Compare nuclear fission and fusion and explain the energy released using binding energy

What this dot point is asking

WACE wants you to compare fission and fusion, explain the energy release through binding energy, and describe chain reactions and fusion conditions. Both are applications of the binding-energy curve.

Nuclear fission

In fission a heavy nucleus absorbs a neutron, becomes unstable and splits into two medium-sized daughter nuclei, releasing two or three neutrons and a large amount of energy. A typical example is

The fragments lie nearer the binding-energy peak than uranium, so they are more tightly bound and the mass defect emerges as energy.

Chain reactions

The neutrons released by one fission can be absorbed by other nuclei, causing further fissions. If on average more than one neutron from each fission triggers another, the reaction grows: this is a chain reaction. A reactor uses control rods to absorb surplus neutrons and a moderator to slow them, keeping the rate steady at one new fission per fission (criticality).

Nuclear fusion

In fusion, light nuclei combine to form a heavier nucleus, for example deuterium and tritium fusing to helium:

The helium nucleus is far more tightly bound per nucleon than the hydrogen isotopes, so fusion releases more energy per nucleon than fission. Fusion powers the Sun and stars.

Why fusion is hard on Earth

Fusing nuclei must approach closely enough for the short-range strong force to bind them, but both are positively charged and repel strongly. Overcoming this electrostatic barrier needs extremely high temperatures and pressures, conditions found in stellar cores but difficult to sustain in a controlled reactor. This is why fission is currently used in power stations while controlled fusion remains a research challenge.

Balancing nuclear equations

A routine skill is to complete or check a nuclear equation by conserving both nucleon number (the top number) and proton number (the bottom number) across the arrow. The sum of the top numbers on the left must equal the sum on the right, and likewise for the bottom numbers. This lets you find a missing particle: for example, if the left side has total nucleon number $236$ and the named products account for $233$, the missing neutrons make up the difference of $3$. The same bookkeeping identifies an unknown daughter nucleus by its proton number (which fixes the element) and nucleon number. Always state which conservation law you used, because the marks are awarded for the reasoning, not just the final particle.

Conditions and consequences of each process

Fission and fusion differ sharply in their practical conditions, which is worth stating in a comparison answer. Fission can be triggered at ordinary temperatures by a slow neutron being absorbed, which is why it is used in current power stations, but it produces radioactive waste because the medium-mass fragments are themselves often unstable and decay over long periods. Fusion needs the colliding nuclei to overcome their mutual electrostatic repulsion, which demands the enormous temperatures and pressures found in stellar cores or, on Earth, only briefly in experimental reactors; its main product (helium) is not radioactive, so it promises cleaner energy if it can be sustained. Both release energy because the products sit higher on the binding-energy-per-nucleon curve, but the engineering challenge is the opposite: containing a runaway chain reaction for fission, versus achieving and holding extreme conditions for fusion.

Explaining the energy source

In any answer, anchor the energy release to the binding-energy curve: both processes move nuclei toward the iron peak, increasing binding energy per nucleon, so the products have less mass than the reactants and the difference is released as energy. Quote $E=mc^2$ as the conversion.