Thermal-neutron-induced fission

A suitable heavy nucleus can absorb a slow neutron and split into two smaller nuclei. This process is called thermal-neutron-induced fission. A thermal neutron is a slow neutron, with energy comparable to that of particles in its surroundings.

When the heavy nucleus absorbs the neutron, it becomes an excited, unstable nucleus. It can then split into two smaller nuclei, called fission products, releasing energy and typically several neutrons.

Most of the released energy appears as kinetic energy of the fission products: the two nuclei move apart at high speeds. The emitted neutrons can cause further fission if they are subsequently absorbed by suitable nuclei.

Fission chain reactions and critical mass

When a fissile nucleus splits, it releases neutrons. If one of these neutrons causes another nucleus to split, that fission releases more neutrons. The process can continue as a fission chain reaction.

Not every released neutron causes another fission. Some escape from the material, while others are absorbed without causing fission. The balance between these outcomes determines whether the chain reaction continues.

When a chain reaction is self-sustaining

The behaviour of the reaction depends on the average number of further fissions caused by each fission:

• One further fission: the reaction is self-sustaining; the number of fissions remains steady on average.
• Fewer than one: the number of fissions decreases and the reaction dies away.
• More than one: the number of fissions increases, so the reaction grows.

These are averages: an individual fission may lead to no further fissions or to several.

Critical mass

The critical mass is the minimum mass of fissile material needed to sustain a chain reaction under specified conditions. At this threshold, each fission causes one further fission on average.

Critical mass is not a fixed value for a material alone. It depends on how likely neutrons are to escape or to be absorbed without causing fission. The relevant conditions include:

• Shape: a more compact shape generally allows fewer neutrons to escape than a spread-out shape.
• Density: at higher density, neutrons are more likely to interact with nuclei before leaving the material.
• Surroundings: surrounding material can affect how many neutrons return to the fissile material or are absorbed.

Changing these conditions can therefore change the minimum mass needed for a self-sustaining chain reaction.

Neutron moderation

Neutrons released by fission are usually fast. A moderator slows them to thermal energies, making them more likely to cause further fission in suitable fuel. At thermal energies, the neutrons have energies comparable to those associated with the random thermal motion of the surrounding material.

How a moderator slows neutrons

In a simple model, a fast neutron collides elastically with a moderator nucleus that is initially at rest. The neutron and nucleus move apart after the collision. Total momentum and total kinetic energy are conserved, but the nucleus gains some kinetic energy from the neutron. The neutron therefore usually leaves with less kinetic energy.

A neutron undergoes repeated collisions as it travels through the moderator. These transfers gradually reduce its energy towards thermal levels. A light nucleus can receive a relatively large fraction of the neutron’s energy in one collision, so light nuclei are effective at slowing neutrons.

Suitable moderator materials

A good moderator contains light nuclei and absorbs relatively few neutrons. Absorbed neutrons are no longer available to cause fission. Water and graphite are examples of moderator materials.

Nuclear reactor control rods

In a thermal reactor, fission releases neutrons that can cause further fissions. Control rods regulate the fission rate by absorbing some of these neutrons before they can cause further fissions.

Inserting the rods further into the reactor generally causes more neutrons to be absorbed. Fewer neutrons are then available to cause further fissions, so the fission rate decreases. Withdrawing the rods generally has the opposite effect: fewer neutrons are absorbed by the rods, leaving more available to cause fission.

Control rods are made using materials that absorb neutrons strongly and are suitable for reactor conditions. Boron and cadmium are examples of neutron-absorbing materials used in control rods.

Nuclear reactor coolant

The reactor core releases thermal energy as nuclear reactions occur. A coolant flows through or around the core, absorbs some of this energy and carries it away. This helps prevent the core from overheating and transfers energy for use outside the core.

Choosing a reactor coolant

A suitable coolant must transfer thermal energy effectively and remain stable under the temperatures and pressures at which the reactor operates. It must also be compatible with reactor materials: for example, it should not cause excessive corrosion of the parts it contacts.

The coolant’s tendency to absorb neutrons also matters. Neutrons sustain the chain reaction, so neutrons absorbed by the coolant are no longer available to continue it. The effect of this absorption must be considered when choosing a coolant.

Water and carbon dioxide are examples of reactor coolants.