One of the reactors at the crippled Fukushima Daiichi power plant did suffer a nuclear meltdown, Japanese officials admitted for the first time today, describing a pool of molten fuel at the bottom of the reactor's containment vessel. Engineers from the Tokyo Electric Power company (Tepco) entered the No.1 reactor at the end of last week for the first time and saw the top five feet or so of the core's 13ft-long fuel rods had been exposed to the air and melted down. Now the company is worried that the molten pool of radioactive fuel may have burned a hole through the bottom of the containment vessel, causing water to leak. Tepco has not clarified what other barriers there are to stop radioactive fuel leaking if the steel containment vessel has been breached. Greenpeace said the situation could escalate rapidly if the lava melts through the vessel". However, an initial plan to flood the entire reactor core with water to keep its temperature from rising has now been abandoned because it might exacerbate the leak. Tepco said ... that it had sealed a leak of radioactive water from the No.3 reactor after water was reportedly discovered to be flowing into the ocean. A similar leak had discharged radioactive water into the sea in April from the No.2 reactor. Greenpeace said significant amounts of radioactive material had been released into the sea and that samples of seaweed taken from as far as 40 miles of the Fukushima plant had been found to contain radiation well above legal limits.