Science 101: How a Chain Reaction Works in a U.S. Nuclear Reactor

Paul Rebstock
Senior Instrumentation and Control Systems Engineer


science_101_squeakychalkThe primary active ingredient in nuclear reactor fuel is a particular variety, or “isotope,” of uranium, called U235. U235 is relatively rare — only about 0.7% of uranium as it exists in nature is U235. Uranium must be enriched to contain about 5% U235 to function properly as fuel for a U.S. commercial nuclear power plant.

U235 has 92 protons and 143 neutrons. Protons and neutrons are some of the almost unimaginably tiny particles that make up the nucleus of an atom — see Science 101 Blog #1. All other isotopes of uranium also have 92 protons, but different isotopes have slightly different numbers of neutrons.

Uranium is a radioactive element. Uranium atoms break apart, or disintegrate, into smaller atoms, releasing energy and a few leftover neutrons in the process. This happens very slowly for U235. If you have some U235 today, in about 700 million years you will have only half as much. You will have the remaining U235, plus the smaller atoms. The energy released will have gone into the environment too slowly to be noticed, and the extra neutrons will have been absorbed by other atoms.

While this happens very slowly, the disintegration of each individual atom happens very quickly, and the fragments are ejected at a very high speed. Those high-speed fragments are the source of the heat generated by the reactor. Under the right man-made conditions, the number of U235 atoms that disintegrate each second can be increased.

When a U235 atom disintegrates, it releases some neutrons. Some of those neutrons can be made to interact with other U235 atoms, causing them to disintegrate as well. Those “target” atoms release more neutrons when they disintegrate, and then those neutrons interact with still other U235 atoms, and so on. This is called a “chain reaction.” This process does not work well for other isotopes of uranium, which is why the uranium needs to be enriched in U235 for use as nuclear fuel.

Most of the energy released when a U235 atom disintegrates is in the form of kinetic energy — the energy of physical motion. The fragments of the disintegrated atom collide with nearby atoms and set them vibrating. That vibration constitutes heat. The fuel rods get hot as the reaction progresses. The faster the chain reaction — that is, the larger the number of U235 atoms that disintegrate each second — the faster energy is released and the hotter the fuel rods become.

The uranium in a U.S. commercial nuclear reactor is thoroughly mixed with neutral material and formed into pellets about half inch wide and three-quarters of an inch long. The pellets are stacked tightly in metal tubes, forming “fuel rods” that are several feet long. Each fuel rod is just wide enough to hold a single column of pellets. The fuel rods are sealed, to keep all of the radioactive materials inside. There are thousands of these fuel rods in a typical reactor. They contain around 60 tons of uranium – but only about three tons are U235. (The majority of the uranium in the reactor is in the form of the most abundant naturally occurring isotope of uranium, U238, which cannot sustain the fission process without the help of an elevated concentration of the isotope U235.)

The people in charge of the reactor can control the chain reaction by preventing some or all of the released neutrons from interacting with U235 atoms. The physical arrangement of the fuel rods, the low U235 concentration, and other design factors, also limit the number of neutrons that can interact with U235 atoms.

The heat generated by the chain reaction is used to make steam, and that steam powers specialized machinery that drives an electrical generator, generating electricity. Science 101 will look at how that works in more detail in a later issue.

The author has a BS in Electrical Engineering from Carnegie-Mellon University.


Author: Moderator

Public Affairs Officer for the U.S. Nuclear Regulatory Commission

5 thoughts on “Science 101: How a Chain Reaction Works in a U.S. Nuclear Reactor”

  1. Aye Captain, the airborne monitoring also shows increased radiation readings after earthquakes 4 and larger in the Fukushima area. I have coined this the “Shake and Bake” effect of joustling the coriums. Admittedly, for me it is a tough one to model. But it should be modeled to determine risk and costs, as compared to the herculean task of “Lift and Seperate” the coriums. Especially important and in light of just week, Oak Ridge research proving beyond a doubt that elements can reorder themselves even in a solid matrix (like a mostly cooled corium) and that heavy elements in particular, like plutonium, are even betteer at moving in solids than lighter elements. Beside the gravity weight separation from beingin a pretty fluid state at the initial meltdown, and subsequent heavy element reordereding, what are the risks that a high enough mass of plutonium is shoved together with another mass pf mostly plutonium during a shake and bake earthquake, and the whole kit and kaboodle goes prompt critical sky high.

    They are just beginning to understand some of this from the WIPP radiation explosion, but the risks from out of vessel coria are dramatically higher.

  2. I’d like to see the NRC post something about the spontaneous fissioning like those that are occurring at Fukushima, due to the corms interactions with ground/seawater water, which I call The Fuky Effect (which is the on-again, off-again fissioning of one of more of the corium(s), as they interact with water below Fukushima).

  3. Thanks for your comment. We’ve revised the post to specify that fuel must be enriched for use in current U.S. commercial nuclear power plants. As for your other comment, yes, all Uranium isotopes decay (the definition of radioactive). I used “disintegrate” as a reasonable synonym for “fission” in this post, which was intentionally written at a middle-school level. For that reason, it does leave out some of the finer points that experts know.

    Paul Rebstock

  4. This piece contains an egregious error in the second sentence, “Uranium must be enriched to contain about 5% U235 to function properly as fuel for a nuclear power plant.”. As most students of nuclear engineering surely know, reactors can be designed to operate successfully using natural uranium fuel, e.g, the first reactor built by Fermi, the early production reactors at Hanford and the CANDU reactors in Canada and elsewhere. The piece is also confusing in its reference to uranium being a “radioactive element” and references to “disintegration” in its failure to distinguish between radioactive decay, spontaneous fission and and the fission chain reaction in a reactor.

    Who writes this stuff? Who at NRC reviews it before posting?

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