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And Richard mentioned spent fuel. Some of the problems at the Fukushima Daiichi plant appear to be related to at least one of its spent fuel storage pools. Spent fuel is highly radioactive.

NPR science correspondent Joe Palca explains how nuclear power plants normally deal with this substance.

JOE PALCA: The fuel in nuclear reactors contains material that can undergo something called nuclear fission.

Professor FRANCIS LIVENS (Research Director, Dalton Nuclear Institute, University of Manchester): Most of the fuel starts out as a uranium material.

PALCA: Francis Livens is research director of the Dalton Nuclear Institute at the University of Manchester.

Prof. LIVENS: And that's pressed into little pellets. It's like pottery. And you put it into long, metal tubes, which are sealed - are called the fuel pins.

PALCA: These fuel pins are also called fuel rods. Assemblies of these rods are what's inside the reactor core. In a fission reaction, a uranium atom splits apart, releasing a lot of energy in the process. That energy, in the form of heat, is what makes the steam that powers the turbines that makes the electricity. But as the fission reaction proceeds, the uranium fuel gets used up.

Prof. LIVENS: There comes a point when actually, the fuel becomes inefficient.

PALCA: When that happens, plant operators turn off the fission reaction, and then take the spent fuel out of the reactor.

Prof. LIVENS: It's thermally very hot when it comes out of the reactor.

PALCA: And to cool it off, it's put into a large pool of water, called a spent fuel pool.

Dr. CHARLES FORSBERG (Nuclear Fuel Cycle Project Executive Director, MIT): The water does two things. The water provides cooling but the other thing it does, it also provides radiation shielding.

PALCA: Charles Forsberg is executive director of the MIT nuclear fuel cycle project.

The shielding the water provides is important because not only are these spent rods thermally hot, they're also very radioactive.

Now, you may be thinking, wait a minute: I thought you needed lead or something like that to protect you from something that's highly radioactive. Not so, says Forsberg.

Dr. FORSBERG: Water's a perfectly fine shield and it has, of course, the advantage, from a practical point of view, is you can see through it.

PALCA: Once the fuel rods are in the pool of water, their radioactivity begins to drop.

Dr. FORSBERG: The radioactivity drops very dramatically in the first couple of days, and then it goes down by another factor of a hundred over the next several years.

PALCA: The concern in Japan is that the water levels in the spent fuel pools at the Fukushima power plant may be dropping. That's not good. Remember, the fuel is contained in metal rods. If they get too hot, the metal tube could melt, letting out the highly radioactive material inside.

Dr. FORSBERG: So the name of the game is to get a fire hose on top of it, and make sure you have water on top.

PALCA: Forsberg says it doesn't matter what kind of water you use. Since the damaged reactor in Japan is near the ocean, there's plenty of water at hand.

The spent fuel will typically sit quietly in these pools for a couple of years. Once they become less radioactive, and less hot, they can either be shipped to a disposal site or, in the case of the Japanese power plant, shipped to a reprocessing plant.

Francis Livens says it's unusual for these spent fuel pools to make news headlines.

Prof. LIVENS: Normally, nuclear fuel storage ponds are very quiet and very boring places, actually.

PALCA: Until things stabilize at the Fukushima plant, it's hard to say when or if the pools there will once again become quiet and boring.

Joe Palca, NPR News, Washington.

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