What is Nuclear Fission?

Nuclear fission is a type of nuclear reaction in which heavier nuclei split into lighter nuclei by releasing a large quantity of energy. The elements like uranium-235 and plutonium-239 isotope undergo nuclear fission releasing energy. When nuclei undergo fission, some quantity of mass is lost. The lost mass is converted to nuclear energy. Nuclear fission reaction is used in nuclear power plants and atomic bombs. In nuclear power plants, nuclear reactors are used to generate electricity.

Theory of Nuclear Fission

Nuclear fission reaction is observed in elements with heavier nuclei. Usually, the high-speed neutron is made to collide with the heavy nucleus. The nucleus splits into two or more stable nuclei. The protons and neutrons present in the nucleus of an atom are bound together with nuclear force. The energy associated with this force is called binding energy. The binding energy is the energy required to split the constituents of the nuclei into its nucleons.

The binding energy depends upon the stability of the nuclei. Stable nuclei have more binding energy than unstable nuclei. Hence, nuclei with less binding energy undergo nuclear fission easily. During fission, some amount of mass is lost this mass difference is called the mass defect. The energy released can be calculated by multiplying the square of the speed of light with the mass defect.

Every heavy nucleus cannot undergo nuclear fission. For fission to occur, the binding energy of the daughter nuclei should be greater than the binding energy of the parent nuclei. Some isotopes can undergo fission on their own by releasing certain radiation. This process is called radioactivity.

Fission of Uranium-235

Let us study nuclear fission with the most common example, U-235 nuclei which are bombarded with a high-speed neutron. U-235, has got 143 neutrons and 92 protons. Hence, the atomic mass of the nuclei is 235. The nuclear reaction occurs as follows.

U23592+n10→B14456a+K8936r+3n10 

The U-235 nucleus undergoes fission to form two lighter nuclei, they are Ba-144 isotope (barium) and Kr-89 isotope (krypton). Also, three neutrons are formed as a byproduct. During this process, a huge quantity of energy is released in the form of radiation and heat.

During this process, three neutrons are formed as byproduct. These neutrons can induce further nuclear fission. These neutrons are bombarded with the daughter nuclei isotopes and the daughter nuclei undergo further nuclear fission. This process is called nuclear chain reaction. The nuclear chain reaction is uncontrollable. A huge quantity of energy is produced during the chain reaction.

The chain reaction can be controlled using various techniques. These type of nuclear fission reaction is known as controlled nuclear reaction. This type of reaction is used in nuclear power plants. The nuclear energy produced in the controlled nuclear reaction can be used to generate electricity.

The energy released during fission

The energy released during the fission reaction can be calculated using,

E=Δmc2 

Where,

E is the nuclear energy (in MeV).

Δm  is the mass defect i.e. the mass difference between the parent nuclei and daughter nuclei (in kg or amu).

c is the velocity of light.

Nuclear Fission reactor

The nuclear fission reaction can be controlled so that the energy produced during the reaction is utilized to generate electricity. In nuclear power plants, electric energy is generated by extracting the heat energy released from the nuclear reactors. About 10 percent of the world’s electric energy is generated by nuclear reactors. The following diagram represents a typical nuclear reactor.

In nuclear reactors, the energy produced during nuclear reactions is extracted using water. The water is converted to steam which is used to run the turbine to generate electricity.

The nuclear fission reactors consist of the following important components:

• Fuel: Fuel is the most important component in a nuclear reactor. Uranium is the common fuel used in nuclear reactors. U-238 isotope is the most abundant isotope. But this isotope is not used as fuel. U-235 is the most common isotope which is used as nuclear fuel.
• Coolants: Coolants are fluids used to carry heat from the reactor to the energy generating unit. Usually, water is used as coolant.
• Moderators: Moderators are the substance that are used to slow down the fast-moving neutron. Graphite or heavy water is used as moderators. Heavy water is formed by reaction isotope of hydrogen with oxygen.
• Control rods: Control rods help in controlling the nuclear chain reaction by absorbing the neutron formed as byproducts. Control rods are important to control the energy produced in the reactors.
• Lead-coated walls: During nuclear fission, dangerous and power radiations are emitted. To prevent the exposure of the radiations. The nuclear reactor is placed in a container with thick concrete walls coated with lead. Lead is a good absorber. It absorbs most of the harmful radiations.

Advantages and disadvantages of nuclear fission

Nuclear fission reaction has got its own merits and demerits.

Advantages

• Nuclear fission is cost-effective.
• Nuclear fission produces less waste hence it is eco-friendly.
• It is used in long-term energy production.
• The quantity of fuel required is less compared to other sources.
• Efficient waste management.

Disadvantages

• Nuclear reactors require several safety precautions.
• Production of radioactive waste.
• Risk of failure and explosion of nuclear reactors.
• Nuclear fuel is expensive.

Formulas

The energy released during the fission reaction can be calculated using,

$E=\mathrm{\Delta m}{c}^2$

Where,

E is the nuclear energy (in MeV).

$\mathrm{\Delta m}$ is the mass defect i.e. the mass difference between the parent nuclei and daughter nuclei (in kg or amu).

c is the velocity of light.

Context and Applications

This topic is significant in physics for both undergraduate and graduate courses, especially for Masters in physics, and Bachelors in physics.

Practice Problems

Question 1: Which of the following is a fission fuel?

(a) Barium

(b) Lead

(c) Uranium

(d) Hydrogen

Answer: Option (c) is correct.

Explanation: Uranium is the most common nuclear fission fuel. It is one of the abundant nuclear fuels.

Question 2: In nuclear fission, ____ is used as nuclear fuel.

(a) Uranium-233

(b) Uranium-236

(c) Uranium-235

(d) Uranium-238

Answer: Option (c) is correct.

Explanation: The isotope Uranium-235 is the main fuel source used in nuclear fission. Compared to other isotopes very less energy is required to split the nucleus of Uranium-235. Hence the reaction is feasible.

Question 3: The control rods are used in nuclear reactors to ____.

(a) Slow down the neutrons

(b) Absorb neurons

(c) Initiate reaction

(d) Absorb protons

Answer: Option (b) is correct.

Explanation: The purpose of the control rod is to control the nuclear reaction. The control rods prevent the chain reaction by absorbing excess neutrons.

Question 4: Heavy water is used as ____.

(a) Moderators

(b) Coolants

(c) Absorbents

(d) Fuels

Answer: Option (a) is correct.

Explanation: Control rods are used to slow down the fast-moving neutrons. Usually, heavy water or graphite is used as moderators.

Question 5: Uncontrollable nuclear fission reaction occurs due to ___.

(a) Mass defect

(b) Increase in temperature

(c) proton-proton reaction

(d) Chain reaction

Answer: Option (d) is correct.

Explanation: In the uncontrolled reaction, the neutrons formed as byproduct collides with the daughter nuclei to initiate a chain reaction.