TOKYO/NEW YORK (Reuters) - One of the reactors at Japan’s crippled Fukushima nuclear power plant has a hole in its main vessel following a meltdown of fuel rods, leading to a leakage of radioactive water, its operator said on Thursday.

The disclosure by Tokyo Electric Power Co (TEPCO) is the latest indication that the disaster was worse than previously disclosed, making it more difficult to stabilize the plant.

The discovery of the leak provides new insight into the sequence of events that triggered a partial meltdown of the uranium fuel in the No. 1 reactor at Fukushima after the plant was struck by a massive earthquake and tsunami on March 11, officials said.

The battle to bring Fukushima under control has been complicated by repeated leaks of radioactive water, threatening both the Pacific Ocean and nearby groundwater.

Workers at the Fukushima Daiichi nuclear plant have been pumping water into at least three of the six reactors on the site to bring their nuclear fuel rods to a “cold shutdown” state by January.

But after repairing a gauge in the No. 1 reactor earlier this week, TEPCO discovered that the water level in the pressure vessel that contains its uranium fuel rods had dropped about 5 meters (16 ft) below the targeted level to cover the fuel under normal operating conditions.

“There must be a large leak,” Junichi Matsumoto, a general manager at the utility told a news conference.

“The fuel pellets likely melted and fell, and in the process may have damaged...the pressure vessel itself and created a hole,” he added.

Since the surface temperature of the pressure vessel has been holding steady between 100 and 120 degrees Celsius, Matsumoto said the effort to cool the melted uranium fuel by pumping in water was working and would continue.

VESSEL HAS A HOLE

Based on the amount of water that is remaining around the partially melted and collapsed fuel, Matsumoto estimated that the pressure vessel had developed a hole of several centimeters in diameter.

The finding makes it likely that at one point in the immediate wake of the disaster the 4-meter-high stack of uranium-rich rods at the core of the reactor had been entirely exposed to the air, he said. Boiling water reactors like those at Fukushima rely on water as both a coolant and a barrier to radiation.

U.S. nuclear experts said that the company may have to build a concrete wall around the unit because of the breach, and that this could now take years.

“If it is assumed the fuel did melt through the reactor, then the most likely solution is to encapsulate the entire unit. This may include constructing a concrete wall around the unit and building a protective cover over it,” W. Gene Corley, senior vice president of CTL Group in Skokie, Illinois, said on Thursday.

“Because of the high radiation that would be present if this has happened, the construction will take many months and may stretch into years,” Corley said.

TEPCO should consider digging a trench around reactors 1-3 all the way down to the bedrock, which is about 50 feet below the surface, said Arnie Gundersen, Chief Engineer at Fairewinds Associates Inc of Burlington, Vermont, who once worked on reactors of similar design to the Fukushima plant.

He said this should be filled with zeolite, which can absorb radioactive cesium to stop more poisons from leaking into the groundwater around the plant.

Workers wearing protective suits stand after water stopped flowing at the pit near the water intake canal of the Fukushima Daiichi Nuclear Power Station No.3 reactor, May 11, 2011. REUTERS/Tokyo Electric Power Co

“TEPCO seems to be going backwards in getting the situation under control and things may well be slowly eroding with all the units having problems,” said Tom Clements with Friends of the Earth, a U.S.-based environmental group.

“At this point, TEPCO still finds itself in unchartered waters and is not able to carry out any plan to get the situation under control,” he said.

Matsumoto said the utility would study whether to increase the amount of water it was injecting to overcome the leak and raise the level of water covering the fuel, at the risk of allowing more radioactive water to leak out of the facility.

Nearly 10,400 tonnes of water has been pumped into the reactor so far, but it is unclear where the leaked water has been going. The high radiation levels makes it difficult for workers to check the site, Matsumoto said.

TEPCO announced a timetable last month for addressing the crisis, saying it aimed to cool reactors to a stable level and reduce the leakage of radiation within the first three months, then bring the reactors to a cold shutdown in another three to six months.

TIMETABLE COULD SLIP

TEPCO is set to review its timetable for stabilizing Fukushima on May 17 and officials indicated that the initial progress targets could slip.

Officials had planned to use the same set of steps to stabilize reactors No. 2 and No. 3 that are under way at No. 1, which workers re-entered last week for the first time since the earthquake.

But Matsumoto said it was likely that the pressure vessels in the other two reactors could be leaking as well if fuel rods had collapsed and melted after the earthquake and tsunami.

“It is necessary to make a reassessment of the condition of the nuclear reactor,” Chief Cabinet Secretary Yukio Edano told a news conference.

On Wednesday, TEPCO sealed a fresh leak of contaminated water found near the No. 3 reactor that may have seeped into the Pacific Ocean from the coastal plant. A previous ocean leak sparked international concern about the impact of the disaster on the environment. [nL3E7GB261]

Traces of radioactive cesium were detected in sewage treatment centers in Ibaraki and Kanagawa prefectures, both to the south of Fukushima, Japanese media reported on Thursday.