Kansai Electric said its Unit 3 reactor at the Ohi nuclear plant shut down automatically at 11:56pm on August 8, after a "loss of field" alarm sounded on the unit's generator. The reactor had been running at a steady, rated thermal output right up to the trip, and the automatic reactor stop followed directly from the generator's own automatic shutdown.
A loss-of-field alarm sounds when a generator's rotor loses the direct-current magnetism it needs to induce power in the surrounding stator windings, the excitation system that keeps a generator's output voltage steady. Kansai Electric's supplementary filing includes a diagram of that circuit, showing how a permanent magnet feeds the exciter, which energizes the rotor coil and, through it, the stator coils that send power to the grid. The company has not said which part of that chain triggered the alarm. The same schematic separately lists a steam generator, pressurizer and primary-coolant pump on the reactor side of the plant, distinguishing that loop from the turbine-generator equipment where the fault occurred.
The cause is still under investigation, Kansai Electric said, without offering a timeline for identifying it. The utility did address the question most likely to worry the public first: the shutdown caused no radioactive impact on the surrounding environment, it said.
The filing is otherwise thin on consequence. Kansai Electric committed only to disclose promptly "if matters requiring disclosure, including effects on business results, become clear". That leaves open how long Unit 3 will sit idle and what it will cost the utility to cover the lost output while engineers trace the fault inside the excitation system.
For now, the record is narrow: a generator field-loss alarm, an automatic trip of both generator and reactor, no radiological release, and an unresolved question about which piece of equipment failed. Kansai Electric, listed on the Tokyo Stock Exchange's Prime Market under ticker 9503, has given no restart date for Unit 3.