The first alarm sounded around 6:20 p.m. Sunday evening, and indicated a rise in pressure in the pressurizer relief tank for the primary coolant system.
The primary coolant system (also called the Reactor Coolant System) consists of the reactor vessel, the steam generators, the reactor coolant pumps, a pressurizer, and the connecting piping. The primary function of the reactor coolant system is to transfer the heat from the fuel to the steam generators.
The second alarm went off around 1:20 a.m. Monday morning, and alerted operators that an auxiliary motor for the reactor’s emergency diesel generator was malfunctioning, Kansai Electric Power Co. said.
Emergency diesel generators are one of the most important contributors to the core damage failure rate of nuclear power plants.
The Nuclear and Industrial Safety Agency stated that the incidents won’t impact the Reactor 4 restart schedule, as the rise in pressure was “marginal”, and the emergency generator is functioning, although its auxiliary motor may now be useless.
Source: The Japan Times