Fission induced by thermal neutrons; possibility of a chain reaction;…

The idea

Induced fission begins when a slow neutron is absorbed by a heavy nucleus such as uranium‑235, forming an unstable uranium‑236 nucleus that splits into two smaller nuclei plus two or three neutrons. Because these neutrons are fast, they must be slowed by a moderator, for example graphite or water, to become thermal neutrons with energies comparable to the thermal motion of surrounding particles, so that further absorption and fission are likely. Each fission can trigger more fissions, giving a chain reaction. Whether the reaction is sustained depends on the critical mass: the smallest mass of fissile material for which, on average, each fission produces at least one further fission. Below the critical mass too many neutrons escape, and the chain reaction dies out; at or above it the reaction can be sustained, which is the basis of controlled reactors and, when uncontrolled, of weapons.

What earns the marks

• Describe absorption of a thermal neutron by a nucleus such as uranium‑235, forming an unstable nucleus that undergoes fission.
• State that fission releases two or three neutrons and a large amount of energy, and that these neutrons can induce further fission.
• Explain the role of a moderator in slowing fast neutrons to thermal energies so that they are more likely to be absorbed.
• Define critical mass as the minimum mass of fissile material needed for a self-sustaining chain reaction.
• Explain that below the critical mass, neutron leakage dominates and the chain reaction stops, while at or above it the reaction can be sustained.

Common mistakes and how to put them right

• Thinking that any neutron can easily induce fission.

  Put it right: Slow thermal neutrons are much more likely to be absorbed than fast ones.

• Believing that critical mass is a fixed property of a material.

  Put it right: It depends on shape, purity and the presence of a moderator or reflector.

• Assuming that a chain reaction always explodes.

  Put it right: In a reactor it is controlled so that on average one neutron from each fission causes one further fission.

Exam tips

• Use the phrase thermal neutron and explain that it means a neutron in thermal equilibrium with the moderator, not simply a 'slow' neutron.
• When describing a chain reaction, refer to the average number of further fissions per fission, often called the multiplication factor.
• For critical mass questions, mention neutron loss from the surface and how a larger mass reduces the fraction lost.