Nuclear Reactor - JEE Main Important Topic

Nuclear reactors are the devices which can initiate and control neutron-induced nuclear fission reactions. A nuclear reactor definition can be “It is a reactor which controls the chain reactions initiated by bombarding large fissile atomic nuclei such as Uranium-$235$ or Plutonium-$239$ with neutrons and produces a large amount of energy”. Nuclear reactors produce radioactive isotopes for research purposes, and they are majorly used in nuclear power plants as energy sources. Around $11\%$ of the world's total electricity is produced using nuclear reactors.

Working Principle of Nuclear Reactor

The fissile atomic nuclei like uranium-$235$ or plutonium-$239$ when bombarded with neutrons initiates a nuclear fission chain reaction. During this reaction, the fissile atom splits into two or more lighter nuclei and releases a large amount of kinetic energy, gamma radiation, and free neutrons. A part of these free neutrons is absorbed by other fissile atoms and triggers a nuclear chain reaction with the release of more neutrons and energy. These reactions can be controlled by absorbing/ changing a portion of neutrons released using control rods containing neutron poisons and neutron moderators. Heat is generated in the reactor core by:

• Conversion of kinetic energy produced by fission products to thermal energy when these products collide with nearby atoms.

• The gamma rays produced during fission get absorbed by the reactor and produce heat.

• Heat is also produced during the fission chain reaction.

A kilogram of uranium-$235$ $(U-235)$ converted via nuclear processes releases approximately $7.2 \times 10^{13}$ Joules of energy.

Reactions

As an example, we can see the fission reaction of Uranium-$235$ when bombarded with a neutron.

\[\begin{align} & {}_{92}^{235}U+{}_{0}^{1}n\to {}_{92}^{236}U\to {}_{56}^{144}Ba+{}_{36}^{89}Kr+3{}_{0}^{1}n \\ & or \\ & {}_{92}^{235}U+{}_{0}^{1}n\to {}_{54}^{140}Xe+{}_{38}^{94}Sr+2{}_{0}^{1}n \\ \end{align}\]

The products of fission are radioactive nuclei and they emit $\beta$ particles to achieve stable end products. The energy released is in the order of $200~MeV$ per fissioning nucleus.

As we can see the extra neutrons released during the fission reaction, in turn, initiates another fission reaction, producing more neutrons, and so on. This leads to a chain reaction. This chain reaction happens in a nuclear reactor but in a controlled manner. The uncontrolled chain reaction leads to an explosion, e.g., the nuclear bomb.

Construction of Nuclear Reactor

The components of a nuclear reactor are,

1. Fuel Rods: Nuclear fuel is the fissionable material used in nuclear reactors for conducting nuclear fission reactions to produce energy. Uranium enriched with $U-235$ or Plutonium is generally used as fuel. This fuel in pellet form is packed into fuel rods. These rods are surrounded by cladding to avoid leaking into the coolant. The assembly of fuel rods is called a fuel bundle. In a reactor, there may be hundreds of fuel bundles.

2. Moderator: The fast-moving neutrons produced, initiates an uncontrollable chain reaction in a nuclear reactor. Moderators are connected along with the fissionable nuclei for slowing down fast neutrons. They are molecules like water, heavy water, graphite and deuterium. As we can see the coolant like water also can act as a neutron moderator. A moderator causes the fast neutrons to lose energy and become thermal neutrons. Thermal neutrons cannot start a fission reaction.

3. Control Rods: Rods of boron or cadmium are inserted inside the reactor to capture the slow neutrons which cause further fission reaction. The slow neutrons lead to a violent chain reaction. These rods are called control roads. 

4. Coolant: The heat energy produced during the fission reaction in a nuclear reactor is transferred to the coolant. The coolant then gets heated and it transfers this energy to water in a heat exchanger and the generated steam is then used to produce electricity. In coolants used are liquid sodium metal, carbon dioxide gas or water. 

5. Shielding: The whole reactor is shielded using $2$ to $2.5$ meters thick concrete walls. This prevents the radiation emitted during the nuclear reaction from coming outside the reactor. This helps to prevent harmful effects on the persons working on the reactor.

Nuclear Reactor Uses

The following are the some of the nuclear reactor uses,

• The main application of nuclear reaction is in energy production in the form of electricity.

• The energy produced in this reactor is used for propulsion in vehicles such as submarines or naval vessels, rockets etc.

• The heat generated is also used for desalination, industrial heating, hydrogen production etc.

• Radioactive isotopes, like americium (used in smoke detectors), cobalt-$60$, molybdenum-$99$ etc. used for imaging and medical treatment are produced in nuclear reactors.

• Weapons-grade plutonium used in nuclear weapons is produced in nuclear reactors.

• Nuclear Reactors are used to generate neutron beams of high intensity. This neutron beam is used for the treatment of cancer and for nuclear research.

Conclusion

The nuclear reactor, are reactors which can initiate and control a self-sustaining chain of nuclear fission reactions. Nuclear reactors are used mainly for electricity generation, energy sources, production of radioactive isotopes etc. The initial capital cost for the building of nuclear reactors is very high. But they produce large amounts of energy in a small volume of fuel. Proper safety measures and control of such reactors are very essential. The major catastrophic accident happened in the Chernobyl reactor in Ukraine in 1986. The nuclear wastes generated, need special treatment techniques and disposal methods, as they are radioactive and hazardous.