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TOKYO — The operator of the Fukushima Daiichi nuclear power plant stood ready Thursday to inject boric acid into one of its most heavily damaged reactors after it found steam emanating from the reactor building. The preventive measure would stave off sustained nuclear reactions in the reactor’s damaged core, though officials stressed that such reactions were a remote possibility.
TOKYO — A damaged reactor at the Fukushima Daiichi nuclear power plant suddenly began releasing steam again, but the operator of the plant said Thursday it did not appear to be a result of renewed nuclear reactions — a worst-case situation that could lead to a large new release of radioactive materials.
The Tokyo Electric Power Company, or Tepco, stressed that it continued to safely cool the reactor core and that vital temperature and radiation readings were stable. It said that it had not detected any signs of criticality, or sustained nuclear reactions. But Tepco said that it had halted all work to remove debris from the top floors of the reactor building, also as a precaution.
Still, the steam was another indicator of the fragile state of the plant more than two years after the worst nuclear disaster after the accident at Chernobyl. The latest news adds to revelations of a slew of other problems at the plant, including indications that radioactive water may have been leaking into the ocean since the disaster.
The incident has brought the Fukushima plant’s vulnerable state into sharp relief, more than two years after its reactors suffered multiple meltdowns when its cooling systems were overwhelmed by a powerful earthquake and tsunami. A recent jump in levels of radioactive cesium and tritium in the groundwater at the coastal plant, along with suggestions that the groundwater is leaking into the Pacific Ocean, has also raised alarms over the continued environmental threat posed by the plant.
The issues have led critics to question the stewardship of Tokyo Electric Power Company, or Tepco, which operated Fukushima Daiichi before the triple meltdowns there and is now in charge of its cleanup. Distrust has also run high because of Tepco’s continued tendency to play down or hide problems at the plant.
Remote camera footage Thursday showed steam escaping from the top of the No. 3 reactor’s primary containment structure, which houses its fuel vessel, according to Tepco. A worker who checked the footage Thursday morning noticed the steam, said Hiroki Kawamata, a spokesman for the operator.
Tepco said it based its conclusion that there was no new chain reaction at Reactor No. 3 on its failure to find xenon, a byproduct of fission that lingers for only a few hours and would be an indication of new nuclear activity. Tepco also said the temperature remained stable.
Mr. Kawamata said officials were unsure what was generating the steam, and hypothesized that rainwater seeping into the containment vessel may have turned to vapor because of elevated temperatures there. Extremely high levels of radiation in the now roofless upper sections of the No. 3 reactor building — which was heavily damaged in a hydrogen explosion that rocked the reactor during the early days of the 2011 disaster — make it too dangerous for workers to approach. Remotely operated cranes are used to remove debris from the site.
Hiroki Kawamata, a spokesman for Tepco, said officials were unsure what was generating the steam and hypothesized that rainwater seeping into the reactor’s damaged containment vessel may have turned to vapor because of heat inside the vessel.
Video footage seemed to show less steam Thursday evening, and after sundown it became too dark to accurately check for any vapor, Masayuki Ono, acting general manager of Tepco’s nuclear power and plant siting division, later said at an emergency news conference.
David Lochbaum, a nuclear analyst at the Union of Concerned Scientists, a group based in Cambridge, Mass., which is generally critical of nuclear power, said that there were many phenomena that could produce steam, including something as simple as a live electrical cable falling into a puddle.
Still, workers were ready to inject water containing boric acid into the reactor from the outside at any signs of further trouble, like a rapid rise in temperature or radiation parameters, the company said in an e-mailed statement.
In an indication of the complexities of the cleanup effort, the company is still struggling to get a covering over the reactor building to protect the reactor and the remaining nuclear fuel. Workers cannot replace the roof until they have removed highly radioactive debris from the upper sections of the building, a job that requires using remotely controlled cranes.
Such spikes would raise the chilling possibility of criticality in the reactor’s damaged fuel, most which is thought to have melted and slumped to the bottom of its containment structure after the hydrogen explosion, one of several at the site in 2011. Boric acid would slow that rate of fission, preventing the worst-case scenario of uncontrolled nuclear chain reactions in the core.
Video images seemed to show less steam on Thursday evening, but after sundown it became too dark to accurately check for any vapor, Masayuki Ono, acting general manager of Tepco’s nuclear power and plant siting division, said at a news conference.
Temperature and radiation levels at the reactor appear stable so far, Mr. Ono stressed. Tepco has also not detected any xenon gas at the reactor, which would be produced in an event of criticality, he said.
Workers were ready to inject water containing boric acid into the reactor from the outside at any signs of further trouble, like a rapid rise in temperature, the company said in an e-mailed statement.
He added that Tepco was working on a cover for the reactor building that would prevent rainwater from seeping in.
Such spikes would raise the possibility of criticality in the reactor’s damaged fuel, most of which is thought to have melted and slumped to the bottom of its containment structure after a hydrogen explosion, one of several after an earthquake and tsunami in March 2011 knocked out crucial cooling systems. Boric acid would be expected to choke off a nuclear chain reaction.
Fresh trouble at the No. 3 reactor is especially worrying because it contains mixed uranium-plutonium oxide fuel. The upper floors of the reactor also house its fuel pool, which stores over 500 fuel assemblies. The reactor complex’s basement is flooded with highly radioactive water. Studies show that an accident like a meltdown or containment failure in a reactor that holds such fuel would result in more cancer deaths than one in a reactor fueled only with uranium.
The No. 3 reactor’s damaged core, like the cores of two other crippled reactors at the site, is being cooled by water that is pumped into the reactor, filtered and recycled. Among the recent mishaps at the site, the cooling system for the reactor shut down for hours in April. Tepco later said a rat had somehow short-circuited a vital switchboard, possibly by gnawing on cables.
The No. 3 reactor’s damaged core, like the cores of two other crippled reactors at the site, is being cooled by water that is pumped into the reactor, filtered and recycled. But in April, the cooling system at No. 3 shut down for hours. Tepco later said a rat had somehow short-circuited a vital switchboard, possibly by gnawing on cables.
More than 100,000 people fled their homes after the meltdowns at Fukushima. While many areas not immediately surrounding the plant are beginning to be repopulated, the news of the residents’ return has been overshadowed by the continued problems at the plant.
More than 100,000 people fled their homes after the meltdowns at Fukushima, the world’s worst nuclear crisis since the Chernobyl disaster in 1986. While many areas are beginning to be repopulated, the residents’ return has been overshadowed by the continued mishaps at the plant.
