INES-event
INES 1

Fast stop of the main circulation pumps and simultaneous loss of their fly wheel systems during reactor scram.

There was a reactor scram and trip of main circulating pumps during start -up after the annual outage at Olkiluoto unit 1 on Friday, 30 May 2008 at 6:31 am. The power of the unit 2 was also reduced later in the Friday evening to ensure adequate core cooling in potential transients with similar sequence of events.

The event did not cause any on-site or off-site radiation consequences. The event was classified as level 1 on the INES scale.

The chain of events: Olkiluoto unit 1 was to be started after the annual outage. When the power was 60% the excitation system of the main generator started to increase generator voltage. The 115 % / 6 s over voltage protection opened the 400 kV grid breaker and the plant entered to the island operation mode. The voltage of the generator busbar jumped approximately to the 150 %, when the unit was disconnected from the 400 kV grid. The main and house load transformers saturated. The saturation triggered the differential current protections of the transformers and the generator breaker was opened after 100 ms time. The 110 kV back-up power supply was connected automatically after 2 s black-out time. All the dieselgenerators started to idle run.

A reactor and turbine trip followed and safety systems stabilized the plant in hot stand-by mode. Over voltage however caused all six main circulation pumps to trip when the 400 kV grid breaker opened. The pumps were tripped by the intermediate circuit over the voltage protection of the frequency converters. Simultaneously the protection disconnected the respective fly wheel motors which are also connected to the intermediate circuit of the frequency converters. The fly wheels are a separate set of generators which use their inertia to produce electricity for the main coolant pumps in case of loss of normal power feed. The fly wheels are intended to provide additional inertia in order to ensure sufficient cooling of the core during electrical transients.

The over voltage tolerance of these safety classified main circulation pump drive systems was thus inadequate. Two main circulation pumps could not be started later on due to failures in the brake resistor units of the frequency converters.

Since the pumps were at minimum speed in the beginning of the transient, the fly wheels were not needed to slow down the pump ramps, and the transient had no consequences for fuel coolability.

This kind of event may challenge the over voltage protection in similar systems or any other power electronics (eg. inverters, rectifiers, UPS systems).

Location: OLKILUOTO-1
Event date: Fri, 30-05-2008
Nuclear event report
Legenda & explanation