(CNN) -- A third reactor at Japan's Fukushima Daiichi nuclear plant encountered problems with its cooling mechanism Monday, triggering fresh fears of a meltdown that could leak dangerous amounts of radiation into the atmosphere.
Blasts occurred at two other reactors at the plant over the weekend after Friday's devastating earthquake and tsunami led to similar cooling issues.
Are we facing a Chernobyl-scale disaster?
Experts say this is highly unlikely. The authorities are seeing -- and will continue to see -- much lower levels of radioactive material being released at Fukushima than at Chernobyl, according to Malcolm Grimston, Associate Fellow for Energy, Environment and Development at London's Chatham House think tank.
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"It won't be a Chernobyl because at the point of that accident the reactor was still operating," he told CNN.
He said a fatal design flaw in the Chernobyl reactor -- the tendency to run away with itself -- meant it was generating something like 500 times the amount of energy it should have been. "Not only was the place blown apart, the explosion was right in the center of the fuel," he added.
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As a result this superheated fuel was sent spewing into the air, forming a massive cloud of harmful radioactive material which drifted across much of the Soviet Union and Europe.
What happened at the Fukushima plant?
Three of its six reactors were in operation when the earthquake hit. 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 the International Atomic Energy Agency (IAEA).
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On Saturday an explosion blew away the roof and walls of the building housing the No. 1 reactor, though the blast did not damage the reactor or result in significant radiation leakage, officials said.
The cooling system in the plant's No.3 reactor then malfunctioned, prompting the authorities to start pumping in seawater in a last-ditch effort to cool down fuel rods and prevent a full meltdown -- a move one analyst called a "Hail Mary pass."
However the building housing the No.3 reactor was rocked by a second hydrogen explosion on Monday which injured 11 people.
What caused the explosions?
Grimston said a build-up of pressure inside the inner containment of the reactor was the likely cause. "My suspicion is that as the temperature inside the reactor was rising, some of the metal cans that surround the fuel may have burst and at high temperature, that fuel cladding can react with water to produce zirconium oxide and hydrogen.
"That hydrogen then will be part of the gases that need to be vented. That hydrogen then mixes with the surrounding air. Hydrogen and oxygen can then recombine explosively.
"So it seems while the explosion wasn't directly connected with the nuclear processes, it was indirectly connected, because the hydrogen was only present because of what was going on in the reactor core."
Now No.2 reactor has reported similar problems with its cooling system.
How serious is the situation?
"Three days into the crisis we've certainly got a serious situation because there is not enough water in the core to keep the fuel cold," said Grimston.
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"But it is also three days since the nuclear fission processes were shut down safely, so three days of the radioactivity and heat has decayed away. And despite the challenges the fuel remains within its inner containment."
Japanese nuclear regulators have said there is a "possibility" a partial meltdown has occurred in the reactors.
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.
The temperature of the molten metal could also melt the core, releasing radioactive material into containment vessel of the nuclear plant. While minor damage to small amounts of fuel in a core is known to occur, an occasion where most of the fuel actually melts and breaches the containment vessel is extremely rare. However the reactor is likely to be rendered unusable after a partial meltdown.
Has radioactive material already been released?
Radioactive material will have escaped when some of the metal fuel cans ruptured in the high temperatures, according to Grimston.
The current levels of detectable radioactive material are thousands of times less than those seen at Chernobyl, he said, but they still represent doses people will want to avoid.
Japan's Chief Cabinet Secretary Yukio Edano said radiation around the plant 250 kilometers (155 miles) north of Tokyo was at a tolerable level for humans, Agence France-Presse reported.
On Monday the U.S. Navy in the region repositioned its vessels -- which had been conducting disaster response missions -- away from the Fukushima plant after detecting low-level contamination in the air and on its aircraft operating in the area.
But it cautioned that the maximum potential radiation dose received by any of its personnel when the ships passed through the area was less than the radiation exposure received from about one month of exposure to natural background radiation from sources such as rocks, soil, and the sun.
How dangerous is it?
According to Grimston, the single biggest issue at Chernobyl was radioactive iodine getting into the thyroid gland and causing cancer.
However he was confident 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 said.
The Japanese government has evacuated more than 200,000 residents from homes within a 20-kilometer (12.4-mile) radius of the plant and tested 160 people for radiation exposure, authorities said Sunday.
CNN's Dylan Reynolds contributed to this report.
