After today's earthquake in Japan, there was concern over the country's nuclear power plants: though 13 of the affected plants automatically shut down, two caused a scare. This raises the question: how does a nuclear plant stay safe during an earthquake, and why were there problems in Japan?
Japan is a big proponent of nuclear energy, with 55 power plants spread across the island, and 15 located within the region affected by the earthquake. But Japan is also located between four tectonic plates, which could potentially cause a nuclear meltdown if the quake is strong enough to disrupt the reactor core.
The main worry with nuclear power plants and earthquakes is the structural integrity of the plant containing the nuclear reactors and the temperature of the reactor core. When the reactor core overheats, it melts down. And when it melts down, it releases radioactive steam and other materials into the containment room surrounding it. And I'm pretty sure you know what happens after that. Thus, much effort is put into the cooling systems designed to keep those cores within a stable temperature range.
Of course, those who design power plants are cognizant of this fact and integrate safeguards, to help prevent meltdowns as a result of seismic shock.
The first (and primary) line of defense against the threat of earthquake-related nuclear incidents are seismic sensors which pickup oncoming earthquakes, and immediately shutdown nuclear reactors, working to neutralize any overheating. Seismic sensors work by detecting peak ground acceleration (PGA), which is the force at which an earthquake shakes the ground around the sensor.
Most nuclear plants around the world are designed to withstand a PGA under .2g (2 m/s^2). When PGA exceeds that number, the sensors will generally alert the nuclear reactors, forcing a shutdown. In general, seismic activity registering over a 6.0 on a richter scale can generate a PGA well over .2g. But magnitude alone isn't the sole determining factor of PGA.
For example, Alaska had a 9.2 magnitude earthquake that was 23km deep, and only generated a PGA of 1.8g. The recent New Zealand earthquake, on the other had, was a lesser 6.3 magnitude quake, but the depth was only 5km deep and had a staggering PGA of 2.2g. Today's earthquake in Sendai was a 9.0 magnitude, with a depth of 24km, and it generated a PGA of .82g
According to the New Scientist, once the seismic sensors go to work, the nuclear reactor is shut down from active duty and emergency cooling systems kick in, working double time to keep the core at a safe temperature:
After a nuclear plant is shut down, control rods are normally inserted into the reactor core to quench the fission reaction. But the reactor remains hot and still needs cooling.
While most of the affected nuclear plants in Japan shut down and resumed operation with minimal issues, two of them experienced larger problems. In the Fukushima power plant, which was most affected by the quake, the power systems were compromised by the earthquake and ensuing tsunami. After a shut down, the cooling systems rely on both standard and backup cooling systems to keep the core stable. As a result, the cores remained hot as the tsunami consumed everything around it. Here's a detailed explanation from All Things Nuclear:
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.
Apparently, the U.S. Secretary of State Hilary Clinton offered to have the Air Force fly in a giant batch of coolant to help quell the threat of a nuclear leak, but Japan turned it down. Instead, the power plant operators are venting some of the slightly radioactive gas inside of the reactor to reduce pressure around the core (and they're evacuating anyone within a 3 kilometer range, because there's no such thing as a safe vent). But apparently, all the cooling mechanisms at the plant have cut out, and some experts are very concerned it could cause a meltdown within hours.
The other plant in Onagawa (which was closest to the epicenter) fell victim to a fire in its turbine generator, which didn't pose a direct threat to the reactor, but it does generate more heat and steam, which could have potentially been a HUGE issue. Luckily, reports indicate the fire is out and that plant is safe for the time being.
While this nuclear power plant cooling failure could still be an issue, it looks like some anticipatory planning in previous years leading up to this disaster may help Japan dodge the bullet. Let's keep our fingers crossed.