Nuclear Fission vs. Nuclear Fusion
CSEC Physics: Nuclear Energy Release
Essential Understanding: Nuclear reactions release enormous amounts of energy by changing the structure of atomic nuclei. Fission splits heavy nuclei into lighter ones, while fusion combines light nuclei into heavier ones. Both processes convert mass into energy according to Einstein's famous equation, but they have very different characteristics and applications.
What is Nuclear Fission?
Nuclear fission is the process of splitting a heavy atomic nucleus into two or more smaller nuclei, along with the release of energy. The word "fission" comes from the Latin word meaning "to split apart." This process was first discovered in 1938 by German scientists Otto Hahn and Fritz Strassmann, and was explained theoretically by Lise Meitner and Otto Frisch.
The Fission Process
Target Nucleus: Typically uranium-235 or plutonium-239, which have large, unstable nuclei with many protons and neutrons.
Trigger: A neutron is absorbed by the heavy nucleus, causing it to become unstable and begin to deform.
Splitting: The nucleus splits into two smaller nuclei called fission fragments, along with 2-3 neutrons and a significant amount of energy.
Energy Release: The total mass of the products is less than the original mass. This "missing" mass (mass defect) is converted to energy according to \(E = mc^2\).
Fission Equation Example
When uranium-235 absorbs a neutron, it splits into barium-141, krypton-92, and releases 3 neutrons plus approximately 200 MeV of energy per reaction!
The Chain Reaction
One of the most important aspects of nuclear fission is that each reaction releases 2-3 neutrons. If at least one of these neutrons causes another fission reaction, a self-sustaining chain reaction occurs. This is the principle behind both nuclear reactors and nuclear weapons.
What is Nuclear Fusion?
Nuclear fusion is the process of combining two light atomic nuclei to form a heavier nucleus, releasing energy in the process. The word "fusion" comes from the Latin word meaning "to melt together." This is the process that powers the Sun and all stars in the universe.
The Fusion Process
Fuel: Light elements like hydrogen isotopes (deuterium and tritium) or hydrogen itself.
Requirements: Extremely high temperatures (millions of degrees) and pressures to overcome electrostatic repulsion between positively charged nuclei.
Combination: When nuclei get close enough (within nuclear force range), they fuse together to form a heavier nucleus.
Energy Release: The resulting nucleus has less mass than the combined original masses. This mass defect is converted to energy according to \(E = mc^2\).
Fusion Equation Examples
Deuterium-Tritium Fusion: The most achievable fusion reaction on Earth, releasing 17.6 MeV per reaction.
Proton-Proton Chain (Sun): The main fusion process in the Sun, converting hydrogen to helium over millions of years.
Why Do These Reactions Release Energy?
Binding Energy per Nucleon: Both fission and fusion release energy because they move nuclei toward the peak of the binding energy curve (around iron-56).
Fission: Heavy nuclei (uranium) have lower binding energy per nucleon than medium-mass nuclei. Splitting them converts some mass to energy.
Fusion: Very light nuclei (hydrogen) also have lower binding energy per nucleon than medium-mass nuclei. Combining them releases energy.
Key Point: Energy is released when nuclei move toward iron-56 on the binding energy curve, regardless of whether they are splitting or combining.
Key Differences: Fission vs. Fusion
| Property | Nuclear Fission | Nuclear Fusion |
|---|---|---|
| Process | Splitting heavy nuclei into lighter ones | Combining light nuclei into heavier ones |
| Fuel | Uranium-235, Plutonium-239 | Hydrogen isotopes (D, T), Hydrogen |
| Energy per Reaction | ~200 MeV | ~17.6 MeV (D-T), ~26.7 MeV (p-p) |
| Conditions Required | Critical mass, neutron moderator | Extremely high temperature (100 million °C) |
| Natural Occurrence | Does not occur naturally on Earth | Powers all stars including the Sun |
| Reaction Control | Moderately controllable with control rods | Extremely difficult; no fully successful reactor yet |
| Radioactive Waste | High-level long-lived waste produced | Minimal short-lived radioactive waste |
| Risk of Meltdown | Possible; requires containment systems | Inherently safe; stops if containment fails |
| Fuel Availability | Limited uranium deposits | Virtually unlimited (seawater deuterium) |
Energy Release Comparison
Note: Fusion releases approximately 4 times more energy per kilogram of fuel than fission!
The Energy Advantage
This is because fusion involves lighter elements where the binding energy per nucleon increases more dramatically as you move toward iron.
Applications of Nuclear Reactions
Nuclear Fission Applications
Nuclear Power Plants: Generate approximately 10% of world electricity. Use controlled fission chain reactions to produce heat, which generates steam to drive turbines.
Nuclear Submarines: Use compact fission reactors to provide propulsion and electricity for months without refueling.
Medical Isotopes: Fission produces radioisotopes like molybdenum-99 (used in medical imaging) and cobalt-60 (cancer treatment).
Space Exploration: Radioisotope thermoelectric generators (RTGs) use heat from radioactive decay (not fission) to power spacecraft.
Nuclear Fusion Applications
The Sun: Every second, the Sun fuses 600 million tons of hydrogen into helium, releasing enough energy to power Earth for millions of years.
Hydrogen Bombs: Uncontrolled fusion reaction using a fission bomb to provide the necessary heat and pressure.
Fusion Research: Projects like ITER (International Thermonuclear Experimental Reactor) are working toward practical fusion power generation.
Future Potential: Clean, abundant energy with minimal radioactive waste could revolutionize global power production.
Interactive Reaction Comparison
Visualize Nuclear Reactions
Objective: Compare the fission and fusion processes. Watch how nuclei split or combine to release energy.
Select a reaction type to see it in action
Click the buttons above to visualize how nuclear fission and fusion work.
Energy Release Chart
Advantages and Disadvantages
Nuclear Fission
Advantages:
- Proven, reliable technology
- High energy density
- Low greenhouse gas emissions
- Stable base-load power
Disadvantages:
- Long-lived radioactive waste
- Risk of nuclear proliferation
- Potential for accidents (meltdown)
- Limited fuel reserves
Nuclear Fusion
Advantages:
- Abundant fuel (deuterium from seawater)
- Minimal radioactive waste
- Inherently safe (no chain reaction)
- Higher energy output per mass
Disadvantages:
- Not yet commercially viable
- Requires extreme conditions
- Technical challenges with containment
- High initial costs
CSEC Exam Tip
Understanding the Binding Energy Curve is essential for CSEC Physics.
When answering questions about why fission and fusion release energy, remember:
- Fission: Heavy nuclei like uranium have lower binding energy per nucleon than medium-mass nuclei. Splitting them moves products toward the peak of the curve.
- Fusion: Light nuclei like hydrogen also have lower binding energy per nucleon. Combining them forms helium, which is closer to the peak.
- Energy Release: In both cases, mass decreases (mass defect) and converts to energy via \(E = mc^2\).
CSEC Practice Arena
Test Your Understanding
Nuclear fission is the splitting of a heavy nucleus (like uranium-235) into lighter nuclei. This requires a neutron to initiate the reaction.
Nuclear fusion is the combining of two light nuclei (like hydrogen isotopes) to form a heavier nucleus. This requires extremely high temperatures and pressures.
Key Difference: Fission splits heavy nuclei; fusion combines light nuclei. Both release energy by converting mass to energy according to \(E = mc^2\).
Fusion: Light nuclei are positively charged. At normal temperatures, they repel each other due to electrostatic repulsion. To force them close enough for the strong nuclear force to take over (causing fusion), temperatures of 100 million °C or more are needed. At these temperatures, atoms exist as plasma with atoms moving at extremely high speeds.
Fission: The uranium nucleus is already unstable and heavy. Simply adding a neutron (which has no charge) is enough to destabilize it and cause it to split. No high temperatures are required.
Step 1: Calculate number of atoms in 1 kg of U-235:
Number of atoms = \( \frac{1000}{235} \times 6.02 \times 10^{23} = 2.56 \times 10^{24} \) atoms
Step 2: Calculate total energy:
Energy = \( 2.56 \times 10^{24} \times 200 \text{ MeV} = 5.12 \times 10^{26} \text{ MeV} \)
Step 3: Convert to joules (1 MeV = 1.6 × 10⁻¹³ J):
Energy = \( 5.12 \times 10^{26} \times 1.6 \times 10^{-13} = 8.2 \times 10^{13} \text{ J} \)
Answer: Approximately \( 8.2 \times 10^{13} \) joules or 82 terajoules.
Inherent Safety: Fusion reactions require precise conditions of extreme temperature and pressure. If anything goes wrong (e.g., containment failure), these conditions cannot be maintained and the reaction stops immediately. There is no possibility of a "runaway" reaction or meltdown.
No Chain Reaction: Fusion does not involve a neutron chain reaction. Each fusion event requires specific conditions to occur. Without these conditions, fusion cannot continue.
Limited Fuel: Only small amounts of fuel exist in the reaction chamber at any time. Even if all fuel fused instantly (impossible), the energy release would be limited and not comparable to a fission meltdown.
Contrast with Fission: Fission involves a self-sustaining chain reaction. If control systems fail, the reaction can escalate rapidly, leading to overheating and potential meltdown with catastrophic consequences.
Chapter Summary
Key Takeaways - Fission
- Splitting heavy nuclei (U-235, Pu-239)
- Requires neutron to initiate
- Releases ~200 MeV per reaction
- Produces long-lived radioactive waste
- Used in current nuclear power plants
Key Takeaways - Fusion
- Combining light nuclei (H isotopes)
- Requires extreme temperature/pressure
- Releases ~17.6 MeV per reaction (D-T)
- Produces minimal short-lived waste
- Powers the Sun and future reactors
Remember!
Fusion releases more energy per kilogram but requires extreme conditions. Fission is currently used for power generation.