Steam Detected at Damaged Fukushima Reactor 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, 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 the Tokyo Electric Power Company, or 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 - destroyed 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. 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 told an emergency news conference. Still, workers were ready to inject water containing boric acid into the reactor from the outside at any signs of further trouble, such as a rapid rise in temperature or radiation parameters, the company said in an e-mailed statement. 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. 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. He added that Tepco was working on a cover for the reactor building that would prevent rainwater from seeping in. Fresh trouble at the No. 3 reactor is especially worrying because it contains highly lethal mixed uranium-plutonium oxide fuel. The upper floors of the reactor also house its fuel pool, which stores over 500 fuel rods. The reactor complex's basement is flooded with highly radioactive water. 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, 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.