The No. 1 reactor at the Tsuruga nuclear power plant in Fukui Prefecture does not have a vent to release excess pressure during a reactor emergency, sources said Sunday.
Of the 30 boiling-water reactors in Japan, the one at Tsuruga is the only one without a venting system.
The anomaly arose after the plant’s manager, Japan Atomic Power Co., deemed the containment vessel in reactor No. 1 less likely to be damaged by a pressure buildup, making installation of a venting system a low priority.
But Japan Atomic Power has since done an about-face in recent checkups because venting was needed to get the crippled reactors at the Fukushima No. 1 power plant under control, the sources said.
The probability of pressure damaging the rogue reactor at Tsuruga was estimated at around 1 percent — which is exactly the same probability given to the No. 1 reactor at Fukushima, Japan Atomic said.
The No. 1 reactor at Tsuruga is the oldest in Japan.
Japan Atomic announced in 2002 that it would suspend the reactor in 2010 and eventually decommission it, but later decided to keep running it through 2016 anyway.