Decommissioning and decontamination

Fluency in effluent

25 March 2010

Babcock Nuclear Division created detailed designs, built and commissioned a facility to process highly caustic, radioactive liquids from Dounreay’s Prototype Fast Reactor (PFR). And all this was carried out over an 18-month period.

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 neutralising the effluent’s pH.

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. Babcock chief engineer Nigel Parkin said: “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.”

Tim Brafield, consultant engineer at Babcock Nuclear, said: “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 International Group recently completed its acquisition of UKAEA Ltd, the commercial arm of the United Kingdom Atomic Energy Authority. UKAEA project manager Mick Moore said, “The teamwork between UKAEA and Babcock was critical, because teams from both organisations worked so closely together.” Prime contractor Babcock based six site engineers at Dounreay full-time for the duration of the GBP3.4 million (EUR3.8 million) project.

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. 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 provided the interface service connections to the new facility. Managing the ventilation, and low active drain connections were some of the most challenging areas of the project. 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. Square fire dampers measuring 1.5m 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 April 2005.

Since then, the facility has applied to the regulator to process liquid waste with a higher activity than previously expected. After revising its safety case and running tests, it is currently awaiting final approval.

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