By Jay O'Leary

Nothing lasts forever, especially in a network of nuclear power plants where aging and obsolescence pose a very serious threat. Planning well ahead, industrial firms and utilities in France and Germany have been busy designing a longer-lasting and safer reactor that both countries can use themselves and export to other nations.

To put a nuclear reactor in service in Germany, operators must demonstrate that sites neighboring the plant grounds will not be affected by ionized radiation, not even in the event of a core meltdown.

The European pressurized-water reactor (EPR), which recently entered the detailed-design phase, has four design partners: Framatome in Paris; Siemens/KWU in Munich, Germany; their Paris-based joint subsidiary, Nuclear Power International; and Electricité de France, the French national electric utility. In addition, the German utilities and Siemens have assigned a team of engineers to review the design. Approximately 250 engineers are working on the new reactor. They are using a three-dimensional computer-assisted design program for layout work. The timetable calls for the EPR's design to be completed in 1999 or 2000, and for the first unit to be commissioned around 2005.

Because of a scant supply of natural resources, France depends on nuclear power for three-fourths of its electricity. Fifty-four pressurized-water reactors are currently operating there, and two more are under construction. The total generating capacity is 71,800 megawatts. In Germany, 19 nuclear power plants, all built by Siemens/KWU and its predecessors, supply about 30 percent of the country's demand for electricity. (The nuclear power plants in the former East Germany, which were of a Russian design known as VVER, were decommissioned by the reunified German government.)

Both Framatome and Siemens are banking on being able to export the EPR design. Framatome has already built nine nuclear islands using its existing design for export customers, and recently signed a contract for two more islands for the planned Ling Ao nuclear power station in China. Siemens/KWU has exported four plants and has two more under construction.

The EPR is a descendant of the French N4 and German Konvoi nuclear reactors, both models currently in service. From the N4, the new reactor derives its designs for containment and the primary system, its instrumentation and control system, and its control room. The EPR's in-core measurement system and four-train architecture are taken from the Konvoi.

The new plant is designed to have a nominal net output of 1,450 megawatts. (Output could be as high as 1,525 megawatts.) The nominal net power output for the N4 is 1,450 megawatts; for the Konvoi, 1,350 megawatts. The EPR's core thermal power will be 4,250 megawatts. Its power level is said to be well adapted to European grids. The latest estimates give an approximate production cost of 3.5 U.S. cents per kilowatt-hour.

The most highly touted features of the EPR design are its safety systems. From the outset, the design team has considered the possibility of a core meltdown and has tried to engineer the plant to withstand such a catastrophe. The systems for safety injection, steam-generator emergency feedwater, component cooling, and backup electric power will be divided into four independent and physically separated trains. The reactor, fuel, and safety-system buildings will share the same foundation raft. The buildings housing two of the safety trains will be bunkered to withstand aircraft crashes; their internal structure will be uncoupled from the external structure to minimize induced vibrations. The buildings housing the other two safety trains, which are not bunkered, will be located on opposite sides of the reactor building to reduce the risk of both being disabled by the same hypothetical accident.

The reactor building will have two containment walls to provide reinforced protection from the reactor and the environment. The inner prestressed-concrete containment is similar to that in the N4, but the design pressure has been increased to 6.5 bars absolute. The concrete is designed to contain the vaporized primary fluid with leakage of less than 1 percent of the total confined volume per day. The outer reinforced-concrete containment is designed, as in the Konvoi, to resist shocks from outside such as crashes of military aircraft. The annular space between the walls is designed to recover leakage from the inner containment.

The strategy for recovering a molten core is known as bleed and feed. In addition to the normal relief and safety valves, a dedicated bleed system is designed to maintain low pressure and allow water to be injected before or during the degradation of the core. The reactor pit will be equipped to recover liquid metal, which will fall by gravity toward a lower area made of refractory material where the metal can spread out. The reserve supply of reactor coolant water that normally serves for refueling will be stored inside the reactor building, where it will cool the core. Hydrogen generated by decomposition of the water on the Zircaloy—an anticorrosive alloy of zirconiumÑof the fuel-rod cladding will be recombined with oxygen to reduce the risk of an explosion.

The internal free volume of the primary components has been increased to reduce transient effects and give operators more time to react, especially to the loss of reactor coolant. By its buffer effect, the increased volume of the pressurizer will in some cases prevent unintentional opening of the safety valves. The increased steam-generator volume provides an added margin in case of loss of feedwater. The reactor coolant system is protected against overpressure by a set of pilot-operated safety valves that have been optimized from the N4 and Konvoi designs.

The new reactor will have several features designed to lower its operating and maintenance costs. Its core, for example, will be rounder and heavier than in previous designs, reducing the necessary fuel enrichment. The EPR's discharge-fuel burn-up will be 60 gigawatt-days per metric ton, compared to 50 for the N4 and Konvoi. The core of the new reactor is designed to accept mixed-oxide assemblies as well as conventional uranium fuel assemblies. This flexibility will make it possible to recycle plutonium in the EPR. Depending on which fuel the plant managers select, the EPR could save as much as 20 percent in fuel costs compared with the N4 and Konvoi. The new reactor's fuel inventory will be 15 percent greater than that of its predecessors.

A descendant of the French N4 and German Konvoi reactors, the European pressurized-water reactor will have an inner prestressed-concrete containment like the N4's and an outer reinforced-concrete containment, like the Konvoi's, that resists shocks from outside, such as airplane crashes.

The EPR is supposed to have a minimum availability of 87 percent, which is the maximum target availability of the N4. Some Konvoi plants have already reached an availability above 90 percent. The EPR is designed to shorten the duration of the outages required for refueling, in-service inspection, and maintenance to an average of 25 days per year. The normal outage time for the N4 is 34 days. In 1995 the Neckarwestheim II plant, a Konvoi model, performed its refueling outage in just 17 days. To simplify and speed maintenance, the EPR's pressurizer spray system and heaters have been designed for easy replacement. Space between the reactor vessel and its heat insulation allows the vessel to be inspected from the outside to help refine a diagnosis made on the basis of the normal inspection from inside the vessel.

The EPR's reactor pressure vessel, the component that ultimately limits the reactor's service life, is designed to last 60 years, 20 years longer than the vessel in the N4 and the Konvoi. The EPR's more massive core barrel and greater water gap between the core barrel and the inner wall of the vessel will give it greater protection against neutron bombardment and leakage.

The reactor core of the EPR will be enlarged to contain 241 fuel assemblies using a 17-by-17 array; the N4 has 205 of those assemblies, while the Konvoi has 193 assemblies with an 18-by-18 array. The EPR's steam generators will be equipped with an axial economizer, thereby increasing the secondary pressure by 3 bars and raising the efficiency accordingly. The reactor coolant pump assembly will be of the same design as that used in existing French and German pressurized-water reactors.

The EPR's design phase will need an estimated 1 million hours of engineering work. The cost is expected to reach 120 million European currency units, the equivalent of approximately $132 million.

Jay O'Leary is a former editor-in-chief of Mechanical Engineering magazine.


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