Why can't nuclear reactor fuel explode like a bomb?

Reactor fuel cannot explode like a bomb because it contains far too little of the atom that makes bombs work. Uranium comes from the ground as mostly uranium-238, which does not sustain a chain reaction in ordinary reactors. Only about 0.7 percent is uranium-235, the fissile kind. Power stations need that share raised to a few percent, and that process is called enrichment.

Think of it as diluted versus concentrated. A typical reactor fuel holds about 3.6 percent uranium-235. Low-enriched uranium, anything under 20 percent, powers most of the world's reactors. Highly enriched uranium, 20 percent or more, has been used in weapons, naval reactors and some research reactors. Weapons-grade uranium usually holds 85 percent or more, and the Hiroshima bomb, Little Boy, used 64 kilograms of uranium enriched to 80 percent.

Weapons and power plants are also built to behave in opposite ways. Weapons are engineered to produce a reaction so fast and intense that it cannot be controlled once it starts. A reactor holds a slow, steady reaction, with fuel that is a low-enriched oxide such as uranium dioxide. A power plant cannot produce a blast comparable to a nuclear weapon, but it can still suffer other explosions: Chernobyl's runaway reaction ended in a low-powered steam explosion, and at Fukushima hydrogen gas exploded after the reactors had been shut down.

Enriching is hard because the two kinds of uranium are almost identical chemically. Only a tiny difference in weight sets them apart. Modern plants turn uranium into a gas and spin it in fast centrifuges, which separate the slightly heavier molecules from the lighter ones. That same skill is why enrichment is watched so closely, and why critics of nuclear power raise proliferation as a concern. The technology is shared, but the target percentage is not.