Japan Times confirms that radiation has leaked from the containment domes at the Fukushima nuclear power plants.

Radiation rose to an unusually high level in and near Tokyo Electric Power Co.’s Fukushima No. 1 nuclear plant Saturday following the powerful earthquake that hit northern Japan the previous day, the nuclear safety agency said, making it the first case of an external leak of radioactive substances since the disaster.

While the agency denied the radiation amount will pose an immediate threat to the health of nearby residents, the impact of the quake appeared to widen as the agency added the area close to the Fukushima No. 2 nuclear plant as a zone that requires evacuation.

Given the adjacent No. 2 plant also has quake-triggered malfunctions, the operator of the two plants in Fukushima Prefecture is set to release pressure in containers housing their reactors under an unprecedented government order, so as to avoid the plants sustaining damage and losing their critical containment function.

But the action would involve the release of steam that would likely include radioactive materials.

The Union of Concerned Scientists provides this explanation of what is happening at these two reactors.

This power failure resulted in one of the most serious conditions that can affect a nuclear plant—a “station blackout”—during which off-site power and on-site emergency alternating current (AC) power is lost. Nuclear plants generally need AC power to operate the motors, valves and instruments that control the systems that provide cooling water to the radioactive core. If all AC power is lost, the options to cool the core are limited.

The boiling water reactors at Fukushima are protected by a Reactor Core Isolation Cooling (RCIC) system, which can operate without AC power because it is steam-driven and therefore does not require electric pumps. However, it does require DC power from batteries for its valves and controls to function.

If battery power is depleted before AC power is restored, however, the RCIC will stop supplying water to the core and the water level in the reactor core could drop. If it drops far enough, the core would overheat and the fuel would become damaged. Ultimately, a “meltdown” could occur: The core could become so hot that it forms a molten mass that melts through the steel reactor vessel. This would release a large amount of radioactivity from the vessel into the containment building that surrounds the vessel.

The containment building’s purpose is to keep radioactivity from being released into the environment. A meltdown would build up pressure in the containment building. At this point we do not know if the earthquake damaged the containment building enough to undermine its ability to contain the pressure and allow radioactivity to leak out.

US military have flown in coolant to help keep the fuel rods covered but it is now a race against time before the core overheats. Reuters reports that radiation levels 8 times normal levels near main gate of Fukushima Daiichi nuclear plant in Japan – Kyodo. More than 20,000 residents within 10km of the power plants have been evacuated as a safety precaution.

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