Nuclear Energy
2. Nuclear Reactors
Learning outcomes
- I can describe the main components of a nuclear reactor.
- I can explain the function of control rods.
- I can explain the role of the moderator and coolant.
- I can describe how electricity is generated from nuclear energy.
- I can identify safety features of modern reactors.
Nuclear Reactors
A nuclear reactor is a system designed to maintain and control a nuclear fission chain reaction.
In a nuclear power station, the reactor's main purpose is to produce thermal energy. This energy is eventually used to generate steam, rotate a turbine, and drive an electrical generator.
The basic energy pathway is:
nuclear energy → thermal energy → mechanical energy → electrical energy
Important reactor components include:
- nuclear fuel
- fuel rods and fuel assemblies
- moderator
- control rods
- coolant
- reactor vessel
- steam generator or boiling system
- turbine
- generator
- condenser
- containment structure
The Reactor Core
The reactor core is the region where nuclear fission occurs.
It contains the nuclear fuel and, depending on reactor design, components associated with:
- neutron moderation
- neutron control
- heat removal
A controlled chain reaction occurs inside the fuel.
The reactor is designed so that the energy released by fission can be safely transferred away from the core.
Nuclear Fuel
A reactor requires nuclei capable of undergoing fission.
An important reactor fuel is:
uranium-235 (²³⁵U)
Other fissile materials can include:
plutonium-239 (²³⁹Pu)
Many commercial reactors use uranium fuel in which the proportion of uranium-235 has been increased above its natural concentration.
This is called:
enriched uranium
Fuel Pellets
Nuclear fuel is commonly manufactured as small ceramic pellets.
A widely used fuel material is:
uranium dioxide (UO₂)
Fuel pellets are stacked inside long metal tubes.
These form:
fuel rods
Fuel Rods and Fuel Assemblies
Many fuel rods are arranged together to form:
fuel assemblies
Many fuel assemblies are then positioned inside the reactor core.
Therefore:
fuel pellets → fuel rods → fuel assemblies → reactor core
Fission occurs primarily within the fuel pellets.
Fission in the Reactor
A neutron can be absorbed by a uranium-235 nucleus.
The nucleus becomes unstable and undergoes fission.
A possible reaction is:
²³⁵₉₂U + ¹₀n → fission fragments + neutrons + energy
The emitted neutrons may cause additional fission reactions.
This produces a:
chain reaction
A Controlled Chain Reaction
A power reactor requires a controlled chain reaction.
For steady operation:
approximately one neutron from each fission must successfully produce another fission, on average.
This corresponds approximately to:
k = 1
where k is the effective neutron multiplication factor.
The reactor is then:
critical
In reactor physics, "critical" means that the chain reaction is self-sustaining at a steady level.
It does not mean that the reactor is necessarily in an emergency condition.
Control Rods
Control rods regulate the fission chain reaction by absorbing neutrons.
They are made from materials that absorb neutrons effectively.
Examples can include materials containing:
- boron
- cadmium
- hafnium
How Control Rods Work
When control rods are inserted farther into the reactor core:
more neutrons absorbed → fewer neutrons available → fewer fissions → reactor power decreases
When control rods are withdrawn:
fewer neutrons absorbed → more neutrons available → more fissions possible → reactor power can increase
Therefore, control rods allow operators and automatic systems to regulate:
reactor power
Control Rods and Criticality
Control rods help control the value of k.
k < 1
The reactor is:
subcritical
The chain reaction decreases.
k = 1
The reactor is:
critical
The chain reaction remains approximately steady.
k > 1
The reactor is:
supercritical
The neutron population and reactor power increase.
Normal reactor control keeps these changes carefully regulated.
Reactor Shutdown
A reactor can be rapidly shut down by inserting neutron-absorbing control rods or other shutdown systems into the core.
This greatly reduces the number of neutrons available to cause further fission.
The chain reaction becomes:
subcritical
However, stopping the chain reaction does not immediately stop all heat production.
Decay Heat
Fission products are radioactive.
Even after the fission chain reaction has been shut down, these nuclei continue to decay.
Their radioactive decay produces:
decay heat
Therefore:
reactor shutdown ≠ instant loss of all heat
Cooling must continue after shutdown.
This is an extremely important reactor-safety concept.
The Moderator
Many reactor designs use a:
moderator
The moderator's main purpose is to:
slow down neutrons
Neutrons released directly from fission are relatively fast.
In thermal reactors using uranium-235, slower neutrons are particularly effective at causing further fission.
Moderator Materials
Common moderator materials include:
- ordinary water
- heavy water
- graphite
The moderator slows neutrons through repeated collisions.
It should slow neutrons without absorbing too many of them.
Why Slower Neutrons Help
A slow neutron has a greater probability of causing fission in uranium-235 than many faster neutrons.
Therefore:
fast neutron → moderator collisions → slower neutron → greater probability of ²³⁵U fission
The moderator helps maintain an efficient chain reaction.
How a Moderator Slows Neutrons
A neutron collides with nuclei in the moderator.
During each collision, the neutron can transfer some of its kinetic energy.
After many collisions:
neutron speed decreases
Materials containing light nuclei can be particularly effective at slowing neutrons.
Hydrogen in ordinary water is a good example.
Moderator vs Control Rods
These two components should not be confused.
Moderator
slows neutrons
Control rods
absorb neutrons
The moderator helps neutrons become more effective at causing fission.
Control rods reduce the number of neutrons available to cause fission.
The Coolant
Fission releases large amounts of thermal energy.
This heat must be transferred away from the reactor core.
The substance that performs this function is the:
coolant
The coolant flows through or around the reactor core and absorbs thermal energy.
Coolant Materials
Different reactor designs use different coolants.
Examples include:
- ordinary water
- heavy water
- carbon dioxide
- helium
- liquid sodium
The coolant chosen depends on reactor design.
Water as Moderator and Coolant
In many reactors, ordinary water performs two functions.
It acts as:
moderator
by slowing neutrons.
It also acts as:
coolant
by carrying thermal energy away from the reactor core.
These functions are related but different.
Reactor Vessel
The reactor core is housed inside a strong:
reactor pressure vessel
in many common reactor designs.
The vessel is designed to withstand:
- high temperatures
- high pressures
- radiation
- demanding operating conditions
It provides an important physical boundary around the reactor coolant and core.
Pressurized Water Reactor
One common reactor design is the:
Pressurized Water Reactor (PWR)
In a PWR:
- water flows through the reactor core
- the water is kept under high pressure
- high pressure prevents it from boiling in the reactor vessel during normal operation
- the hot primary water transfers thermal energy to a separate water system
The Primary Coolant Loop
In a PWR, water in the primary circuit flows through the reactor core.
It absorbs thermal energy.
The hot water then flows to a:
steam generator
The primary water transfers thermal energy to water in a separate secondary circuit.
The two water systems remain physically separated during normal operation.
The Steam Generator
The steam generator is a:
heat exchanger
Thermal energy passes:
primary coolant → secondary water
The secondary water boils and becomes:
steam
The steam then travels toward the turbine.
Why Separate Coolant Loops Are Useful
Keeping the primary and secondary circuits separate helps prevent reactor coolant from directly entering the turbine system during normal operation.
The primary circuit remains part of the reactor cooling system.
The secondary circuit carries steam to the turbine.
This provides an additional physical separation between the reactor core and the turbine equipment.
Boiling Water Reactors
Another reactor design is the:
Boiling Water Reactor (BWR)
In a BWR, water boils directly inside the reactor vessel.
The steam produced travels to the turbine.
Therefore, unlike a PWR:
BWR: steam produced in reactor vessel
PWR: steam produced in separate steam generator
Both systems ultimately use steam to turn a turbine.
From Nuclear Energy to Electricity
The overall process in a nuclear power station is:
fission → thermal energy → steam → turbine → generator → electricity
Each stage involves an energy transfer or transformation.
Fission Produces Thermal Energy
Fission fragments leave the nucleus at high speeds.
They collide with surrounding atoms in the fuel.
Their kinetic energy is transferred to the material.
This increases the:
thermal energy
of the fuel.
Heat Transfer to the Coolant
Thermal energy moves from the hot fuel to the coolant.
The coolant carries this energy away from the reactor core.
In a PWR, the coolant transfers this energy to a separate water circuit through the steam generator.
Producing Steam
Water in the steam-producing system receives thermal energy.
It boils.
The resulting high-pressure steam contains energy that can be used to rotate a turbine.
The Turbine
Steam flows across the blades of a:
turbine
The turbine rotates.
The energy transformation is approximately:
thermal/internal energy → mechanical energy
The Generator
The turbine shaft is connected to an electrical:
generator
As the generator rotates, electromagnetic induction is used to produce an electric current.
The energy transformation is:
mechanical energy → electrical energy
The electricity can then be transformed to suitable voltages and transmitted through the electrical grid.
The Condenser
After passing through the turbine, the steam must be converted back into liquid water.
This occurs in the:
condenser
The steam transfers thermal energy to a cooling system and condenses.
The water can then be pumped back through the steam cycle.
Cooling Systems
The unwanted thermal energy must eventually be transferred to the environment.
Cooling systems may use:
- rivers
- lakes
- seawater
- cooling towers
Cooling towers release thermal energy primarily through:
- evaporation
- convection
The visible cloud above many cooling towers is mainly:
condensed water droplets
not smoke.
Cooling Towers Are Not the Reactor
A common misconception is that the large cooling tower is the nuclear reactor.
It is not.
The reactor is housed inside the nuclear facility.
The cooling tower is part of the:
heat-rejection system
Many non-nuclear thermal power stations can also use cooling towers.
Energy Transformations
The full energy pathway can be summarized as:
nuclear energy in fuel
↓
kinetic energy of fission fragments
↓
thermal energy in fuel and coolant
↓
thermal/internal energy of steam
↓
mechanical energy of turbine
↓
electrical energy from generator
Not all the original nuclear energy becomes useful electrical energy.
Some energy is transferred to the surroundings as waste heat.
Reactor Efficiency
The efficiency of a power station can be calculated using:
efficiency = useful energy output / total energy input
or:
efficiency = useful power output / total power input
As a percentage:
efficiency (%) = (useful output / total input) × 100
No thermal power station can convert all thermal energy into electrical energy.
Worked Efficiency Example
Suppose a reactor provides:
3000 MW
of thermal power.
The power station generates:
1000 MW
of electrical power.
Efficiency:
= 1000/3000 × 100
= 33.3%
The remaining energy is mainly transferred to the environment as thermal energy.
Safety in Nuclear Reactors
Modern reactor safety uses multiple layers of protection.
This approach is often described as:
defence in depth
Instead of depending on one safety system, several independent or overlapping barriers and systems are used.
Fuel Matrix
The first physical barrier is the nuclear fuel itself.
Many radioactive fission products remain trapped within the ceramic fuel material during normal operation.
This helps limit their release.
Fuel Cladding
Fuel pellets are enclosed within:
fuel cladding
The cladding separates the fuel from the reactor coolant and helps contain radioactive fission products.
It forms another important safety barrier.
Reactor Pressure Boundary
The reactor vessel and connected primary system provide another barrier.
They are engineered to withstand:
- high pressure
- high temperature
- radiation
- long operating periods
They help keep reactor coolant and radioactive material contained.
Containment Building
Many reactors are surrounded by a strong:
containment structure
This is designed to provide an additional barrier between reactor systems and the external environment.
Containment structures may use:
- reinforced concrete
- steel liners
- thick structural walls
Their purpose includes limiting radioactive release during serious accidents.
Multiple Physical Barriers
A simplified sequence of barriers is:
fuel material → fuel cladding → reactor pressure boundary → containment structure
If one barrier fails, additional barriers remain.
This is a major principle of:
defence in depth
Emergency Shutdown Systems
Reactors have systems designed to rapidly reduce the fission chain reaction.
A rapid reactor shutdown is sometimes called a:
scram
The exact mechanism depends on reactor design.
A shutdown system can quickly insert neutron-absorbing material into the core.
This makes the reactor:
subcritical
Emergency Core Cooling
Even after fission is stopped:
decay heat continues
Therefore, reactors need systems capable of maintaining cooling.
Emergency core cooling systems are designed to provide or maintain coolant under specified accident conditions.
The goal is to prevent excessive fuel temperatures.
Redundant Safety Systems
Important reactor safety systems may use:
redundancy
This means more than one system can perform an important safety function.
For example, there may be:
- multiple pumps
- multiple power supplies
- multiple sensors
- multiple shutdown systems
If one component fails, another may still function.
Diverse Safety Systems
Safety systems may also use:
diversity
This means different methods or technologies are used to perform similar safety functions.
This reduces the possibility that one type of failure disables every backup system.
Passive Safety
Some modern reactor designs include:
passive safety systems
These rely partly on natural physical processes rather than requiring continuous active operation.
Examples may use:
- gravity
- natural convection
- pressure differences
- stored water
Passive systems can continue providing some safety functions even when external electrical power is unavailable.
Backup Power
Cooling and monitoring equipment may require electricity.
Nuclear plants therefore have backup electrical systems such as:
- emergency generators
- batteries
- multiple electrical connections
These help maintain critical safety functions if normal electrical power is lost.
Radiation Monitoring
Nuclear facilities continuously monitor radiation in important areas.
Monitoring systems can detect:
- abnormal radiation levels
- contamination
- radioactive releases
- changes in reactor conditions
Workers may also wear:
personal dosimeters
to measure occupational exposure.
Control Room
Operators monitor reactor systems from a:
control room
Information can include:
- reactor power
- neutron flux
- temperature
- pressure
- coolant flow
- water levels
- radiation measurements
Automated systems can also respond to abnormal conditions.
Controlling Reactor Temperature
A reactor must continuously balance:
energy produced by fission
with:
energy removed by the coolant
If heat production exceeds heat removal significantly, fuel temperature can rise.
Therefore, reactor safety depends on reliable:
- chain-reaction control
- coolant circulation
- heat removal
Loss of Coolant
A serious reactor concern is:
loss of coolant
Without sufficient cooling, decay heat and any remaining reactor heat can cause fuel temperatures to increase.
Safety systems are designed to:
- shut down fission
- maintain or restore coolant
- remove decay heat
- protect containment barriers
Why Shutdown Alone Is Not Enough
Suppose control rods are fully inserted.
The fission chain reaction rapidly decreases.
However:
radioactive fission products remain
They continue undergoing radioactive decay.
Therefore:
decay heat continues
This means cooling remains essential after shutdown.
Nuclear Reactor vs Nuclear Bomb
A nuclear power reactor is not simply a nuclear bomb operating slowly.
They differ fundamentally in:
- fuel configuration
- enrichment
- geometry
- neutron behaviour
- engineering design
- control systems
Power reactors are designed to maintain a:
controlled chain reaction
for energy production.
Reactor Safety and Human Factors
Safety does not depend only on machinery.
Important human factors include:
- operator training
- clear procedures
- maintenance
- communication
- safety culture
- emergency planning
- independent regulation
Engineering systems and human systems must work together.
Radioactive Waste
Nuclear reactors produce radioactive materials, including fission products and spent fuel.
Spent fuel must be:
- cooled
- shielded
- securely stored
- monitored
- eventually managed according to long-term waste strategies
Radioactive waste management is therefore part of the complete nuclear fuel cycle.
Spent Fuel Pools
Recently removed spent fuel produces significant:
- radiation
- decay heat
It can initially be stored underwater in:
spent fuel pools
Water provides:
- cooling
- radiation shielding
As decay heat decreases over time, other storage methods may become appropriate.
Dry Cask Storage
After sufficient cooling, some spent nuclear fuel can be transferred to:
dry storage casks
These use robust containers made from materials such as:
- steel
- concrete
The systems provide:
- shielding
- containment
- passive heat removal
Advantages of Nuclear Power
Potential advantages include:
- very high energy density
- large-scale electricity production
- reliable power generation
- low direct carbon dioxide emissions during operation
- relatively small fuel quantities
- reduced dependence on combustion for electricity generation
Challenges of Nuclear Power
Challenges include:
- radioactive waste management
- high construction costs
- complex safety requirements
- decommissioning
- accident consequences
- security requirements
- long construction times for some projects
Scientific evaluation should consider both:
benefits and risks
Worked Example: Control Rods
A reactor is producing more power than required.
What should the control system do?
Insert control rods farther into the core.
This causes:
more neutron absorption
↓
fewer neutrons available for fission
↓
fewer fission reactions
↓
lower reactor power
Worked Example: Moderator
Fast neutrons are produced by fission.
They collide repeatedly with nuclei in the moderator.
Their kinetic energy decreases.
The neutrons become slower.
For uranium-235 in a thermal reactor:
slower neutrons → greater probability of further fission
Therefore, the moderator helps maintain the chain reaction efficiently.
Worked Example: Energy Conversion
Suppose a nuclear power station produces:
3600 MW
of thermal power and:
1200 MW
of electrical power.
Efficiency:
= 1200/3600 × 100
= 33.3%
Approximately two-thirds of the energy is ultimately transferred to the environment, mainly as thermal energy.
Comparing Important Reactor Components
| Component | Main Function |
|---|---|
| Fuel | Undergoes nuclear fission |
| Fuel rods | Contain nuclear fuel |
| Moderator | Slows neutrons |
| Control rods | Absorb neutrons |
| Coolant | Transfers thermal energy away from core |
| Reactor vessel | Contains core and primary coolant in many designs |
| Steam generator | Transfers heat to produce steam in PWRs |
| Turbine | Converts steam energy into mechanical rotation |
| Generator | Converts mechanical energy into electrical energy |
| Condenser | Condenses exhaust steam |
| Containment | Provides major barrier against radioactive release |
Moderator, Coolant and Control Rods
These three are commonly confused.
Remember:
Moderator → controls neutron SPEED
Control rods → control neutron NUMBER
Coolant → controls HEAT TRANSFER
This is a useful way to distinguish their functions.
Common Mistakes
Mistake: Saying control rods slow neutrons.
Control rods primarily absorb neutrons.
Mistake: Saying the moderator absorbs neutrons.
The moderator's primary purpose is to slow neutrons without absorbing too many.
Mistake: Saying coolant controls the chain reaction directly.
The coolant's main function is to transfer thermal energy away from the core, although coolant conditions can also influence reactor physics in some designs.
Mistake: Saying the reactor directly generates electricity.
The reactor produces thermal energy. A turbine and generator ultimately produce electricity.
Mistake: Saying control rods stop all heat immediately.
Decay heat continues after the fission chain reaction is stopped.
Mistake: Saying cooling towers release radioactive smoke.
The visible plume from a normal cooling tower is primarily condensed water droplets.
Mistake: Saying the cooling tower is the reactor.
The reactor is located within the nuclear facility; cooling towers are part of the heat-rejection system.
Mistake: Thinking "critical" means the reactor is in danger.
A critical reactor has a steady self-sustaining chain reaction.
Mistake: Assuming nuclear safety depends on one protective wall.
Modern reactor safety uses multiple barriers and overlapping safety systems.
Key Terms
Nuclear reactor — system designed to maintain and control a nuclear fission chain reaction.
Reactor core — region containing nuclear fuel where fission occurs.
Fuel rod — tube containing nuclear fuel pellets.
Fuel assembly — group of fuel rods arranged together.
Control rod — neutron-absorbing component used to regulate the fission chain reaction.
Moderator — material used to slow neutrons.
Coolant — substance that transfers thermal energy away from the reactor core.
Critical — condition where a fission chain reaction remains approximately steady.
Reactor pressure vessel — strong vessel containing the reactor core and coolant in many reactor designs.
Steam generator — heat exchanger that produces steam in the secondary circuit of a PWR.
Turbine — device rotated by moving steam.
Generator — device that converts mechanical energy into electrical energy.
Condenser — equipment that converts exhaust steam back into liquid water.
Containment structure — strong barrier designed to limit radioactive release.
Decay heat — thermal energy produced by continuing radioactive decay after reactor shutdown.
Defence in depth — safety strategy using multiple independent or overlapping protective layers.
Redundancy — use of multiple systems capable of performing the same safety function.
Passive safety — safety systems that rely partly on natural physical processes such as gravity or convection.
Scram — rapid reactor shutdown.
Key Takeaways
- A nuclear reactor maintains and controls a nuclear fission chain reaction.
- Nuclear fuel is located inside the reactor core.
- Uranium-235 is an important fissile reactor fuel.
- Nuclear fuel is commonly formed into pellets contained in fuel rods.
- Control rods absorb neutrons.
- Inserting control rods generally decreases reactor power.
- Withdrawing control rods can increase reactor power.
- Moderators slow neutrons.
- Slow neutrons are particularly effective at causing uranium-235 fission in thermal reactors.
- Water, heavy water and graphite can be used as moderators.
- Coolant transfers thermal energy away from the reactor core.
- Water can act as both moderator and coolant in many reactor designs.
- A PWR keeps its primary water under high pressure and produces steam in a separate secondary circuit.
- A BWR produces steam directly in the reactor vessel.
- Nuclear power stations transform nuclear → thermal → mechanical → electrical energy.
- Steam rotates a turbine.
- The turbine drives a generator.
- A condenser converts exhaust steam back into liquid water.
- Nuclear reactors continue producing decay heat after the fission chain reaction is shut down.
- Continued cooling is therefore essential.
- Modern reactors use multiple physical barriers against radioactive release.
- Safety systems can include rapid shutdown, emergency cooling, containment, backup power and radiation monitoring.
- Defence in depth means relying on multiple layers of protection rather than a single safety system.
- Spent nuclear fuel remains radioactive and requires cooling, shielding and secure management.
Check Your Understanding
- What is the main purpose of a nuclear reactor?
- Name four important components found in or associated with a reactor core.
- What nuclear fuel is commonly used in commercial reactors?
- Describe the relationship between fuel pellets, fuel rods and fuel assemblies.
- What is the main function of control rods?
- What happens to reactor power when control rods are inserted farther into the core?
- Why are materials such as boron useful in control rods?
- What is the purpose of a moderator?
- Name three materials that can be used as moderators.
- Explain why slowing neutrons is useful in a uranium-235 thermal reactor.
- What is the main purpose of reactor coolant?
- Explain how water can act as both moderator and coolant.
- Describe the difference between the primary and secondary circuits of a PWR.
- What is the purpose of the steam generator?
- Describe the energy transformations from nuclear fission to electrical energy.
- What is the function of the turbine?
- What is the function of the generator?
- Why does a nuclear reactor still require cooling after the fission chain reaction has been stopped?
- Explain what is meant by defence in depth.
- Identify four safety features used in modern nuclear power plants and explain how each improves reactor safety.