E.4.2—Chain reactions

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.

How it is assessed

Questions explain why neutron energy is reduced or evaluate possible neutron-loss values in a reactor model.

Command terms

Outline / Determine

What earns marks

Mention fast neutrons, greater probability for thermal neutrons, and distinguish absorbed, escaping and fission-causing neutrons.

Watch for

Saying moderation increases neutron energy or treating every absorbed neutron as causing fission.

Retrieve the chain

Fission converts nuclear binding and mass defect into energy. A controlled chain reaction depends on neutron energy and losses; moderator, control rods, heat exchanger and shielding perform different jobs.

Retrieve the safety boundary

Fission products can be radioactive and require containment, shielding and long-term waste management.

E.4.2—Chain reactions

Model Chain Reactions

E.4.2 Exam Analysis

Assessment in practice

Retrieve the Fission Model