The report says Maanshan 1 PWR suffered a loss of external power after accumulated salt deposits caused transmission instabilities on incoming power lines. These instabilities caused short circuiting, leading to an electrical fire inside the plant, preventing one diesel generator from picking up the load. The total blackout ensued when the second diesel failed to start.

AEC found that personnel abandoned efforts to start the second diesel when it failed to immediately respond. The blackout was ended after two hours when a swing diesel was brought into service. It was later discovered that the second diesel would have ben operable.

The event left the unit in blackout for two hours, but it had been in hot standby for the previous 21 hours, which minimised the core heat load. AEC confirmed that an auxiliary feedwater pump started up, keeping the temperature and pressure in the core decreasing until full power was restored.

Shen Li, AEC’s director of nuclear regulation, said: ‘Had the feedwater system failed to function, and had the core temperature and pressure increased or decreased, then this would have been a different story.’

The event was said to be the world’s first station blackout at a western-designed PWR.