E.4 Fission

Model fission

A heavy nucleus can split into two lighter nuclei after absorbing a neutron, or spontaneously in an unstable state. The products have a greater binding energy per nucleon than the original heavy nucleus.

Track the release

The increase in total binding energy appears as kinetic energy of the fission products, neutron energy and radiation. The mass of the products is slightly smaller, with the mass difference converted to energy.

Understand fissile material

Enrichment increases the fraction of uranium-235 relative to uranium-238, making a sustained fission process more feasible.

Common trap

Do not say energy is created from nothing. It comes from the mass defect and the change in nuclear binding energy.

Start the chain

A fission event can emit neutrons. If one of them causes another fission, the process becomes a chain reaction.

Control the multiplication

A self-sustaining reactor requires, on average, one effective neutron from each fission to cause the next fission. Neutrons can instead escape, be absorbed by control rods, or be absorbed without causing fission.

Explain moderation

Fast neutrons are slowed by collisions with a moderator because low-energy neutrons have a higher probability of causing the relevant fission in this reactor model.

Common trap

Do not say every emitted neutron continues the chain. Losses and absorption determine whether the reaction dies out, stays critical or grows.

Moderator

The moderator slows neutrons through collisions, increasing the chance that they cause the desired fission.

Control rods

Control rods absorb neutrons and regulate the chain-reaction rate, allowing operators to control power output.

Heat exchanger and shielding

A heat exchanger transfers internal energy from the reactor to a working fluid that drives turbines. Shielding reduces radiation exposure outside the reactor.

Common trap

Do not assign power regulation to the moderator or heat exchanger. Each component has a distinct job in the energy and safety chain.

Identify fission products

Fission produces two medium-mass daughter nuclei, free neutrons and energy. The daughter nuclei may be radioactive because they have neutron-to-proton ratios away from the stable region.

Treat the waste

Fission products can remain hazardous for different timescales. Waste management therefore requires containment, shielding, monitored storage and long-term isolation according to the material and activity.

Separate the engineering issue

The high energy density of fission does not remove the need to manage radioactive products. Safety decisions must account for radiation, heat and containment.