Nuke plant suffers jolt greater than designed in aftershock
The No. 1 reactor of the Onagawa nuclear power plant in Miyagi Prefecture on April 7 sustained a jolt greater than what it was designed to withstand during a strong aftershock from the powerful March 11 earthquake, according to nuclear safety officials.
The finding raises further doubts about the viability of the assumed quake resistance at the Tohoku Electric Power Co. complex, even though it had been shut down safely after the deadly quake last month.
The Nuclear and Industrial Safety Agency has instructed the regional utility serving northeastern Japan to analyze the impact of such a jolt on key facilities at the three-reactor plant, the officials said.
During the aftershock, the biggest after March 11, measuring upper 6 in Miyagi on the Japanese seismic scale of 7, a seismometer at the building housing the No. 1 reactor registered a quake acceleration of 476.3 gal vertically, against the 451 gal assumed for the facility.
The assumed level of the jolt ''shouldn't be exceeded in principle,'' said agency spokesman Hidehiko Nishiyama. ''While I intuitively think that if it is this much, it shouldn't be a cause for concern, but we still have to evaluate its safety.''
None of the reactors at the plant was operating when the aftershock struck. But as a result of the temblor, the plant lost part of its external power supply and saw a cooling system for pools storing spent nuclear fuel briefly lose power.
Both the troubled Fukushima Daiichi nuclear power plant in Fukushima Prefecture and the Onagawa plant were hit by the March 11 quake and tsunami, but the latter has been largely under control with its key cooling functions kept alive.