Nuclear fission is a core idea in IB Chemistry Topic 12 (Atomic Structure) and connects to nuclear energy, radioactivity, mass defect, and binding energy. Fission involves splitting large, unstable nuclei into smaller ones, releasing huge amounts of energy. Understanding fission helps students grasp how nuclear reactors operate, why chain reactions occur, and how mass converts into energy according to Einstein’s equation. It also clarifies the difference between fission and fusion—two processes students often confuse.
What Is Nuclear Fission?
Nuclear fission is the process in which a heavy atomic nucleus splits into two smaller nuclei, releasing energy, neutrons, and radiation.
The nuclei involved are usually very large and unstable, such as:
- Uranium-235
- Plutonium-239
These isotopes can undergo fission when struck by neutrons.
The Basic Fission Reaction
A classic example used in IB Chemistry involves uranium-235:
n + ²³⁵U → ²³⁶U* → ¹⁴¹Ba + ⁹²Kr + 3n + energy
The steps:
- A slow-moving neutron is absorbed by uranium-235
- The nucleus becomes unstable
- It splits into two smaller nuclei (fission fragments)
- Extra neutrons are released
- A large amount of energy is produced
The asterisk () in ²³⁶U indicates an excited, unstable state.
Why Fission Releases Energy
Fission releases energy because:
- The product nuclei are more stable
- The total mass of products is less than the original mass
The mass difference (mass defect) is converted into energy:
E = mc²
This energy appears as:
- Kinetic energy of fragments
- Gamma radiation
- Heat (used in power generation)
The mass difference is tiny but, due to c², the energy is enormous.
Chain Reactions
The released neutrons can initiate further fission events.
1. Uncontrolled chain reaction
Fission accelerates rapidly.
Used in nuclear weapons.
2. Controlled chain reaction
Neutrons are moderated and absorbed to maintain steady reaction rates.
Used in nuclear reactors.
This ability to control the reaction makes fission usable for generating electricity.
Nuclear Reactors and Fission
A nuclear power plant uses controlled fission to produce heat, which then generates electricity.
Key components:
1. Fuel rods
Contain fissionable isotopes (U-235 or Pu-239).
2. Control rods
Made of boron or cadmium; absorb excess neutrons to regulate reaction rate.
3. Moderator
Slows neutrons to make fission more likely.
Common moderators: water, heavy water, graphite.
4. Coolant
Transfers heat from the reactor core to turbines.
The heat turns water to steam, which drives turbines to produce electricity.
Fission vs Fusion
IB students often confuse these, but they differ significantly:
Feature Fission Fusion Process Splits heavy nuclei Joins light nuclei Energy Very high Even higher Waste Significant radioactive waste Minimal waste Conditions Room temperature possible Very high temp & pressure Use Nuclear reactors, bombs Stars, experimental reactors
Fission is easier to achieve but produces long-lived radioactive waste.
Advantages of Nuclear Fission
1. Massive energy production
A small amount of uranium generates huge energy.
2. No carbon emissions during operation
Nuclear plants do not release CO₂.
3. Reliable baseload power
Provides continuous electricity, unlike some renewables.
4. Long-term fuel availability
Uranium supplies are significant worldwide.
Disadvantages of Nuclear Fission
1. Radioactive waste
Long-lived isotopes require secure storage.
2. Risk of accidents
Meltdowns (e.g., Chernobyl, Fukushima) have severe consequences.
3. Proliferation concerns
Fissionable material can be weaponized.
4. High costs
Reactors are expensive to build and maintain.
IB questions often explore these environmental and social implications.
Common IB Misunderstandings
“Fission and fusion both split atoms.”
Fusion combines nuclei; fission splits them.
“Fission always needs high temperature.”
No—only fusion requires extreme conditions.
“Fission releases electrons.”
It releases neutrons and radiation, not electrons.
“A single neutron causes only one fission event.”
It often triggers a chain reaction with multiple neutrons.
FAQs
Why does U-235 undergo fission easily?
Because its nucleus is large, unstable, and easily deformed by a neutron.
What happens to the neutrons released?
They may cause further fission, be absorbed by control rods, or escape the reactor.
Is nuclear fission renewable?
No, uranium is finite—but supplies are long-lasting.
Conclusion
Nuclear fission is the process by which heavy nuclei split into smaller ones, releasing energy due to mass–energy conversion. With controlled chain reactions, reactors harness fission to produce large amounts of electricity. Understanding fission—its mechanism, energy release, advantages, and hazards—is essential for mastering nuclear chemistry in the IB syllabus.
