How does a Nuclear Reactor Works- Sciencedoze

What is a Nuclear Reactor

A Nuclear reactor (also known as atomic pile), is a device used to initiate, terminate and control a nuclear chain reaction. It is based on the principle of controlled chain reaction and provides energy at a constant rate.

Nuclear reactors are mainly used at the nuclear power plant for electricity generation. Energy (Heat) from nuclear fission is passed to a fluid, which turns into steam and runs a turbine to generate alternating current. Some nuclear reactors are also used for producing isotopes for medical and research purposes.

Main Parts of Nuclear Reactor


Nuclear Reactor Diagram

1. Nuclear Fuel

It is a material that can be fissioned by neutrons. The isotopes such as U-235, Th-232, and Pu-239 can be used in a reactor as a fuel. A certain amount of the fuel is taken in the form of rods, kept in aluminum containers. The rods which are separated by the moderator are placed in the core of the reactor.

2. Moderator

During the fission of Uranium, fast-moving neutrons of energy 2 MeV are released. These fast-moving neutrons have more tendency to escape instead of initiating another fission reaction. Moreover, slow-moving neutrons are more efficient in initiating fission in U-235 nuclei than fast-moving neutrons.

By using a moderator, the fast-moving neutrons can be slowed down to thermal velocities. Heavy water, graphite, and Beryllium oxide are commonly used moderators in a nuclear reactor.

3. Control rods

Control rods are used in nuclear reactors to initiate, terminate or control the chain reaction. They are made up of neutron absorbing material like Cadmium or Boron. The rate of production of neutrons is controlled by changing the depth of the control rods.

4. Coolant

Coolant is a material that is used in a nuclear reactor to cool down the moderator and fuel rods. It is capable to drive away a large amount of heat produced during fission. The coolant transfers heat to liquid like water and produces steam. This steam drives a turbine that runs a generator to generate electric current.

Properties of coolant:

  • High boiling point.
  • High specific heat.
Heavy water and liquid sodium are some good examples of coolants.

5. Shielding

The neutrons and gamma radiations produced in a nuclear reactor are hazardous for the human body. To protect the workers from these radiations, the core of the nuclear reactor is surrounded by a concrete wall known as the reactor shield.

Working of Nuclear Reactor

First of all, some neutrons are produced by the collision of alpha particles on polonium or beryllium. They are slowed down and are used to start fission of U-235 nuclei. Fast-moving neutrons are released in these fissions which are further slowed to thermal velocities by moderators. These slow-moving neutrons cause fission of more U-235, thus initiating a chain reaction. By changing the depth of the control rods, the chain reaction is suitably controlled.

Classifications of Nuclear Reactor

1. By Nuclear Reaction

All nuclear reactors are based on the principle of nuclear fission and contain U-235 as nuclear fuel.

Nuclear reactors can be divided into two parts, depending upon the energy of moving neutrons that control the nuclear chain reaction:
  • Thermal neutron reactors used slow-moving or thermal neutrons to sustain the fission chain reaction. These contain moderators that slow down neutrons until their temperature is thermalized.
  • Fast neutron reactors use fast-moving neutrons to initiate fission reactions. They do not contain moderator and use less or no moderating coolants. Sustainability of chain reaction requires radioactive fuel to be more enriched in fissile material due to lower chances of fission.

2. By Moderator Material

These are the following moderator material used by thermal reactor:
  • Graphite moderator reactors
  • Water moderator reactorsThese are further classified into two types:
    • Heavy water reactors, commonly used in Canada, India, China, etc.
    • Light water reactors (the most common type of thermal reactor).
  • Light element moderator reactors
    • Molten salt reactors (MSRs) are moderated through light elements like beryllium and lithium, which are parts of coolant salts.
  • Liquid metal cooled reactors contain coolant mixture of lead and bismuth.
  • Organically moderator reactors use biphenyl and terphenyl as coolant.

3. By Coolant

  • Water-cooled nuclear reactor: Many operational nuclear reactors are based on the water-cooled reactor. As of the year 2014, 93% of the world's nuclear reactors are water coolant based and they are responsible for 95% of the total nuclear generation.
  • Liquid metal cooled nuclear reactor: Water is a moderator and cannot be used as a coolant in many fast reactors. So, Liquid metal coolants based reactor is used which contains sodium, lead, and mercury as coolants.
  • Gas-cooled reactors: In these reactors, an inert gas is used as a coolant. Helium, carbon dioxide and nitrogen used as a coolant in these reactors.
  • Molten salt reactors (MSRs): These reactors are cooled by passing a molten salt (fluoride salts eutectic mixture) like FLiBe.

4. By Generations

  • Generation 1 Reactor: They are research reactors, non-commercial current producing reactors.
  • Generation 2 Reactor: They are currently in usage.
  • Generation 3 Reactor: They have undergone improvements in existing designs (1996-present).
  • Generation 4 Reactor: Technologies are under development.

5. By Phase of Fuel

  • Use solid as fuel.
  • Use the liquid as fuel.
  • Use Gas as fuel (Practically not developed yet).

6. By Shape of the Core

  • Cubical in shape.
  • Cylindrical in shape.
  • Octagonal in shape.
  • Spherical in shape.

Nuclear Waste

The remains of nuclear fuel used in a nuclear reactor are called Nuclear waste. This waste sometimes called "used fuel", is hazardous and can remain as it is in nature for 1000 years.

Nuclear Waste

Nuclear waste is kept underwater (excellent shield) for a few years until nuclear radiation decays to a low level and then it is shielded by large concrete casks. Finland is the leader in nuclear waste disposal.

Nuclear power plants keep all produced waste on their site, so the environment around it is considered as nuclear positive.

Uses of Nuclear Reactor

  • In the preparation of radioisotopes which are used in scientific research, medicine, agriculture, and industries.
  • In the generation of Electric Current.
  • In the production of fast neutrons which are used in the nuclear bombardment.
  • In producing fissile material like plutonium which is extensively used in atomic bombs.

References


Simplified Physics: S.L Arora

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