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Fukushima Daiichi and Daini

Nuclear Monitor Issue: 
WISE Amsterdam

The 9.0-magnitude earthquake and resultant tsunami in northeastern Japan on March 11, affected more than 31,800 megawatts (MW) of generating capacity. In the immediate aftermath of the earthquake 11 nuclear reactors with 9,674 MW of capacity at four sites shut down automatically, while three other reactors with 2,700 MW of capacity which were closed for maintenance were also affected.

The Japan Atomic Power Company’s 1,100-MW Tokai Daini boiling water reactor (BWR) in Ibaraki prefecture shut down without apparent problems, although JAPC said on March 13, that two of three diesel generators used for emergency cooling had failed.

Meanwhile a fire occurred immediately after the disaster in a turbine building at one of the three BWRs at Tohoku Electric Power Company’s 2,174-MW Onagawa plant in Miyagi prefecture. It was extinguished without indications at the time of radioactive leakage.

Tohoku Electric subsequently said on March 12 that radiation levels at Onagawa had surged. But by March 14 radiation had fallen to normal levels, with the International Atomic Energy Agency (IAEA) saying that “the current assumption of the Japanese authorities is that the increased level may have been due to a release of radioactive material from the Fukushima Daiichi nuclear power plant.”

Fukushima, which has experienced by far the worst problems, comprises two plants located 11.5 kilometers apart. Fukushima Daiichi (Fukushima-I) and Fukushima Daini (Fukushima-II) are both owned and operated by the Tokyo Electric Power Company (Tepco), with the Fukushima-I complex comprising six BWRs with 4,700 MW of capacity, while Fukushima-II comprises four BWRs with 4,400 MW of capacity.

All four Fukushima-II reactors were operating at the time of the earthquake and shut down automatically, as did three units at Fukushima-I. The remaining three reactors at Fukushima-1 were already shut for scheduled maintenance.

The automatic shutdown of the Fukushima-II reactors ran into cooling problems when emergency generators failed, apparently as a result of the impact of the tsunami on the generators or their diesel stocks. But much worse loss of coolant incidents occurred at Fukushima-I. Nevertheless, early may Tepco, perhaps bowing to reality, said that it may never restart its four Fukushima II (Daini) reactors.

Fukushima I (Daiichi)

Reactor 1 [BWR, 439MWe, March 1971] - Possible hydrogen explosion March 12, outer building is damaged and there was a partial meltdown. When fuel rods heat up due to insufficient cooling, the zirconium alloy in the fuel rods reacts with steam and produces a large amount of hydrogen. Radioactivity has been vented and leaked. Probably 70% of fuel rods are damaged. Operators have trouble cooling down the reactor. The reactor has 400 fuel assemblies and the spent fuel pool has 292. Update May 12: possible 100% of fuel rods damaged

Reactor 2 [BWR, 760MWe, July 1974] - The fuel and the reactor core severely damaged. Some fuel may have leaked out of the reactor vessel into the primary containment vessel, which was damaged in an explosion on March 15. Broken fuel rods have been found outside the reactor, probably from the spent fuel pool. The reactor has 548 fuel assemblies and the spent fuel pool has 587. Probably 30-40% of the fuel rods have been damaged.

Reactor 3 [BWR, 760MWe, March 1976] - The reactor used uranium and plutonium (MOx), which may produce more toxic radioactivity. The reactor containment vessel may have been damaged due to the March 14 explosion, and the spent fuel pool may have become uncovered. The reactor had 548 fuel assemblies and the spent fuel pool has 514. About 30% of fuel rods have been damaged. A remarkable early May video of the fuel pool at Unit 3 has been released. It shows the pool is now underwater, but also a picture of complete devastation. There is no actual visual evidence any fuel remains in the pool -certainly not in racks as designed. However, some fuel must remain, as NHK TV reports on May 11, radiation readings taken May 8, inside the pool of “140,000 becquerels of radioactive cesium-134 per cubic centimeter, 150,000 becquerels of cesium-137, and 11,000 becquerels of iodine-131.” The presence of short-lived Iodine-131 indicates that either the pool has become contaminated from melting fuel in the Unit 3 reactor or there has been inadvertent fissioning inside the fuel pool itself. An inadvertent criticality is believed by many to have caused the enormous explosion at Unit 3.

Reactor 4 [BWR, 439MWe, March 1971] - Spent fuel rods in a water pool may have become exposed to air, emitting radioactive gases. On March 15, a hydrogen explosion created by chemical reactions with the spent fuel rods, and fire have damaged the building and probably also the spent fuel pool.

There are no fuel assemblies in the reactor; 548 were removed for maintenance and are part of 1,535 in the spent fuel pool.

Reactor 5 [BWR, 760MWe, October 1978] - The reactor is shut down at the time of the earthquake and the building is not damaged. But the concern had been about spent fuel in the building becoming exposed to air. With power restored to the building, that concern has abated. The reactor has 548 fuel assemblies and the spent fuel pool has 946.

Reactor 6 [BWR, 760MWe, April 1978] - The reactor was shut down at the time of the earthquake and the building is not damaged. But the concern had been about spent fuel in the building becoming exposed to air. With power restored to the building, that concern has abated. The reactor has 764 fuel assemblies and there are 876 in spent fuel pools.

General: New joint U.S.-Japanese aerial monitoring results of the area have been posted and show significant Cesium contamination well beyond the government’s evacuation zone. Cesium levels above 600,000 becquerels per square meter are indicated more than 60 kilometers (30 miles) northwest of the Fukushima Daiichi site. After Chernobyl, the Soviet Union evacuated areas above 550,000 becquerels per square meter. Maps are posted on the DOE website at

Sources: Wim Turkenburg, Power point presentation Copernicus Institute Utrecht, NL; April 26, 2011); NIRS Updates; TEPCO updates; Japan, coming to terms with the power crisis (Platts, April 2011)

Nuclear reactor residual heat generation over time from shut down

Time after reactor stop        Residual power (% of operating power)

1 second                                             17%

1 minute                                             5%

1 hour                                                 1.5%

1 day                                                  0.5%

1 week                                               0.3%

1 month                                              0.15%

1 year                                                 0.03%

Source: Autorité de Sûreté Nucléaire (ASN)