Japan’s nuclear authority said Aug. 21 a radioactive water leak at the stricken Fukushima nuclear plant represents a “serious incident” under an international scale, the latest blow in the struggle against contaminated water accumulating at the site.
The Nuclear Regulation Authority appraised the leak of 300 tons of highly radioactive water from a surface tank as the equivalent of Level 3 on the eight-level International Nuclear and Radiological Event Scale (INES). The decision came after the NRA estimated the total amount of radioactive materials in the leaked water at 24 trillion becquerels.
INES has been compiled by the International Atomic Energy Agency and other bodies.
Tokyo Electric Power Co., operator of the Fukushima No. 1 nuclear power plant, has yet to determine what caused the leak or where specifically the water had escaped from the tank.
TEPCO officials have moved contaminated water that remained in the tank to another tank on Aug. 21. Sandbags have been placed around the problem tank because water is still believed to be spilling out.
“There is the possibility that a small volume of water may have been leaking for a long period of time,” Masayuki Ono, acting general manager of TEPCO’s Nuclear Power and Plant Siting Division, has said.
Most of the leaked water is believed to have seeped into the ground.
The tank in question is a so-called flange type that features steel sheets connected by steel bolts. It has been in use since October 2011.
Flange-type tanks have a relatively short durability of five years, and the rubber packing used to prevent leaks wears out quickly.
But TEPCO officials said it is still too early to determine if the problem was caused by degradation due to age.
The water may have leaked from areas where the sheets were poorly joined or because the bolts were fastened unevenly.
Of all 1,000 or so surface tanks at the crippled Fukushima plant, about 350 are flange types. TEPCO officials said they will check all tanks of that structure.
The utility also plans to install tanks that have been welded together, but those take longer to build.
On Aug. 19, TEPCO officials informed the NRA that at least 120 liters of contaminated water had leaked, based on pools of water found at two locations.
At that time, the NRA provisionally evaluated the incident as an “anomaly,” or a Level 1 event on INES.
However, the following day, TEPCO workers checked a 1,000-ton cylindrical tank measuring about 11 meters high and 12 meters in diameter and found that the water level had dropped by nearly 3 meters.
That showed that the leak totaled 300 tons, equivalent to what can be held in 1,500 barrels. The NRA then raised its severity assessment on Aug. 21.
The accident at the Fukushima No. 1 plant following the 2011 Great East Japan Earthquake and tsunami was judged to be a “major accident,” or a highest Level 7 event.
NRA officials will ask their counterparts at the IAEA if it is appropriate to use the same INES evaluation methods for events at nuclear plants operating normally for an incident at the Fukushima nuclear plant, such as the latest leak.
After confirmation is made, the NRA will officially raise its evaluation of the latest leak to Level 3.
It is clear that the accident is far from under control at the Fukushima No. 1 nuclear plant.
TEPCO has long struggled with contaminated water leaking on or out of the plant compound. It is also running out of space to store the radioactive water used to cool the melted nuclear fuel.
The latest incident represents the largest amount to leak from the tanks.
The NRA instructed TEPCO to confirm if the water from the leak flowed into the ocean through drainage ditches.
TEPCO officials said they measured for radiation in water within a drainage ditch close to the latest leak and the seawater near the outlet about 700 to 800 meters downstream.
So far, no major changes in radiation levels have been recorded, they said.
Many of the surface tanks were installed because contaminated water was found to be leaking from underground storage facilities in April. Now that the dependability of those tanks has been called into question, TEPCO has lost another measure for storing the contaminated water, which is increasing by 400 tons a day.
According to TEPCO officials, the total capacity of all tanks on the plant site was 412,000 tons as of Aug. 6. About 346,000 tons, or more than 80 percent of capacity, have already been stored.
The utility plans to increase the capacity of the tanks to 700,000 tons in 2015 and to 800,000 tons by the end of fiscal 2016.
In June, TEPCO was forced to suspend trial operations of the Alps purifying unit, which is capable of removing 62 types of radioactive materials from highly contaminated water, after untreated water was found leaking from welded parts of one of its tanks.
TEPCO also planned to pump groundwater to prevent it from entering buildings at the plant and becoming contaminated. The clean water would be dumped into the ocean.
However, after TEPCO acknowledged in July that contaminated water had already been flowing into the ocean, local fishermen objected to the release of any water into the sea.
They said they could have trouble selling their seafood if rumors spread that all marine products off Fukushima Prefecture were contaminated with radioactive substances.
Various media reports about the recent leaks at the Fukushima plant led Asiana Airlines of South Korea to cancel charter flights between Seoul and Fukushima from October. The charter flights had only resumed in July.
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