Many countries in Eastern Europeespecially those that were part of the USSRstill rely on Russian-designed nuclear reactors for much of their electricity output. Bulgariafor examplehas four such reactors that generate nearly half of its electricity requirements.
The country’s interest in nuclear power generation began in the mid 1950s with the construction of a research reactor. Then in 1966 Bulgaria signed an agreement with the USSR for commercial unitssome of which still form the basis of its electricity supply programme today.
According to the Uranium Information Centre (UIC)the first pair of pressurised water reactors (PWRs) were installed at the Kozloduy plantclose to the River Danube border with Romaniashortly after this deal was signed. These were WER440/230 models. A second pairalso WER440s but incorporating the much-improved safety features of the 213 modelwere then installed. A third pair of much larger WER1000/320 units were added to the site later.
Between 1991 and 1997 a great number of safety improvements were carried out at Kozloduy in consultation with the International Atomic Energy Agency (IAEA)the EU and the World Association of Nuclear Operators (WANO). This short-term project cost E129m and was designed to bring standards closer to international norms. From 1998-2002 further modifications were carried out in line with IAEA safety criteria at a cost of E120m/y.
The UIC says that the promise of EU accession accompanied by the offer of E200m from the EC led to Kozloduy reactors one and two being closed at the end of 2002. Units three and four were originally to close in 2006. Howeveras these have achieved levels of safety comparable with reactors of a similar age in Western Europethe Bulgarian parliament ruled that they should stay open for the time being.
Since thenan IAEA mission has reported favourably on the units and a two-week scrutiny by 18 international inspectors from the WANO found that the units met all necessary international standards for safe operation. The Bulgarian government is now aiming to renegotiate the agreed 2006 shutdown and gain a reprieve until the licences expire in 2011 and 2013giving a 30-year operating life.
An upgrade and modernisation programme for units five and six is ongoingbut the UIC says there is no great concern about the safety of these unitswhich conform well to international standards.
PWR control upgrade
All PWR reactors work in a similar fashion. Firstwater is highly pressurised to prevent it boiling. Within the primary loopit is then pumped through the nuclear core where it is heated and passed though thousands of tubes within two or more steam generators.
There the heat of the water is conducted across the tubes to a secondary supply of water at lower pressurewhich boils to make steam. This steam spins the turbinedriving the electric generator and so producing electricity.
Similar to fossil fired boilersa system of turbine exhaust condensersair ejectorsheaters and pumps recover the condensed steam and return it to the steam generator. An automatic control system ensures that the correct amount of feedwater is returnedbalancing the exiting steam flow and maintaining a constant inventory of water within the reactor vessel.
It is this feedwater control system that is at the heart of a successful control project which Triconexpart of Invensyshas just completed successfully at Kozloduy.
Each of the Russian designed WER440 PWRs has 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.
Howeverwhile providing a degree of fault tolerancesuch flexibility demands a complex control regime. Feed water control in particular is critical. If the water levelor inventorydecreases significantlyreactor cooling is jeopardised. If the level increases too muchwater passes through the steam header causing damage to the reactor’s turbines.
In other wordsthere is no acceptable failure mode. Any control solution has to offer high and proven levels of reliabilityavailability 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 Urutchevchief engineer at Kozloduy. “We eventually selected the Tricon9 platform from Triconex because it has a very good architecture and met our price requirements” he added.
Although Tricons are used in thousands of critical applications worldwidethis 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 reliabilitymatching 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 turbinefeed 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 shutdownwhen flow measurement is inaccurate and unreliablethe program uses diverse measurements to estimate flow. Three-element control is used over the full range of power operationfrom
0-100percent and back.
In additionthe 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 statusan operator simply uses the touch sensitive screen to select and view grouped parameters. Meanwhilemultiple security levels prevent unauthorised access to program code and/or stored data.
Triconex’s contractwhich was won with the help of DICS Intertrade Limitedalso included all equipment such as transmittersvalves and wiringtogether with programmingsimulation testing and project implementation services.
So far the system has operated flawlessly and Triconex is confident that it will win future business at the Kozloduy plant.
“We are now considering using Tricon systems in other safety-related aspects such as reactor controlreactor protection systemsturbo generation control and total unit control” confirmed Urutchev.
Update in Ukraine
Building on its Bulgarian successInvensys Nuclear has just won a major contract from DICS Intertrade to supply four Tricon safety controllers to the Rovno nuclear power plant at Kuznetsovsk in Ukraine.
Rovno also operates a Russian-designed WER440 PWRwhich along with 14 other similar reactors provides over half of Ukraine’s electricity output.
This contract covers the installation of a Tricon-based cold overpressure protection system (COPS) on each of the two 400MW reactor units at Rovno. Such protection is vital to the safe operation of PWRswhich are vulnerable to vessel damage when below normal operating temperatures.
A COPS compares pressure and temperature inputs against a preset setpoint curve and relieves the pressure when this setpoint is reached by opening power-operated relief valves (PORVs). This relief is important because at the low temperatures experienced during plant start-up and shutdown the reactor vessel is at risk from brittle fractures.
Each of the Rovno units has two PORVs that are capable of working independently and each will be linked to a dedicatedelectrically isolated and geographically separated Tricon controller. The four Tricons will monitor three temperature detectors in each of six coolant loops and three primary system pressure transmittersopening redundant PORVs to prevent pressure limits being exceeded.
Compared with other solutionsInvensys Nuclear says that Tricon-based COPS applications offer great reliabilityease of online maintenanceinherent self-test diagnostics and enhanced operator awareness.
According to the companythe accurate balance provided by Tricon technology at Rovno will increase the coefficient of demand on the systemits reliability and therefore its overall availability."