Babcock Nuclear commissions Dounreay effluent treatment plant

London - Babcock’s Nuclear Division has created detailed designs, manufactured and commissioned a new build facility capable of processing caustic, radioactive liquids from the PFR reactor, Dounreay. The contract with UKAEA Dounreay was worth £3.4 million, according to Babcock International, which recently acquied UKAEA Ltd, the commercial arm of the UK Atomic Energy Authority.
 
“The new PFR ETP completed active commissioning trails prior to operations being taken over by the facility personnel from the project team”, said Mick Moore, UKAEA project manager. The test results of the recent effluent through the facility processing train were very positive.
 
“Throughout this project there has been a philosophy of raising issues in an open manner, then implementing an immediate action plan to provide efficient resolution … The teamwork between UKAEA and Babcock was critical, because teams from both organisations worked so closely together; we already have an excellent working relationship on which to build for the future.”
 
Dounreay’s PFR reactor is undergoing decommissioning.  While in operation, it was cooled by sodium. As part of the decommissioning activities sodium wetted components are cleaned producing a caustic, radioactive effluent.  The purpose of the PFR Effluent Treatment Plant (ETP) is to process this effluent, removing the radioactivity and returning the pH to neutral.
 
Tim Brafield, consultant engineer at Babcock Nuclear, said: “We were appointed principal contractor in a ring-fenced site and were given ultimate responsibility for a diverse range of specialists working to a tight deadline.  As the scheme designs for the building, the process plant and the ventilation system had been developed separately, the Babcock team took six months to produce the manufacturing design using advanced modelling techniques to remove clashes and conflicts.”

Babcock based six site engineers at Dounreay full-time for the duration of the project. The detailed designs feature bespoke lead shielding and numerous interlocks.  AutoCAD’s Inventor software was used by the team on a large scale for the first time.  The 3D model prevented many potential design and installation problems, allowing them to be solved virtually, and will aid future operations, maintenance and even, ultimately, decommissioning, said Babcock.

The batch process, automated pH correction system consists of a pumped recirculation loop providing continuous pH monitoring and correction of effluent in the 10m3 capacity neutralisation vessel.  The allied nitric acid facility features a 30m3 capacity tank, pumps with interlocks and a ventilation scrubber chimney that ensures only clean air leaves the nitric acid storage tank.  Babcock used Proportional, Integral and Derivative (PID) Controls, for the control system it fitted to the pH neutralisation part of the process.  This ensures that the minute quantities of acid that have to be added to the liquid as it approaches neutral pH are constantly monitored, so there is little danger of adding too much nitric acid.  Tests have shown that the end result is consistent, regardless of the starting point.  In addition, the entire system is linked to PFR’s SCADA system for added safety.
 
Once the liquid is pH neutral, it is passed through an ion exchange and filtration system.  The specialist Ion Exchange Columns and resins for removal of cobalt and caesium were supplied by UKAEA.  Particulates bigger than one micron are removed by filtration at this stage.  Processed effluent is then passed to the effluent sentencing vessel.  Extra safety features incorporated, include shielding to the effluent sentencing vessel and a further filter to ensure none of the resins from the ion exchange process are contained within the liquid. Samples are drawn off in an automated cabinet once the process is complete and analysed by the Station’s chemists.  If the liquid is found to be acceptable, it is then pumped to the Low Level Liquid Effluent Treatment Plant (LLLETP) for final discharge.
 
The plant ventilation system was the complete responsibility of Babcock.  Fans extracting air from the plant pass it through HEPA filters before discharge through the PFR stack.  Creating interlocks and interfaces with the existing system, as well as coping with space constraints led to the need for a highly specialised engineering design.
 
The UKAEA project team worked closely with the Babcock team provided the interface service connections to the new facility.  This involved managing the radiological hazards, providing ventilation, low active drain, fibre optics, telephone, fire alarm, water services, drains, electrical supplies and low active drain connections.

Two major challenges were in providing ventilation and low active drains connections.  A ventilation ducting was designed and installed using local contractors. Connection was made at the fan outlet and a new route provided over the top of the PFR containment building joining into a an existing active duct within the PFR vent annex.  1.5m square fire dampers and non return valves were installed to an active duct at the connection point.  

The connection into the LLLETP active drain line was made by construction of a 3m deep by 3m diameter chamber over the low active drain connection point, outside the new facility.

This construction was used in order to provide containment boundaries for the classified radiological area.  The new coaxial stainless pipe from the facility was welded into the existing low active drain system.  On completion of the service connections, the facility commenced active commissioning in the week forecast at start of the construction phase.
 
Space constraints within the plant meant that logistically, the installation was highly complex, with the order of operations being crucial.  The 3D model proved to be vital when the team needed to determine the order of operation.  For example, the local control room cubicle, which is sited on a mezzanine floor in the Plant, had to be fitted before the end wall of the control room was constructed.
 
Nigel Parkin, chief engineer at Babcock commented: “Integration of the entire process plant to PFR’s SCADA allows full remote control from the PFR control room.  This involved a great deal of complex electrical design, power distribution, motor controls and instrumentation measuring levels and pressures.  Using the same team throughout the Project resulted in great team spirit and the build up and dissemination of a formidable body of knowledge.  There were zero lost time accidents thanks to a proactive safety culture.”