A nuclear power plant on the Sea of Japan coast had been vulnerable to unauthorized entry in around a dozen locations since March last year, after its security system lost partial functionality and secondary measures were not effective, the country's nuclear regulatory body said Tuesday.

Intruder detection systems were defective in 15 locations at the Kashiwazaki-Kariwa plant in Niigata Prefecture, according to its operator Tokyo Electric Power Company Holdings Inc. The Nuclear Regulation Authority deemed backup systems were insufficient in 10 of the locations.

The NRA has provisionally rated the breach at the plant at the worst level in terms of safety and severity, marking the first time it has given such an assessment. It will notify Tepco of the rating, finalizing it if there are no objections, and consider what disciplinary action should be taken.