Nuclear Fission and FusionAQA GCSE Physics: Revision notes

Nuclear fission is the process where a large, unstable nucleus splits into two smaller nuclei. This splitting releases:

• Energy (in the form of heat and kinetic energy)
• Neutrons (typically 2–3 neutrons per fission event)

Fission usually occurs when a nucleus is bombarded with a slow-moving neutron, causing it to become unstable and split apart. The total mass of the products is slightly less than the original nucleus; this missing mass is converted into energy according to Einstein's equation E = mc². This is why fission releases enormous amounts of energy from a small amount of mass.

Common examples include the fission of uranium-235 and plutonium-239, which are used in nuclear weapons and power stations.

A chain reaction occurs when the neutrons released from one fission event go on to cause further fission events in neighbouring nuclei. This leads to an exponential increase in the number of fissions and energy released.

Setting up a chain reaction:

1. One nucleus undergoes fission, releasing 2–3 neutrons
2. Each of these neutrons splits another nucleus, releasing more neutrons
3. The number of neutrons (and fissions) doubles with each generation, creating a runaway reaction

Controlling a chain reaction in a reactor:

In a nuclear reactor, the chain reaction must be controlled so that exactly one neutron from each fission goes on to cause the next fission. This is achieved using three key components:

If too many neutrons are absorbed, the reaction slows and stops. If too few are absorbed, the reaction becomes uncontrolled. The control system maintains a stable, continuous reaction.

Nuclear fusion is the process where two light nuclei combine (fuse) to form a single heavier nucleus. This process releases large amounts of energy.

Fusion is the opposite of fission and typically occurs when two nuclei (such as isotopes of hydrogen—deuterium and tritium) collide at very high speeds. The products of fusion have a lower total mass than the original nuclei; the missing mass is converted into energy according to E = mc².

Key differences from fission:

• Requires extremely high temperatures and pressures to force nuclei close enough together
• Does not release neutrons in the same way fission does
• Releases energy through the binding of nucleons in a heavier, more stable nucleus
• Powers the Sun and other stars

Fusion is considered a clean energy source because:

• The fuel is abundant (hydrogen isotopes)
• It produces no long-lived radioactive waste
• Runaway reactions are impossible (if temperature drops, the reaction stops)

For fusion to occur, two light nuclei must be brought very close together—within about 10⁻¹⁵ m of each other. This is extremely difficult because:

The Coulomb Repulsion Problem:

• Both nuclei have positive charges (protons) that repel each other with a very strong electrostatic force
• This repulsive force increases dramatically as the nuclei get closer
• Overcoming this repulsion requires enormous kinetic energy

Why high temperature is essential:

• Temperature is a measure of the average kinetic energy of particles
• At very high temperatures (millions of Kelvin), nuclei move at speeds fast enough to overcome Coulomb repulsion
• They collide with sufficient energy to allow the strong nuclear force to take over and bind them together
• Without adequate temperature, the nuclei will simply bounce apart

Why high pressure is essential:

• Pressure forces the nuclei closer together, increasing collision frequency
• High pressure concentrates the nuclei in a small volume, making collisions more likely
• This dramatically increases the probability of successful fusion events

Combined effect:

• Millions of Kelvin and enormous pressures are needed simultaneously
• These conditions ensure enough nuclei reach the required proximity and speed for fusion to occur
• Even with these conditions, only a small fraction of nuclei actually fuse in any given time interval

Why fusion is difficult on Earth:

1. Extreme conditions are hard to create and maintain:

   • Temperatures of millions of Kelvin require enormous energy input
   • Sustaining these temperatures in a controlled way is technologically challenging

2. Plasma containment:

   • At fusion temperatures, matter becomes a plasma (ionised gas)
   • Plasma is extremely unstable and difficult to confine
   • It will rapidly cool if it touches the reactor walls

3. Energy balance:

   • The energy required to heat the plasma and maintain fusion conditions is very large
   • To date, fusion reactors have consumed more energy than they produce
   • Achieving a net energy gain (where output exceeds input) remains a major goal

4. Instability and control:

   • Small perturbations can cause the plasma to become unstable and lose confinement
   • Precise control systems are needed

Current research into fusion reactors:

Recent progress:

• NIF's achievement of net energy gain is a major breakthrough showing fusion energy gain is possible
• ITER construction continues, aiming to demonstrate sustained fusion with net gain
• Private companies and governments are investing heavily in fusion research
• Realistic timeline for commercial fusion power plants is still decades away