How a nuclear reactor works: fission, chain reactions and control

Fission in one paragraph

When a uranium-235 nucleus absorbs a slow neutron it becomes unstable and splits into two lighter nuclei, the fission fragments. Each fission releases about 200 MeV of energy — tens of millions of times more than burning a carbon atom — plus two or three new neutrons. Most of that energy appears as the motion of the fragments, which stop within the fuel pellet and heat it. The coolant carries the heat away to make steam or to supply industrial heat.

Because each fission releases more than one neutron, a chain reaction is possible. Some neutrons leak out of the core or are captured without causing fission; the design goal is for, on average, exactly one neutron from each fission to cause another. That balance is measured by the effective multiplication factor, k-eff.

Moderator, coolant and control

Fission neutrons are born fast. In a thermal reactor a moderator — ordinary water, heavy water or graphite — slows them down so that U-235 captures them more readily. In the light-water reactors that make up the US fleet, the same water is both moderator and coolant. Fast reactors skip the moderator entirely and use a higher fissile content instead.

Operators and automatic systems control reactivity with neutron absorbers: control rods containing boron, cadmium or hafnium, and in pressurized-water reactors, boric acid dissolved in the coolant. A small fraction of fission neutrons are delayed by seconds to minutes, which stretches the neutron generation time enough for power to change slowly and controllably. Well-designed cores also have negative temperature feedback: if they heat up, reactivity falls.

Why decay heat matters

Shutting a reactor down stops the chain reaction but not the radioactive decay of the fission products. Decay heat starts at about 6 to 7 percent of the previous power and falls to around 1 percent within hours. Removing it reliably — with pumps and diesel generators in older designs, or with passive natural circulation and large water pools in many SMRs — is the central safety task after shutdown.

Sample quiz

Practice questions for this section are being reviewed.

More in this topic

• Reactor types
• SMR concepts
• Advanced designs & fuels
• Types of radiation
• Radioactive decay
• Units & dose quantities
• Dose limits & health effects
• Shielding & ALARA
• US regulatory framework

Frequently asked questions

What does it mean when a reactor is "critical"?

Critical means the chain reaction is exactly self-sustaining (k-eff = 1), so power stays constant. It is the normal operating state and does not mean something has gone wrong.

Why can't a power reactor explode like a bomb?

Power reactor fuel is low-enriched (typically 3 to 5 percent U-235, or up to 20 percent HALEU), and physical feedback such as negative temperature coefficients limits power excursions. A nuclear explosion needs highly enriched material assembled very rapidly, which a power reactor cannot do.

What is the difference between fissile and fertile material?

Fissile nuclides (U-235, Pu-239, U-233) can sustain a chain reaction with slow neutrons. Fertile nuclides (U-238, Th-232) cannot, but they turn into fissile ones after absorbing a neutron.

How much natural uranium is U-235?

About 0.7 percent. The rest is almost entirely U-238, which is why light-water reactor fuel is enriched.

Sources

1. DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory, DOE-HDBK-1019/1-93 and DOE-HDBK-1019/2-93
2. U.S. NRC Glossary (NRC Basic References) www.nrc.gov
3. U.S. Energy Information Administration, Nuclear explained www.eia.gov
4. U.S. DOE Office of Nuclear Energy, What is High-Assay Low-Enriched Uranium (HALEU)?

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