Explainer: What Are Spent Fuel Rods? During a nuclear reaction, fuel rods generate a tremendous amount of heat. After most of the fuel has been used, the rods are removed from the reactor and kept in a separate cooling pool nearby. Problems cooling these pools have officials worried that the spent rods could overheat and melt.

Explainer: What Are Spent Fuel Rods?

Explainer: What Are Spent Fuel Rods?

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Inside The Nuclear Reactors

Some of the problems afflicting the Fukushima Daiichi nuclear power plant appear to be related to one or more of the spent fuel storage pools at the plant. In Japan on Tuesday, a fire broke out in the No. 4 reactor building near an area where spent fuel is stored. The material is still highly radioactive, and all nuclear power plants have facilities to deal with this material.

First, some background on how nuclear reactors work.

The fuel in a nuclear reactor contains material that can undergo something called nuclear fission.

"Most of the fuel starts out as uranium material," says Francis Livens, research director of the Dalton Nuclear Institute at the University of Manchester. In the Japanese plants, the uranium material is uranium oxide that's pressed into little pellets. "It's kind of like pottery, and you put it into long metal tubes, which are sealed, and those tubes are called the fuel pins."

These fuel pins are also called fuel rods.

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. "There comes a point when actually, the fuel becomes inefficient," says Livens.

When that happens, plant operators use control rods to turn off the fission reaction, and then they take the spent fuel out of the reactor. When the pins come out, Livens says, they are hot. But the heat is not just lingering heat from the fission reaction.

"These are radioisotopes that come from the products of the fission," says Jeffrey Binder, director of the fuel cycle and isotope program at the Department of Energy's Oak Ridge National Laboratory.

These fission byproducts are also very radioactive, but they're not permanently radioactive.

"The radioactivity drops very dramatically over the first couple of days, and then it goes down by a factor of a hundred over the next several years," says Charles Forsberg, executive director of the MIT nuclear fuel cycle project. He says to cool the fuel rods that have come out of a reactor, they're submerged in water in what's called a spent fuel pool. "The water does two things — the water provides cooling, but the other thing it does, it also provides radiation shielding."

Why Water Is So Key

The concern in Japan is that the water levels in the pools may be dropping. That's not good because these spent fuel rods are still producing heat.

"If you have any kind of heat source — it doesn't matter what the heat source is — take a look at a burner on a stove with a teapot," says Forsberg. "If you don't have any cooling, and you let the teapot evaporate dry, it ruins the teapot."

In the case of nuclear fuel, it would be the metal surrounding the fuel that would melt, letting out the highly radioactive material inside. And some of that might come out as radioactive gas. So the name of the game, says Forsberg, is to keep the rods covered with water. Forsberg says it doesn't matter what kind of water you use. Since the damaged reactor is near the ocean, there's plenty of water at hand.

The spent fuel rods 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 Japanese power plant, shipped to a reprocessing plant.

Livens says it's unusual for these spent fuel pools to make news headlines. "Normally nuclear fuel storage ponds are very quiet, very boring places actually," he says.