Nuclear Fission: Spontaneous Fission, Induced Fission, Chain Reactions, and Energy Release
Spontaneous Fission: When Nuclei Split on Their Own
Spontaneous fission
Spontaneous fission occurs when a heavy nucleus splits into smaller nuclei without any external influence.
This process is rare because most heavy nuclei are stable enough to resist splitting on their own.Note
Some isotopes, such as uranium-238 and californium-252, can undergo spontaneous fission under the right conditions.
Why Does Spontaneous Fission Happen?
- The nucleus of an atom is held together by the strong nuclear force, which binds protons and neutrons tightly.
- However, in very heavy nuclei, the repulsive electromagnetic force between the positively charged protons becomes significant.
- If the nucleus is large enough, this repulsion can overcome the strong nuclear force, causing the nucleus to split into smaller fragments.
Spontaneous fission is much less common than other forms of radioactive decay, such as alpha or beta decay, because the strong nuclear force typically stabilizes the nucleus.
Induced Fission: Splitting Nuclei with Neutrons
Induced fission
Induced fission occurs when a nucleus splits after absorbing a neutron.
Unlike spontaneous fission, this process requires an external trigger.
ExampleWhen a uranium-235 nucleus absorbs a neutron, it becomes uranium-236, an unstable isotope.
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: $$
^{1}_{0}n + ^{235}_{92}U \rightarrow ^{236}_{92}U \rightarrow ^{144}_{56}Ba + ^{89}_{36}Kr + 3 ^{1}_{0}n
$$ - Here’s what happens step by step:
- A neutron is absorbed by a uranium-235 nucleus, forming uranium-236.
- The uranium-236 nucleus becomes unstable and quickly splits into two smaller nuclei (barium-144 and krypton-89 in this example).
- Three free neutrons are released, along with a significant amount of energy.
- Consider you’re observing a uranium-235 reaction.
- The three neutrons released can collide with other uranium-235 nuclei, potentially causing more fission reactions.
- This cascading effect forms the basis of a chain reaction.
- Don’t confuse induced fission with spontaneous fission.
- Induced fission requires an external neutron to trigger the process, while spontaneous fission occurs without any external influence.
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.
