Experts have warned of a potentially dangerous radiation leak if plans to flood a damaged reactor containment vessel at the Fukushima Dai-ichi nuclear plant go ahead. The facility’s operator has admitted uranium fuel rods in the No 1 reactor partially melted after being fully exposed in the aftermath of the March 11 tsunami.
Tokyo Electric Power Co (TEPCO) said water levels had fallen to at least 1m below 4m long fuel rods inside the reactor core and melted fuel had slumped to the bottom of the reactor’s containment vessel.
The damage is more severe than TEPCO had previously reported and is almost certain to frustrate its quest to bring the plant under control within six to nine months.
Officials said that the leaked fuel was being kept cool and there was no risk of an explosion of the kind that blew the roof off the reactor in March.
The discovery was made after engineers were able to enter the reactor building, where they adjusted water gauges for the first time since the plant was crippled by the earthquake and tsunami.
Officials said initial findings indicated a large leak in the pressure vessel, but temperatures remained well below dangerous levels.
“There must be a large leak,” Junichi Matsumoto, a general manager at the utility, told reporters. “The fuel pellets likely melted and fell, and in the process may have damaged the pressure vessel itself and created a hole.”
Nuclear safety official Takashi Sakurai said: “The situation in the core hasn’t changed [since early in the crisis] and the fuel rods are being cooled by water continuously being injected into the core.”
Japan’s nuclear safety agency said it believed partially melted fuel had fallen to the bottom of the pressure vessel, which holds the reactor core together, and may have leaked into its concrete base, known as the dry well.
Greenpeace has urged TEPCO to abandon plans to flood the container with water, given the likelihood that melted fuel has damaged it.
“Flooding a reactor that has fuel [that has fallen] through the pressure vessel is not a good idea,” said Shaun Burnie, nuclear adviser to Greenpeace Germany.
Outlining a worst-case scenario, Burnie said very large amounts of cold water hitting the melted fuel could cause an explosion, trigger substantial damage to the reactor and create a “high risk of atmospheric release running for days, if not weeks.”
“I think [the flooding option] will now be scrapped,” he added
Greenpeace said problems could escalate rapidly if the fuel melted through the reactor vessel.
“As the fuel rods were fully exposed and subsequently melted, it is highly likely that the core’s integrity is compromised and that there is larger amount of melted fuel at the bottom of the reactor pressure vessel,” it said.
John Large, an independent nuclear engineering consultant in London, said TEPCO’s plan to flood the reactor was riddled with “potential risks.”
It appeared not to have factored in the extent of damage to the fuel rods and the structural state of the containment vessel, including whether it was watertight, he said.