E.4.1 Mechanisms of nuclear fission

Nuclear Fission: Spontaneous Fission, Induced Fission, Chain Reactions, and Energy Release

1. Imagine holding a tightly coiled spring in your hands.
2. The spring stores potential energy, and with a small nudge, it can release this energy in an instant.
3. Nuclear fission works in a similar way: inside heavy nuclei like uranium-235, an enormous amount of energy is stored.
4. When these nuclei split, this energy is unleashed.

Spontaneous Fission: When Nuclei Split on Their Own

This process is rare because most heavy nuclei are stable enough to resist splitting on their own.

Why Does Spontaneous Fission Happen?

1. The nucleus of an atom is held together by the strong nuclear force, which binds protons and neutrons tightly.
2. However, in very heavy nuclei, the repulsive electromagnetic force between the positively charged protons becomes significant.
3. If the nucleus is large enough, this repulsion can overcome the strong nuclear force, causing the nucleus to split into smaller fragments.

Induced Fission: Splitting Nuclei with Neutrons

Unlike spontaneous fission, this process requires an external trigger.

This instability leads to the nucleus splitting into smaller nuclei, releasing energy and additional neutrons.

A Typical Induced Fission Reaction

Consider this common induced fission reaction involving uranium-235:

$${}_0^{1}n{+}_{92}^{235}U{\to }_{92}^{236}U{\to }_{56}^{144}Ba{+}_{36}^{89}Kr+3_0^{1}n$$

Here’s what happens step by step:

1. A neutron is absorbed by a uranium-235 nucleus, forming uranium-236.
2. The uranium-236 nucleus becomes unstable and quickly splits into two smaller nuclei (barium-144 and krypton-89 in this example).
3. Three free neutrons are released, along with a significant amount of energy.

Chain Reactions: The Self-Sustaining Cascade

The neutrons released during fission can collide with other fissile nuclei, such as uranium-235 or plutonium-239, causing them to undergo fission as well.

$⟹$ This creates a chain reaction.

Critical Mass and Chain Reactions

1. For a chain reaction to occur, there must be a minimum amount of fissile material, known as the critical mass.
2. If the mass is too small, neutrons escape without triggering additional fission reactions, and the chain reaction stops.
3. If the mass is large enough, the reaction continues, releasing a steady stream of energy.

Energy Release: The Source of Nuclear Power

1. When a nucleus undergoes fission, the total mass of the resulting products is slightly less than the mass of the original nucleus.
2. This “missing” mass is converted into energy, as described by Einstein’s famous equation:

$$E=\mathrm{\Delta }m{c}^2$$

Here:

• $E$ is the energy released,
• $\mathrm{\Delta }m$ is the mass defect (the difference in mass between the reactants and products),
• $c$ is the speed of light ($3.00\times {10}^8\text{m/s}$).

Why Does Fission Release Energy?

1. The energy release in fission can also be understood using the binding energy per nucleon curve.
2. This curve shows that nuclei with intermediate mass numbers (like krypton and barium) have higher binding energy per nucleon than very heavy nuclei (like uranium).

Applications of Fission Energy

The energy released in fission can be harnessed for various purposes, such as:

1. Nuclear Power Plants: Controlled chain reactions produce heat, which is used to generate electricity.
2. Nuclear Weapons: Uncontrolled chain reactions release energy explosively.

Reflection

Nuclear fission unlocks the vast energy stored within atoms, offering both immense potential and significant responsibility.