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.
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.
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.
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.
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.
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