Feedback on the N4 series3 April 2002
The N4 series had a chequered commissioning, but now units are providing more power than expected. By Francis Cances
France's N4 series of reactors is the sixth series of PWRs developed during the country's nuclear power programme. The earlier series were:
• CP0: six 900MWe units, connected to the grid in 1977-1979.
• CP1: eighteen 900MWe units, connected in 1980-1985.
• CP2: ten 900MWe units, connected in 1981-1987.
• P4: eight 1300MWe units, connected in 1984-1986.
• P'4: twelve 1300MWe units, connected in 1986-1993.
The N4 series was designed in the early 1980s, increasing unit rating to 1500MWe, and the first units to go into operation were Chooz B1 and B2, which started up in 1996 and 1997, respectively. Two further units went into operation at Civaux in 1997 and 1999.
The N4 development was driven by four main factors. First, licensing agreements between the vendors of the major components and their designers were reaching their end, most importantly those of Framatome for the nuclear steam supply system and Alstom for the turbine generator. Meanwhile, most of the 900MWe and half the 1300MWe units were under construction and some were already operating. This had developed design skills in both the vendors and utility capable of supporting a new design, especially with regard to investment costs, plant efficiency and simplification. The utility EdF had also acquired enough operating experience to feed back improvements into a new design.
Most importantly, lessons had been learned from the Three Mile Island accident. Accidents not foreseen in the Safety Analysis Report might happen, and the man-machine interface should be improved. While some 'beyond design' safety features and some improvements in the man-machine interface were made in the 900MWe and 1300MWe units, optimising the man-machine interface was only possible in a new design.
One of the main developments of the N4 series (compared to the 1300MWe series) is an increase of about 12% in the rated thermal power of the core (to 4270MW). This was made possible by increasing the number of assemblies by 6% and the specific power by 6%.
This higher power level is available in baseload operation with automatic control of the reactor coolant temperature (A mode). Load-following operation is possible at reduced power (4076MW), using automatic control of the reactor coolant temperature and of the axial-offset (X mode).
Increasing the rated power also required developments in the major components. The diameter of the reactor vessel was increased by 10cm, using shells forged from hollow ingots. The diameter of the in-core bottom nozzles was reduced. The hydraulic part of the main coolant pump was fully redesigned, reducing to four the number of vanes on the impeller. Using a hydrostatic bearing around the impeller cut down vibrations on the shaft, improving the performance of the sealing components.
The N4 steam generators used U-tubes made of Inconel 690 arranged in a triangular pattern. In the secondary stage, an axial preheater increased the efficiency of the steam generators. The upper part of the secondary stage of the steam generator was also redesigned so that water is separated from steam through 129 two-stage swirl vane separators. A new dryer design has12 cells in a compact star array.
The most extensive changes were in the turbine. The single 1530MWe N4 turbine, called Arabelle, has one high pressure and one medium pressure cylinder and three low pressure cylinders. The medium part gathers the first stages of the three low pressure parts, so the number of wheels in the low pressure section can be reduced from ten to five. The 'direct action' design of the blades also helps reduce the number of blade wheels needed for the same power. The design of the high and medium pressure diffusers was also improved.
These features together reduce the overall number of wheels from 60 (in the 1300MWe series) to 28. This decreases the bypass rate, improves turbine efficiency and makes the structure simpler and lighter. Other design changes, like parietal suction diffusers on the exhaust sections, have further improved efficiency. The resulting machine is 7m shorter, 12% lighter and 2% more efficient.
INSTRUMENTATION & CONTROL
Digital automatic processing at level 1 was first used in 1300MWe plants and we now have more than 150 unit-years of experience.
The N4 level 1 I&C follows the 1300MWe experience, using Framatome and Schneider Electric technology for the protection system. Industrial programmable controllers are used for the other level 1 functions (mainly provided by Sema Group and Hartmann Braun).
Most development in the N4 I&C has been in level 2, the computerised operation system. The design development had four main objectives:
• To improve interactivity and the control displays.
• To use the same system under normal and accident conditions.
• To improve alarm processing and provide efficient aids to diagnosis.
• To carry out cyclic monitoring of plant status parameters and aid implementation of emergency operating procedures.
These objectives were met by making the entire man-machine interface computer-based, simplifying the architecture of the main control room. The N4 has four operation workstations, each with three graphic screens, three touch screens, one keyboard and one trackball, while a large wall-mounted mimic panel presents the main parameters and the status of the main actuators. An auxiliary panel has conventional controls in case or failure of the computer system.
Implementing the level 2 I&C system was a long project, extending over some 15 years in co-operation with Sema Group. The long timescale was a result of the ambitious nature of the project, the fact that during the specifications phase ergonomic and functional validation was being run on the control room and man-machine interface, and the need to replace the level 1 controller in 1990-1991. The project was implemented stepwise: there were three test versions (V1-3); V4, with a core of basic functions, was ready for the first Chooz startup; V5 and V6 have incorporated experience feedback. V5 is installed at Chooz B2 and Civaux 1, while V6 is installed at Civaux 2.
After initial delivery to the site in 1993-1995 dynamic system performance problems began to appear. They were traced to underestimation of the data flow being transmitted from level 1. The solution was to use more powerful equipment and install filter mechanisms to meet the needs of the operator.
System convergence problems were harder to solve. They caused anomalies on site, system malfunctions and slow operation under certain operating conditions, an above-normal rate of undetected faults and problems with debugging the incremental data input mechanisms. In response, EdF altered the upstream development process. It tightened specifications, using strict stabilisation (including correcting anomalies) and longer intervals between design versions to ensure the design base remained stable. It reinforced the validation and acceptance process, in the light of problems encountered on site, and altered the system to make it sturdier and improve its ability to detect its failures.
V5 at Chooz B has now met availability targets for five equivalent reactor years of operation and there have been no system-related events compromising the safety of the unit, and no wrong orders since the system was started up on the site.
Further improvements are being made to control room lighting conditions, data finishing (images, technical data and alarm sheets) and to the ergonomics of some functions and alarm processing.
The following features have proved very valuable:
• For each component — such as sensors, actuator and so on — detailed diagnostics are given so possible causes of malfunctions can be found after a control action.
• Alarms are validated from a functional standpoint and according to unit status.
• Follow-up formats provide information relating to any component, system or functional device to be monitored.
In comparison to the previous design, all this diagnostic information makes the commissioning task easier.
Normal operating procedures were intensively used during the commissioning period; this contributed, in addition to training periods on simulators, to improving operators' knowledge of the system. In the same way, tests such as loss of external power supply, loss of DC supply and safety injection, were performed during hot functional tests using incidental procedures based on state approach procedures; it led to calm testing periods, with good cooperation between operation and startup teams.
The behaviour of the main new components is quite satisfactory. From the performance perspective the net output power of 1500MWe is 2% higher than expected under normal operating conditions. It allows each unit to produce more than 1TWh per month and to reach a net output power of 1530MWe under cold weather conditions.
CHOOZ AND CIVAUX
The Chooz units underwent a year-long outage after startup during which design and construction upgrades were made to the residual heat removal (RHR) systems and the main turbines.
Arabelle's HP and MP cylinder diaphragms were replaced, and the RHR system was reconditioned following the Civaux 1 incident. Since then, the units have been operating satisfactorily at full power. Chooz B1's first fuel cycle was completed in mid-October 1999 and that of Chooz B2 in spring 2000.
The two N4 units at Civaux have some features not found at Chooz B. The safety auxiliaries are cooled using a closed-flow system aided by forced-circulation air coolants. The pipes and fittings for the raw water systems are made of composites. Finally, high-performance concrete is used for the reactor building's inner containment. The last N4 unit, Civaux 2, reached full power in June 2000.
Considerable attention was given to the matter of amoebas in some of the nuclear units in 1999. These micro-organisms can develop only when water temperatures rise above 40°C and when no copper is present. This restricts their proliferation to the summer and a few sites, including Civaux. In March1999 work began to implement a system to treat the secondary system's water cooling processing system using ultraviolet rays. The system, still in the prototype stage, should eliminate all releases of processing products. The system went into service in August 1999.