Between 1972 and 1985 twelve nuclear power stations were commissioned in Sweden. Three of these are located at Oskarshamn; unit 1, with an output of 465MWe was officially opened on 8 May, 1972; the 620MWe unit 2 opened in 1974; and finally unit 3 (1200MWe) opened in 1985. The Swedish spent fuel interim storage facility is also located at the Oskarshamn site.
Oskarshamn 1 has a couple of features that distinguish it from other Swedish nuclear power stations. It has a radial high-pressure turbine with two counter-rotating shafts, each connected to a generator; and it also has a support condenser. The small size of the building itself forced process and safety equipment to be placed close to each other.
Pipework problems
The upgrade currently taking place is the third and last phase in a modernisation programme that can be traced back almost ten years. In September 1992 the Swedish nuclear authority withdrew the commissions for five nuclear power stations, among them Oskarshamn 1. This was done to investigate and rectify a construction weakness in the shutdown cooling system.
“Whilst carrying out inspections we found cracks in parts of the pipework,” said Gösta Carlsson, head of Oskarshamn 1. “The pipes had become frail in the manufacturing process and all the damaged pipework had to be replaced. During the spring of 1993 more flaws were found, this time on internal parts of the reactor tank. In the summer of 1993 the company board decided to undertake a full investigation of the entire station.”
The investigation project was named Fenix, and attracted quite a bit of international attention as both inspections and work was carried out in the reactor vessel.
“The decontamination of the reactor vessel was very successful,” said Carlsson. “We used oxalic acid and achieved better results than we could have dreamed of. The project also gave us a lot of new information. For instance we learned that the reactor vessel was in very good condition and that made us confident about the future of Oskarshamn 1. But the investigation also showed some weak spots elsewhere in the station and, in 1995, a modernisation plan was established to ensure that Oskarshamn 1 could stand up to future demands, regarding both safety and availability. At this point we estimated that the final upgrade was going to be completed during 1999.”
Modernisation begins
The formal decision to continue the modernisation plan was taken in 1997 and, in 1998, project MAX was started. During this project several internal reactor parts, such as the moderator tank and lid were replaced to ensure that all internal parts could stand up to firmness requirements. The isolation valves in the main recirculation system were also replaced during project MAX. In 1998 several preparations for the third and final modernisation project, project MOD were also made.
As already mentioned, Oskarshamn 1 is a small power station. An important part of the modernisation programme has therefore been to implement a new safety concept where process and safety systems are separated. To make this concept possible a new building – the electric control building (EKB) – was constructed in 1998, wall-to-wall with the original power station. Today, the EKB holds the emergency power systems and the I&C equipment, all separated into four zones. Two diesel generators for emergency power were purchased and placed in the EKB. The EKB also holds an emergency control room and the building meets seismic requirements.
Rector protection system
Project manager Anders Helmersson said: “The new safety concept also includes a reactor protection system. The reactor protection is part of a new computer-based control system called Advant, developed by Westinghouse Atom. Advant is going to change the work of both control room operators and maintenance technicians in many ways. The fact that Advant is computer-based means that we can get access to far more process information than before, and this will make troubleshooting less complicated.”
Advant consists of four main components. Computer screens that make up the operator interface and control boards where process data is received and operations are transmitted. Process data is treated by function processors and is stored and calculated by a main processor. These separate parts are linked together by a network. Safety-related functions also have a hardwired backup, as it is important to always have a traditional backup.
Developing Advant proved to be the single most difficult and time-consuming part of the whole modernisation programme. It may seem strange to some as computer-based control systems are not unusual in modern industries; in fact they have been common on the market for many years.
“But qualifying a computer-based control system for the safety demands put up by the nuclear industry is definitely something else,” said Helmersson. “On the other hand the whole project was dependent on it, so we chose to postpone the start of project MOD. Initially we planned to start in 1999, but it actually took until 2001 before the final factory acceptance test was approved by the company.”
The new safety concept and the computer-based control system meant that a number of changes in the control room environment had to be implemented. An extensive reconstruction of the control room is now taking place.
“We realised at an early stage that the modernisation programme was going to radically change the control room,” said Helmersson. “A control room team had the task of developing ideas for a new environment inside the same four walls as before. Opinions and experience were gathered from fellow control room operators and even experts on human behaviour were consulted. A number of layouts using virtual reality software were presented and all these ideas have been distilled into a final layout that is both practical and safe.”
Control room simulator
Parallel with the development of a new control room, a simulator for Oskarshamn 1 was built on an industrial site in the town of Oskarshamn, some 30km from the power station site. The simulator is intended to meet the needs of future training for the control room operators but is also an essential part of the preparation for the restart of the power station. Building a simulator of a control room that doesn’t yet exist has of course been quite a bit of a challenge, but as of 1 January the simulator is being used for preparatory training. Control room operators will follow a training programme that ends in October, just before the restart of Oskarshamn 1.
“The simulator project has consistently been one step ahead of the rebuild of the real control room and, at times, that has forced us to look into a future that we know nothing about. On the other hand, we gathered a lot of experience of the new functions while starting up the simulator and a number of construction errors were actually identified and corrected before the rebuild even started. That will save us a lot of time and trouble during the test and start-up period later on this year,” said Helmersson.
Turbine replacement
Most of the upgrade work at Oskarshamn 1 is in one way or another related to safety issues and the ability to meet the demands outlined by the Swedish nuclear authorities. But one part of the upgrade project is aiming at other goals, namely the turbine replacement.
“The turbine at Oskarshamn 1 has been sensitive to vibrations since day one and through the years that has caused a number of undesired turbine trips. The turbine has also been subject to extensive wear and it simply needed an upgrade to cope with the company’s availability goals. Most of the major turbine components will be replaced,” said Helmersson.
The modern turbine components need less maintenance since they are built of stronger materials. Before the turbine is assembled the condenser will be strengthened by a system of reinforcement beams. This is another important measure in dealing with vibration problems. The reheater system will also be replaced by a new one. Not only the availability is improved. The new shape of the low-pressure turbine is going to raise the output even though the thermal power remains the same.
“Alstom Power, the turbine supplier, says that the output will be improved by at least 15MWe, maybe even more. It is hard to make an exact estimate at this point,” said Carlsson.
Pump housings
During the upgrade at Oskarshamn 1 the pump housings of the main recirculation system will be replaced. Project Fenix investigations showed that there were flaws on all four pump housings in the system – the same kind of manufacturing damage that was found on the pipework. The flaws were corrected during Fenix but, ever since, extensive tests of the system have been required, a lengthy and dose-accumulating process. The new pump housings are made of better materials that require less frequent testing.
Dose rates
A large scale upgrade of this kind requires good dose management. So far, the dose accumulation among the project staff follows the expected rates. This would not have been possible without the work by the company’s own decontamination group.
“As the project started we soon realised that the dose was higher than we had expected in several systems. Extensive decontamination was carried out and that consequently brought a higher dose to those involved in it. We saw that as an investment to avoid high dose accumulation to a large number of people later on in the project. We are still working hard to decontaminate further and so far we have been successful. In some areas the dose has been reduced by as much as 70%,” said Helmersson.
Project MOD
Project MOD started on 7 December last year. The first month was dominated by regular overhaul jobs like maintenance and refuelling. In January the dismantling of old equipment began and, during February, the project gradually entered the installation phase, which it is still underway. The test and start-up period is due to start in late June and, on 23 October, Oskarshamn 1 will be restarted if everything works out according to the initial plan. And so far, it does.
According to Helmersson: “The upgrade of Oskarshamn 1 is a huge project. Six hundred workers from external suppliers are currently working alongside our own personnel. A lot of work is being done every day inside a very limited area. Needless to say, that requires a lot of planning and cooperation. I look forward to the second half of this project and, on 23 October, I intend to hand over a much upgraded power station to Gösta Carlsson.”
