Highly radioactive water found in a turbine building at the Fukushima No. 1 nuclear power plant probably leaked from a reactor containment vessel where it had been in contact with melted nuclear fuel, the Nuclear Safety Commission of Japan said Monday.
Highly radioactive water found in a turbine building at the Fukushima No. 1 nuclear power plant probably leaked from a reactor containment vessel where it had been in contact with melted nuclear fuel, the Nuclear Safety Commission of Japan said Monday.
The commission said the water, which on Saturday recorded more than 1,000 millisieverts per hour of radiation near its surface, most likely flowed into the turbine building from the containment vessel.
The measurement, the highest level so far recorded in leaked material at the facility, was taken near the surface of a puddle in the basement of the No. 2 reactor's turbine building, according to the Nuclear and Industrial Safety Agency and Tokyo Electric Power Co. (TEPCO). TEPCO said Monday afternoon that water in a trench outside the building also registered more than 1,000 millisieverts of radiation per hour. Just 15 minutes exposure would reach the limit of 250 millisieverts for workers at the stricken plant.
TEPCO, the plant's operator, repeatedly corrected its statements about radioactivity readings in water taken from the puddle on Saturday.
On Sunday morning, the utility reported that the amount of radioactive iodine-134 in the water was 10 million times the normal level, raising concerns that partial fission reactions were still continuing in the reactor.
After questions were raised about the figures, the company found that the level was in fact 100 times less than it had first reported. The water had 100,000 times the normal level.
TEPCO said it would also re-examine polluted water from the turbine buildings of the No. 1 and No. 3 reactors.
Three workers were exposed to very high levels of radiation at the No. 3 reactor's turbine building last Thursday.
After the accident, the water had a reading of about 10,000 times the normal level of radioactivity in water within the reactor.
On Saturday, the levels of radioactivity in puddles at the turbine buildings of the No. 1 and No. 3 reactors were about 1,000 times the normal level.
At the No. 2 reactor, the water levels in the reactor core remained low. It is thought possible that the suppression pool, which is connected to the containment vessel, was damaged due to an explosion on March 15, increasing the risk of radiation leaks.
The water in the puddle at the No. 2 reactor's turbine building contained high levels of radioactive iodine-131 and cesium-137. Both substances are produced by fission of nuclear fuel.
Experts say the contaminated water likely came from the reactor itself rather than a cooling pool for spent nuclear fuel.
"I think the fuel in the core had surely collapsed," said Keiji Miyazaki, a professor emeritus of nuclear reactor engineering at Osaka University.
"If a long time has passed since spent fuel was placed in the cooling pool, then the water should not contain high levels of iodine-131."
Fumiya Tanabe, director of the Sociotechnical Systems Safety Research Institute, said "much of the core should not retain its original shape." He said fuel rods would be deformed if left uncovered by water for a long time.
Even after a reactor is stopped, its fuel rods remain very hot. Even if the heat is reduced to 0.1 percent the normal operating level at the No. 2 reactor, the rods would still be expected to be putting out 2,380 kilowatts of energy, enough to evaporate more than 3 tons of water in an hour.
Fuel rods not covered by water heat up further, causing the metal alloy in their covering to react with water vapor and produce hydrogen.
This chemical reaction in turn raises their temperature. At 2,800 degrees, pellets inside the rods melt, leading to radioactive leaks.
Work to repair systems to cool the reactor cores and fuel pools has been hampered by problems with removing the contaminated water from the turbine buildings.
Workers had been planning to pump the water into steam condensers, but the condensers at the No. 2 and No. 3 reactors were found to be already full. That water has to be shifted to other tanks before the standing water in the buildings can be drained.
The work started at the No. 1 reactor on Thursday and the number of pumps was increased from one to three on Sunday, but water still remained on Monday morning, according to TEPCO officials.