122 Fission

Nuclear fission is a nuclear reaction in which a heavy atomic nucleus splits into two smaller nuclei. Along with the daughter nuclei, the reaction typically produces several free neutrons and a large amount of energy. This energy comes from the conversion of a small amount of mass into energy according to Einstein's equation, [latex]E=\Delta mc^2[/latex]. Because heavy nuclei are less tightly bound than the medium-mass nuclei produced during fission, the products have a greater binding energy per nucleon and therefore a lower total mass.

Unlike nuclear fusion, which combines light nuclei, fission breaks apart heavy nuclei such as uranium or plutonium. Controlled nuclear fission has been used for decades to generate electricity in commercial nuclear power plants around the world. These facilities provide a significant fraction of the world's low-carbon electrical energy and play an important role in the energy mix of many countries. Advances in reactor design continue to improve safety, efficiency, and waste management while reducing greenhouse gas emissions associated with electricity generation.

Fission releases energy because the nuclei produced are more tightly bound than the original heavy nucleus. As discussed in the previous section on binding energy, energy is released whenever a nuclear reaction produces nuclei with a larger binding energy per nucleon, [latex]\mathrm{BE}/A[/latex], than the starting nucleus.

Heavy nuclei with mass numbers near [latex]A\approx240[/latex] have binding energies of approximately 7.6 MeV per nucleon, whereas medium-mass nuclei near [latex]A\approx120[/latex] have binding energies closer to 8.6 MeV per nucleon. When a heavy nucleus splits into two medium-mass nuclei, the increase of about 1 MeV per nucleon corresponds to a total energy release of roughly 200–240 MeV for a single fission event. This is an enormous amount of energy compared with ordinary chemical reactions, which typically involve only a few electron volts per atom.

The energy released during fission appears primarily as the kinetic energy of the two fission fragments, with smaller amounts carried away by emitted neutrons and gamma rays. These neutrons are especially important because they can induce additional fission events in neighboring nuclei. If enough neutrons continue to produce new fissions, a chain reaction develops. The ability to control this chain reaction is what allows nuclear reactors to generate electricity safely and continuously.

Neutron-Induced Fission and Nuclear Chain Reactions

Although some heavy nuclei undergo spontaneous fission, this is usually a relatively rare decay mode. For example, uranium-238 can split spontaneously, but it is far more likely to decay by emitting an alpha particle. In practice, most nuclear reactors rely on neutron-induced fission, in which an incoming neutron is absorbed by a heavy nucleus and causes it to split.

Neutrons are especially effective at initiating fission because they carry no electric charge. Unlike positively charged particles, they are not repelled by the positively charged nucleus. Even slow-moving neutrons can approach a heavy nucleus closely enough for the attractive strong nuclear force to capture them.

A useful way to visualize this process is the liquid drop model of the nucleus. In this model, a large nucleus behaves somewhat like a vibrating liquid droplet. When a neutron is absorbed, it deposits energy into the nucleus, causing it to oscillate and become distorted. As the nucleus stretches, it develops a narrow "neck" between two larger regions. At this point, the attractive strong nuclear force holding the nucleus together weakens, while the electrostatic repulsion between the many protons becomes increasingly important. If the deformation becomes large enough, the nucleus separates into two smaller nuclei, releasing additional neutrons and a large amount of energy.

A general neutron-induced fission reaction can be written as

where [latex]\mathrm{FF}_1[/latex] and [latex]\mathrm{FF}_2[/latex] are the two fission fragments, and [latex]x[/latex] is the number of neutrons emitted. The two fragments usually have different masses rather than splitting exactly in half. Most of the released energy appears as the kinetic energy of the fission fragments, while the remainder is carried by the emitted neutrons and gamma rays.

The emitted neutrons are particularly important because they can trigger additional fission events. If each fission produces, on average, more than one neutron that successfully induces another fission, a self-sustaining chain reaction becomes possible.

A typical neutron-induced fission reaction is

As with every nuclear reaction, both electric charge and nucleon number are conserved. In this example, the total charge remains

and the total number of nucleons is also conserved:

Although these whole numbers balance exactly, the total mass of the products is slightly smaller than the mass of the reactants. This small mass difference is converted into the large amount of energy released during fission.

Critical Mass and Fission Fuels

Not every neutron released during fission causes another nucleus to split. Some neutrons escape from the material, while others are absorbed without producing fission. For a chain reaction to continue, enough neutrons must remain available to trigger additional fission events.

The smallest quantity of fissile material capable of sustaining a chain reaction is called the critical mass. Below the critical mass, too many neutrons escape and the reaction quickly dies out. Above the critical mass, enough neutrons remain inside the material for the chain reaction to continue.

Different isotopes require different critical masses because they differ in both the number of neutrons emitted during fission and the probability that an incoming neutron will produce another fission event. For example, plutonium-239 generally releases slightly more neutrons per fission than uranium-235 and therefore requires a smaller critical mass.

The isotopes [latex]{}^{235}\mathrm{U}[/latex] and [latex]{}^{239}\mathrm{Pu}[/latex] are much easier to fission than the much more abundant [latex]{}^{238}\mathrm{U}[/latex]. One reason is related to the number of neutrons in the nucleus. Uranium-235 and plutonium-239 each contain an odd number of neutrons, whereas uranium-238 contains an even number. When an additional neutron is absorbed by a nucleus with an odd number of neutrons, the resulting even-neutron nucleus gains extra binding energy, making it easier for the nucleus to deform and undergo fission.

Naturally occurring uranium contains about 99.3% uranium-238 but only about 0.7% uranium-235. Because uranium-235 is the isotope most useful for sustaining a chain reaction, reactor fuel is usually enriched to increase its concentration before use. Modern enrichment methods primarily use gas centrifuges, which are significantly more energy-efficient than the older gaseous diffusion process.

Another important property of uranium-235 is that it fissions most readily after absorbing slow (thermal) neutrons. However, the neutrons produced by fission are fast, with energies of roughly 1 MeV or more. Nuclear reactors therefore include a moderator, usually ordinary water, which slows the neutrons through repeated collisions with hydrogen nuclei. Once slowed, these thermal neutrons are much more likely to induce additional fission events in uranium-235, allowing a controlled chain reaction to continue.

Controlling Fission in a Nuclear Reactor

Nuclear reactors use control rods made from materials that readily absorb neutrons. By inserting or withdrawing these rods from the reactor core, operators adjust the number of neutrons available to produce additional fission reactions. This makes it possible to increase, decrease, or maintain the reactor's power output.

A reactor operates normally in a condition called criticality. In this state, each fission event causes, on average, one additional fission, so the reaction rate and power output remain approximately constant. The term critical does not mean that the reactor is in danger; it describes a stable, self-sustaining chain reaction.

If too few neutrons cause further fissions, the reactor is subcritical, and the chain reaction decreases. If more than one neutron from each fission produces another fission, the system is supercritical, and the reaction rate increases. Controlled changes in reactivity are part of routine reactor operation, but a rapid or unintended increase must be prevented.

In many reactors, water serves two important purposes. First, it acts as a moderator, slowing fast fission neutrons so that they are more likely to induce fission in uranium-235. Second, it acts as a coolant, carrying thermal energy away from the reactor core. Changes in the temperature and density of the water can also influence the rate of the chain reaction, providing an important form of feedback.

Stopping the fission chain reaction does not immediately eliminate all heat production. The fission fragments created during reactor operation are radioactive and continue to decay after the control rods have been inserted. This decay heat decreases with time but can initially remain large enough to damage the fuel if cooling is lost. For this reason, reactors include emergency cooling systems, backup power supplies, reinforced containment structures, and other safety systems designed to remove residual heat during a shutdown or loss-of-coolant accident.

Breeding Fissionable Fuel

Plutonium-239 is another important fissile isotope. It has a half-life of approximately 24,100 years and is produced artificially from uranium-238 inside nuclear reactors. This process makes it possible to convert a nuclide that does not readily undergo fission with thermal neutrons into one that can serve as reactor fuel.

The production of a fissile nuclide from another nuclide is called breeding. The production of plutonium-239 begins when uranium-238 captures a neutron:

The uranium-239 nucleus is radioactive and undergoes beta-minus decay:

During beta-minus decay, a neutron in the nucleus changes into a proton, so the atomic number increases from 92 to 93 while the mass number remains 239. The resulting nucleus is neptunium-239.

Neptunium-239 then undergoes another beta-minus decay:

This second decay increases the atomic number from 93 to 94, producing plutonium-239. The complete breeding sequence can therefore be summarized as

Plutonium-239 accumulates in uranium-based reactor fuel because uranium-238 nuclei absorb some of the neutrons produced during fission. Some of the resulting plutonium-239 later undergoes fission and contributes to the reactor's energy output.

A breeder reactor is designed to produce fissile material from more abundant nonfissile, or fertile, isotopes. In principle, breeding can make more efficient use of natural uranium because uranium-238 represents nearly all naturally occurring uranium. Breeder systems, however, require careful fuel processing, reactor control, waste management, and safeguards because they produce and handle materials with significant radiological and security concerns.

Plutonium-239 has several properties that make it useful as a reactor fuel. It can undergo fission after absorbing a thermal neutron, and each fission releases enough neutrons to support a chain reaction. It is also chemically different from uranium, which allows the two elements to be separated through chemical processing of used reactor fuel.

Interactive Exploration: Nuclear Fission

Nuclear fission occurs when a heavy nucleus absorbs a neutron and splits into two smaller nuclei. This process releases a large amount of energy along with two or three additional neutrons. If these neutrons induce fission in nearby nuclei, the process can continue as a chain reaction. Whether the reaction dies out, remains steady, or grows rapidly depends on how many neutrons continue to produce additional fission events.

In this interactive simulation, you will investigate how chain reactions begin, why a critical amount of fuel is required, and how nuclear reactors safely regulate fission. You will also explore the effects of neutron-absorbing control rods and non-fissionable materials on the behavior of the chain reaction.