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In this photo taken on Oct. 3, 2008, the No. 1 reactor, center, of the Fukushima Daiichi Nuclear Power Plant in Okumamachi, Fukushima Prefecture, is seen. A strong earthquake on Friday, March 11, 2011 knocked out power at the plant, and because a backup generator failed, the cooling system was unable to supply water to cool the 460-megawatt reactor. An official with Japan's nuclear safety commission said Saturday, March 12, 2011 that a meltdown at the plant is possible. (AP Photo/Kyodo News)

RICHARD T. PIENCIAK, Associated Press

The nuclear crisis in Japan has developed rapidly on many fronts, making it difficult to track the threads. What are the dangers? Will the situation improve? Can the reactors be cooled?

The crisis began Friday when a magnitude-8.9 earthquake and tsunami cut off regular electricity to the oldest unit at the Fukushima Dai-ichi nuclear complex, 170 miles (270 kilometers) northeast of Tokyo.

Since then more reactors - at more nuclear sites - have lost at least some cooling ability, increasing concerns about possible meltdowns.

Some questions and answers about the crisis in Japan:

Q: What is the status of nuclear reactors as of Monday?

A: There are nine units under states of emergency - three at Fukushima Dai-ichi, three at Fukushima Daini and three at Onagawa. All are north-northeast of Tokyo, along the eastern coast, and all are boiling water reactors.

The other three reactors at Fukushima Dai-ichi, operated by Tokyo Electric Power Co., were shut before the earthquake. A fourth reactor at Tokyo Electric's Daini site appears fine. There are only three units at the Onagawa facility, owned by Tohoku Electric Power Co. Most concern has been directed at Dai-ichi units 1 and 3.

Q: What are the worries?

A: At Unit 1, which began operating in 1971, workers are trying to prevent a meltdown, complicated by the fact that a need to release a pressure buildup in the reactor vessel led to a hydrogen explosion that blew off the roof and walls of the containment building. Officials say the reactor vessel is intact, but worry about the overheated uranium fuel. In a desperate move, officials have piped large amounts of seawater into the reactor vessel to try cooling the severely overheated uranium core.

On Monday, a hydrogen explosion also hit Unit 3. It was not immediately clear how much, if any, radiation was released. Officials were using seawater to cool the unit, where they believe there has been a partial meltdown.

Shortly after Monday's explosion, Tokyo Electric warned it had lost the ability to cool Fukushima Dai-ichi's Unit 2. Hours later, the company said fuel rods in that reactor were fully exposed, at least twice.

As with the other troubled reactors, the key is to cool the nuclear fuel by circulating new cool water around the fuel rods. If the rods are fully exposed, that increases the temperature of the rods and could hasten the path to complete meltdown.

The company was trying to channel sea water into the reactor to cover the rods, cool them down and prevent another explosion at the stricken plant.

Q: What is the significance of using seawater?

A: With so many equipment failures, plant operators face challenges using mobile generators powered by batteries. They also need a dependable high-volume water source. The Pacific Ocean solves the supply problem. But using it assures that these very expensive reactors will never be used again to generate power. The salty sea water, accompanied by a boron mix, is very corrosive.

Q: What is the situation at the nearby Fukushima Daini facility?

A: Japanese officials say units 1, 2 and 4 retained offsite power after the earthquake and tsunami, but were experiencing increased pressure inside their containment vessels and equipment failures. As a result, plant operators vented steam at each unit and were considering additional venting to alleviate pressure increases.

Q: And now there are concerns about a third complex?

A: Yes, as of late Sunday, there are states of emergency at each of the three reactors at the Onagawa nuclear site. Officials have said only they've detected higher than permitted radiation levels there.

Q: Has radiation already escaped from any of the other reactor sites?

A: An official from Japan's Nuclear and Industry Safety Agency said Saturday that a small amount of radioactive cesium was detected outside Fukushima Dai-ichi Unit 1 the day before. That would have been before the containment building explosion, but after some venting of steam had occurred. The official said the presence of cesium did not necessarily indicate a partial meltdown; he said it could have been from a mechanical failure.

Q: Any indications of radiation exposure to humans yet?

A: Of the more than 180,000 people evacuated from around the two Fukushima complexes, up to 160 may have been exposed. And at one point, officials said the radiation detected outside the Dai-ichi Unit 1 in a one-hour period represented the allowed rate for an entire year.

Q: Exactly what is a meltdown, and why is it potentially dangerous?

A: A meltdown occurs when a reactor's radioactive core, which holds its uranium fuel, gets so hot that it begins to melt. A complete meltdown can breach a reactor's steel pressure vessel and other protective barriers - and spread radioactive byproducts like iodine and cesium into the surroundings. That endangers the environment and nearby residents. However, a reactor will not explode like an atomic bomb.

Q: What steps can be taken to prevent a meltdown?

A: The immediate key is reducing temperatures in all the reactor vessels. Another critical goal would be restoring regular electrical power.

Q: Why did the containment building at Dai-ichi Unit 1 explode?

A: When officials decided to vent steam from the reactor vessel to reduce the pressure, the hydrogen in the steam interacted with available oxygen. They knew it could cause a blast, but felt they had no choice. If the pressure kept building, the reactor vessel could have exploded, likely starting a meltdown scenario.

Q: How likely is it that one or more total meltdowns will occur?

A: That is very difficult to predict without detailed real-time measurements from inside the nuclear facilities. But admissions from Japanese officials that a partial meltdown may have already occurred are troubling.

Q: Why was the official announcement made late Sunday about something that occurred Friday?

A: Officials in Japan have been slow to provide information about the status of the nuclear plants. (There are 55 reactors on 17 sites throughout the country. Japan gets one-third of its electricity from nuclear plants.) The belated disclosures are often clouded in generalities. At times, new information has been available sooner from the International Atomic Energy Agency in Vienna. The UN organization posts updates here.

Q: How long will the crisis last?

A: One expert said cooling down all the reactors will "take days, not hours." But even if circumstances improve, conditions can still turn negative again.

Q: How about personal health danger?

A: Exposure to radioactive iodine released in a nuclear power accident can cause thyroid cancer.

Q: Is there a way to protect against the effects of radiation exposure?

A: Potassium iodide pills can help prevent thyroid cancer.

Q: So what is the worst-case scenario?

A: The attempts to cool the reactors fail, resulting in meltdowns and widespread radioactive contamination. If that occurs, everyone will be hoping the wind blows east, into the Pacific, as it usually does.

Q: The best-case?

A: Officials gain complete control of the temperature and pressure at the troubled reactors; then conditions will need to improve enough so it will be safe for workers to get close to assess the damage and restore normalcy.