Tokyo Electric Power Co. (TEPCO) in May discovered—after calibrating water gauges—that the water level in the reactor pressure vessel of Unit 1 at the quake- and tsunami-ravaged Fukushima Daiichi nuclear plant may have dropped to such low levels that the fuel was completely uncovered. This caused almost all the fuel pellets to melt and fall to the bottom of the vessel at a relatively early stage in the accident—roughly 15 hours after the March 11 earthquake that killed an estimated 28,000.
“Most… of the fuel is considered to be submerged in the bottom of [the] reactor pressure vessel and some part [was] exposed,” the company said in an official report, though it added that actual damage to the reactor pressure vessel is limited, based on temperatures around it. The utility is expected to release similar findings about fuel meltdown at Units 2 and 3. Meanwhile, computer simulations of the damaged units suggest that Unit 1 has one hole, Unit 2 may have two breaches, and Unit 3’s cooling systems may have been breached, TEPCO said.
Japan’s response to the dire crisis was exemplary under the circumstances, but the country’s regulation of its nuclear power sector and safety preparedness was faulty, the International Atomic Energy Agency (IAEA) said in a preliminary report released in June (Figure 4). Specifically, the IAEA said that officials had underestimated the threat from tsunamis to coastal nuclear plants. “Nuclear regulatory systems should address extreme external events adequately, including their periodic review, and should ensure that regulatory independence and clarity of roles are preserved in all circumstances in line with IAEA safety standards,” it says.
|4. Battered down. The International Atomic Energy Agency (IAEA) said in a preliminary report that Japan’s response to the Fukushima Daiichi crisis was exemplary, but regulators underestimated the threat from tsunamis to coastal nuclear plants. IAEA representatives are pictured near the battered plant’s Unit 3. Courtesy: TEPCO|
Japan has responded to the report by admitting that poor oversight contributed to the disaster and that it would overhaul regulation of the country’s nuclear power sector. Japan’s Nuclear and Industrial Safety Agency (NISA) has so far already announced new safety measures. Only 19 of the country’s 54 reactors are in operation, and local governments have reportedly been waiting for the new standards before approving restart of the remaining reactors.
Prime Minister Naoto Kan and NISA officials have come under fire for failing to completely disclose key information about the crisis and ensuing radiation. In early June, for example, NISA officials revealed that the Fukushima accident had generated 770,000 terabecquerels of radiation—more than twice the radiation previously estimated. That figure is reportedly seven times the radiation emitted by the accident at Three Mile Island but just 15% that from Chernobyl.
Crews continue scrambling to control the situation at the Daiichi plant, which suffered critical power loss after a magnitude 9.0 earthquake and 14-meter (46-foot) wave—more than twice the height of the protective wall at Fukushima—and saw subsequent explosions at Units 1, 2, 3, and 4. TEPCO has begun preparatory work for installing a cover over the Unit 1 reactor building as an emergency measure to prevent the dispersion of radioactive substances until mid- to long-term measures, including radiation shielding, are implemented. Nitrogen gas is still being injected into the Unit 1 containment vessel to reduce the possibility of hydrogen combustion inside the vessel.
As of early June, closed-loop cooling had not yet been established, and freshwater was being continually injected both via the feedwater system lines and the fire extinguisher lines into the reactor pressure vessels at Units 1, 2, and 3. TEPCO in May also began installing a supporting structure for the floor of the damaged spent fuel pool of Unit 4 (Figure 5). TEPCO believes that the damage to the Unit 4 building could have been caused by “hydrogen generated at Unit 3 that flowed into Unit 4.”
|5. Dirty pool. The spent fuel pool at Daiichi 4 lost massive amounts of water in the aftermath of the accident, resulting in a hydrogen explosion and flames. This image from a video shows the damage sustained by the fuel in the pool. Courtesy: TEPCO|
Meanwhile, TEPCO continues to deal with massive volumes of stagnant water in the basement of the turbine buildings of Units 1 and 3 that has high levels of radioactivity. It is trying to transfer the water into condensers, a radioactive waste treatment facility, the high-temperature incinerator building, and temporary storage tanks.
—Sonal Patel is POWER’s senior writer.