Physics · nuclei · NEET
| Meaning | Fission: one heavy nucleus splits into two lighter nuclei | Fusion: two light nuclei join into one heavier nucleus |
| Fuel example | Heavy nuclei like U-235 | Light nuclei like hydrogen (deuterium, tritium) |
| How it starts | Triggered by a neutron (neutral, no barrier) | Needs very high temperature (~10^8 K) to beat Coulomb repulsion |
| Energy per event | Large, about 200 MeV per U-235 fission | Small, a few MeV per reaction (but more per kg of fuel) |
| Where it happens | Nuclear reactors and atom bombs on Earth | Stars and the Sun (proton-proton cycle) |
| Why energy is released | Fragments are more tightly bound than uranium | Product is more tightly bound than the light nuclei |
Fission = splitting. One heavy nucleus breaks into two lighter fragments (plus a few neutrons). Fusion = joining. Two light nuclei combine into one heavier nucleus. Simple trick: fisSION sounds like SCISSORS (cut apart), fuSION sounds like FUSE (stick together).
The answer is the binding energy per nucleon graph. It rises to a peak near iron (A around 56) and falls on both sides. Heavy nuclei (like uranium) are on the right slope, so splitting them moves the pieces UP toward the peak (more tightly bound = energy released). Light nuclei (like hydrogen) are on the left slope, so joining them also moves UP toward the peak. In both cases the product is more tightly bound, so energy comes out.
Per single reaction (per event), fission releases much more: about 200 MeV for one U-235 fission. Fusion of two light nuclei releases only a few MeV. BUT per unit mass of fuel, fusion releases much more energy, because hydrogen atoms are so light that a small mass has a huge number of nuclei. So: per event fission wins, per kilogram fusion wins.
To fuse, two positively charged nuclei must come very close so the short-range nuclear force can act. But both are positive, so they repel each other (Coulomb repulsion). They need very high kinetic energy to overcome this Coulomb barrier, which means temperatures around 10^8 K (as in stars). Fission is triggered by a slow neutron, which has no charge, so there is no Coulomb barrier to cross. That is why fission works at ordinary temperature.
Fusion. Stars, including our Sun, generate energy by fusing hydrogen nuclei into helium (the proton-proton cycle). Nuclear reactors on Earth use fission of uranium. So: stars = fusion, power plants = fission.
Fission. U-235 is the heavy nucleus that splits when hit by a neutron. Fusion uses very LIGHT nuclei, mainly isotopes of hydrogen (deuterium and tritium). Never pair uranium with fusion in an answer.
Fission splits one heavy nucleus into two lighter nuclei; fusion joins two light nuclei into one heavier nucleus. Both release energy because the product is more tightly bound.
In both, the products have higher binding energy per nucleon than the reactants. Higher binding energy means lower mass, and the lost mass appears as energy through E = mc^2.
About 200 MeV per fissioning nucleus, according to NCERT. This first appears as kinetic energy of the fragments and neutrons, then turns into heat.
The two light nuclei are both positive and repel each other. Only at very high temperature (about 10^8 K) do they have enough energy to overcome this Coulomb barrier and get close enough to fuse.
Nuclear power plants use controlled fission of uranium. Controlled fusion on Earth is still under development because confining plasma at 10^8 K is very hard.
