Learning outcomes
- Describe fission of uranium-235 after neutron absorption.
- Explain release of daughter nuclei, neutrons and energy.
- Explain a chain reaction.
- State the functions of moderator, control rods and coolant.
- Distinguish controlled and uncontrolled chain reactions.
13.1 The fission process
Nuclear fission is the splitting of a heavy nucleus into two smaller daughter nuclei. In the common syllabus example, uranium-235 absorbs a neutron and becomes unstable. It splits, releasing energy and two or more additional neutrons.
The daughter nuclei are radioactive and move apart with high kinetic energy. Their kinetic energy is transferred to surrounding material as thermal energy. Additional energy appears in emitted radiation and neutrons.
13.2 Why energy is released
The total mass of the fission products is slightly less than the initial mass of the uranium nucleus plus neutron. The missing mass corresponds to released energy. O Level questions normally require the statement that energy is released rather than a numerical mass–energy calculation.
One fission event releases a tiny amount, but an enormous number of nuclei reacting each second produces substantial power.
13.3 Chain reaction
Neutrons released by one fission may be absorbed by other uranium-235 nuclei, causing further fissions and releasing more neutrons. If, on average, more than one neutron from each fission causes another fission, the number of reactions grows rapidly.
In an uncontrolled chain reaction, the rate rises very quickly. In a reactor, the aim is a steady controlled chain reaction where approximately one neutron per fission continues the sequence.
13.4 Moderator
Fission neutrons are initially fast. A moderator, such as water or graphite in simplified descriptions, slows them through collisions without absorbing too many. Slower neutrons are more likely to cause fission in uranium-235.
The moderator does not control the reaction by absorbing most neutrons; that is the role of control rods. Mixing these functions is a common examination error.
13.5 Control rods
Control rods are made from materials that absorb neutrons. Inserting them farther absorbs more neutrons and reduces the reaction rate. Withdrawing them absorbs fewer, allowing the rate to increase. They can be inserted fully in an emergency to stop the chain reaction rapidly.
Control systems monitor reactor conditions and move the rods to maintain a steady rate. Even after shutdown, radioactive products continue producing decay heat, so cooling remains important.
13.6 Coolant
The coolant flows through the reactor core and carries thermal energy away. That energy can be used to heat water, produce steam and drive turbines in a power station. The coolant’s main syllabus function is energy transfer, not neutron absorption.
Reactor designs use different coolants and arrangements, but the conceptual roles remain: fuel undergoes fission, moderator slows neutrons, control rods absorb neutrons, and coolant removes thermal energy.
Worked examples
Control response
If reactor power rises above the target, control rods are inserted farther. More neutrons are absorbed, so fewer additional fissions occur and power falls.
Moderator function
Fast neutrons collide with moderator nuclei and lose kinetic energy. Slower neutrons are more likely to trigger further U-235 fission.
Coolant failure
If coolant flow decreases, thermal energy is removed less effectively, so core temperature may rise even if the fission rate is unchanged initially.
Practical focus
Investigation
Use counters or dominoes to model a chain reaction. Compare an unrestricted arrangement with one containing “control-rod” barriers that remove moving counters. State clearly which features the model represents and which nuclear features it cannot reproduce.
Examination guidance
- State that U-235 absorbs a neutron before splitting.
- Fission releases daughter nuclei, two or more neutrons and energy.
- Moderator slows neutrons; control rods absorb neutrons; coolant removes thermal energy.
- A controlled reactor maintains a steady chain reaction.
- Do not say control rods cool the reactor.
Check your understanding
- What starts a U-235 fission event?
- How do control rods reduce reactor power?
- What is the function of the coolant?
Answers
- The U-235 nucleus absorbs a neutron and becomes unstable.
- They absorb more neutrons, so fewer neutrons cause further fissions.
- It carries thermal energy away from the reactor core.
