Fire at a generator of a power plant

The incident started with the cracking of the low-pressure turbine blades. This caused an unbalance to turbine generator rotor which led to the failure of the bearings and the loss of confinement of the hydrogen used as a coolant, which ignited. Also the oil pipes connected to the turbine generator were teared open due to the vibrations. The fire spread to cable trays in the turbine building and control equipment room, which triggered a total loss of power supply which lasted for 17 hours. The secondary cooling system of the reactor was stopped until operators started up the diesel-driven fire pumps in order to feed the steam generators with fire water.
There were no injuries, no damage to the nuclear reactor, and no spread of radioactivity.

INITIATING CAUSES
The failure of the low-pressure turbine started the series of events. The initial event was the breaking of some turbine blades.
ROOT CAUSE
The fire could spread up to the control equipment room due to lack of measures for fire propagation and escalation: (i) lack of proper fire-retarding provisions, (ii) inadequacy in fire-barriers, and (iii) insufficient physical separation in redundant safety.

turbine, collant system, cables, control room

The release of hydrogen was preceded by the failure of the low-pressure turbine.

DESCRIPTION of the FACILITY
The nuclear power plant was a pressurised water reactor, consisting of a twin - unit of 220 Mwe each. It had been commissioned in the year 1989
and 1992 respectively.

The reactor was immediately shut-down and could be maintained in a safe shut-down state with adequate sub-criticality at all stages. Crash cool down of the primary heat transport system was done and subsequently fire fighting water was injected into the steam generators for maintaining thermosiphon cooling of the core.

The most risk-significant auxiliary systems are, in a decreasing order:
(1) Turbine-generator lubricant oil systems,
(2) Hydrogen cooling system,
(3) Pump lubricant oil,
(4) The electrical components (cables, motors etc.)
The relative importance of the fire consequences is based on the masses involved, which generate very different fire loads. Therefore a hydrogen fire load is much less that the one from oil, however the possibility of explosion raises the attention to be given to the hydrogen system. In this incident, apparently, hydrogen ignited first, assisting or triggering the oil fire.

Several measures were implemented on both units of the nuclear plant, aiming at improving the following aspects:
(1) Passive and active fire protection
(2) fire isolation
(3) Fire protection management
(4) Minimisation of combustible materials
(5) Elimination of ignition source
(6) Control Room Habitability