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Explaining Japan's Nuclear Crisis

A disaster unfolding at the Fukushima nuclear power plant includes four reactors with major problems and two without.

photo of Amy Harder
March 17, 2011

In light of Japan’s nuclear crisis, the entire world is either getting an impromptu nuclear physics lesson or completely confused about what’s going on inside the four rocked reactors at the Fukushima Daiichi plant. More accurately, it’s probably both.

Here’s a primer on what is—and is not—going on with the reactors (with the understanding that major developments are happening constantly).

The disaster unfolding at the plant, which includes four reactors with major problems and two without, is at the start of the nuclear power generation cycle and at the end. Partial nuclear meltdowns and coinciding explosions have occurred in at least two of the reactors after the double whammy of the earthquake and tsunami knocked out the plant’s electricity. Those problems have arisen with the actual generation of the nuclear power. The news of the day Wednesday focused on problems with the fourth reactor’s spent-fuel pool, the nuclear waste left over once power has been generated.

 

The terms nuclear meltdown and spent fuel—and the dozens of others also cropping up in media reports—can be complex on paper and even more confusing to imagine. But one basic point is that the most important piece of the puzzle is water. And in Japan’s case, that piece of the puzzle has been missing.

What’s a meltdown?

Former Nuclear Regulatory Commission Chairman Dale Klein described a nuclear meltdown like a car accident: It can be as minor as a fender bender or as major as a head-on collision. Three of the four damaged reactors at Japan’s plant have experienced what can best be described as “partial meltdowns.” A nuclear meltdown is a bad thing, to be sure, but it’s bad only if the barriers encasing the reactor’s core break down. That has appeared to have happened in at least two of the four crippled reactors, which caused explosions.

A nuclear meltdown occurs within a reactor’s core when pellets about the size of a pencil eraser containing the radioactive material heat up, melt, and form a molten blob. The pellets are inside thousands of 10-feet long, pencil-thin, shiny metal tubes called fuel rods. And those fuel rods make up the reactor’s core.

An explosion occurs if that molten blob—which includes hydrogen—escapes some or all of the reactor’s barriers and combusts with air. The reactors at Japan’s plant have three major barriers.

So what causes a reactor’s core to heat up in the first place? The lack of water moving around the reactor’s core. In Japan’s case, the earthquake and tsunami knocked out electricity and prevented the plant from using its backup diesel generators to ensure water can keep getting pumped into the core to keep the rods cool.

In a simplified analogy, producing nuclear power is like boiling water on a hot stove, where the flame underneath the tea kettle is the source of energy. In a normal scenario water boils and steam goes out of the kettle. At a normal nuclear plant, nuclear fission in the fuel is the source of energy that heats the water to produce steam, which generates electricity.

But even if the nuclear plant stops, the radioactive material within the nuclear reactor’s core continues to produce energy and requires water to cool it down. “They [fuel rods] have a mind of their own,” said Gilbert Brown, a nuclear engineering professor at the University of Massachusetts. “They’re still producing heat.” It’s like a tea kettle on a stove where the flame is only slowly turned down, but not entirely off after you tried to turn it off.  In normal generation of nuclear power, “you constantly have to be putting water in and taking energy out” to cool down the fuel, Brown said. But if there is no water (because there is no electricity) that’s not an option and the fuel can become dangerously overheated. 

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Infographic

 

What’s the spent-fuel pool?

It’s essentially a massive swimming pool full of water and spent fuel enclosed in a concrete and steel container. “It’s a very robust swimming pool,” Klein said. In Japan’s case, the container was elevated next to the reactors, Klein said. Current NRC Chairman Greg Jaczko told lawmakers Wednesday that he believed the pool had run dry of water and was left with the exposed fuel rods. Klein and other industry experts who spoke with National Journal Wednesday said the reports are mixed. But the consequences of losing water in a spent pool are bad, Klein said.

“If you lose water then the spent-fuel pools are outside containment and so you could have radioactive release if you have a problem with losing water in those pools,” Klein said. “I think the worst-case scenario is the inability to keep the spent-fuel pool covered with water.”

How does it stack up to other nuclear disasters?

“I would imagine that after everything stabilizes this will be more significant than Three Mile Island,” Klein said, referencing the partial meltdown that occurred at a Pennsylvania plant in 1979. The Three Mile Island disaster resulted from reactor malfunctions and operator error, while Japan’s crisis stems from just not being prepared to respond to the natural disaster double-header. “I think they know what they want to do, it’s not having the ability to do it,” Klein said.

The Chernobyl disaster in the former Soviet Union, considered the worst nuclear disaster in history, was vastly different and Japan’s crisis isn’t likely to rise to that level, Klein and other experts said. That reactor didn’t have any major secondary containment, which allowed radioactive material to be released into the atmosphere freely after an explosion.

The big picture

The reactors at the Japanese plant were built by General Electric and faced what the nuclear power industry describes as a “beyond-design” catastrophe. In other words, the double whammy of an earthquake and a tsunami larger than anticipated was not something the reactor design was capable of withstanding.

Radioactive material that has been intermittently released from the crippled reactors is not in and of itself a great cause of public concern, experts say. It depends on whether the material goes anywhere other than right near it and how much is released. “Just because there is radiation, unless you’re sitting in a hot air balloon over the top of the building, you’re probably not going to get radiated,” Brown said.

But scientific assessments aside, the idea of radiation is not a good one and that’s what got the entire world stirred up.

“You would not know you were being radiated,” said Arthur Motta, chair of the Nuclear Engineering program at Penn State University. “That’s the scary part. I could be sitting here and I wouldn’t know it.”

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