Some of the spent nuclear fuel rods stored in the No. 4 reactor building of the crisis-hit Fukushima Daiichi power plant were confirmed to be damaged, but most of them are believed to be in sound condition, plant operator Tokyo Electric Power Co. said Wednesday.
The firm known as TEPCO said its analysis of a 400-milliliter water sample taken Tuesday from the No. 4 unit's spent nuclear fuel pool revealed the damage to some fuel rods in such a pool for the first time, as it detected higher-than-usual levels of radioactive iodine-131, cesium-134 and cesium-137.
The No. 4 reactor, halted for a regular inspection before last month's earthquake and tsunami disaster, had all of its 1,331 spent fuel rods and 204 unused fuel rods stored in the pool for the maintenance work and the fuel was feared to have sustained damage from overheating.
The cooling period for 548 of the 1,331 rods was shorter than that for others and the volume of decay heat emitted from the fuel in the No. 4 unit pool is larger compared with pools at other reactor buildings.
According to TEPCO, radioactive iodine-131 amounting to 220 becquerels per cubic centimeter, cesium-134 of 88 becquerels and cesium-137 of 93 becquerels were detected in the pool water. Those substances are generated by nuclear fission.
The government's Nuclear and Industrial Safety Agency said the confirmed radioactive materials were up to 100,000 times higher than normal but that the higher readings may have also been caused by the pouring of rainwater containing much radioactivity or particles of radiation-emitting rubble in the pool.
The roof and the upper walls of the No. 4 reactor building have been blown away by a hydrogen explosion and damaged by fires since the disaster struck the plant. The water level in the spent fuel pool is believed to have temporarily dropped.
TEPCO said the fuel rods may have also been damaged by steel frames that fell into the pool in addition to overheating caused by the loss of cooling functions after the twin disasters.
The utility plans to examine the condition of the plant's reactor buildings by deploying a small unmanned helicopter to see whether it is possible to extract spent fuel from pools.
The nuclear agency said now that the condition of the No. 4 unit pool is partially known, workers can better prepare for recovery works there.
Earlier in the day, the government's nuclear regulatory agency ordered TEPCO to check the quake resistance of reactor buildings at the Fukushima plant, which have been rocked by strong aftershocks from the magnitude-9.0 earthquake that wrecked the site and triggered tsunami on March 11.
The agency told the utility to immediately examine the buildings and consider reinforcement work if they are judged as not sufficiently quakeproof.
In addition to the No. 4 unit, the Nos. 1 and 3 reactor buildings have also been severely damaged by hydrogen explosions in the early days of the crisis.
''As strong aftershocks occur almost daily, we have to consider what will happen to buildings already damaged by blasts,'' said Hidehiko Nishiyama, a spokesman for the nuclear agency.
He acknowledged the difficulties involved in the work to reinforce the quake resistance of the buildings, where radiation levels are high, but said, ''We must devise some ways.'' The agency urged TEPCO to report back to it on the matter as soon as possible.
Meanwhile, Yoko Komiyama, senior vice minister of health, labor and welfare, said Wednesday at a Diet session that a total of 22 workers at the plant have been exposed to radiation exceeding 100 millisieverts as of early Wednesday and that the highest level of exposure among them is 198.24 millisieverts.
Exposure to 100 millisieverts is the permissible level for nuclear plant workers dealing with an emergency, but the limit has been raised to 250 millisieverts for the ongoing crisis.
Workers continued Wednesday to remove highly radioactive water in the plant as part of efforts to put an end to the emergency, which is now acknowledged as one of the world's worst nuclear disasters.
TEPCO had pumped out 700 tons of highly polluted water by Wednesday evening from an underground tunnel-like trench to a ''condenser,'' where in normal operations steam from the reactor is converted into water.
Eventually, the operator plans to remove a total of 60,000 tons of contaminated water, found in the basements of the Nos. 1 to 3 reactor turbine buildings as well as the trenches connected to them, and to store it in nearby tanks and other areas.
As a result of the operation, the level of highly radioactive water that had been filling up the trench connected to the No. 2 reactor's turbine building was lowered. Nishiyama said it will likely take several weeks before the tainted water removal operation ends.
The highly toxic water is believed to originate from the No. 2 reactor's core, where fuel rods have partially melted. The water, which has also affected other parts of the plant, is hampering efforts to restore the reactors' key cooling functions, lost in the March 11 earthquake and tsunami.
The nuclear agency also said TEPCO has installed three steel sheets near a seawater intake for the No. 2 reactor and set up ''silt fence'' curtain barriers near intakes for the Nos. 3-4 reactors at the six-reactor plant to block the spread of radioactive substances in water.
Massive amounts of water have been poured into the reactors and their spent nuclear fuel pools as a stopgap measure to cool them down at the Fukushima plant.
But pools of contaminated water have been detected in various parts of the nuclear complex on the Pacific coast, with some water leaking into the sea, as an apparent side effect of the emergency measure. TEPCO successfully stopped the leak of highly radioactive water from a cracked pit on April 6.
==Kyodo
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