Fission: a slow neutron is absorbed by a heavy uranium-235 nucleus, making it unstable. It splits into two lighter nuclei (here barium-141 and krypton-92) and releases three more neutrons plus energy.
Fusion: two very light nuclei — deuterium (²H) and tritium (³H) — join together at extremely high temperature to form helium-4 and a neutron, releasing energy.
In both reactions the total mass of the products is less than the total mass before. The "missing" mass (the mass defect) has been converted into energy.
The energy released is calculated from the mass defect using E = Δmc². For this fission: Δm = 0.18603 u = 3.089×10⁻²⁸ kg, so E = 3.089×10⁻²⁸ × (3.00×10⁸)² ≈ 2.78×10⁻¹¹ J ≈ 173 MeV. For D–T fusion: Δm = 0.01888 u = 3.136×10⁻²⁹ kg, giving E ≈ 2.82×10⁻¹² J ≈ 17.6 MeV.
The products are more tightly bound than the reactants — their binding energy per nucleon is higher. The extra binding energy is released as kinetic energy of the fragments and gamma radiation.
Useful conversions: 1 u = 1.6605×10⁻²⁷ kg, and 1 u of mass defect corresponds to 931.5 MeV.
The binding energy curve explains why both reactions release energy even though they are opposites: iron-56 sits at the peak (~8.8 MeV per nucleon), so any reaction that moves nuclei toward iron — fusing light nuclei or splitting heavy ones — increases binding energy per nucleon and releases the difference.
Fusion needs temperatures around 10⁸ K because both nuclei are positively charged: they must have enough kinetic energy to overcome their electrostatic (Coulomb) repulsion before the short-range strong nuclear force can bind them. Fission needs no such barrier — the neutron is uncharged, so it enters the nucleus easily, which is also why the released neutrons can sustain a chain reaction.
Per nucleon, fusion releases about five times more energy than fission (3.5 vs 0.73 MeV), and both release roughly a million times more energy per kilogram than chemical reactions, because they rearrange the nucleus rather than the electrons.
One fusion event is not a power station. For a self-sustaining burn the plasma must satisfy the Lawson criterion: the triple product of density, temperature and energy confinement time must exceed roughly n·T·τE ≥ 3×10²¹ keV s m⁻³ for D–T. Each variable plays a distinct role: temperature gets individual nuclei over the Coulomb barrier, density sets how often they collide, and confinement time sets how long the plasma keeps its heat before leaking it to the walls. In the sim, a hot but thin or leaky plasma still fuses once — then the burn dies out.
The probability readout combines all three: P = 1 − e^(−n·⟨σv⟩·τE), the chance a given ion fuses before the plasma loses its heat. Here ⟨σv⟩ is the reactivity — how quickly fusion collisions happen at a given temperature (approximated as ⟨σv⟩ ≈ 1.1×10⁻²⁴ T² m³ s⁻¹ for D–T near 10–20 keV). Doubling density doubles the collision rate; doubling confinement doubles the time available; raising temperature increases the reactivity roughly quadratically in this range.