The massive aftershock Thursday compromised power supply systems and emergency generators at nuclear power facilities in northeastern Japan, sparking concern that plant operators are still unprepared to avert another crisis.
The massive aftershock Thursday compromised power supply systems and emergency generators at nuclear power facilities in northeastern Japan, sparking concern that plant operators are still unprepared to avert another crisis.
As the nation races to deal with the accident at the Fukushima No. 1 nuclear power plant following the March 11 Great East Japan Earthquake, experts foresee a series of aftershocks, including at least one major seismic event.
According to Tohoku Electric Power Co., its Higashidori nuclear power plant in Aomori Prefecture switched to an emergency diesel generator to cool a storage pool for spent fuel rods after its external power was knocked offline by an area-wide power blackout caused by the magnitude-7.1 temblor.
The utility continued to use its diesel generator even after external power was restored at 3:30 a.m. Friday.
The reactor was shut down for maintenance and there were no fuel rods in the reactor when the magnitude-9.0 quake and subsequent tsunami struck.
The plant is equipped with three emergency diesel generators but two are unavailable due to maintenance.
At Tohoku Electric's Onagawa nuclear power plant in Miyagi Prefecture, one of two emergency diesel generators was unavailable for a week at the No. 1 reactor, the Nuclear and Industrial Safety Agency said Friday.
NISA said plant personnel discovered the critical component was unusable during a scheduled check of the No. 1 reactor conducted April 1, when the generator failed to connect to power sources inside the plant.
The utility reported the incident to NISA as required by law Friday, saying an extra generator was not deployed until the problem was solved.
The aftershock around 11:30 p.m. Thursday left the Onagawa plant with only one of three power sources working, forcing the plant operator to walk a tightrope to cool the spent fuel rods with only one generator.
The situation at both plants has raised concerns about the operator's ability to deal with a loss of external power if a powerful aftershock were to sever the facilities from the power grid.
Tohoku Electric told NISA it is capable of carrying out a minimum level of cooling with power source vehicles it has deployed after the nuclear crisis at the Fukushima No. 1 plant.
At the Onagawa plant, the systems to cool pools holding spent fuel rods at its three reactors were shut down for about an hour after the quake Thursday.
The utility said the shutdown was caused when cooling system pump motors were overloaded by the rapid movement of water in the storage pools.
About 3.8 liters of slightly radioactive water escaped the pools but was quickly wiped up with special fabrics.
The utility said there was no change in readings of radiation in surrounding areas.
The government's Earthquake Research Committee concluded Friday that the quake Thursday night was an aftershock of the March 11 temblor.
The committee warned against a powerful aftershock in the coming months, saying it believes the tectonic imbalance caused by the main temblor last month will continue.
Seismologists said the public should brace for a magnitude-7 class quake or greater in the coming six months.
Yasuhiro Umeda, seismologist and professor emeritus at Kyoto University, said a magnitude-8.0 quake was possible.
According to observations by Geospatial Information Authority of Japan, powerful tectonic activities in the Tohoku region and the Tokyo metropolitan area are under way following the huge quake last month.