OCR Specification focus:
‘Explain induced fission and chain reactions; outline reactor fuel rods, control rods, and moderator.’
Induced nuclear fission underpins modern nuclear power generation, enabling large quantities of energy to be released from heavy nuclei under controlled conditions. These notes outline the core physical ideas governing induced fission, chain reactions, and the essential reactor components that regulate the process.
Induced nuclear fission occurs when a suitable nucleus absorbs a neutron, becomes unstable, and splits into smaller nuclei, releasing energy, neutrons, and radiation in a controlled chain process.
Induced Nuclear Fission
Induced nuclear fission is central to both power reactors and certain research applications, and understanding its mechanism is crucial for interpreting reactor behaviour and safety features.
Induced fission: The process in which a heavy nucleus absorbs an incoming neutron, becomes unstable, and splits into two smaller nuclei, releasing energy and additional neutrons.
When a heavy nucleus such as uranium-235 or plutonium-239 captures a slow-moving neutron, it forms a short-lived, highly excited compound nucleus. This nucleus subsequently splits into two medium-mass fission fragments, typically accompanied by 2–3 fast neutrons and a large amount of binding energy released as kinetic energy and gamma radiation.
A key requirement for induced fission is that the incident neutron must often be thermal, meaning low-energy, to maximise the probability of absorption. This dependence on neutron speed is an essential factor in reactor design.
The energy released per fission event is on the order of 200 MeV, far exceeding chemical energy scales. This enormous energy density allows nuclear reactors to operate using only small amounts of fuel relative to their output.
Conditions for a Chain Reaction
The neutrons released in fission can trigger further fission events if they are absorbed by nearby fissile nuclei.
Diagram of an induced fission chain reaction in U-235, illustrating fission fragments, emitted neutrons, and propagation of further fission events. Source.
Chain reaction: A self-sustaining sequence of fission events in which neutrons from each fission cause further fissions.
For a chain reaction to be sustained, the effective neutron multiplication factor must be at least one, meaning each fission event leads on average to one more fission. If the factor is less than one, the reaction dies out; if greater than one, the reaction escalates rapidly, forming the basis of both reactor control challenges and the underlying principle of nuclear weapons. In a reactor, the chain reaction must be kept at a steady, controlled level.
The typical sequence in an induced fission chain reaction involves:
A neutron being absorbed by a fissile nucleus.
Formation of an unstable compound nucleus.
Splitting into two fission fragments and emission of multiple neutrons.
Slowing of the emitted neutrons to thermal energies.
Subsequent fission events triggered by these moderated neutrons.
Ensuring that the number and energies of neutrons remain within narrow bounds is essential for safe operation.
Reactor Structure and Components
A nuclear reactor is engineered to maintain a controlled chain reaction by regulating neutron populations, fuel arrangement, and energy removal systems.
Diagram of a pressurised water reactor showing the reactor core, coolant loops, and heat-transfer pathway to the turbine; includes plant-level components not assessed in this subsubtopic. Source.
The OCR specification highlights three essential components: fuel rods, control rods, and the moderator.
Fuel Rods
Fuel rods contain the fissile material necessary for induced fission. They are typically composed of uranium dioxide pellets housed in zirconium alloy tubes.
Fuel rods: Cylindrical assemblies that contain fissile nuclear fuel, providing the nuclei required for sustained fission within a reactor core.
The arrangement of fuel rods dictates the geometric probability of neutron capture and therefore influences the likelihood of maintaining a stable chain reaction. Fuel gradually becomes spent as fissile isotopes are consumed and fission products accumulate.
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Control Rods
Control rods are inserted or withdrawn from the reactor core to adjust the neutron population. They are made from materials that efficiently absorb neutrons, such as cadmium, hafnium, or boron carbide.
Control rods: Neutron-absorbing rods used to regulate the rate of fission by capturing excess neutrons within the reactor core.
By altering their position, operators can:
Reduce power output by absorbing more neutrons.
Increase power output by withdrawing rods, enabling more fission.
Achieve an emergency shutdown (SCRAM) by fully inserting all control rods to halt the chain reaction.
This precise control capability makes reactors fundamentally different from explosive, uncontrolled fission systems.
Schematic showing how control rod height affects reactor criticality, demonstrating how neutron absorption controls the stability of the chain reaction. Source.
The Moderator
The moderator plays a vital role in sustaining induced fission by slowing fast neutrons produced in fission to thermal speeds.
Moderator: A material within the reactor core that reduces the kinetic energy of fast neutrons, increasing their probability of being absorbed by fissile nuclei.
Common moderator materials include light water, heavy water, and graphite. Moderation enhances the effectiveness of fuel utilisation and allows reactors to extract steady energy from fissile material.
The interaction between the moderator, fuel rods, and control rods ensures that neutrons remain in the correct energy regime for a controlled chain reaction. Without moderation, many fissile isotopes would not absorb neutrons efficiently enough to sustain fission.
Summary of Key Processes in Reactor Operation
Neutron absorption by fissile nuclei initiates induced fission.
Fission fragments and fast neutrons are emitted, releasing large amounts of energy.
Moderation slows neutrons, making further fission more likely.
Control rods regulate neutron numbers to maintain steady reactor output.
Fuel rods supply fissile material and determine the core’s geometric arrangement.
These elements collectively ensure that induced fission in reactors remains safe, stable, and productive for energy generation.
FAQ
A neutron’s ability to trigger further fission depends mainly on its energy, its likelihood of being absorbed by a fissile nucleus, and whether it avoids leakage from the reactor core.
Thermal neutrons are far more effective at causing fission in uranium-235, so moderation is crucial.
Core geometry, fuel arrangement, and neutron-absorbing materials also influence how many neutrons remain available to sustain the chain reaction.
These materials have exceptionally high neutron absorption cross-sections across a wide energy range, making them effective at regulating neutron populations.
They also remain stable under high temperatures and intense radiation, avoiding swelling or chemical reactions that could compromise reactor control.
Mechanical strength and resistance to embrittlement further ensure reliable insertion and withdrawal throughout reactor operation.
Higher enrichment means a higher proportion of fissile uranium-235, increasing the probability that neutrons will encounter nuclei capable of fission.
This leads to:
Higher reactivity
Reduced need for moderation adjustments
Lower control rod insertion levels to maintain criticality
Enrichment also affects fuel lifetime, as a more fissile-rich core can sustain an effective chain reaction for longer before becoming spent.
Fission fragments typically have a neutron-to-proton ratio that places them far from the line of stability, making them highly unstable.
As a result, they undergo a series of beta decays, often accompanied by gamma emission, to reach more stable isotopes.
Their short half-lives and high decay energies make fresh fission products significant contributors to reactor radioactivity and decay heat.
Engineers incorporate multiple, overlapping systems to ensure reactivity cannot rise uncontrollably.
These include:
Precise placement of control rods with rapid insertion capability
Use of neutron absorbers dissolved in coolant in some reactor types
Negative temperature and void coefficients that naturally reduce reactivity when temperature rises
Core geometry designed to minimise accidental increases in neutron density
Together, these features stabilise neutron behaviour and keep the reactor operating at safe critical levels.
Practice Questions
Question 1 (2 marks)
State what is meant by induced nuclear fission and explain why a thermal neutron is typically required for the process to occur.
Mark scheme:
• Induced fission is when a heavy nucleus absorbs a neutron and splits into two smaller nuclei, releasing energy and additional neutrons. (1)
• A thermal (slow) neutron is required because it has a higher probability of being absorbed by the nucleus than a fast neutron. (1)
Question 2 (5 marks)
A nuclear power station uses a controlled fission chain reaction within its reactor core.
(a) Describe the role of the moderator and explain why it is essential for sustaining the chain reaction.
(b) Explain how the control rods are used to maintain a steady power output in the reactor.
(c) Suggest what would happen to the rate of reaction if all the control rods were suddenly fully inserted, and explain why.
Mark scheme:
(a)
• Moderator slows down fast neutrons produced in fission. (1)
• Thermal neutrons are more likely to cause further fission, so moderation is essential for sustaining the chain reaction. (1)
(b)
• Control rods absorb excess neutrons. (1)
• Raising or lowering them changes the neutron population, allowing operators to maintain a steady rate of fission and therefore steady power output. (1)
(c)
• Inserting all control rods would greatly reduce the number of free neutrons. (1)
• Reaction rate drops rapidly and the chain reaction becomes subcritical or stops. (1)
