Steam has been spotted in the reactor 3 building at the Fukushima No. 1 power plant, but there was no sign yet of increased radiation, Tepco said Thursday, speculating the vapor was just evaporated rainwater hitting a hot metal surface.
The incident, which Tokyo Electric Power Co. said was not “an emergency situation,” is the latest event underlining how precarious the plant remains more than two years after it was wrecked by tsunami and subsequently suffered three meltdowns. One of them, reactor 3, is the only one at the plant to use the highly lethal mixed uranium-plutonium oxide (MOX) fuel, some of which is in its spent-fuel pool near the top of the reactor.
“Steam has been seen around the fifth floor of the reactor 3 building,” a Tepco spokesman said. The spent-fuel pool is on that floor.
The roof of the building was blown away in a hydrogen explosion at the beginning of the crisis in March 2011, sparked when cooling systems were flooded by tsunami after the huge March 11 undersea quake.
“(The steam) was drifting thinly in the air and it’s not like a big column of steam is spurting up,” the spokesman said. “Neither the temperature of the reactor nor readings at radiation monitoring posts have gone up.
“We do not believe an emergency situation is breaking out, although we are still investigating what caused this,” he said.
The temperature readings on reactor 3 pretty much stayed the same as before the steam was found, as have the radiation figures around it.
As of 6 p.m. Thursday, the steam was still coming out, Tepco said.
What is creating the steam is unclear, but Tepco said the best explanation is that rainwater, which had fallen from Wednesday night, dropped onto the containment vessel’s lid and evaporated.
The steam appears to be emanating from a seal over the top of the reactor.
The lid of the containment vessel is just below that seal and the rainwater is dropping onto the lid from some inner space, Tepco speculated.
It added that the temperature of the containment vessel’s lid is probably around 40 degrees, so if rainwater reaches it, steam could form depending on atmospheric conditions outside.
The utility said it had actually witnessed the steam last July, too, but it was very short-lived, unlike this time.
Tepco said it had confirmed the reactor remained subcritical at 9:20 a.m., one hour after the steam was first spotted. Criticality is the term used for reactors in which there is a self-sustaining nuclear chain reaction.
Workers were continuing to pump water into the reactor and spent-fuel pool as part of on-going cooling efforts, Tepco said, adding it would measure dust near the building as well as the air above it to gauge radiation levels.
The steam is the latest in a growing catalogue of mishaps that have cast doubt on Tepco’s ability to fix the world’s worst nuclear disaster in a generation.
A series of leaks of water contaminated with radiation have shaken confidence, as did a blackout caused by a rat that left cooling pools without power for more than a day. The company has admitted in recent weeks that water and soil samples taken at the plant are showing high readings for potentially dangerous isotopes, including cesium-137, tritium and strontium-90.
The Nuclear Regulation Authority said last week the reactors are very likely leaking highly radioactive substances into the Pacific Ocean.
NRA commissioners voiced frustration with Tepco, which has failed to identify the source and the cause of the groundwater radiation spikes.
NRA officials are urging Tepco to offer more detailed and credible data and make efforts to better explain to the public what it knows.
Almost all of the Japan’s reactors remain offline, but Prime Minister Shinzo Abe’s Liberal Democratic Party, as well as utilities around the nation, are hoping to restart them