Nuclear Fission: Release of Prodigious Amounts of Energy


By Robert Hazen, Ph.D.George Mason University

In a process known as nuclear fission, some heavy isotopes, most notably uranium-235 and plutonium-239, release very high amounts of energy when they split apart. In most cases, fission is triggered by a collision with a particle such as a neutron. Uranium-235 is a very important example of this process.

An image of a nuclear power plant
Nuclear power plants use nuclear fuel to produce energy. (Image: Meryll/Shutterstock)

Radioactive Isotopes in the Periodic Table

The uranium-235 isotope can be split apart when a neutron comes along and hits that nucleus, and 235 just spontaneously breaks apart. The products from that original nucleus are two large chunks of roughly equal size, plus you produce energetic neutrons that carry energy away; fast-moving neutrons then turn into heat, as they collide with other particles. The products of the original nucleus also include two large chunks, which are essentially isotopes themselves. 

They’re new isotopes that are very commonly radioactive. If you look at the periodic table, you can sort of track where some of these are. For example, on the left side of the periodic table, there is strontium-90, a radioactive isotope, a member of the alkaline earth group. If you look at the right side of the periodic table, you see other radioactive isotopes produced by uranium-235 fission; these include iodine-129. 

Krypton is one of the inert gases, so krypton radioactive atoms go flying off into the atmosphere as isolated, individual radioactive atoms. The energetic neutrons that are produced by one of these reactions are available to split other uranium-235 isotopes, so if you have a high enough concentration of uranium-235, you get a chain reaction. Each fission reaction triggers new reactions in the neighboring 235 atoms, and you can have a chain reaction in this way.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

How Nuclear Reactors Work 

An image of the fragment fuel of a nuclear reactor
Nuclear reactors can control the chain reaction caused by nuclear fission. (Image: Parilov/Shutterstock)

A nuclear reactor is nothing more than a device that controls this nuclear-fission chain reaction. It’s basically a big machine that converts one kind of energy into other kinds of energy. In modern commercial reactors, uranium fuel is used. Sometimes that fuel is just normal uranium out of the ground, and sometimes it’s enriched in uranium-235. It’s shaped into long, pencil-thin rods, and those rods are then encased in zirconium metal. 

The metal casing prevents new radioactive isotopes from escaping into the environment. The fuel rods are arranged in a regular pattern, so they can irradiate each other with neutrons, and a typical nuclear power plant may have several hundred, even a few thousand of these individual fuel rods that are arranged in easily installed units. 

There’s a fluid, either normal water or heavy water—that is, deuterium water, water with the deuterium isotope of hydrogen—that surrounds the fuel. This water absorbs some of that energy from the neutrons, and the water gets very hot. There are also, in many nuclear reactors, graphite control rods that can absorb neutrons and be used to shut down the reaction if you want to cool things off.

Learn more about nuclear fission and fusion reactions.

Nuclear Power Plant Disasters

In 1979, Three Mile Island, near Harrisburg, Pennsylvania, had a partial meltdown. This is a case when reactor water escaped from the core, so the core kept getting hotter and hotter, but the computer test systems failed. The result was that the whole inner core became unusable. Fortunately, there was very little—if any—radioactive leak, so this was not a danger to the surrounding community.

A much more serious accident occurred in 1986, in Chernobyl, in Ukraine. There was a complete meltdown there; it produced incredibly high temperatures, and the confinement structure blew up. There were huge fires and release of huge amounts of radioactive elements into the air that were carried downwind, contaminating hundreds of square miles with intense amounts of radioactivity. Many people died as a result of radiation poisoning and many more became sick. It devastated the ecosystem in that area.

Natural Nuclear Fission

Under unusual natural circumstances, there might be hot water deep in the Earth that dissolves uranium atoms out of the rock and concentrates them, and then they can be re-deposited in a uranium-rich ore deposit. Billions of years ago, when uranium-235 was many times more concentrated than it is now, it turns out that a few of these deposits actually became natural fission reactors in the ground. 

That is, the neutrons emitted by one 235 actually triggered the decay of other uranium-235 in a sustained nuclear reaction. We now know of 17 separate natural-reactor sites near Oklo and Bangombe, in equatorial West Africa, in the Republic of Gabon. These were discovered in 1972, quite by accident, when uranium ore was shipped from these mines; and when people analyzed the ore, they said, these have been depleted in uranium-235, which is the valuable material. 

Photo of the explosion during a nuclear test.
Only a small amount of concentrated uranium-235 can cause an atomic explosion. (Image: United States Department of Energy/Public domain)

Upon further examination, it was realized that the uranium-235 was depleted because these had been natural reactors that reacted for perhaps 100,000 years, raising the local temperature by 350 degrees centigrade for all that time.

Learn more about atoms.

How Atomic Bombs Work

If nuclear fuel, such as uranium-235, is sufficiently concentrated, you can get a chain reaction that proceeds faster and faster in an uncontrolled explosion. That’s the way atomic bombs work. They use a few pounds of concentrated uranium-235, or plutonium-239, typically in a grapefruit-size mass, and you can split that mass into several pieces. 

Then that charge is surrounded by precisely shaped conventional explosives, like a plastic explosive. When that conventional explosive is fired, it mashes together all the fissionable material, and so you get a critical mass, and suddenly it explodes in an atomic bomb explosion. That uncontrolled chain reaction releases heat, light, radiation, and also huge amounts of radioactive isotopes from the split uranium atoms.

Common Questions about Nuclear Fission and Release of Prodigious Amounts of Energy

Q: What is nuclear fission? 

Some heavy isotopes, most notably uranium-235 and plutonium-239, release very high amounts of energy when they split apart. This process is known as nuclear fission.

Q: What’s the function of a nuclear reactor? 

The most important function of a nuclear reactor is to control the chain reactions caused by nuclear fission. A nuclear reactor is actually a massive machine that converts energy into other types of energy.

Q: How many natural fission reactors are currently known?  

We know of 17 separate natural-reactor sites near Oklo and Bangombe, in equatorial West Africa, in the Republic of Gabon. These were discovered in 1972.

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