Nuclear Fusion vs Fission

Nuclear Binding Energy — The Source of All Nuclear Power

The nucleus of an atom is held together by the strong nuclear force, which overcomes the electromagnetic repulsion between protons at short range (<1 fm = 10⁻¹⁵ m). The energy required to completely separate a nucleus into its constituent protons and neutrons is the binding energy.

A plot of binding energy per nucleon (BE/A) vs atomic mass number (A) is the key to understanding both fusion and fission:

• BE/A peaks at approximately 8.8 MeV/nucleon for iron-56 (⁵⁶Fe) — the most tightly bound nucleus. Iron is the "bottom of the energy well."
• Light nuclei (H, He, Li…) have lower BE/A — they can release energy by fusing into heavier nuclei, moving toward the iron peak.
• Heavy nuclei (U, Th, Pu…) also have lower BE/A — they can release energy by splitting into lighter nuclei, also moving toward iron.

This is why iron is the endpoint of stellar nucleosynthesis — stars can no longer extract energy from nuclear reactions once their cores are iron.

Nuclear Fission Explained

Nuclear fission occurs when a heavy nucleus absorbs a neutron and becomes unstable, splitting into two smaller "daughter" nuclei plus 2–3 neutrons plus energy (mostly as kinetic energy of the fragments, plus gamma rays).

Example — the key reaction in nuclear power plants:

Energy release: approximately 200 MeV per fission event. 1 gram of U-235 fully fissioned releases ~83 GJ — equivalent to burning ~2,700 tonnes of coal.

Chain Reaction

The 2–3 neutrons released can trigger further fissions. If on average ≥1 neutron triggers a new fission (criticality), a chain reaction sustains itself:

• Subcritical: keff < 1 — reaction dies out
• Critical: keff = 1 — steady power output (reactor mode)
• Supercritical: keff > 1 — exponential growth (weapons mode or reactor excursion)

Nuclear reactors maintain criticality via control rods (boron or hafnium, which absorb neutrons) and moderators (water, graphite — slow neutrons to "thermal" energies where U-235 fission is more probable).

Nuclear Fusion Explained

Nuclear fusion occurs when two light nuclei fuse into a heavier nucleus, releasing energy. The most accessible fusion reaction (lowest ignition temperature) is D-T fusion:

Deuterium (²H) + Tritium (³H) → Helium-4 + neutron + 17.6 MeV. The helium nucleus carries 3.5 MeV; the neutron carries 14.1 MeV.

The Lawson criterion defines the conditions for a fusion plasma to be self-sustaining (ignition): nτE ≥ ~10²⁰ m⁻³·s (where n = plasma density, τE = energy confinement time). Equivalently, the fusion triple product must satisfy: nTτE ≥ 3 × 10²¹ keV·s·m⁻³.

The challenge: to overcome electrostatic repulsion between nuclei, the plasma must reach 10–15 keV (100–150 million Kelvin) — 10× hotter than the Sun's core. At these temperatures, any material wall would vaporise, requiring magnetic or inertial confinement.

Side-by-Side Comparison

Property	Fission	Fusion
Reaction	Split heavy nucleus	Join light nuclei
Fuel	Uranium-235, Plutonium-239	Deuterium (sea water) + Tritium (from Li)
Energy/reaction	~200 MeV	~17.6 MeV
Energy/kg fuel	~80 GJ/kg	~340 GJ/kg (>4× more)
Radioactive waste	Long-lived (10,000+ years)	Short-lived (<100 years)
Runaway risk	Yes (requires active control)	No (plasma extinguishes naturally)
Fuel abundance	Limited uranium reserves	Deuterium unlimited; tritium bred from Li
Current status	Commercial power worldwide	R&D; not yet commercial
COâ‚‚ emissions	Near zero (lifecycle)	Near zero (lifecycle)

E = mc² and the Mass Defect

Both fission and fusion release energy via Einstein's famous mass-energy equivalence. The products of the reaction are slightly less massive than the reactants. This mass defect (Δm) converts to energy:

For D-T fusion: Δm = (m_D + m_T) − (m_He + m_n) = 0.01886 u. E = 0.01886 × 931.5 MeV/u = 17.56 MeV ✓

The fraction of mass converted to energy: fission ~0.09%, fusion ~0.38% — 4× more efficient per unit mass.

Current Nuclear Power (Fission)

As of 2026, approximately 440 nuclear fission reactors operate in 32 countries, supplying ~10% of global electricity (and ~25% of low-carbon electricity). Three reactor generations are in operation:

• Generation II (most current): Pressurised Water Reactors (PWR), Boiling Water Reactors (BWR). Operating since 1970s.
• Generation III/III+: AP1000, EPR — advanced passive safety features.
• Generation IV (emerging): Molten salt reactors, fast breeder reactors, high-temperature gas reactors — still being demonstrated.

The IPCC consistently identifies nuclear power as essential for achieving net-zero carbon targets — it produces ~12 g CO₂-eq/kWh over lifecycle (solar: ~40, wind: ~12, gas: ~490, coal: ~820).

The Race to Achieve Fusion Power

ITER (International Thermonuclear Experimental Reactor)

Under construction in Cadarache, France, ITER will be the world's largest tokamak — a donut-shaped magnetic confinement device. Design specifications: 500 MW fusion output from 50 MW heating input (Q = 10). Plasma volume: 840 m³. First plasma planned for 2025 (delayed to 2027–2028). ITER will not generate electricity — it is a proof-of-concept machine.

NIF — First Ignition (December 2022)

The National Ignition Facility (Lawrence Livermore, California) achieved fusion ignition in December 2022 — the first time a fusion reaction produced more energy than the laser energy delivered to the target (3.15 MJ out from 2.05 MJ in). This was a landmark scientific milestone. Wall-plug efficiency (total electrical energy input vs fusion output) remains far below unity, but the principle is proven.

Private Fusion Ventures

• Commonwealth Fusion Systems (MIT spin-off): Developing SPARC tokamak using high-temperature superconducting (HTS) magnets providing 20 T fields — allowing a much smaller and cheaper device. Aims for Q>2 by 2025–2026.
• TAE Technologies: Field-reversed configuration approach, targeting a proton-boron (p-¹¹B) fuel cycle — aneutronic, no radioactive neutrons.
• Helion Energy: Pulsed approach; Microsoft agreed to purchase fusion electricity by 2028.

Weapons: Atomic vs Hydrogen Bombs

Atomic bomb (fission): Supercritical mass of U-235 or Pu-239 assembled to keff >> 1 in microseconds. "Little Boy" (Hiroshima, 1945): ~64 kg of U-235, ~15 kt yield. Releases roughly 6.3 × 10¹³ J in <1 second.

Hydrogen bomb / thermonuclear bomb (fission + fusion): A fission bomb (the "primary") creates the extreme temperature (~10⁸ K) and pressure needed to ignite a fusion secondary stage. The most powerful test was Tsar Bomba (1961): ~57 Mt yield — roughly 3,800 Hiroshima bombs. Fusion is used to amplify yield; the fission trigger is essential.

Frequently Asked Questions

What is the main difference between fusion and fission?

Fission splits heavy atoms (uranium), fusing joins light atoms (hydrogen isotopes). Both release energy from the mass defect via E=mc², but fusion releases ~4× more per kilogram of fuel and produces no long-lived radioactive waste.

Which is safer — fusion or fission?

Fusion is inherently safer. A fusion reactor cannot sustain a runaway chain reaction — if plasma conditions are disrupted, the reaction extinguishes in seconds. Fission reactors require active control systems to prevent criticality accidents (though modern designs have passive safety features).

How does the Sun produce energy?

The Sun fuses hydrogen into helium via the proton-proton chain: 4¹H → ⁴He + 2e⁺ + 2νe + 2γ + 26.7 MeV. At the Sun's core (T~15×10⁶ K), quantum tunnelling allows protons to overcome electrostatic repulsion despite missing the classical threshold.