(CNN) -- A cloud of white smoke rose above Japan's earthquake-damaged Fukushima Daiichi nuclear plant Wednesday, as radiation levels were reported to have increased again.
Earlier in the day, a fire was discovered in the No. 4 reactor building at the plant, a Tokyo Electric Power Company official told reporters. It renewed concerns over spent fuel rods sitting in an uncovered pool inside, which would release dangerous radiation if they caught fire. This followed explosions in the past few days at No.1, 2 and 3 reactors.
How dangerous are radiation levels now?
Workers trying to stabilize three overheating reactors at the plant were "asked to withdraw to a safe area," Japan's Chief Cabinet Secretary Yukio Edano said. Authorities later allowed them to return after radiation levels dropped, the Tokyo Electric Power Company said.
A Japanese self defense forces helicopter also aborted its mission to head to the site to drop cooling water over the reactors because of radiation levels, Japanese public broadcaster NHK reported.
Radiation levels at the plant have surged and dropped repeatedly over the past few days. The most recent spike "probably" occurred "because the containment vessel in reactor No. 3 has been damaged," a spokesman for Japan's nuclear safety agency later told reporters.
Another fire had broken out Tuesday in the No. 4 reactor. While it burned, radiation levels at the plant increased to about 167 times the average dose, the International Atomic Energy Agency (IAEA) said. That dose quickly diminished with distance from the plant, and radiation fell back to levels where it posed no immediate public health threat, Edano said.
But concerns about a potential shift in wind direction that could send increased levels of radiation toward populated areas prompted authorities to warn people as far as 18.6 miles (30 kilometers) from the plant to stay inside.
What caused the smoke?
Edano said the smoke or vapor above the plant may have been caused by breach in the containment structure around the No. 3 reactor's containment vessel -- the steel and concrete shell that insulates radioactive material inside.
Do we know the extent of the damage to the reactors?
Three of the plant's six reactors -- No. 1, 2 and 3 -- experienced problems with their cooling systems following Friday's devastating 9.0 earthquake. A subsequent build up of hydrogen resulted in explosions in each of the buildings housing the reactors. The authorities now say the cooling systems at reactors No. 5 and No. 6 are "not functioning well."
The diesel back-up generators used to pump in water to cool the reactor core if its own system fails were knocked out by the tsunami which followed the quake. Workers have spent the last five days trying to flood the reactors with seawater to cool them down.
Tuesday's blast at reactor No. 2 is thought to have damaged a water chamber at the base of the reactor. "My understanding is that the damage was associated with the torus structure underneath the reactor's containment vessel," said Professor Andrew Sherry, Director of the Dalton Nuclear Institute at the University of Manchester.
This donut-shaped structure, also known as a wet well, helps to manage the pressure inside the reactor by condensing the steam into water.
"If there was a pressure drop at the time then this suggests it has been damaged and there may be a leak that led to more hydrogen and steam escaping in a less controlled manner," said Sherry. "That will have increased local radiation levels."
How is radiation leaking from the reactors?
Initially slightly elevated levels at the site were recorded because workers at the plant were venting small pumps from the reactor buildings to release pressure.
But according to Glenn Sjoden, a professor of Nuclear Engineering at Georgia Tech in the United States, the situation is now very different as a fraction of the core in at least one of the reactors could have melted releasing some fission products to the atmosphere as they continue to relieve the pressure in the system.
What about the spent fuel rods?
Fuel rods are long metal tubes containing pellets of fissionable material which provide fuel for nuclear reactors. Once these fuel rods have been used, they are submerged in a reinforced steel and concrete container nearby filled with water -- not unlike a large swimming pool -- to cool them and allow their radioactivity to decay.
As long as the spent fuel rods remain covered they are perfectly safe. If this changes then the likelihood of a problem developing increases.
"There's still long-lived radioactivity in the rods from the spent fuel," said Paddy Regan, Professor of Physics at Surrey University in England. This consists of a mixture of residues from the nuclear reaction that take a long time to decay.
"There are not many of them but they do take a lot of time to decay and they generate a lot of heat. That's why they're put in these cooling ponds."
Has a reactor meltdown occurred?
Radiation levels indicate that only a partial meltdown has occurred in the affected reactors, Sjoden told CNN Tuesday. "The levels would be significantly higher if a total meltdown had occurred," he said.
A nuclear meltdown is where the fuel in the core of the reactor starts to melt. This happens when there is a loss of coolant (water) in the core. A fuel melt is a serious problem as it releases a lot of the radioactive material that is contained in that fuel.
What is being released into the atmosphere?
According to Sjoden, the main radioactive components likely to escape when a reactor's fuel rods are breached are Iodine-131 and Caesium-137. Damaging levels of both components were emitted during the Chernobyl reactor disaster in the Ukraine in 1986.
What is radiation?
In the context of nuclear energy we are referring to ionizing radiation, which passes through matter causing it to become electrically charged or ionized. In living tissues, the electrical ions produced by radiation can affect normal biological processes.
What are the effects on humans?
Malcolm Grimston, Associate Fellow for Energy, Environment and Development at London's Chatham House, said the single biggest issue following the Chernobyl disaster was radioactive iodine getting into the thyroid gland and causing cancer.
However he said it was likely those living near the Fukushima plant will have been offered iodine tablets which flood the thyroid gland with non-radioactive iodine so there is no space for radioactive iodine to penetrate.
"At Chernobyl they had trouble getting hold of these tablets because it was such a poor area," he told CNN.
The IAEA said the Japanese authorities were distributing iodine to residents in the area near the plant.
How did the crisis at the plant begin?
Three reactors were in operation when Friday's 9.0 magnitude quake struck. The reactors -- which went into service between 1970 and 1979 -- are designed to shut down automatically when a quake strikes, and emergency diesel generators began the task of pumping water around the reactors to cool them down. However, these stopped about an hour later. The failure of the back-up generators has been blamed on tsunami flooding by IAEA.