On Tuesday, Japan’s Nuclear and Industrial Safety Agency (NISA) provisionally raised the accident rating for three reactors at the crippled six-unit Daiichi nuclear plant in Fukushima Prefecture to Level 7—making it a “major accident” and putting in on par with the 1986 Chernobyl accident in the Ukraine. And today the Tokyo Electric Power Co. (TEPCO) confirmed damage to spent nuclear fuel rods stored in the Unit 4 building.

Rating nuclear accidents on a scale from 0 to 7, the International Nuclear and Radiological Event Scale (INES) defines a Level 7 as an “event resulting in an environmental release corresponding to a quantity of radioactivity radiologically equivalent to a release to the atmosphere of more than several tens of thousands of terabequerels of I-131 [radioactive iodine].” NISA had previously given the crisis at Daiichi Units 1, 2, and 3—rated as a singe event—a Level 5.

NISA raised the INES rating after estimating the total release of radioactive substances to the atmosphere after it received results of an analysis conducted by the Japan Nuclear Energy Safety Organization.

The agency said, however, that the release of radioactive material to the atmosphere is about 10% that of the Chernobyl accident, the only other accident rated a Level 7 on the INES scale.

Damage to Unit 4 Spent Fuel Rods

An analysis of a water sample taken from Daiichi 4’s spent nuclear fuel pool detected high levels of radioactive iodine and cesium—indicating that some spent fuel rods had been damaged. That unit had been shut down on March 11, when a catastrophic quake and subsequent tsunami hit northeastern Japan, and all of its 1,331 spent fuel rods and 204 unused fuel rods were being stored in the spent fuel pool.

TEPCO told Japanese news agency Kyodo that substances generated by nuclear fission—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. NISA said that the higher readings could also have been contributed to by radioactive rainwater or rubble.

Daiichi 4, a unit with fewer problems than Daiichi 1, 2, and 3, has experienced both a hydrogen explosion and fires since March 11. The reactor buildings of Units 1 and 3 have also been severely damaged.

Work Continues to Stabilize Reactors, Amid Continued Turmoil

Today, TEPCO workers were reportedly continuing efforts to restore key cooling functions, and operations are under way to pump contaminated water from a flooded trench at Daiichi 2, where fuel rods are believed to have partially melted. Similar measures are planned for Daiichi 1 and 3.

The underground trenches, typically used to run piping and cabling, and turbine halls were deluged by the 14-meter tsunami on March 11. More than 60,000 tons of highly radioactive water will reportedly be pumped to a condenser.

Workers, meanwhile, manually extinguished a fire that broke out in the housing outlet structure for cooling water for Daiichi 1, 2, 3, and 4.

Freshwater continues to be injected into Daiichi 1, 2, and 3. Nitrogen gas is also being injected into Daiichi 1’s containment vessel to thwart another hydrogen explosion. TEPCO said the pressure in the containment vessel has stabilized. The reactor vessels and drywells of Units 2 and 3 are also reportedly at atmospheric pressure.

Aftershocks Rattle Daiichi

The workers are battling to stabilize the reactors amid a series of strong aftershocks. On April 11, a magnitude 6.2 quake whose epicenter was 229 kilometers (km) away, rocked the region, and on April 12, another magnitude 6.0 tremor hit, its epicenter only 46 km away.

According to NISA, the April 12 quake did not affect the four nuclear plants near Fukushima Prefecture: Daiichi, Daiini, Tokai Daiini, and Onagawa.

Long-Term Damages

The Japanese government reported that radiation levels today in Tokyo fell to normal ranges seen before the quake crippled Daiichi, falling to 0.079 microsieverts per hour from the maximum level of 0.093 microsieverts detected Monday to Tuesday. At Sendai, levels dropped to 0.080 microsieverts. In Fukushima, in Fukushima Prefecture, radiation was recorded at 2.100 microsieverts (against pre-disaster ranges of 0.037 to 0.046 microsieverts).

In an important development, TEPCO President Masataka Shimizu today told reporters that the utility was considering paying damages as soon as possible to residents living close to the devastated nuclear plant. Details about how much will be offered were being finalized, he said.

Reuters said, quoting the Yomiuri newspaper, that the rough draft of a compensation plan being considered by TEPCO and the government could cap the utility’s liability at 2 trillion yen to 3.8 trillion yen (US$24 billion to $45 billion). Other nuclear plant utilities may be asked to contribute to the fund, which could ultimately contribute up to 2.7 trillion yen.

Sources: POWERnews, TEPCO, IAEA, NISA, JNES, Kyodo, Reuters