Nuclear Fission

Nuclear fission is the splitting of a large, unstable atomic nucleus into two smaller daughter nuclei. The most common fissile materials are uranium-235 and plutonium-239, used in nuclear reactors and weapons.

Fission can occur in two ways: spontaneous fission (rare, nucleus splits without external trigger) and induced fission (a neutron is absorbed, making the nucleus unstable and causing it to split).

When a neutron hits a uranium-235 nucleus, the nucleus absorbs it and becomes uranium-236, which is highly unstable. This unstable nucleus immediately splits into two smaller daughter nuclei (such as barium-144 and krypton-89), releasing 2-3 neutrons and gamma rays.

Each fission event releases approximately 200 MeV (200 million electron volts) of energy—mostly as kinetic energy of the products, plus heat and gamma radiation.

The total mass of the fission products is slightly less than the original uranium nucleus plus neutron. This "missing" mass is called the mass defect. According to Einstein's famous equation, this mass is converted directly into energy:

The 2-3 neutrons released from each fission can trigger more fissions in neighboring uranium nuclei. This creates a chain reaction with exponential growth: 1 → 2 → 4 → 8 → 16 → 32...

• Controlled chain reaction: In nuclear reactors, control rods absorb excess neutrons to maintain a steady reaction rate for power generation.
• Uncontrolled chain reaction: In nuclear weapons, the chain reaction is allowed to proceed unchecked, releasing massive energy in microseconds.

The minimum amount of fissile material needed to sustain a chain reaction is called the critical mass.