TOKYO - Tokyo Electric Power Co. is struggling to find out what exactly caused the massive radioactive water leak from a storage tank at its wrecked Fukushima No. 1 nuclear power plant after the utility announced the leak Tuesday, and there were no indications that the leak had stopped.
TEPCO said it recovered only four tons of the 300 tons of the leaked, contaminated water before it seeped into the ground. The radioactive water leak, equivalent to the volume of a 25-meter swimming pool, is the largest from a storage tank at the facility.
The Nuclear Regulation Authority, meanwhile, is considering raising the seriousness rating of the leak. On the 0-to-7 international scale adopted by the International Atomic Energy Agency to indicate the seriousness of accidents and problems at nuclear power plants, the NRA has provisionally graded the leak as Level 1, or "anomaly," but now it is studying whether it should be raised to Level 3, or "serious incident."
"We have identified the leaking storage tank, but have yet to pinpoint the exact location where the water is leaking from the tank," Masayuki Ono, acting general manager of TEPCO's Nuclear Power and Plant Siting Division, said at a press conference Tuesday. "The leak has not stopped."
TEPCO said it will remove the contaminated soil, while swiftly removing radioactive water from the tank and inspecting about 350 storage tanks of the same type.
The leakage occurred from one of the tanks assembled from steel parts and installed on a hill near the No. 4 reactor in the wake of the nuclear crisis that started in March 2011. The tanks, each measuring 12 meters in diameter and 11 meters in height and capable of holding 1,000 tons of water, have been used to store high-level radioactive water, with 80 million becquerels per liter, retrieved from the reactor buildings.
During a regular checkup on Monday morning, a pool of roughly 0.1 ton of water was found near a group of tanks. Workers subsequently conducted checks on storage tanks and found a tank in which the water level was down nearly three meters.
Accumulated radioactive water from the plant has reached close to 430,000 tons. It is kept in 1,060 storage tanks. Such tanks are indispensable for advancing work to decommission the plant's reactors.
In previous leak cases, TEPCO discovered radioactive water leaking from an underground tank in April and high-level radioactive water mixed with groundwater leaking into the sea in July. TEPCO is preparing to pump up uncontaminated groundwater and release it into the sea, provoking the ire of local fishermen.
The utility said it has ruled out the possibility of contaminated water from the storage tank having leaked into the sea through a nearby drainage ditch in this leakage.
"I'm afraid it is possible a small amount of leakage continued for a long period, and we may have failed to spot that," Ono said, admitting insufficient checking routines for the storage tanks.
Workers patrol around the tanks twice a day to visually check for leakage. As the leaking tank is located deep within a cluster of tanks, it is possible that the leak was missed during the routine checks.
The tank in question is made of several steel parts bolted together and sealed with resin to prevent leaks. Such tanks can be set up faster than welded tanks, but the resin, which has a five-year lifespan, can swell in summer heat and shrink in winter cold, making it easy for the attached parts to loosen up.
Up to 10 liters of contaminated water have seeped out of tanks on four occasions before, but such a large-scale leak had not been foreseen by the Secretariat of the Nuclear Regulation Authority. The cause of the leak has yet to be identified.
A fail-safe structure installed to prevent the spread of leaks failed to function as designed. Each group of tanks was set up on a concrete foundation and surrounded by a 30-centimeter-tall dike. But the drainage vent was left open all the time so that rainwater would not accumulate inside the dike. Keeping the area within the dike dry is essential for spotting leaks, TEPCO says.
Outside each dike is a bank of sandbags. High-level radioactivity was confirmed outside of the bank.
"We cannot absolutely rule out the contaminated water having leaked out to the ocean," the secretariat's spokesman Hideka Morimoto said. "We are urging TEPCO to inspect further."