Question
Compare nuclear fission and nuclear fusion. Give one example of each, and explain which releases more energy per gram of fuel.
Solution — Step by Step
Nuclear fission is the splitting of a heavy nucleus into two or more lighter nuclei when struck by a neutron, releasing a large amount of energy and additional neutrons.
Example:
The three neutrons released can each trigger further fissions, creating a chain reaction. Uncontrolled chain reaction = atomic bomb. Controlled chain reaction = nuclear reactor.
Energy released per fission of one U-235 nucleus ≈ 200 MeV (about J).
Nuclear fusion is the combining of two light nuclei to form a heavier nucleus, releasing energy.
Example (hydrogen fusion in the Sun):
(Deuterium + Tritium → Helium-4 + neutron + energy)
This reaction requires extremely high temperatures (≥10⁷ K) to overcome the electrostatic repulsion between positively charged nuclei — this is why fusion is called a thermonuclear reaction. The Sun’s core (~15 million K) sustains fusion naturally.
| Property | Fission | Fusion |
|---|---|---|
| Definition | Heavy nucleus splits | Light nuclei combine |
| Fuel | U-235, Pu-239 (heavy elements) | Deuterium, Tritium (hydrogen isotopes) |
| Products | Radioactive waste | Helium (non-radioactive, mostly) |
| Condition | Neutron bombardment at room temperature | Temperature ≥ 10⁷ K (extreme conditions) |
| Chain reaction | Yes | No |
| Current technology | Nuclear power plants (operational) | Experimental (ITER, NIF) |
| Radioactive waste | Significant long-lived waste | Minimal |
Fusion produces more energy per gram than fission.
- Fission of 1 g U-235: ~ J
- Fusion of 1 g D-T mixture: ~ J
Fusion releases about 4 times more energy per gram than fission. Additionally, fusion fuel (deuterium) is abundant (obtained from seawater), while uranium is a limited resource.
The physics reason: in fusion, the binding energy per nucleon of the product (He-4) is significantly higher than the reactants (H-2, H-3). The increase in binding energy per nucleon × number of nucleons = energy released. Iron (Fe-56) has the highest binding energy per nucleon — both fission and fusion release energy by approaching this maximum.
Why This Works
Both reactions release energy through mass defect (Einstein’s ). The products have slightly less mass than the reactants — this “missing mass” is converted to energy. Even tiny mass differences correspond to enormous energies because J/kg.
Common Mistake
Students often say “fusion is better because it produces no radiation.” This is partially true — fusion produces far less long-lived radioactive waste than fission. However, the D-T fusion reaction does produce high-energy neutrons, which can activate the reactor vessel materials over time. The statement “fusion is clean” means “cleaner than fission” not “completely clean.”
A memory trick: Fission = Split (both have an “s” sound and mean breaking). Fusion = Join (fusion comes from “fuse” = merge). This helps when you’re reading quickly and might mix them up. In an exam, always double-check: does the reaction shown split or combine nuclei?