original design for the PFR, like most other older nuclear plants, did not include many facilities for decommissioning.

This has imposed some significant design constraints on the equipment used for the liquid metals disposal project.

Typically, the equipment to extract sodium from the reactor vessel can only be deployed through existing penetrations in the structure and will only be used once.

Mitsubishi Control products were chosen because they were fully compliant with their programming package for their Q4AR process systems to IEC61131-3.

This worldwide programming standard allowed the systems programmes to be created in a standard format, using coding terminology and symbols that are universally recognisable.

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