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Compare the energy input and output for nuclear fission and nuclear fusion.
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Short answer
- Fusion releases more energy per kilogram of fuel than fission (for common reactions like D–T fusion by roughly a factor of ~4), and a larger fraction of the fuel mass is converted to energy.
- However, fusion requires far larger external energy (very high temperatures and/or pressures and good confinement) to get the reaction going; fission can be started and sustained much more easily (neutron-induced chain reaction) so the net practical energy output from fission is currently much easier to obtain.
Key numbers and why
- Mechanism: both convert a small amount of nuclear mass (binding-energy differences) to energy via E = mc^2. Fission moves very heavy nuclei toward the binding-energy-per-nucleon peak (around iron); fusion moves light nuclei up that curve toward iron. The small per-nucleon energy changes add up over many nucleons.
Typical energy per elementary reaction
- U-235 fission (typical): ≈ 200 MeV per fission ≈ 3.2×10^−11 J per nucleus.
- Per kilogram of U-235 this is ≈ 8×10^13 J/kg.
- Fraction of mass converted ≈ 0.09% (≈10^−3 of the mass).
- D–T fusion (most practical laboratory fusion): 17.6 MeV per fusion ≈ 2.82×10^−12 J per reaction.
- Per kilogram of D+T fuel this is ≈ 3.4×10^14 J/kg (≈4× the fission value).
- Fraction of mass converted ≈ 0.38% (≈3.8×10^−3 of the reactant mass).
- For comparison chemical fuels: gasoline ≈ 4×10^7 J/kg — many orders of magnitude lower than nuclear.
Energy-input (initiation and sustainment)
- Fission:
- Initiation: a neutron (or spontaneous fission) is sufficient to induce fission of many heavy isotopes because neutrons see no Coulomb barrier.
- Sustaining a chain reaction requires reaching and maintaining criticality (correct geometry, purity, moderators), but the external energy input (to start and control a power reactor) is small compared with the nuclear output. Weapons use conventional explosives to assemble a supercritical configuration (MJ-scale inputs).
- Practical reactors convert the released nuclear energy to heat and then electricity; thermal-to-electric efficiencies are typically 30–40%.
- Fusion:
- Initiation: you must overcome the Coulomb repulsion between positive nuclei → very high temperatures (tens to hundreds of millions of kelvin) and/or very high pressures/densities and confinement time (the Lawson criterion).
- External energy to heat and confine the plasma is large; experimental devices often have Q (fusion power out / input heating power) < 1, though milestones toward Q > 1 have been reached in specific senses. In inertial confinement experiments, laser systems deliver megajoules of input; in magnetic confinement the heating power and confinement must meet strict n·τ·T criteria.
- Much of D–T fusion energy is carried by fast neutrons, so in practical reactors you capture neutron energy as heat (like fission) and convert it to electricity; some designs hope to recover charged-particle energy more directly.
Practical implications
- Energy density: fusion > fission >> chemical fuels. Fusion gives more energy per kg of fuel, but engineering to reach and sustain fusion conditions is much harder, so fusion plants have not yet produced routine net electricity commercially. Fission reactors, by contrast, produce large net power with modest external input and are a mature technology.
- Waste and neutrons: both produce neutrons and activation; D–T fusion produces very high-energy neutrons (14.1 MeV) that cause material damage and activation; fission produces many radioactive fission products and long-lived waste issues.
Bottom line
- Per unit mass: fusion (e.g., D–T) releases more energy than fission (~3.4×10^14 J/kg vs ~8×10^13 J/kg).
- Required input: fission needs little external energy to start/maintain a chain reaction; fusion requires very large input energy to heat and confine the fuel, making it much harder to exploit despite higher theoretical energy yield.
- Fusion releases more energy per kilogram of fuel than fission (for common reactions like D–T fusion by roughly a factor of ~4), and a larger fraction of the fuel mass is converted to energy.
- However, fusion requires far larger external energy (very high temperatures and/or pressures and good confinement) to get the reaction going; fission can be started and sustained much more easily (neutron-induced chain reaction) so the net practical energy output from fission is currently much easier to obtain.
Key numbers and why
- Mechanism: both convert a small amount of nuclear mass (binding-energy differences) to energy via E = mc^2. Fission moves very heavy nuclei toward the binding-energy-per-nucleon peak (around iron); fusion moves light nuclei up that curve toward iron. The small per-nucleon energy changes add up over many nucleons.
Typical energy per elementary reaction
- U-235 fission (typical): ≈ 200 MeV per fission ≈ 3.2×10^−11 J per nucleus.
- Per kilogram of U-235 this is ≈ 8×10^13 J/kg.
- Fraction of mass converted ≈ 0.09% (≈10^−3 of the mass).
- D–T fusion (most practical laboratory fusion): 17.6 MeV per fusion ≈ 2.82×10^−12 J per reaction.
- Per kilogram of D+T fuel this is ≈ 3.4×10^14 J/kg (≈4× the fission value).
- Fraction of mass converted ≈ 0.38% (≈3.8×10^−3 of the reactant mass).
- For comparison chemical fuels: gasoline ≈ 4×10^7 J/kg — many orders of magnitude lower than nuclear.
Energy-input (initiation and sustainment)
- Fission:
- Initiation: a neutron (or spontaneous fission) is sufficient to induce fission of many heavy isotopes because neutrons see no Coulomb barrier.
- Sustaining a chain reaction requires reaching and maintaining criticality (correct geometry, purity, moderators), but the external energy input (to start and control a power reactor) is small compared with the nuclear output. Weapons use conventional explosives to assemble a supercritical configuration (MJ-scale inputs).
- Practical reactors convert the released nuclear energy to heat and then electricity; thermal-to-electric efficiencies are typically 30–40%.
- Fusion:
- Initiation: you must overcome the Coulomb repulsion between positive nuclei → very high temperatures (tens to hundreds of millions of kelvin) and/or very high pressures/densities and confinement time (the Lawson criterion).
- External energy to heat and confine the plasma is large; experimental devices often have Q (fusion power out / input heating power) < 1, though milestones toward Q > 1 have been reached in specific senses. In inertial confinement experiments, laser systems deliver megajoules of input; in magnetic confinement the heating power and confinement must meet strict n·τ·T criteria.
- Much of D–T fusion energy is carried by fast neutrons, so in practical reactors you capture neutron energy as heat (like fission) and convert it to electricity; some designs hope to recover charged-particle energy more directly.
Practical implications
- Energy density: fusion > fission >> chemical fuels. Fusion gives more energy per kg of fuel, but engineering to reach and sustain fusion conditions is much harder, so fusion plants have not yet produced routine net electricity commercially. Fission reactors, by contrast, produce large net power with modest external input and are a mature technology.
- Waste and neutrons: both produce neutrons and activation; D–T fusion produces very high-energy neutrons (14.1 MeV) that cause material damage and activation; fission produces many radioactive fission products and long-lived waste issues.
Bottom line
- Per unit mass: fusion (e.g., D–T) releases more energy than fission (~3.4×10^14 J/kg vs ~8×10^13 J/kg).
- Required input: fission needs little external energy to start/maintain a chain reaction; fusion requires very large input energy to heat and confine the fuel, making it much harder to exploit despite higher theoretical energy yield.
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