Nuclear fission and fusionIB MYP Physics: Revision notes

Nuclear fission is the splitting of a large nucleus into two smaller nuclei, releasing energy.

In a uranium-235 reactor, a slow-moving neutron is absorbed by a uranium-235 nucleus. The nucleus becomes unstable and splits into two smaller nuclei, releasing two or three more neutrons and a large amount of energy as heat.

The released neutrons can be absorbed by other uranium-235 nuclei, which split and release more neutrons. This is a chain reaction. If it is not controlled, the energy release grows rapidly.

• Fuel rods: contain uranium-235.
• Moderator (for example water or graphite): slows down the neutrons so they are more likely to be absorbed and cause fission.
• Control rods (for example boron): absorb neutrons. Lowering them slows the reaction; raising them speeds it up.
• Coolant: carries heat away from the reactor.
• The heat turns water into steam, which drives turbines connected to generators.
• Thick concrete shielding protects workers from radiation.

Fission produces radioactive waste, including used fuel rods and contaminated materials. Some waste remains dangerous for thousands of years, because it gives out ionising radiation.

Waste must be stored safely. Used fuel is cooled in water ponds, then sealed in glass inside steel containers, and may be stored deep underground in geologically stable rock. Finding safe long-term storage is one of the main arguments against nuclear fission.

Nuclear fusion is the joining of two small nuclei to form a larger nucleus, releasing energy. Stars such as the Sun release energy by fusing hydrogen nuclei to form helium.

Nuclei are positive and repel each other, so fusion needs very high temperatures (millions of degrees) and very high pressure to push the nuclei close enough to join.

Challenges of fusion power on Earth: it is very difficult to heat and contain hot material, and so far reactors have not produced more energy than they use. Benefits: the fuel is plentiful, there is little long-lived waste, and there is no runaway chain reaction.

• Fission: large nucleus splits; started by a neutron; used in power stations now; long-lived radioactive waste; chain reaction needs control.
• Fusion: small nuclei join; needs very high temperature and pressure; occurs in stars; still experimental on Earth; much less long-lived waste.

Both release large amounts of energy, and both can produce electricity with low carbon dioxide emissions while running.

When evaluating energy options, consider reliability, cost, safety, waste and the environment, and then give a justified conclusion.