Nuclear Crisis at Fukushima

, senior scientist | March 11, 2011, 2:45 pm EDT
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As of 2:30 pm EST Friday 3/11/11:

The massive earthquake off the northeast coast of Japan has caused a potentially catastrophic situation at one of Japan’s nuclear power plants. The situation is still evolving, but here is a preliminary assessment based on the facts as we currently understand them.

The plant’s owner, Tokyo Electric Power Company (TEPCO), reported that at 2:46 p.m. local time (12:46 a.m. EST) “turbines and reactors of Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Station Unit 1 … and Units 2 and 3 … automatically shut down due to the Miyagiken-oki Earthquake.”

These reactors are 3 of the 6 operating reactors at the Fukushima I nuclear facility. All are boiling water reactors. Unit 1 has a rated output of 460 megawatts, and Units 2 and 3 each have a rated output of 784 megawatts.

TEPCO went on to state the shutdowns were caused by the loss of off-site power “due to malfunction of one out of two off-site power systems.” This loss of power triggered emergency diesel generators, which automatically started to provide backup power to the reactors.

However, at 3:41 p.m. local time (1:46 a.m. EST), the emergency diesel generators shut down “due to malfunction, resulting in the complete loss of alternating current for all three units,” according to TEPCO. The failure of the diesel generators was most likely due to the arrival of the tsunami, which caused flooding in the area. The earthquake was centered 240 kilometers from Japan, and it would have taken the tsunami approximately an hour to reach the Japanese islands.

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.

According to technical documents translated by Aileen Mioko Smith of Green Action in Japan, if the coolant level dropped to the top of the active fuel rods in the core, damage to the core would begin about 40 minutes later, and damage to the reactor vessel would occur 90 minutes after that.

Concern about a serious accident is high enough that while TEPCO is trying to restore cooling the government has evacuated a 3-km (2-mile) radius area around the reactor.

Bloomberg News reported that the battery life for the RCIC system is eight hours. This means that the batteries would have been depleted before 10 a.m. EST today. It is unclear if this report is accurate, since it suggests that several hours have elapsed without any core cooling. Bloomberg also reported that Japan had secured six backup batteries and planned to transport them to the site, possibly by military helicopter. It is unclear how long this operation would take.

There also have been news reports that Fukushima Unit 2 has lost its core cooling, suggesting its RCIC stopped working, but that the situation “has been stabilized,” although it is not publicly known what the situation is. TEPCO reportedly plans to release steam from the reactor to reduce the pressure, which had risen 50% higher than normal. This venting will release some radioactivity.

More information about the cooling issue is available in this New York Times story.

We will post updates as more information becomes available.

Posted in: Japan, Nuclear Power Safety Tags: , ,

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