WASHINGTON — An explosion at a nuclear power plant in northern Japan on Saturday blew the roof off one building, brought down walls and caused a radiation leak of unspecified proportions, Japanese officials said, after Friday’s huge earthquake caused critical failures in the plant’s cooling system.

Television images showed a huge cloud of white-gray smoke from the explosion. Soon afterward, government officials said an evacuation zone around the plant had been doubled, to 12 miles.

The chief cabinet secretary, Yukio Edano, confirmed earlier news reports of an explosion at the Fukishima Daiichi nuclear plant, 15o miles north of Tokyo, saying: “We are looking into the cause and the situation and we’ll make that public when we have further information.” He was speaking amid fears that a disastrous meltdown could be imminent after critical cooling failures at that plant and another nearby, Daini, after both were shut down.

Images on Japanese television showed that the walls of one building had crumbled, leaving only a skeletal metal frame standing with smoke billowing from the plant. The Associated Press reported that the damaged building housed a nuclear reactor, though that report was not immediately verified by nuclear officials. The cause of the explosion was unclear, with some experts speculating that it may have resulted from a hydrogen build-up.

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Officials said even before the explosion that they had detected cesium, an indication that some of the fuel was already damaged.

In the form found in reactors, radioactive cesium is a fragment of a uranium atom that has been split. In normal operations, some radioactivity in the cooling water is inevitable, because neutrons, the sub-atomic particles that carry on the chain reaction, hit hydrogen and oxygen atoms in the water and make those radioactive. But cesium, which persists far longer in the environment, comes from the fuel itself.

Naoto Sekimura, a professor at Tokyo University, told NHK, Japan’s public broadcaster, that “only a small portion of the fuel has been melted. But the plant is shut down already, and being cooled down. Most of the fuel is contained in the plant case, so I would like to ask people to be calm.”

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Radioactive material release

Once the first line of containment is breached during a reactor plant accident, there is a possibility that the fuel or the fission products held inside can be released into the environment. Although the zirconium fuel cladding has been breached in other nuclear reactors without generating a release to the environment, at TMI-2 operators permitted fission products to leave the other containment barriers in a matter of minutes.[citation needed] However, since very little of the fission products released were solids at room temperature, very little radiological contamination was reported in the environment. No significant level of radiation was attributed to the TMI-2 accident outside of the TMI-2 facility. Noble gases made up the bulk of the release of radioactive materials from TMI-2, with the next most abundant element being iodine.

Within hours of the accident the United States Environmental Protection Agency (EPA) began daily sampling of the environment at the three stations closest to the plant. By April 1, continuous monitoring at 11 stations was established and was expanded to 31 stations two days later. An inter-agency analysis concluded that the accident did not raise radioactivity far enough above background levels to cause even one additional cancer death among the people in the area. The EPA found no contamination in water, soil, sediment or plant samples.[28]

Researchers at nearby Dickinson College, which had radiation monitoring equipment sensitive enough to detect Chinese atmospheric atomic weapons testing, collected soil samples from the area for the ensuing two weeks and detected no elevated levels of radioactivity, except after rainfalls (likely due to natural radon plate out, not the accident).[29] Also, white-tailed deer tongues harvested over 50 mi (80 km) from the reactor subsequent to the accident were found to have significantly higher levels of Cs-137 than in deer in the counties immediately surrounding the power plant. Even then, the elevated levels were still below those seen in deer in other parts of the country during the height of atmospheric weapons testing.[30] Had there been elevated releases of radioactivity, increased levels of Iodine-131 and Cesium-137 would have been expected to be detected in cattle and goat's milk samples. Yet elevated levels were not found.[31]

All reactors' temperature is now under control and the residual heat reactions inside them continue to die away; soon, no further cooling will be required.

The plant’s operator, Tokyo Electric Power, said late Monday that efforts to inject seawater into the reactor had failed.

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The company said water levels were not immediately rising to the desired level, possibly because of a leak in the containment vessel. Still, a Tokyo Electric official said the situation was improving.

"We do not feel that a critical event is imminent," he told a press conference.

"So there's some advantages to the BWR in terms of severe accidents. But one of the disadvantages is that the containment structure is a lightbulb-shaped steel shell that's only about 30 or 40 feet [nine to 12 meters] across—thick steel, but relatively small compared to large, dry containments like TMI. And it doesn't provide as much of an extra layer of defense from reactor accidents as containments like TMI [do]. So there is a great deal of concern that if the core does melt, the containment will not be able to survive. And if the containment doesn't survive, we have a worst-case situation."

And just what is that worst-case scenario? "They're venting in order to keep the containment vessel from failing. But if a core melts, it will slump to the bottom of the reactor vessel, probably melt through the reactor vessel onto the containment floor. It's likely to spread as a molten pool—like lava—to the edge of the steel shell and melt through. That would result in a containment failure in a matter of less than a day. It's good that it's got a better containment system than Chernobyl, but it's not as strong as most of the reactors in this country."

Finally, Bergeron summed up the events so far: "Based on what we understand, the reactor has been shut down, in the sense that all of the control rods have been inserted—which means there's no longer a nuclear reaction. But what you have to worry about is the decay heat that's still in the core—that will last for many days.

"And to keep that decay heat of the uranium from melting the core, you have to keep water on it. And the conventional sources of water, the electricity that provides the power for pumps, have failed. So they are using some very unusual methods of getting water into the core, they're using steam-driven turbines—they're operating off of the steam generated by the reactor itself.

"But even that system requires electricity in the form of batteries. And the batteries aren't designed to last this long, so they have failed by now. So we don't know exactly how they're getting water to the core or if they're getting enough water to the core. We believe, because of the release of cesium, that the core has been exposed above the water level, at least for a portion of time, and has overheated. What we really need to know is how long can they keep that water flowing. And it needs to be days to keep the core from melting.

"The containment, I believe, is still intact. But if the core does melt, that insult will probably not be sustained and the containment vessel will fail. All this, if it were to occur, would take a matter of days. What's crucial is restoring AC power. They've got to get AC power back to the plant to be able to control it. And I'm sure they're working on it."

When the cooling system at the reactor broke down, TEPCO used fire engines to pump seawater into the reactor to cool the rods, but the pump malfunctioned, causing most of the fuel rods to be exposed. If exposed to air, fuel rods overheat and melt, releasing radioactive materials into the atmosphere.

TEPCO said it cannot rule out a meltdown due to the exposure of the fuel rods but is bringing the temperature down again by injecting seawater. It added since the reactor is contained by an outer structure there is "no possibility of a worst-case accident."

Japanese broadcaster NHK reported the pumps stopped flushing seawater into the reactor after running out of fuel when a plant official who was operating the pumps left his seat to monitor something else.

Even as workers race to prevent the radioactive cores of the damaged nuclear reactors in Japan from melting down, concerns are growing that nearby pools holding spent fuel rods could pose an even greater danger.

The pools, which sit on the top level of the reactor buildings and keep spent fuel submerged in water, have lost their cooling systems and the Japanese have been unable to take emergency steps because of the multiplying crises.

The threat is that the hot fuel will boil away the cooling water and catch fire, spreading radioactive materials far and wide in dangerous clouds.

The good news is that the Japanese have a relatively long time to deal with the problem. Nuclear experts estimate the timeline for serious problems that could lead to a reactor meltdown as minutes to hours, and put the comparable time for cooling pools at days to weeks.

The bad news is that if efforts to deal with the emergency fail, the results could be worse.

The pools are a worry at the stricken reactors at the Fukushima Daiichi plant because at least two of the three have lost their roofs in explosions, exposing the spent fuel pools to the atmosphere. By contrast, reactors have strong containment vessels that stand a better chance of bottling up radiation from a meltdown of the fuel in the reactor core.

Were the spent fuel rods in the pools to catch fire, nuclear experts say, the high heat would loft the radiation in clouds that would spread the radioactivity.

TEPCO: Spraying water from air "difficult"

Tokyo Electric Power has found it difficult to spray water from a helicopter to cool down a storage pool for spent nuclear fuel inside the No.4 reactor at the Fukushima Daiichi nuclear power plant.

The reactor was undergoing an inspection when the quake occurred. The firm says the temperature of the storage pool for spent nuclear fuel was 84 degrees Celsius on Monday morning, more than double the normal level. More recent temperatures are not available due to a technical failure.

On Tuesday morning, an explosion was heard and the roof of the building that houses the No.4 reactor was damaged. Tokyo Electric Power, the operator of the plant, says it appears a lack of coolant caused the fuel rods to be exposed, adding that a hydrogen explosion might have occurred.
If the reactor can't be cooled, the fuel rods may emit hydrogen or melt down. Tokyo Electric Power considering pouring water onto the storage pool in the containment vessel through a hole on the roof created by the blast.

However, the firm concluded that it would be extremely difficult to spray water from a helicopter as the hole is dozens of meters from the storage pool and a helicopter can only carry a limited amount of water on a single flight.

Workers are currently unable to approach the storage pool due to the high radiation levels. Tokyo Electric Company is studying the possibility of using fire engines and other options to inject water into the reactor.

Wednesday, March 16, 2011 03:04 +0900 (JST)

- Increase the capacity of spent fuel pools by re-racking
- Installation of common spent fuel pool
- Installation of dry cask storage facility

Richard T. Lahey Jr., who was General Electric’s chief of safety research for boiling-water reactors when the company installed them at the Fukushima Daiichi plant, said that as seawater was pumped into the reactors and boiled away, it left more and more salt behind.

He estimates that 57,000 pounds of salt have accumulated in Reactor No. 1 and 99,000 pounds apiece in Reactors No. 2 and 3, which are larger.

The big question is how much of that salt is still mixed with water and how much now forms a crust on the reactors’ uranium fuel rods. Chemical crusts on uranium fuel rods have been a problem for years at nuclear plants.

Crusts insulate the rods from the water and allow them to heat up. If the crusts are thick enough, they can block water from circulating between the fuel rods at all. As the rods heat up, their zirconium cladding can ignite, which may cause the uranium inside to melt and release radioactive material.

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Preventing the reactors and storage pools from overheating through radioactive decay would go a long way toward limiting radioactive contamination. But that would require pumping a lot of cold freshwater through them, which is not easily done.

The emergency cooling system pump and motor for a boiling-water reactor are roughly the size and height of a compact hatchback car standing on its back bumper. The powerful system has the capacity to propel thousands of gallons of water a minute throughout a reactor pressure vessel and storage pool. But that very power can also be the system’s Achilles’ heel.

The pump and piping are designed to be kept full of water. But they tend to leak and develop alternating pockets of air and water, said Mr. Friedlander, who said he had performed maintenance on the systems many times in his career.

If the pump is turned on without venting the air and draining the water, the water from the pump would hit the alternating pockets with enough force to blow holes in the piping. Venting the air and draining the water requires a technician to reach a dozen valves, sometimes using a ladder. The water is removed through a hose to the nearest drain, usually in the floor, that leads to machinery designed to remove radiation from the water.

The process takes a full 12 hours in a reactor that is operating normally, Mr. Friedlander said. But even then, the water in the pipes tends to become radioactively contaminated because the valves that separate it from the reactor are never entirely tight.

It is likely to be an even bigger problem when the water inside the reactor is much more radioactive than usual and is under extremely high pressure, as it has been in all three reactors at the Fukushima Daiichi plant at various times since the earthquake and tsunami.

Japanese government and power company officials expressed optimism on Wednesday morning that the crisis was close to being brought under control, only to encounter two reminders in the afternoon of the unpredictable difficulties that lie ahead.

Fukushima Daiichi’s Reactor No. 3 began belching black smoke for an hour late in the afternoon, leading its operator, Tokyo Electric Power Company, to evacuate workers. No. 3 is considered one of the most dangerous of the reactors because of its fuel — mixed oxides, or mox, which contain a mixture of uranium and plutonium and can produce a more dangerous radioactive plume if scattered by fire or explosions. The cooling system at Reactor No. 5, which was shut down at the time of the earthquake and has shown few problems since, also abruptly stopped working on Wednesday afternoon, said Hiro Hasegawa, a spokesman for Tokyo Electric.

“When we switched from the temporary pump, it automatically switched off,” he said. “We’ll try again with a new pump in the morning.”