E.4 FissionIB Physics SL: Revision notes

Nuclear fission is the splitting of a heavy nucleus into two medium-mass nuclei (fission fragments) plus a few neutrons. It can be spontaneous (rare, e.g. in some heavy nuclides such as californium-252) or neutron-induced: uranium-235 absorbs a slow neutron, becomes uranium-236, and splits.

The products have a higher binding energy per nucleon than the original nucleus, so the total mass decreases and energy is released: about 200 MeV per fission, mostly as the kinetic energy of the fragments. Find it from the mass decrease × 931.5 MeV u⁻¹.

Each fission releases 2–3 neutrons. If on average one of them causes another fission, the reaction is steady (critical), as in a power station. If more than one does, the rate grows exponentially (as in a weapon). If fewer, it dies out.

Neutrons can also be lost by escaping from the fuel or being absorbed without fission, so a minimum amount of fuel, the critical mass, is needed to sustain a chain reaction.

• Fuel rods: enriched uranium (a higher proportion of uranium-235).
• Moderator (water or graphite): slows fast neutrons to thermal speeds by collisions, so they are more likely to cause fission of uranium-235.
• Control rods (boron or cadmium): absorb neutrons; lowering them reduces the fission rate, and raising them increases it.
• Coolant and heat exchanger: carry thermal energy from the core to a separate circuit that makes steam for the turbines, keeping radioactive coolant in a closed loop.
• Shielding: steel and thick concrete absorb neutrons and gamma radiation to protect workers.

Fission products are neutron-rich, so they are radioactive, mainly β⁻ and gamma emitters. Spent fuel has very high activity and keeps producing heat after removal. Half-lives range from days (iodine-131) to decades (strontium-90, caesium-137). Plutonium-239, formed from uranium-238, lasts tens of thousands of years.

Management: store spent fuel in cooling ponds (cooling and shielding), then in sealed dry casks, and ultimately in deep geological repositories in stable rock. Some fuel can be reprocessed to recover uranium and plutonium.