Fire at a generator of a power plant

This event regards hydrogen used as coolant of the stator of a turbine-generator system.
A loss of the sealing oil pump and other problems in the sealing oil system resulted in the coolant hydrogen leaking from the generator. The hydrogen leaked for several minutes before it ignited and exploded. A fire followed the large explosion, which burned for five to eight minutes.

THE DETAILED ACCIDENTAL SEQUENCE
The plant was operating at 92% power when a ground fault on a land steam exhauster caused the supply breaker to a motor control centre (MCC) to trip. This supply breaker could not be reclosed.
The loss of the MCC resulted in a loss of the hydrogen side seal oil pump for the main generator that is powered from that MCC. Nevertheless, the plant continued operation as the licensee procedures and technical manual of the turbine-generator manufacturer allowed full-load operation with the hydrogen side seal oil pump shut down.
When operators initiated implementation of a procedure to restore the seal oil system, a small hydrogen explosion occurred and a reactor power reduction was initiated.
Shortly thereafter, with the plant at 85% power, a large explosion and fire occurred and the turbine was immediately tripped.
Some of the equipment damaged during the fire and explosion caused several grounds in the i nth electrical power system, resulting in voltage degradation which hindered the fighting of the emergency.

INITIATING CAUSE
The loss of the pump serving the hydrogen side seal oil was at the basis of the incidents.
ROOT CAUSE
The fact that the operation was allowed even without the seal oil pump was probably a contributing or root cause, because it was the responsible for the hydrogen leak and accumulation before ignition. The fighting of the emergency resulted in additional deficiencies, probably due to loss of voltage capacity because of the fire. This suggests design or management root causes.

steam exhauster, turbine-compressor system

The plant was operating at 92% power when a ground fault on a land steam exhauster caused the supply breaker to a motor control centre (MCC) to trip. This supply breaker could not be reclosed. The loss of the MCC resulted in a loss of the hydrogen side seal oil pump for the main generator that is powered from that MCC. The plant continued operation as the licensee procedures and technical manual of the turbine-generator manufacturer (Westinghouse) allow full-load operation with the hydrogen side seal oil pump shut down.

Following the large explosion, the fire burned for several minutes and was extinguished by the CO2 system.
The explosion resulted in the generator housing lower skirts being thrown into some neighbouring equipment. The missiles also broke two large hand wheels for valves and damaged some small test valves and non-safety-related level instruments.

US.NRC issued an information notice with suggestions but no NRC requirements. Therefore, no specific action or written response is required.
The notice is to alert recipients of potentially significant problems caused by failure of non-nuclear instrumentation (NNI) power supplies at B&W designed reactor facilities.
It is expected that recipients will review the information for applicability to their facilities and consider actions, if appropriate, to preclude similar problems occurring at their facilities.

0. Following shut down of the hydrogen side seal oil pump, operators initiated a procedure to restore the seal oil system. There is a time correlation between this event and the first small hydrogen explosion.
1. The reactor was automatically tripped by the recently installed anticipatory trip circuit.
2. Following the large explosion, the fire burned for several minutes and was extinguished by the CO2 system
3. Shortly after the reactor trip, the operators proceeded to start auxiliary boilers to provide for the auxiliary steam loads. The small boiler was successfully fired. When the control power for the big boiler was turned on, in preparation for its start, the small boiler tripped and control power to both boilers was lost.

The national nuclear authority issued suggestions but no legal requirements.
The suggestions regarded the need to consider and adopt measure regarding the potentially severe problems caused by failure of non-nuclear instrumentation power supplies at reactor facilities of the same type as the one affected.
For a generic , see HIAD ID = 042