Tokyo Electric Power Co. (9501.TO) said Thursday that it stopped the cooling system for a reactor at the Fukushima Daini power plant in Fukushima Prefecture for three and a half hours due to an electrical system problem, Kyodo News reported.
Cooling operations were suspended for the No. 1 reactor and an adjacent pool for spent nuclear fuel from around 5:30 p.m. to shortly past 9 p.m. while the operator, known as TEPCO, looked into why sparks came from a power panel at a building adjacent to that housing the reactor.
Temperatures for the reactor and the fuel storage pool are believed to have risen slightly from about 25 C to 35 C and from 26 C to 27 C, respectively, during the suspension.
The reactor and the pool must be kept cooled because nuclear fuel inside them emits heat as it decays, even though the reactor has gone offline and has been brought into a stable condition called 'cold shutdown' following the March earthquake and tsunami.
The other three reactors at the Fukushima Daini plant, located to the south of the crisis-hit Fukushima Daiichi nuclear power complex, have also been in cold shutdown-mode since the disasters.
A plant worker found sparks coming out of a power panel in the basement of the No. 1 reactor's adjoining facility shortly after 2 p.m., prompting the company to cut power to the facility so that it could be inspected, TEPCO said.
The reactor has two sets of cooling systems for sustaining cold shutdown, but one of them has remained crippled since the tsunami in March