32.6 Fission

Nuclear Fission

Nuclear fission is the splitting of heavy atomic nuclei, releasing enormous amounts of energy and additional neutrons. This process is the basis for nuclear reactors (controlled energy production) and nuclear weapons (uncontrolled energy release). Understanding fission connects directly to how nuclear technology shapes modern energy production, medical isotope generation, and global security.

Process of Nuclear Fission

Fission occurs when a neutron strikes a heavy, fissile nucleus like $^{235}U$ or $^{239}Pu$. The nucleus absorbs the neutron, becomes unstable, and splits into two lighter nuclei called fission fragments. Here's the step-by-step process:

1. A free neutron approaches and is absorbed by a fissile nucleus (e.g., $^{235}U$).
2. The nucleus becomes excited and deforms, stretching into an elongated shape.
3. The nuclear forces can no longer hold the deformed nucleus together, and it splits into two lighter nuclei (fission fragments).
4. The split releases 2–3 additional prompt neutrons along with gamma radiation.
5. Those prompt neutrons can then collide with other nearby fissile nuclei, potentially triggering more fission events.

Key components involved:

• Fissile material: A nucleus capable of undergoing fission when it absorbs a neutron ($^{235}U$, $^{239}Pu$)
• Prompt neutrons: The extra neutrons released during fission that make chain reactions possible
• Energy output: Released as kinetic energy of the fission fragments and as gamma radiation

Products of Fission Reactions

A single fission event produces several things at once. A common example is the fission of $^{235}U$:

$^{235}U + n \rightarrow ^{141}Ba + ^{92}Kr + 3n$

Barium-141 and krypton-92 are the fission fragments here, along with 3 neutrons. The exact fragments vary from event to event, but the general pattern holds.

Energy released per fission event: approximately 200 MeV.

• About 168 MeV appears as kinetic energy of the fission fragments (they fly apart at high speed).
• The remaining energy is carried by prompt gamma rays and the kinetic energy of the released neutrons.

This energy comes from a real loss of mass. If you add up the masses of all the products, they weigh slightly less than the original uranium nucleus plus the incoming neutron. That "missing" mass has been converted to energy according to Einstein's equation $E = mc^2$. Even a tiny amount of mass converts to a huge amount of energy because $c^2$ (the speed of light squared) is such an enormous number.

Another way to think about it: the binding energy per nucleon is higher for the medium-mass fission fragments than for the original heavy nucleus. That difference in binding energy is what gets released.

Controlled vs. Uncontrolled Chain Reactions

Because each fission event releases 2–3 neutrons, those neutrons can trigger additional fission events, which release more neutrons, and so on. This is a chain reaction. Whether it's useful or destructive depends entirely on whether the reaction rate is regulated.

Controlled chain reaction (nuclear power plants):

• The goal is to keep exactly one neutron per fission event going on to cause another fission, maintaining a steady reaction rate.
• Control rods (made of neutron-absorbing materials like cadmium or boron) are inserted into or withdrawn from the reactor core to absorb excess neutrons.
• A moderator (often water or graphite) slows neutrons down. Slower neutrons are more likely to be absorbed by $^{235}U$ and cause fission, making the reaction more efficient.
• The result is a steady, sustained energy output that can be used to generate electricity.

Uncontrolled chain reaction (nuclear weapons):

• No control mechanisms limit the neutron population.
• Each fission event triggers multiple new fission events, and the reaction rate increases exponentially.
• This leads to a massive, near-instantaneous release of energy, along with significant destruction and radioactive fallout.

Nuclear Reactors and Safety Considerations

Nuclear reactors convert the heat from controlled fission into electricity. The core components work together:

• Fuel rods contain the fissile material (typically enriched $^{235}U$).
• Control rods regulate the fission rate by absorbing neutrons.
• Moderator slows neutrons to the right speed for efficient fission.
• Coolant (often water) carries heat away from the core and transfers it to generate steam, which drives turbines to produce electricity.

Safety is a central concern because fission produces radioactive materials:

• Containment structures surround the reactor to prevent any release of radioactive material.
• Reactor core temperature and pressure are continuously monitored.
• Emergency shutdown systems (called SCRAM systems) can rapidly insert all control rods to halt the chain reaction.
• Radioactive waste management is a long-term challenge. Many fission products have long half-lives, meaning they remain radioactive for thousands of years and require secure storage.