The blast injured 11 people, one seriously.
The string of earthquake- and tsunami-triggered troubles at the Fukushima Daiichi plant began with the failure of the primary and back-up cooling systems, necessary to keep reactors from overheating.
On Saturday, a similar explosion occurred at unit 1. Trace amounts of radioactive elements cesium-137 and iodine-131 have been detected outside the plant.
The U.S. Seventh Fleet said on Monday that some of its personnel, who are stationed 100 miles offshore from the Fukushima Daiichi plant, had come into contact with radioactive contamination. The airborne radioactivity prompted the fleet to reposition its ships and aircraft.
Using sensitive instruments, precautionary measurements were conducted on three helicopter aircrews returning to USS Ronald Reagan after conducting disaster relief missions near Sendai. Those measurements identified low levels of radioactivity on 17 air crew members.
The low level radioactivity was easily removed from affected personnel by washing with soap and water, and later tests detected no further contamination.
Like the Saturday explosion at unit 1, the blast at unit 3 took place after a buildup of hydrogen was vented by the reactor. The hydrogen was produced by the exposure of the reactor’s fuel rods and their zirconium alloy casing to hot steam.
In normal conditions, the fuel rods would be covered and cooled by water.
The explosion occurred as Tokyo Electric entered day four of its battle against a cascade of failures at its two Fukushima nuclear complexes, using fire pumps to inject tens of thousands of gallons of seawater into two reactors to contain partial meltdowns of ultra-hot fuel rods.
The tactic produced high pressures and vapors that the company vented into its containment structures and then into the air, raising concerns about radioactivity levels in the surrounding area where people have already been evacuated. The utility said that at one of the huge, complicated reactors, a safety relief valve was opened manually to lower the pressure levels in a containment vessel.
But the limited vapor emissions were seen as far less dire than the consequences of failure in the fight against a more far-reaching partial or complete meltdown that would occur if the rods blazed their way through the reactor’s layers of steel and concrete walls.
The potential size of the area affected by radioactive emissions could be large. A state of emergency was declared briefly at another nuclear facility, the Onagawa plant, after elevated radioactivity levels were detected there. Later, Japanese authorities blamed the measurement on radioactive material that had drifted from the Fukushima plant, more than 75 miles away, according to the International Atomic Energy Agency.
The IAEA noted that forecasts said winds would be blowing to the northeast, away from the Japanese coast, over the next three days.
Tokyo Electric said radioactivity levels inside the plant and at its nearby monitoring post were higher than normal. Although levels had fallen Sunday, the Kyodo News Agency said that radiation at the plant’s premises rose Monday over the benchmark limit of 500 microsievert per hour at two locations, measuring 751 microsievert at the first location at 2:20 a.m. and 650 at the second at 2:40 a.m., according to information Tokyo Electric gave the government. The hourly amounts are more than half the 1,000 microsievert to which people are usually exposed in one year.
In addition to one worker hospitalized for radiation exposure, two others felt ill during stints in the control rooms of Fukushima Daiichi units 1 and 2.
Although Tokyo Electric said it also continued to deal with cooling system failures and high pressures at half a dozen of its 10 reactors in the two Fukushima complexes, fears mounted about the threat posed by the pools of water where years of spent fuel rods are stored.
At the 40-year-old Fukushima Daiichi unit 1, where an explosion Saturday destroyed a building housing the reactor, the spent fuel pool, in accordance with General Electric’s design, is placed above the reactor. Tokyo Electric said it was trying to figure out how to maintain water levels in the pools, indicating that the normal safety systems there had failed, too. Failure to keep adequate water levels in a pool would lead to a catastrophic fire, said nuclear experts, some of whom think that unit 1’s pool may now be outside.
“That would be like Chernobyl on steroids,” said Arnie Gundersen, a nuclear engineer at Fairewinds Associates and a member of the public oversight panel for the Vermont Yankee nuclear plant, which is identical to the Fukushima Daiichi unit 1.
People familiar with the plant said there are seven spent fuel pools at Fukushima Daiichi, many of them densely packed.
Gundersen said the unit 1 pool could have as much as 20 years of spent fuel rods, which are still radioactive.
At Fukushima Daiichi unit 3, the explosion was an indicator of serious problems inside the reactor core.
Victor Gilinsky, a former commissioner at the Nuclear Regulatory Commission, said that to produce hydrogen, temperatures in the reactor core had to be well over 2,000 degrees and as high as 4,000 degrees Fahrenheit. He said a substantial amount of fuel had to be exposed at least at some point.
“That’s the significance of the hydrogen — it means there was serious fuel damage and probably melting,” said Gilinsky, who was at the NRC when Pennsylvania’s Three Mile Island reactor had a partial meltdown in 1979. “How much? We won’t know for a long time. At TMI we didn’t know for five years, until the vessels were opened. It was a shock.”
The Fukushima Daiichi unit 3 was built by Toshiba. Last year, the unit began using some reprocessed fuel known as “mox,” a mixture of plutonium oxide and uranium oxide, produced from recycled material from nuclear weapons as part of a program known as “from megatons to megawatts.” Anti-nuclear activists have called mox more unsafe than enriched uranium. If it escapes the reactor, plutonium even in small quantities can have much graver consequences on human health and the local environment for countless years, much longer than other radioactive materials.
The Kyodo News Agency cited Tokyo Electric as saying that more than three yards of a mox nuclear-fuel rod had been left above the water level, raising concerns that bits of plutonium or its byproducts may already be mixed into vapors or molten material.
The Fukushima Daiichi unit 3, once capable of generating 784 megawatts of power, is substantially bigger than unit 1, which generated about 460 megawatts. As a result, lowering temperatures in its reactor core could prove a much tougher task, experts said.
Japanese officials were also trying to figure out whether Friday’s earthquake, or the subsequent high pressures and temperatures in the reactors, had caused other cracks or leaks in reactors in the region. So far officials have not said that they have found any, though they have noted still unexplained losses of water in some reactor vessels.
Although Fukushima Daiichi units 1 and 3 posed the gravest dangers for now, Tokyo Electric said it was still working on its other units.
Tokyo Electric also said it had released vapors with some radioactive materials at all four of the reactors at its second Fukushima complex — Fukushima Daini — on Saturday. After injecting water into the reactors, the company said that water levels were stable, off-site power restored, and shutdowns complete or in progress. Nonetheless, Japan’s Nuclear and Industrial Safety Agency said Monday that Fukushima Daini units 1, 2 and 4 remained in a nuclear state of emergency.