Unit 3 was in refueling state with reactor vessel head removed and spent fuel pool gates open to the reactor vessel. The 400 kV unit breakers received an unintentional order to open, but only two out of three phases opened causing low-voltage in the station but still over 65%.No EDG got start order.
Forsmark unit-3 had been shut down for yearly maintenance since 2013-05-19.
Forsmark-3 is equipped with 4 safety trains. Two offsite power grid connections, one to the 400 kV grid (double parallel breakers) and one to the regional 70 kV backup grid.
The unit was in refueling state with reactor vessel head removed and spent fuel pool gates open to the reactor vessel. The unit was powered from the 400 kV offsite power grid (one of the two breakers, one was open due to maintenance). The regional 70 kV grid was not available due to maintenance. Safety train A and B were verified for operation, train C and D were operable but maintenance was ongoing.
2013-05-30 at 10.01: During maintenance and testing of relays on the excitation system on the main generator the 400 kV unit breakers received an unintentional signal to open (and one breaker was already open due to maintenance). However one phase of the 400 kV breakers did not open. Due to the main and station auxiliary transformers configurations(Y/D resp. D/Y) a substantial line-voltage (>65%) was still available. One phase on incoming side induces voltage on all three phases on the outgoing side.
The safety bus bars were not disconnected because no undervoltage protection was activated. The undervoltage protection was not activated because >65% voltage was still available (but unbalanced) and the bus bars do not have phase unbalance protection.
And hence no signal to disconnect the safety bus bars and to start the EDG:s was activated.
Safety-grade components with phase imbalance protection were disconnected, at least 146 components, among them components vital for the heat removal from the spent fuel pool. Some non-safety components (motors) were over heated and failed.
The failed phase for the busbar was not identified in the MCR because the surveillance of the busbar is only between two phases.
The information in the MCR was ambiguous. Voltage on the 10 kV busbars seems normal, unit breaker status is ambiguous (nor open or closed) lights in MCR becomes subdued and a lot of objects disconnects from the busbars. The unit was at this time without heat removal for the fuel and the EDG:s.
The loss of two phases is not analyzed in the safety analysis report. The unit was in an unanalyzed condition. Manuel measures had to be taken to power the busbars, and to reset protection relays for tripped safety grade components.
The residual heat exchange system was lost. It would take more than 30 hours before the water in the pool had started to boil, the temperature in the spent fuel pool increased from 33 ˚C to 34 ˚C.
2013-05-30, 10.15: EDG train B was manually started and operators opened the incoming breaker from 10 kV ordinary grid. Disconnected components was reset and started up as intended.
2013-05-30, 10.17: RHR and cooling of EDG train B is restored.
2013-05-30, 10.20: EDG in train A was started in the same way as train B.
2013-05-30, 10.26: EDG in train C was started in the same way as train B.
2013-05-30, 10.37: EDG in train D was not able to start from the MCR due to maintenance. Operators opened the incoming breaker from 10 kV ordinary grid and the EDG in train D started automatically due to undervoltage.
2013-05-30, 10.43: A loose electrical connection in a cabinet was found and was reconnected.
2013-05-30, 10.44: Operability of 400 kV grid was established.
Local resetting of other objects finished after approximately 90 min.
Event date: Thu, 30-05-2013