This article highlights that Electricité de France’s (EDF) N4 nuclear technology has increased European standards of power, efficiency, and safety beyond previous limits. EDF, the Paris-based French national utility, has developed and is operating the N4 reactor, capable of generating 1450 megawatts, at its power plant in Chooz. The N4 was designed to put public safety concerns to rest while providing more power than the previous generation of 1,300-megawatt EDF reactors. This installation represents the next step in French, European, and possibly the world’s nuclear power. Chooz A began operations in 1967 as the first pressurized water reactor (PWR) in France, originally based on the 185-megawatt synchronized PWR Yankee Rowe plant in Massachusetts. Typical reactor safety systems analyze a problem after it occurs. Such a procedure involves painstaking historical, reconstruction that is time-consuming, often difficult to interpret, and less reliable as time passes.
France depends on nuclear power plants for 75 percent of its electricity, making it essential for that nation to be at the forefront of nuclear equipment and plant design. Electricité de France (EDF), the Paris-based French national utility, has developed and is now operating the N4 reactor, capable of generating 1,450 megawatts, at its power plant in Chooz. The N4 was designed to put public safety concerns to rest while providing more power than the previous generation of 1,300- megawatt EDF reactors. This installation represents the next chapter in French, European, and possibly the world's, nuclear power.
The Chooz plant is a fitting location for the latest chapter in French nuclear evolution, because this power station reflects the history of French atomic power. Electricité de France chose this site on the Franco-Belgian border in the early 1960s because its proximity to high-voltage transmission lines would facilitate electrical exports to Belgium.
Chooz A began operations in 1967 as the first pressurized water reactor (PWR) in France, originally based on the 185-megawatt synchronized PWR Yankee Rowe plant in Massachusetts. "Chooz A delivered 240 megawatts to Electricité de France and Centre & Sud, a consortium of Belgian utilities," said Pierre Boussard, a nuclear engineer and president at Electricité de France International North America, Washington. "Further modifications increased the reactor's capacity to 305 megawatts."
The original Chooz A reactor was shut down in 1991 prior to its dismantling. All fuel from Chooz A was removed by December 1995 and sent to the Cogema fuel-reprocessing center in La Hague. Chooz B was constructed on the opposite bank of the Meuse, still on French soil. Its first N4 reactor was connected to the EDF grid on August 30, 1996, and the second unit on April 10, 1997. The second reactor reached full capacity on September 18, 1997.
The new plant is operated by EDF in partnership with the Belgian utilities Electrabel S.A./ N.V. and Société Publique d'Electricité, both based in Brussels. Chooz, B will generate 2,900 megawatts from two new N4 reactors designed by Framatome, head quartered in Paris. The N4 reactors, the first 100-percent French-designed reactors, and their nuclear steam supply system were designed with innovative nuclear power technologies that extend unit performance, power, and safety beyond previous limits.
Framatome engineers developed a new operating mode for the N4 reactors called Mode X, which enables them to adjust their output to match the fluctuations in daily electrical demand. For example, in Mode X, the N4 reactors can vary their output from 30 to 95 percent of full power in less than 30 minutes. Plant engineers accomplish this adjustment by inserting or withdrawing gray control rods from the reactor core. These rods are referred to as gray because they absorb fewer free neutrons than conventional black rods.
The gray rods are positioned by means of automatic controls to optimize the distribution of neutrons within the reactor core. This provides thermal homogeneity, reducing the risk of hot spots, which can wear the reactor walls. Because fuel is consumed evenly, the economy of the fuel cycle is improved.
Typical reactor safety systems analyze a problem after it occurs. Such a procedure involves painstaking historical, reconstruction that is time-consuming, often difficult to interpret, and less reliable as time passes.
The safety system of the N4 reactors is a departure from the norm. Framatome takes what Boussard described as a "state-oriented approach" to N4 safety that was originally developed for the thermal-hydraulic functions of the company's 1,300-megawatt-class PWRs before being adapted to the N4.
The state-oriented approach avoids recreating historical events in favor of producing a diagnosis based on a selection of operating parameters gathered in real time. This is made possible by the automation of the N4 reactors, which enables operators to check data while providing them with the optimum solution to any problems.
EDF commissioned the Sema Group in Stockholm, Sweden, to design a completely computerized command and control system for the N4 control room and all its instruments. The controls are designed to prevent an accident caused by human error, such as that experienced by Three Mile Island in 1979. The computerized control systems at Chooz B are intended to increase the automation of the reactor operation, especially in the first few critical minutes following an incident. The system also clearly presents information, such as reactor parameters and operating procedures, to the reactor operators.
Sema Group designers selected a dual Ethernet local area network (LAN), active hardware redundancy, and the Ada programming language for the N4 command and control system. The system contains self-monitoring procedures to activate backup sequences in the case of an emergency. One hundred fifty programmable logic controllers (designed by Hartmann & Braun of Frankfurt am Main, Germany), and 14 VAX 4000-700 computers (from Digital Equipment Corp. of Maynard, Mass.) pro- vide the hardware for the control system.
There are four identical workstations in the Chooz B control room. Reactor operators at each workstation choose an operating display format and select an item using a trackball. The item then appears on a touch-sensitive screen so that the operator can select a command by checking the appropriate item. The item is confirmed by pressing a validation key. Command history is displayed on-screen, as is an updated graphic presentation.
In addition, the control room is equipped with a wall-mounted mimic diagram that provides an overview of the unit's status. This eliminates the multiple alarms and indicator lights that were activated simultaneously in the past. A wall-mounted auxiliary control panel serves as backup if the central information system fails.
Increasing Steam Supply
Framatome engineers were given the task of increasing the steam power of the N4 reactors approximately 12 percent over the preceding generation of 1,300- megawatt reactors, while minimizing design changes. They accomplished this objective by limiting the evolution ofN4 components. For example, they increased the volume of the new reactor's core by 6 percent, raising the number of fuel assemblies contained within to 205 from 193. At the same time, they increased the entire reactor vessel by only 2 percent, to 13.65 meters tall and 4.65 meters in diameter.
Framatome improved the N4's steam generators by adding feed-water heaters, equipped with a dual housing flow distribution baffle that directs water flows. All the equipment contained inside the steam generators is made of stainless steel, which helps dry the secondary steam more effectively. As a result, operators are able to raise the steam pressure to 72 from 68 bar at its outlet, increasing the overall Nuclear Steam Supply System (NSSS) efficiency.
The N4 design team reduced the steam generators' volume by arranging the 5,600 U-tubes in a triangular pitch configuration and designing the moisture/vapor separators to capture water droplets. This permits only "dry" steam to exit to the reactor turbines.
The U-tubes provide nearly 3 hectares of surface for heat exchange. This ensures the transfer of the reactor core's heat energy, absorbed by the primary fluid system, to the secondary fluid system and the turbine-generator, which uses it to generate up to 1,450 megawatts of electricity.
New materials and designs extend the reactor's operating life. The steam generator U-tubes and pressure head penetrations are made of Inconel 690, a nickel-based alloy, which provides superior corrosion resistance compared to the Inconel 600 previously used. The impeller belt of the reactor coolant pumps is equipped with a hydrostatic bearing, which reduces vibration. Most critically, the cylindrical parts of the reactor vessel are made from forged hollow ingots. This design preserves the ingots' mechanical strength despite irradiation-the single most important factor affecting the reactor's service life.
A Leaner Turbine
EDF selected the 1,500-megawatt Arabelle steam turbine, designed by GEC Alsthom in Paris, to generate electricity from the steam produced by the N 4 reactor. The extra 50 megawatts provided by Arabelle are needed to power the unit auxiliaries at Chooz. GEC Alsthom engineers designed the Arabelle as a leaner, less costly turbine that is 2 percent more efficient than its predecessors. The key to this accomplishment is the Arabelle steam flow arrangement. Unlike most steam turbines that subdivide steam into partial flows when entering the turbine, the Arabelle keeps steam in a single flow until the pressure reaches approximately 3.3 bar.
To support this arrangement, the Arabelle consists of a combined high-pressure/medium-pressure cylinder containing two steam paths. Steam at 71 bar enters the high-pressure cylinders in the Arabelle, where it expands and then exits at a pressure of 10 bar and 15-percent moisture content. The steam is then dried and heated in a dryer-heater before entering the medium pressure cylinder and then exiting at 3 bar. Next, the steam. is routed to one of three pressure cylinders where it is divided into two separate flows upon entry. The steam enters these cylinders at the center point, and then is divided into two flows. As the steam exits the three low-pressure cylinders, it is collected in the condenser, where it is converted back to water. The reheated water is then returned to the reactor.
The Arabelle's steam flow arrangement reduces the number of turbine blades because it minimizes the expansion stages requiring more than one row of blading. This enabled GEC Alsthom engineers to make the Arabelle seven meters shorter and 12 percent lighter than the preceding 1,300-megawatt steam turbines. The lower weight cut the concrete foundation block needed by Arabelle to 1,800 from 2,400 cubic meters, reduced the weight of steel reinforcement to 300 from 415 tons, and lowered the number of spring boxes that support the foundation to 74 from 120.
"Current energy requirements do not justify building any new plants in the immediate future," noted Boussard, "but EDF is planning to replace existing nuclear plants, beginning in 2010. The N4 is our greatest nuclear asset. In addition to the Chooz installations, we' connected an N4 to the grid at the Civaux power plant in December 1997, and will connect the second Civaux N4 unit in February 1999."