TOKYO (Kyodo) --The pressure vessel housing nuclear fuel at the No. 1 reactor of the Fukushima No. 1 power plant or its accessory piping is likely to have been partially damaged immediately after the March 11 earthquake, possibly allowing steam to leak out to the containment vessel encasing it, according to data made public by its operator.
At the reactor, the magnitude 9.0 quake registered an intensity smaller than envisaged under its quake-resistance design. But if the temblor actually caused damage to the critical reactor component, power suppliers across the country might be forced to reconsider the quake resistance designs for their reactors.
A diagram showing temperature changes at the reactor's containment vessel indicates that temperatures and pressure momentarily shot up immediately after the quake.
Mitsuhiko Tanaka, a former nuclear reactor design engineer, says high-temperature steam apparently leaked out to the containment vessel after either the reactor's pressure vessel or its accessory piping was partially damaged.
The operator, Tokyo Electric Power Co., admitted Wednesday that critical cooling piping at the same plant's No. 3 reactor may also have been damaged in the quake.
The utility, known as TEPCO, had earlier suggested that no major damage was done to the reactor, such as ruptures in the facility's main steam piping, until the arrival of massive tsunami after the quake.
"If we do our analysis on the premise that there was a leak in the piping, it matches (data) in reality," a TEPCO official said at a news conference, referring to possible damage at the No. 3 reactor. "We can't deny the possibility."
According to TEPCO, as soon as the reactor's emergency core cooling system was activated shortly after noon on March 12, pressure inside both the reactor's pressure vessel and containment vessel, which encases the pressure vessel, dropped, suggesting that steam was leaking from the cooling pipe.
While measuring devices may have malfunctioned, pressure readings corresponded to an analysis based on the hypothesis that steam did indeed leak from the piping, TEPCO said.
At the No. 3 reactor, the earthquake had an intensity greater than envisioned under resistance guidelines. The cooling piping is housed in a building designed to resist direct damage from tsunami.
"We must fully accept the fact that (the earthquake) has been cited as a possible" cause for the damage to piping, said Goshi Hosono, a special adviser to Prime Minister Naoto Kan, at a news conference.
"A thorough investigation should be carried out on the cause of the incident" after the government's Nuclear and Industrial Safety Agency analyzes related data, he added.
Agency spokesman Hidehiko Nishiyama stressed at the same news conference that steam leaks from the piping have not actually been confirmed.
Another analysis by TEPCO has shown that breaches may have occurred to containment vessels encasing the Nos. 1 and 2 reactors at the power plant in Fukushima Prefecture, possibly causing leaks of highly radioactive water.
The possible ruptures to the containment vessels at the Nos. 1 and 2 reactors are certain to complicate efforts to deal with accumulating contaminated water there, raising questions about the soundness of a TEPCO plan to rebuild a stable cooling system by around mid-July.
In a report submitted to the agency, TEPCO said that if a breach around 3 centimeters in diameter occurred at the No. 1 reactor's containment vessel 18 hours after the quake and it widened to about 7 cm 50 hours after the quake, it would account for changes in pressure readings inside the containment vessel.
TEPCO said it believes that parts used to ensure air tightness in the containment vessel may have broken from overheating, judging from temperatures measured when the leaking possibly occurred.
The company also hypothesized that a rupture roughly 10 cm in diameter occurred to the No. 2 reactor's containment vessel 21 hours after the quake due to elevated temperatures, among other factors, finding that it also corresponds with data obtained.
The same TEPCO report has also shown that massive amounts of hydrogen likely formed at the Nos. 1 to 3 reactors shortly after water levels dropped in their reactor cores and exposed nuclear fuel inside, possibly causing explosions at the buildings housing the reactors.
According to TEPCO's analysis, about 750 kilograms of hydrogen gas was produced at the No. 1 reactor, between 350 and 800 kg at the No. 2 reactor and between 600 and 700 kg at the No. 3 reactor.
Hydrogen is formed when zirconium cladding on fuel rods gets heated and reacts chemically with water.
Between 300 and 400 kg of the gas quickly formed within an hour of exposure at each reactor, the analysis shows, though No. 2 was spared the kind of explosion that blew off the roofs of the buildings for the other two reactors in the days following the massive quake and tsunami.
There is a possibility that the No. 2 reactor escaped an explosion because a ventilating hatch on the upper part of its building was opened in time, unlike at the Nos. 1 and 3 reactors.
TEPCO is struggling to bring the three reactors and a spent fuel storage pool at the No. 4 reactor under control, with the plant's two other reactors having already been brought into a stable condition called "cold shutdown."
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