Thermal Reactor vs Fast Reactor: Difference and Comparison

Key Takeaways

  1. Neutron Speed: The fundamental difference between thermal reactors and fast reactors is the energy level of the neutrons causing fission. Thermal reactors use slow or “thermal” neutrons to sustain the nuclear chain reaction, whereas fast reactors use high-energy or “fast” neutrons.
  2. Fuel and Efficiency: Thermal reactors primarily use uranium-235 or plutonium-239 as fuel, which are fissile with both fast and slow neutrons. However, fast reactors can also efficiently utilize uranium-238, a much more abundant isotope, and can also “breed” more fuel (plutonium-239) from this, leading to better fuel utilization and less waste.
  3. Moderation and Cooling: Thermal reactors require a moderator, like water or graphite, to slow down neutrons to thermal energies, which also acts as a coolant in many designs. Fast reactors, on the other hand, don’t use a moderator, as the objective is to maintain neutron speed. Instead, they use substances like liquid sodium, lead, or molten salt for cooling, which don’t slow down the neutrons.

What is Thermal Reactor?

A thermal reactor is a type of nuclear reactor that relies on thermal neutrons to sustain a fission chain reaction. It is a crucial component of nuclear power plants and research reactors. The term “thermal” refers to the fact that the neutrons used in the fission process have been slowed down, or moderated, to lower energies, making them more effective at causing fission in certain isotopes of uranium and plutonium.


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The basic principle behind a thermal reactor is to control the rate of nuclear fission by using a moderator material, which slows down the fast neutrons produced during fission. The most commonly used moderator in thermal reactors is either water (light water reactors) or graphite (graphite-moderated reactors).

What is Fast Reactor?

A fast reactor is a type of nuclear reactor that uses fast neutrons (neutrons with higher kinetic energy) to sustain a nuclear fission chain reaction. Unlike thermal reactors, which rely on thermalized (slowed down) neutrons, fast reactors use fast neutrons to fission certain isotopes of nuclear fuel, such as plutonium-239 and uranium-233. Fast reactors are designed to efficiently utilize nuclear fuel and produce less radioactive waste compared to conventional thermal reactors.

Fast reactors have several advantages, including the potential for increased fuel efficiency, reduced long-term radioactive waste, and the ability to utilize certain nuclear fuels that are not practical in thermal reactors. However, fast reactors also present technical challenges and safety concerns, such as the risk of rapid power increases (“prompt critical” events) and the corrosive properties of liquid metal coolants.

Difference Between Thermal Reactor and Fast Reactor

  1. Neutron Energy: The main difference between thermal reactors and fast reactors lies in the energy of the neutrons used to sustain the nuclear fission chain reaction. Thermal reactors use thermalized (slowed down) neutrons, while fast reactors use fast neutrons (neutrons with higher kinetic energy).
  2. Fuel Utilization: Thermal reactors use enriched uranium fuel; some use mixed-oxide (MOX) fuel containing a combination of uranium and plutonium isotopes. Fast reactors, on the other hand, can use a broader range of fuel types, including plutonium-239 and uranium-233, and have the potential to achieve greater fuel efficiency and generate less long-lived radioactive waste.
  3. Neutron Moderation: In thermal reactors, a moderator (e.g., water or graphite) is used to slow down neutrons, which enhances the probability of fission in certain isotopes. Fast reactors operate without a moderator or use minimal moderation, relying on fast neutrons for fission reactions.
  4. Breeding Capability: Fast reactors have the potential to be “breeders,” meaning they can produce more fissile material than they consume during operation. By converting non-fissile isotopes (e.g., uranium-238) into fissile isotopes, such as plutonium-239, fast reactors can generate additional nuclear fuel while generating energy. Thermal reactors do not have significant breeding capabilities.
  5. Coolant: Thermal reactors generally use water as a coolant and sometimes as a moderator. In contrast, fast reactors often employ liquid metal coolants, such as sodium or lead, which have good heat transfer properties and can efficiently remove heat from the reactor core.

Comparison Between Thermal Reactor and Fast Reactor

Parameters of ComparisonThermal ReactorFast Reactor
Neutron EnergyThermalized (slowed down) neutronsFast neutrons (higher energy)
Fuel TypesEnriched uranium, MOX fuelPlutonium-239, Uranium-233, and more
Fuel EfficiencyModerate fuel efficiencyHigher fuel efficiency (potential breeder)
Cooling SystemWater coolantLiquid metal coolant (e.g., sodium, lead)
Nuclear WasteProduces long-lived radioactive wasteCan reduce long-lived waste (potential breeder)
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