Nuclear Fission: Splitting Heavy Nuclei for Energy

What fission is

Nuclear fission is the splitting of a heavy atomic nucleus into two lighter nuclei. It happens when a neutron strikes a nucleus of a suitable heavy isotope — most famously uranium-235 — making the nucleus wobble, stretch, and finally break apart.

The break is never clean down the middle. The nucleus splits into two unequal fragments, releases two or three free neutrons, and gives off a burst of energy as gamma rays and as the kinetic energy of the fragments. Each of the released neutrons can go on to strike another uranium-235 nucleus, which is the seed of the chain reaction.

The uranium-235 fission equation

A representative fission of uranium-235 is written as:

$$^{235}_{92}U + ^{1}_{0}n \rightarrow ^{141}_{56}Ba + ^{92}_{36}Kr + 3\,^{1}_{0}n$$

Check the balance: mass numbers $235 + 1 = 236$ on the left, and $141 + 92 + 3 = 236$ on the right ✓. Atomic numbers $92 = 56 + 36$ ✓ (the three neutrons carry no charge).

This is one illustrative outcome; real fission produces a spread of different fragment pairs. Barium-141 and krypton-92 are the textbook example because the arithmetic is clean and the pair is a common one. The fission fragments themselves are usually radioactive and decay further down their own decay chains.

Chain reactions and critical mass

Because each fission releases neutrons that can cause further fissions, one event can trigger a self-sustaining sequence: a chain reaction. Whether the chain grows, holds steady, or dies out depends on what happens to the neutrons.

• If, on average, fewer than one neutron per fission causes another fission (neutrons escape or are absorbed without effect), the chain fizzles out.
• If exactly one neutron per fission continues the chain, the reaction runs at a steady rate — this is the condition maintained inside a power reactor.
• If more than one neutron per fission continues the chain, the rate multiplies rapidly — the uncontrolled case.

Two ideas follow from this. Critical mass is the minimum amount of fissionable material needed for a sustained chain, because in too small a lump most neutrons escape through the surface before hitting another nucleus. Moderators are materials, such as water or graphite, that slow neutrons down; slow neutrons are far more likely to cause fission in uranium-235, so reactors surround the fuel with a moderator.

How a nuclear power plant works

A nuclear power plant is, at heart, a fancy way to boil water. Its main parts each serve the chain reaction or the heat:

• Fuel rods contain uranium (enriched to a few percent uranium-235) formed into pellets. This is where fission happens.
• Moderator and coolant (often ordinary water) slow the neutrons and carry heat away from the fuel.
• Control rods made of neutron-absorbing material slide in and out to tune the reaction rate.
• Heat exchanger and steam system: the hot coolant makes steam, the steam spins a turbine, and the turbine drives a generator that produces electricity.

The reactor vessel itself sits inside thick concrete and steel shielding that absorbs radiation and contains the fuel. Nothing about the electricity is special — a coal plant's turbine works the same way; only the heat source differs.

Where the energy comes from

The energy released in fission comes from a small loss of mass. The fission fragments and neutrons together weigh slightly less than the original nucleus plus the incoming neutron, and the missing mass becomes energy via $E = mc^2$. Since $c^2$ is enormous, converting even a tenth of a percent of the mass releases a tremendous amount of energy per atom — millions of times more than any chemical reaction.

Physicists describe the same idea with binding energy: the energy that holds a nucleus together. Medium-weight nuclei (around iron) have the highest binding energy per nucleon, so splitting a very heavy nucleus into medium-weight fragments moves toward a more tightly bound arrangement, and the difference is released as energy.

Waste and a note on scope

Fission has two practical footnotes worth knowing honestly. First, the fission fragments and some activated materials in the reactor are highly radioactive. Spent fuel is first cooled in deep water pools, then typically moved to massive steel-and-concrete dry casks; because some components remain hazardous for thousands of years, long-term geological storage is the accepted long-range plan.

Second, fission also underlies nuclear weapons through an uncontrolled chain reaction. That engineering is deliberately not covered here — this article addresses only the physics of fission and its peaceful applications in power generation, medicine, and research.

Variables and units

Common mistakes

Real-world applications

• Electricity generation in nuclear power plants using controlled fission

• Production of medical radioisotopes in research reactors

• Naval propulsion in submarines and aircraft carriers

• Scientific study of nuclear structure and decay series