San Onofre Nuclear Generating Station: Unit 3 Turbine Generator Repair Following the Loss of Lubrication Oil Incident

Upon return to service from a refueling outage, the 1150 MW SONGS Unit 3 was at 39% reactor power when a circuit breaker fault resulted in a fire, a partial loss of AC off-site power, and a reactor shutdown. The failure of a circuit breaker to function properly resulted in the unavailability of the DC driven turbine emergency lubricating oil pump, causing extensive damage to the turbine generator. The power supply crisis in California at this time highlighted the need for an urgent understanding of the seriousness of the damage to the turbine generator, the consequences for the repair and timely return of the unit to service. Initial inspections indicated that all rotor line bearings had suffered from loss of white metal and most of the housings were distorted. Rotor journals were scored from solid contact and subject to excessive heat damage. Rotor steam seals had suffered rubbing damage. In view of the component sizes associated with such a large machine, and the need to minimize the outage duration, a large proportion of the repair work was undertaken on site. This enabled SCE to project manage the work themselves ‘in-house’. It was decided that the HP and the three LP turbine rotors would remain on site, with only the Generator rotor and the complete Exciter being shipped to the OEM’s workshops for repair. Specialist sub-contractors were employed for site machining and heat treatment. Consultants were hired for specialist technical matters, primarily related to metallurgical issues, to complement the in-house expertise of both SCE and ALSTOM. The serious nature of the damage required close cooperation between SCE and ALSTOM the OEM. It was important to understand the design margins and therefore the scope for repair, and long term integrity of the solutions. The detailed repair instructions were prepared and controlled jointly between ALSTOM and SCE engineers on site, with extensive technical back-up provided from the OEM engineering design departments. Similar close co-operation on the re-design of components and fast track supply of spare parts was crucial to the success of the project.