Following a switchyard short circuit, two emergency diesel backup power plants failed at Sweden’s 1106 MWe Forsmark 1 boiling water reactor (BWR), an incident ranked at two on the seven level International Nuclear Event Scale (INES) because of the common-cause failure of the emergency power supply system.
The two phase short occurred in a 400 kV switch yard when a section disconnector was opened under full load. The unit line breakers disconnected the generators and the plant, located 200 km north of Stockholm, automatically inserted selected control rods, shortly after the initial event the plant was scrammed and the containment was isolated. However, while the reactor being shut down could be concluded by the surveillance systems, the indication of the position of the control rods was unclear due to lack of power supply. The emergency cooling system did pump water into the reactor vessel and the water spraying system in the containment was also activated as part of the safety system.
While all of the four backup generators started, two of the converters needed for the uninterrupted power supply (UPS) did not work and two of the diesels did not start automatically. Consequently, two of four divisions in the 500 V internal grid were out for approximately 22 minutes before being manually switched, when the main internal grid regained power and the plant could go to a safe state.
Shortly after the initial event, and before the scram, one turbine tripped due to low hydraulic pressure in the control valve system. The turbine speed decreased to 2820 rpm which should have opened the generator circuit breaker due to low frequency. However, the breaker did not open causing the main power supply net, supported by the diesel generators, to disconnect. During the electrical transient two UPS units, which support the speed measurement control of the diesels, failed and the engines failed to start automatically. The loss of power resulted in loss of two auxiliary feed water pumps.
Analysing the failure, Forsmark owner-operators Vattenfall state that the protection system in the 400 kV switchyard did not work as specified and as a result the magnitude of the electrical transient was higher the expected which affected the two UPS units. Furthermore, the generator frequency protection breakers failed due to a design error. Forsmark says that if this system had worked properly, the power supply from the external 70 kV grid would have been connected automatically to the internal power supply system earlier and the loss of power would have been limited to seconds instead of minutes.
Addressing the failure Forsmark is to instigate a design change of the protection units in or above the rectifier in the UPS units and correct installation of the frequency protection unit of the generator breaker. If the problem exists at Forsmark 2, it will be corrected there as well. Forsmark will also install a parallel power supply to the speed measurement device on the diesel generators in two divisions before it is restarted. In the longer-term, the company will also install parallel a power supply to the speed measurement device on the diesel generators in two divisions.
On 2 August OKG chose to stop operations of Oskarshamn 1 and 2 over similar concerns while Forsmark 2 and Ringhals 4 are out of operation due to refuelling outages. SKI, the Swedish Nuclear Power Inspectorate, subsequently asked for safety analysis reports from the Ringhals and Oskarshamn plants to try to determine whether the same problem could occur there. SKI has determined that Forsmark 3, Oskarshamn 3 and Ringhals units 1-4 can continue operation, but further investigations are required before Forsmark 1 can be restarted and as a result of the event Forsmark 2, Oskarshamn 1 and Oskarshamn 2 are currently shut down.
Environmental campaigners Greenpeace called for all of the reactors to be shut down, saying without power the temperature would have been too high after 30 minutes and within two hours there could have been a meltdown. Jan Vande Putte of Greenpeace International said the incident: “proved that a simple power blackout can very easily lead to a catastrophic reactor meltdown.”
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