Internals replacement rejuvenates Forsmark20 February 2001
Replacement of reactor core components and upgrading the control systems is extending the working life of BWRs at Forsmark
Sweden belongs to the deregulated Nordic electricity market. In this market the electricity prices have been very low for four consecutive years, mainly owing to wet weather conditions and large supplies of hydropower in Norway and northern Sweden. Why would anyone invest in new or operating power plants in market conditions like these? One reason is the characteristics of nuclear power – high investment costs and long construction times, but very low fuel costs compared to coal or gas, and almost no emission of greenhouse gases. This makes the existing nuclear power plants in Sweden and elsewhere important assets for the future. Keeping the plants operating long-term at high electricity production will always have a high priority. Reinvestments to keep them reliable and up to modern safety standards need to be made.
Since the referendum in 1980, Sweden has struggled over the issue of premature closing of nuclear power plants. With the closure of Barsebäck 1 in 2000 and the possibility of closing down Barsebäck 2 – the conditions for replacement power are not yet fulfilled – a compromise seems to have been reached. Sweden has 11 nuclear units still in operation, eight BWRs and three PWRs. The public debate over nuclear power has almost ceased to exist. Power companies and safety authorities are now looking ahead and formulating their strategies and requirements for safe and reliable operation of the plants for their remaining technical lifetime.
The Forsmark plant, with its three operating advanced boiling water reactors supplied by Westinghouse Atom (then Asea-Atom), is the most modern nuclear power plant in Sweden. These units now have some 15-20 years of successful operation behind them. In order to secure long-term operation at high performance, the plant owner, FKA, completed major reinvestments in the plant during last year’s outage.The major supplier was Westinghouse Atom, which replaced reactor vessel internals and modernised the control and instrumentation equipment.
During the outage of 2000, Westinghouse Atom replaced internals inside the reactor pressure vessel (RPV) at the twin units Forsmark 1 and 2. The replacement included design, licensing, manufacturing and installation of core shrouds and upper core support grids. Under a separate contract Westinghouse Atom also performed the segmentation of the old internals as a preparation for final disposal.
The core shroud is a cylinder surrounding the BWR reactor core, and the upper core support grid is the top guide of the fuel elements in the reactor core. In all BWR designs by Westinghouse Atom these internals have been designed for easy replacement in the event that cracking or other problems occur. The core shroud is therefore bolted to the supporting structure, as opposed to welded. Westinghouse Atom had in the past replaced several core support grids and one core shroud, the one in Oskarshamn 1, Sweden's oldest nuclear reactor. However, the replacements at Forsmark were unique in several ways:
• The replacement of internals was done in world record time: the first replacement at Forsmark 2 in 9.5 days and the second
at Forsmark 1 in 7.5 days. Allocated time according to the contract was 10.5 days for each unit.
• There were no structural cracks in the old internals, forcing the customer to replace or repair. The decision to replace was made from a life cycle cost/lifetime management consideration, taking into account the cost of future inspections of the internals during outages, as well as the risk of unplanned shutdowns if cracks were detected.
• In order to completely eliminate the cost of future inspections, a design using only forged materials with no structural welds was chosen, although this hardware is more expensive than designs with welds.
Design and manufacturing
The forgings for the new core shrouds consisted of approximately 4 metre high cylinders with a diameter of 5 m and a wall thickness of 100 mm. These forgings were made from castings in which a central hole was bored, with the resulting cylinders being shaped by applying pressure both inside and out. The final dimensions were reached by machining. The wall thickness was thus reduced to 40 mm.
For the upper core support grids two forged pieces were used, a plate with diameter 5 metres and thickness 230 mm and a ring with corresponding size. The geometry of the grid was established by machining with tight tolerances. For the core shrouds and the upper core support grids stainless steel 316L was used, with additional requirements for carbon, sulphur and cobalt concentrations.
The design work for the new internals started in 1997/98. At the same time the procurement process for the forgings and the final machining were set in motion. In addition to detailed quality and manufacturing plans etc, comprehensive stress analyses were performed to verify that the new internals would withstand loads expected during normal operation, as well as in the case of transients and possible accidents.
Installation at Forsmark
Prior to the installation work, Westinghouse Atom developed special tooling and work procedures, as well as training of personnel, by building mock-ups at its LWR service centre in Sweden.
The installation work started at Forsmark 2 in May 2000 by first removing the old core support grid, and then loosening the bolts and removing the old core shroud. Then the new internals were assembled under water inside the RPV. Through the use of advanced measurement and positioning methods during these operations and beforehand, the shifts in position for the new internals compared to the old ones were only tenths of millimetres and well within tolerances. While the total time was 9.5 days at Forsmark 2, the same operations at Forsmark 1 in September 2000 took only 7.5 days.
Segmentation of the old components
After installation of the new internals, the old core shroud was placed on a support structure in the internals handling pool in the reactor building. The old core support grid was put on top of the shroud. Segmentation was performed during normal plant operation. The work took 12 weeks to complete, and stayed on schedule.
Mechanical cutting and sawing equipment was used for segmentation, to avoid producing gaseous products. The cutting chips could be collected from the floor of the refuelling pools. A hydraulic tool was specially developed for cutting the 7 mm thick bars of the core support grid. All the cutting and sawing equipment was tested at Westinghouse Atom underwater testing facility prior to the work at site.
The outcome of this work has verified that mechanical cutting is a viable method of segmentation of radioactive mechanical components, such as BWR internals, as it only creates residual products that are easy to collect. In this respect the method seems to be superior to alternatives such as plasma or water jet cutting.
Modernisation of control equipment
In parallel with replacement of RPV internals, new control equipment was installed during last year's outages at Forsmark units 1 and 2. Other similar replacements have been going on at these units for several years, making them among the most comprehensively upgraded in the world. The strategy behind this modernisation is a step by step approach, based on a long-term co-operation between FKA and Westinghouse Atom, and the positive outcome is the result of close co-operation between the customer and the supplier.
The co-operation between FKA and Westinghouse Atom (at that time ABB Atom) started in 1995 when FKA ordered a new control system infrastructure, process computer, control rod indication and positioning system, as well as a neutron flux calibration system for unit 1 and 2. At the same time, FKA and Westinghouse Atom developed joint guidelines and standard requirements that would govern future modernisation projects, in order to achieve standardisation for the benefit of operators, maintenance personnel and design engineers. The new control system is based on ABB's Advant modules, from which Westinghouse Atom has developed a plant-wide platform called Nuclear Advantage. This has given a common "look and feel" and handling, independent of source of information or which process components are being controlled. Step by step, several process systems have been modernised.
During the outages of 2000 several projects were completed, such as the installation for a new condensate clean-up system. This installation was a complete functional supply, including everything from system design to control room furniture.
Installation of new drive systems has also been carried out on all 16 main recirculation pumps. These pumps are vital components in BWRs, controlling the coolant flow to the fuel and the power level in the reactor core. The control system of the new frequency converters will prevent fuel damage during severe grid disturbances, thus reducing the need for margins in coolant flow, which will improve fuel economy. Similar new drives will also be installed at Forsmark 3 during the outage this summer.
Finally a new turbine process control system was installed at units 1 and 2 during 2000. In a BWR plant, the turbine controller controls the pressure in the reactor pressure vessel, in addition to turbine speed, synchronisation and load variations etc.
Since 1995, the instrumentation in approximately 350 cabinets has been changed or modified during the control system modernisation of Forsmark units 1 and 2.