The Swedish experience3 May 2002
2001 was a record year for Swedish nuclear power plants, with a 25% increase of electricity production over 2000. Production from Oskarshamn was the highest ever, while that from Ringhals was its third highest. The Event Analysis Department at Karnkraftsakerhet and Utbildning has produced a report examining the performance of Swedish reactors.
In terms of safety, 2001 was an excellent year. There were no serious incidents, and Swedish reactors maintained a high safety level in an international perspective.
As far as production is concerned, 2001 was a record year for Swedish nuclear power plants. Total electricity production amounted to 69.2TWh, an increase of 25% compared to the equivalent figure for 2000. Production from the Oskarshamn plant was the highest ever, while that from Ringhals was its third highest. Coastdown operation, which makes better use of reactor fuel prior to a refuelling outage, resulted in unutilised capacity of less than 5%. Unutilised capacity due to output reductions for national power balance reasons amounted to less than 1%.
The three Ringhals PWRs had an average energy availability of 87.9%, while that for the 8 BWRs was 90%. Forsmark 1 and 2 and Oskarshamn 2 and 3 all had average availabilities of over 90%.
Total electricity production in Sweden in 2001 was 157.6TWh. This is the highest annual production to date. Nuclear power production increased from 54.7TWh in 2000 to 69.2TWh in 2001, an increase of 26.4%. This can be compared with output in the record year of 1991 of 73.5TWh. Subtracting the Barsebäck 1 output from the 1991 figure to make it comparable with that for 2001 gives an output of 69.0TWh.
Total electricity use in Sweden was 150.2TWh, or 3% more than in 2000. Most of this increase was due to the continued favourable economy during most of the year, and a high price of oil, which resulted in a switch from oil to electricity for heating.
Barsebäck's annual refuelling outage was planned to take 30 days, but ended up taking six days longer. During this time, the following large items of work were carried out:
• Working to improve the safety by increasing the number of electrical power supply feeds to the auxiliary feedwater system. Using the feeds that had previously supplied Barsebäck 1, there are now four feeds to Barsebäck 2, instead of two.
• The internal parts of the reactor pressure vessel were extensively inspected by remote video inspection. Preparations were made for next year's replacement of the primary systems inside the reactor containment.
The collective dose during the outage was 0.54manSv, which was 0.10manSv more than had been forecast.
Barsebäck 2 underwent two scrams during 2001. On 18 April, during testing of the safety system, two inner and two outer isolation valves closed spuriously, which resulted in a reactor scram. The valve closure was due to high resistances in relay cards.
On 31 July, a reactor scram occurred when restarting after the annual outage. The reactor was in nuclear low power mode at the time, and the system for monitoring neutron flux during power operation had been adjusted in accordance with current procedures. Increasing the power above 5% initiated a scram. It was found that the system becomes too sensitive if calibrated in the low power range.
The annual refuelling outage was planned to take a little over ten days. However, a number of unforeseen items of work had to be carried out, particularly on the isolation valves in the reactor's shutdown cooling system and on a leaking control rod actuator shaft seal in connection with start-up. The final outage time was 11 days, 15 hours. The following large items of work were carried out during the outage:
• Three cases of damage to fuel occurred during the last year. The fuel rods were identified and removed from the core.
• Valve leakage in the shutdown reactor cooling system and in the feedwater system has been rectified.
• Four main recirculation pumps were serviced. Insulation resistance was measured on all pump motors. Two of the motors showed signs of ageing of the stator insulation, and were rewound.
• Resonance testing was carried out in order to determine which pump runs the greatest risk of vibration problems at high speeds. The results showed that four pumps were more sensitive to imbalance and are less rigidly supported than the other pumps.
• Several circuit breakers and disconnectors were replaced in the inverter system that was installed during the previous outage.
• The temperature in the enclosure cupola had been high ever since the work on the toroid during the 1997 outage. A number of defects in the insulation were put right during the 1999 outage; after further work, an acceptable temperature of 59ºC has been achieved.
The collective dose during the outage was 0.25manSv, which was 0.08manSv higher than forecast. The main reason for this difference was the elevated radiation level in the containment.
The annual refuelling outage was planned to take ten days, but actually took 12.5 days. The extra time was mainly involved in correcting a leaking seal on a control rod drive, modifcations of the pilot valves in the residual heat removal system, and an incorrectly operating inspection camera on the fuel charging machine.
The following larger items of work were carried out in addition to the regular work of the refuelling:
• Inspection of the locking rings on the diffusers for the main circulation pumps showed that four pin bolts and a locking tab were missing. One neutron flux detector was replaced due to a high burnout and because it was generating faulty signals during power reductions. Seven control rod drives were replaced.
• The pilot valves for the inner isolation valves in the residual heat removal cooling system were modified. In addition, work was carried out for the replacement of inverters for the main circulation pumps.
During the year, coastdown operation for fuel economy at the end of the operating period resulted in a non-utilisation of a possible output production of 296.2GWh. This is equivalent to about 13 days full power output.
The refuelling outage was devoted to major modernisation work. The outage lasted for over 48 days, which was about two days longer than planned.
Control equipment for both the reactor and the turbine was replaced. The turbine rotors were inspected, confirming crack growth in all three LP rotors. The following larger items of work were carried out:
• A successful decontamination involving replacement of piping in systems connected to the reactor was carried out. The dose rates in the systems concerned were reduced by 99.5%. The piping replacement work was the largest mechanical operation during the outage, and determined the time required for the outage. It proceeded well, keeping to the planned time and generating only low personnel radiation doses. Systems were extensively flushed to prevent any contamination or particles from the pipework from entering the reactor in the future and damaging the fuel.
• The reactor containment steel liner was inspected. The condensation pool was inspected for any corrosion growth on the sealing platework.
• A project to modernise the control rod operation and indication system was concluded, and 30 drive rod actuators were serviced.
• More modern equipment was installed in the neutron flux measurement system.
• The inverters for the main recirculation pumps were replaced, and four mechanical energy storages were installed.
• Service work was carried out on two of the main recirculation pumps, on isolation valves, and on some of the pilot valves for the larger main valves.
The collective dose during the outage was 1.18manSv, which was 0.82manSv below forecast. This was due to the successful decontamination of systems carried out at the start of the outage.
Coastdown operation in the interests of fuel economy during the operating period resulted in non-utilisation of a possible output production of 451.3GWh, equivalent to about 16 days full power operation.
In 2001, Oskarshamn 1 underwent three scrams. The first was on 6 January, when the plant was being operated at reduced power, due to a steam pipe being closed after testing had revealed excessive operating time for the inner isolation valve. During testing of the reactor pressure relief system, a pressure rise occurred that initiated a reactor scram due to high reactor pressure. This was caused by unavailability of the turbine bypass system due to a steam line being unavailable.
On 10 January, high condensate level occurred in a high pressure feedwater preheater when restarting after the stop. As the feedwater preheater temperature was being raised, the feed heater filled with condensed drainage, and had to be pumped out manually. The condensate was not pumped out sufficiently quickly, and so the scram was initiated due to high condensate level.
On 28 March, one of the main steam isolating valves suddenly closed when the reactor was operating at 100% output. This was followed 20 seconds later by a reactor scram due to high reactor pressure. Event analysis showed that a miniature circuit breaker had tripped, due to a short circuit in a newly fitted pilot valve.
Modernisation of Oskarshamn 1
The work on modernising Oskarshamn 1 started in December 2001, and is due to last for 10 months. This is the largest project carried out by the owner OKG since the construction of Oskarshamn 3. It is OKG's policy that its plants should have a potential remaining life of 20 years at any time. The work can be divided into four areas:
• Safety improvements. A new safety concept that enhances and complements the current safety functions is being introduced.
• Computer-based control system. The use of proven technology is an important part of OKG's safety policy. The development of computer-based control systems has now reached the stage where the new technology is capable of meeting the high requirements of the nuclear power sector. Oskarshamn 1 will be the first nuclear power station in Sweden to be controlled by computer-based equipment.
• Upgraded control room. New technology offers new opportunities, such as modern process control from display screens. This is not new outside the nuclear industry, but it is only now that OKG has felt that it can be regarded as proven technology, and thus suitable for use in the nuclear sector. After the modernisation, the control room personnel will monitor and control much of the plant from computer screens instead of control panels as at present. The new equipment to be used has undergone many different tests in order to prove that it meets OKG's high requirements. After the modernisation, Oskarshamn 1 will have Sweden's most modern nuclear power station control room.
• New turbine. OKG intends to improve availability by replacing the turbine with a more efficient one. One of the benefits of the new turbine will be a reduction in maintenance requirements. In addition to higher availability, output will also be increased by 15MWe, to give a unit output of 460MWe.
The replacement of pump casings on the main recirculation pumps is one of the larger jobs to be carried out.
The collective dose during the period 7-31 December was 0.35manSv, which was 0.06manSv less than forecast.
The annual refuelling outage was planned to take 25 days, but finished one day ahead of schedule. Testing of the core spray stays did not reveal any new damage, which meant that further testing was not needed.
The following larger items of work were carried out:
• Remaining work in the boron injection system.
• A separator device was fitted in the auxiliary feedwater pumps‚ suction inlets in the condenser.
The collective dose during the outage was 0.66manSv, which was 0.16manSv more than forecast. Most of the additional dose was due to various problems with service of a main steam valve, and to a greater number of measurements in preparation for future outage work.
There was a reactor scram on 9 August. It had been decided to shut down to the cold shutdown reactor state due to leakage problems with two control rod drive seals. However, when the turbine was disconnected from the grid and partial vacuum breaking had been activated to brake the turbine, a fault occurred in pressure measurement in the condenser. As a consequence, the vacuum breaker valves remained open too long, so that the pressure in the condenser rose to the point where a reactor scram was initiated.
The annual refuelling outage was the shortest in the 16-year history of the unit, and the first one in which the planned times were all held completely. Total outage duration was 16 days and 8 hours. The following larger items of work were carried out:
• Replacement of the pilot valves for the reactor main steam line valves.
• Replacement of control equipment for the mussel filter in the cooling water system for shutdown reactor cooling.
The collective dose during the outage was a record low of 0.19manSv, which was 0.02manSv lower than forecast.
Coastdown operation to improve fuel economics resulted in a loss of output production of 15.1GWh, equivalent to 0.6 days of full power operation.
There was one scram, on 1 December. This scram occurred during testing to verify correct operation of the scram system. When raising the temperature between tests, the power increased too rapidly.
The annual refuelling outage was planned to take 28 days, but in fact took 32 days. There were few modifications and other work items. In addition to preventative maintenance and testing, the following larger items of work were carried out.
• Measurements to prepare for replacing the reactor core shroud head next year.
• Inspection of the core spray.
There was some work carried out on the turbine. This included:
• Inspection and replacement of turbine blades with cracked erosion protection layer on one of the three LP turbine rotors.
• Inspection of the generators, including leakage testing, flow measurement, pressure and vacuum testing of the stator winding, pressure testing of the rotor, cleaning brush chambers and turning sliprings.
• An arrangement was introduced to enable output to be increased, with reactor output raised to 110% and unaltered steam data. This takes the form of a bypass around the HP turbine, so that some of the steam can be supplied directly to the LP turbines, thus increasing turbine output. The collective dose during the outage was 0.80manSv, which was 0.30manSv lower than forecast.
The annual refuelling outage was planned to take 28 days, but actually took 29 days. The first stage of the work of installing a new plant control system was started. Modernisation of the plant will continue in stages over the next annual outages. In addition to the regular routine inspections and refuelling operations, the activity monitoring equipment was modernised. The top plates of a number of fuel elements were replaced.
The collective dose during the outage was 0.33manSv, which was 0.15manSv lower than forecast.
On 17 June, a small oil leak was discovered from one of the main unit transformers. Attempts to tighten a leaking connector only made the leak worse, and the unit was taken out of operation. However, before this was done, the transformer stopped, disconnecting power to parts of the electrical system, and a reactor coolant pump stopped. As all the reactor coolant pumps must be in operation for full power operation, a reactor scram was automatically initiated.
The annual refuelling outage was planned to take 25 days, but actually took 32 days. The main reason for the extra time was problems with welds between the reactor vessel and the reactor cooling loops. Other additional work that extended the length of the outage was repair of a valve in the pressuriser and a valve in the component cooling water system.
In addition to regular routine inspections and refuelling, work during the outage included inspection of the above mentioned welds, inspection of the reactor pressure vessel head and of the filter building for the reactor containment venting system. Work was also carried out on the steam generators in accordance with a special programme, and a rectifier in the electrical power system was replaced.
The collective dose during the outage was 0.27manSv, which was 0.05manSv lower than forecast.
Two crack indications were identified in welds between the reactor vessel nozzle and the hot leg of one of the reactor coolant loops. Three defects were identified on the internal surface of the weld. These three defects were removed by cutting out the surrounding material. The remaining material was checked and found to be free from further defects. In addition, another three internal defects were found in this weld. The origin of the cracks has not been fully explained. It is, however, assumed that the cracks were caused by variations in the material structure originating from the manufacturing process.
Calculations indicate that the cracks that were found have a negligible impact on the strength of the reactor circuit, and that the safety margins have not been affected. Since there is a possibility that new defects can occur in the long run in the existing material, Ringhals is now preparing to restore the affected welds and pipe sections to as-new condition. Ringhals 4 welds will be repaired during the 2002 refuelling outage. Ringhals 3 will be repaired during the 2003 refuelling.
The non-destructive measuring methods that until now have been used for detecting cracks do not measure the depth of the cracks correctly. Ringhals therefore started a research project to improve the measurement equipment and the test methods. When such improved equipment and methods are made available, it is expected that the depth of the defects can be determined without cutting out samples of the material surrounding the indications. Another aim is to have these improved methods qualified by the regulatory authorities. Such a qualification would allow for a return to a previous schedule when these circuits were tested every ten years.
The annual refuelling outage was planned to take 30 days, but in fact took 39 days. The reason for the extra time was problems with welds between the reactor vessel and the reactor cooling loops, similar to that described for Ringhals 3.
In addition to regular routine inspections and refuelling, work during the outage included inspection of the above mentioned welds, the reactor pressure vessel head, the steam generators and switchgear. A rectifier in the electrical power system was replaced, and the reactor containment was leak tested.
The collective dose during the outage was 0.30manSv, which was 0.03manSv lower than forecast.
On 17 April, an operator error resulted in a reactor scram from full load when testing the reactor protection system. When one of the operators performing the testing should have operated a circuit breaker, he operated another similar circuit breaker, which had the result of scramming the plant. All safety systems operated as intended.
Rupture discs at Barsebäck 2
When testing rupture discs that had been mounted in the filtered containment venting system, it was revealed that two discs had been interchanged. The consequence was that the threshold for activating the path between the containment and the venting system was too low, and the activating threshold for an emergency relief pipe, preventing overpressurisation of the filter vessel, was too high.
The two rupture discs were mounted in two identical adapter rings. The only identification was a small metal tag near the disc edge marked with the valve number. The tag is hidden when the disc is mounted in its adapter ring. The contractors that installed the rupture discs in the plant did not know that that the two discs broke at different pressures.
Previously, these discs were not replaced during the same outage and hence it was not possible to exchange the discs. On this occasion, two almost identical discs mounted in their adapter rings should be installed.
New rupture discs have now been mounted in the venting system, and the procedures used have been modified.
Overload protection devices at Ringhals 2 On 20 June, Ringhals 2 was operating at full power. After a load switchover in the non-prioritised part of the electrical supply system, a programmable overload protection device tripped unexpectedly. The device tripped after being subjected to a load of only 60% of the expected tripping level. The overload protection devices were disconnected when it had been demonstrated that these functions were not necessary over the short term. These devices will be reinstated when the software fault has been corrected. A faulty computational algorithm in the device's software caused the trip.
The fault had been introduced during the 2001 annual refuelling outage when finishing off a remaining action. Modern programmable overload protection devices had been installed during the modernisation of the electric power systems of Ringhals 2. These protection devices are located in the relay protection equipment and are intended to protect cables and transformer against thermal overloads.
The Ringhals personnel regarded the planned action as an adjustment of the existing equipment. The intention of the equipment vendor was to replace a component. Due to a misinterpretation, the work was handled according to normal maintenance routines. The tests that were required by these routines were not extensive enough to reveal the software fault.
An investigation of the event revealed that 44 out of a total of 58 overload devices would have tripped at loads lower than anticipated because of this faulty software.
TablesSwedish nuclear power plants