Report: Japan Has Just 'Hours' to Prevent Nuclear Meltdown

Fears of nuclear meltdown are spreading throughout Japan after five nuclear reactors have malfunctioned. Now, reports suggest Japanese officials may have only hours to cool reactors disabled by Friday's massive 8.9 earthquake or face nuclear meltdown.

According to Reuters, Tokyo's Electric Power Co. is struggling to cool down the reactor core after the plant lost power necessary to keep water circulating through the plant to prevent overheating.

Daiichi Units 1, 2 and 3 reactors shut down automatically at 2:46 p.m. local time due to the earthquake. But about an hour later, the on-site diesel back-up generators also shut, leaving the reactors without alternating current (AC) power.

That caused Tepco to declare an emergency and the government to evacuate thousands of people from near the plant. Such a blackout is "one of the most serious conditions that can affect a nuclear plant," according to experts at the Union of Concerned Scientists, a U.S. based nuclear watchdog group.

"If all AC power is lost, the options to cool the core are limited," the group warned.

TEPCO also said it has lost ability to control pressure at some of the reactors at its Daini plant nearby.

The reactors at Fukushima can operate without AC power because they are steam-driven and therefore do not require electric pumps, but the reactors do require direct current (DC) power from batteries for its valves and controls to function.

If battery power is depleted before AC power is restored, the plant would stop supplying water to the core and the cooling water level in the reactor core could drop.

Japan's ambassador to the United States spoke to CNN Friday evening to assess the safety of Japan's nuclear plants:

To try and release some of the reactor pressure which had built up to more than 50 percent higher than normal, nuclear officials have opened valves to release radioactive vapors they say shouldn't harm the surrounding environment. James Acton from the Carnegie Endowment told the BBC that if releasing the radioactive steam is the worst fallout, "there's unlikely to be significant lasting damage to people outside the plant." However, if the structural integrity of the core is lost and the core begins to melt, "we could be in an extremely serious situation."

Edwin Lyman, a senior staff scientist at the Union of Concerned Scientists, also told Reuters: "We don't have all the information but every indication is that the type of event that occurred there is one of the most serious things that can happen to a nuclear reactor."

"In the worst case the entire core could melt through the steel reactor vessel and escape into the containment building, and then the containment is the only thing that is standing between the radiation in the reactor and the atmosphere," he said. "There is a chance if that does occur that there will be over pressure, the containment can fail and you might have a release on the order of the Chernobyl accident."

Meanwhile, nuclear expert Cham Dallas suggests a Chernobyl-like meltdown is unlikely, but claims a Three Mile Island-type incident is a possibility:

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