(Physics only) Describe a chain reaction in terms of repeated fission events.

Within Nuclear fission (physics only), this learning objective asks you to (physics only) describe a chain reaction in terms of repeated fission events. Focus on the approved ideas chain reaction, physics only and connect them clearly to Nuclear fission and fusion. A strong response should state the relevant particle, radiation, isotope, decay, half-life or nuclear-equation idea, then explain how it answers the exact command word. Avoid swapping nearby concepts such as atomic number and mass number, isotope and ion, alpha, beta and gamma radiation, or contamination and irradiation. This is a Physics-only point, so keep the answer tied to the AQA Physics specification.