Control project success at Bulgarian nuclear plant

Paul Boughton

A Russian-designed nuclear power plant in Bulgaria is reaping the benefits of improved feedwater control.

The Kozloduy nuclear power station in Bulgaria operates a Russian-designed WER440 pressurised water reactor with six primary coolant loops providing multiple paths for cooling the reactor. This design allows plant personnel to isolate one or more of these loops for maintenance without the need for a complete plant shutdown. However, while providing a degree of fault tolerance, such flexibility demands a complex control regime. Feed water control in particular is critical. If the water level, or inventory, decreases significantly, reactor cooling is jeopardised. If the level increases too much, water passes through the steam header causing damage to the reactor's turbines.
In other words, there is no acceptable failure mode. Any control solution has to offer high and proven levels of reliability, availability and fault tolerance.
One of the means of achieving this involved developing and installing protection systems for cold over pressurisation of the reactor vessel explained Vladimir Urutchev, chief engineer at Kozloduy. We eventually selected the Tricon 9 platform from Invensys Triconex because it has a very good architecture and met our price requirements he added.
Although Tricons are used in thousands of critical applications, this is the first time the technology has been involved with nuclear feed water control.
Control at Kozloduy now involves a classic three-element system configured for feed forward-feedback performance to control feed water flow. Dual and triple redundant sensors and transmitters are installed to improve reliability, matching fault tolerance in the field components.
Operation of the steam generators within the prescribed limits is important for plant operationso the triple modular redundancy of Tricon9 and the 1E certified basic software and hardware very much suited the importance of this mission critical process control added Urutchev.
The new system dynamically balances steam generator inventory against steam flow to the main turbine, feed water flow and level. Tricon pressure and temperature control compensates for all three elements and uses an adaptive gain control algorithm to maintain water level during steady state and transient operation.
This control algorithm models the static and dynamic components of the process. During low power operations such as start-up and shutdown, when flow measurement is inaccurate and unreliable, the program uses diverse measurements to estimate flow. Three element control is used over the full range of power operation, from 0percent to 100percent and back.
In addition, the development of a plant model and simulation computer during the course of the project allowed installation and start-up of the steam generator water level control system without the need for post start-up tuning.
The project also includes Wonderware InTouch-based operator interfaces. So to review plant status, an operator simply uses the touch sensitive screen to select and view grouped parameters. Meanwhile, multiple security levels prevent unauthorised access to program code and/or stored data.
Triconex's contract, which was won with the help of DICS Intertrade Limited, also included all equipment such as transmitters, valves and wiring, together with programming, simulation testing and project implementation services.

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