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Nuclear Reaction refers to the process by which a nucleus and a subatomic foreign particle collide to give rise to one or more nuclides. This merge may also take place between two nuclei. A nuclear reaction witnesses the production of a massive amount of energy. Nuclear reactions keep occurring in our environment. They are of different types which exhibit an important application in the generation of energy. Let us get a better understanding of nuclear reaction, their types, and their importance in life.
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Key Terms: Nuclear reaction, Nuclear Fission, Nuclear Fusion, Subatomic particles, nuclear reaction, nuclear decay.
What is Nuclear Reaction?
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In Nuclear Physics, a nuclear reaction can be defined as the process where one or more than one nuclides are formed due to the collision between two nuclei, or a single nucleus and a subatomic foreign particle from the exterior environment.
Nuclear Reactions
Now the question is what are nuclides? A nuclide is referred to a class of atoms that are known and characterized by their number of protons(Z), their number of neutrons(N), and their nuclear energy state.
Nuclide
Subatomic particles, in Physics, refer to a kind of particle that makes up an atom. This particle may exist in two different forms. It may be of a composite nature where it holds or is composed of other particles together. For example, protons, and neutrons. On the other hand, it could also exist independently and alone, in the form of elementary particles where it doesn't consist of other particles. For example, electrons and photons.
Subatomic particle
Therefore, we can also conclude that nuclear reaction is the conversion of at least one nuclide into the form of another. It is very essential that it satisfies this condition. In some cases, both of the separate nuclei collide, and then they go different ways without any change in their nature. Here, we cannot call this a nuclear reaction, it would rather be called a process of nuclear scattering.
Non-nuclear reactions
There are certain processes that are very similar to the nature of nuclear reactions. However, these aren't induced externally, rather they happen spontaneously. Hence, these cannot be termed as ‘nuclear reactions’. This is because nuclear reactions involve the atomic nuclei being subjected to external changes induced by the process.
Given below are certain processes that do not qualify as nuclear reactions.
Nuclear scattering: This is a process in which the separate nuclides or atomic nuclei collide with one another without undergoing any changes in their composition. This process involves a huge energy transfer. Transfer of momentum is also noticed here.
Neutron Scattering
Nuclear Decay: Also known as radioactive decay, this is a process through which energy loss occurs smoothly in an unstable atomic nucleus. Any element containing unstable nuclei will be considered as nuclear disintegration. The three primary decay are alpha, beta, and gamma. All of these three are known to give out or release one or more particles.
Radioactive decay
Spontaneous Fission Reactions: These are a certain variety of nuclear fission reactions. Such reactions don't need the assistance of a neutron, in order to move forward. Therefore, these are not induced and cannot be called a nuclear reaction.
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| Nuclei | Magnetic Flux | Unit of Radioactivity |
| Alpha particle scattering | Atomic Spectra | Bohrs model of the Hydrogen atom |
| Radioactive Decay | Electron Emission | Photoelectric Effect |
Types of Nuclear Reaction
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Nuclear reactions are categorised into two major types. Both play an equal and unique role in the production of energy. They are:
Nuclear Fission
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Nuclear fission is defined as the process where an atomic nucleus splits up into two or more parts, accompanied by the release of tremendous energy. Here, usually the larger or the heavier nucleus is called the ‘parent’ nucleus, and when it splits up into parts, the lighter part is known as the daughter nucleus.
Nuclear Fission of U-235
For example, in the case of a large nucleus like uranium-235, when nuclear fission occurs, a huge amount of energy is released. The quantifiable measure of this energy is so much that there is a visible loss in the mass of the nuclei. It can be said that some portion of the mass of the heavy nuclei was converted to energy during the process. The amount of mass which is lost in the nuclear fission process, when measured, is equal to about 3.20×10−11 J of energy.
Nuclear Fission vs Nuclear Fusion
This kind of process is very common in situations when a large nucleus which is unstable by nature, is hit by a low energy thermal neutron. Although less heavy or lighter nuclei are formed, the process of nuclear reaction also releases neutrons. When we say, it is unstable, it usually means that there is a certain imbalance in the forces present inside the nucleus.
Now, why does this fission occur? As we know that it is a kind of energy that disturbs the balance between the positively-charged protons and the nuclear Force which holds together the protons along with neutrons. This disturbance causes the nucleus to start oscillating. The repulsion force present may not be affected by the attraction, which ultimately forces the atom to split up.
Energy released during nuclear fission
The figure shows the nuclear fission of one kg of uranium. It releases the same amount of energy that is released when we burn 4 billion kgs of coal.
Uses of nuclear fission
- Nuclear fission is devised to help generate energy within a nuclear power plant. This process is not very reliable and safe. The reason behind this is a huge amount of nuclear waste is given out which deteriorates the natural quality of the environment. It affects people too which is why it is advised for people to not carelessly step into nuclear power plants. People can resort to fossil fuels for much safer use of energy.
Nuclear Power Plant
- Nuclear fission is also used in nuclear power plants to generate heat. This heat is further used to produce steam from water. This steam is essential for the working of power electrical generators.
- Uranium and plutonium are excellent examples of nuclear fission. These are most commonly used in nuclear power plants and reactors. This is done for creating fission reactions because they are easy to control in magnitude. The energy released in the process makes the water subject to high temperatures. It gives rise to steam as a result. Turbines are able to work smoothly because of this steam and produce carbon-free electricity.
Boiling water reactor
- The mechanism of nuclear fission is also used by the government as a method of rocket propulsion. This was first initiated by the US government, which took this step in the 1960s. However, with regards to the Test Ban Treaty, everything was shut down when it was signed. This was done with a view of protecting the environment and saving mankind from nuclear explosions.
Nuclear Fusion
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In the process of nuclear fusion, two nuclei that are lighter by nature come together to merge and thereby form a heavier nucleus. A tremendous amount of energy is released in this merge. The reason behind this is the mass of the resultant nucleus is very less than the initial two separate nuclei.
Einstein’s equation (E=mc2), explains the major reason nuclear fusion occurs. It shows evidence of how mass and energy can be converted into forms of each other, without any change in composition. If we could devise machines, which would help harness such power of fusions from Earth, a lot of energy could be produced for important purposes.
Nuclear Fusion Reaction
The fusion of deuterium-tritium (DT) falls under the mechanism of such a reaction. DT fusion produces two separate nuclei. One is a neutron and the other one is helium. During the reaction, it releases more energy. These are very promising in producing huge amounts of energy at lower temperatures.
Nuclear Fusion in Sun
We can see the traces of nuclear fusion in the sun. It serves as the best example where when two hydrogen atoms come together and fuse together to form one helium atom. This is the reason behind the energy source in the sun. The temperature at the sun's core happens to be around 15 million degrees Celsius. Nuclear fusion occurs when two isotopes of Hydrogen, Deuterium, and Tritium, fuse to form Helium. This happens when they are subjected to a high temperature for a longer period and give out a lot of energy.
No radioactive products are released here unlike in that of nuclear fission. Also, the energy produced here is more than that produced in Nuclear fission reactions.
Difference between Nuclear Fission & Nuclear Fusion
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The key differences between nuclear fission and nuclear fusion are tabulated below
| Nuclear Fission | Nuclear Fusion |
|---|---|
| In this process of nuclear fission, a heavy nucleus breaks down or splits up so as to form two lighter nuclei. | Nuclear fusion is defined as the process in which two nuclei merge themselves together to form a nucleus that is heavier than them. |
| Nuclear fission involves the mechanism of a chain reaction which can be dangerous. | There is no chain reaction involved here. |
| It is certain that nuclear waste will be produced because of fission. It pollutes and corrodes the environment. | Nuclear fusion does not involve nuclear waste as a byproduct. |
| The nucleus is heavy and is bombarded with neutrons from all sides. | As the nuclei are lighter in nature, they have to be subjected to long periods of high temperature. |
| For example, raw materials like plutonium and uranium are expensive, and scarce at the same time. | For example, raw materials are easy to find. |
Difference between Fission and fusion
Things to Remember
- Nuclear reaction refers to the process of one or more nuclides forming as a result of the collision taking place between two nuclei.
- This merge also may be between a single nucleus and a subatomic particle from outside. The process involves the release of huge amounts of energy which may be used for many purposes.
- Nuclear reactions are broadly classified into two major classes. These are nuclear fission and nuclear fusion.
- Nuclear fission is the process when the heavier atomic nucleus splits up into two or more parts. The parts it is broken into is called the daughter nucleus, where the original and heavier one is known as the parent nucleus. Nuclear power plants use this method to produce energy.
- When two or more atoms combine together to form a single atom, the process is called nuclear fusion. The atoms are subjected to very high temperatures for a longer period of time in this kind of fusion. This reaction usually takes place in the sun and stars.
- Nuclear fission is used in power plants to help generate electricity.
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Sample Questions
Ques. What is a nuclear reaction? (2 marks)
Ans. In Nuclear Physics, the nuclear reaction has been defined as a process in which two nuclei or a singular nucleus collide with a subatomic particle from the external environment, so as to produce nuclides. These nuclides may be one or more than one in number. The process has to have caused the transformation of at least one nuclide to the form of another for the process to be whole. The basic nature of a nuclear reaction always includes two reactive elements: a heavy target particle and a light bombarding particle.
Ques. How is fission used in energy generation? (2 marks)
Ans. Nuclear fission is of great importance when it comes to the production of energy. It is used in nuclear power plants, to help develop heat energy. This heat energy further helps produce steam from water. Steam is an essential element that charges up the turbines to work. The turbines spin and produce electricity that is carbon-free. The heat energy produced is also used to run electrical generators.
Ques. What are the different types of nuclear reactions? (2 marks)
Ans. There are two primary categories of nuclear reaction. These are nuclear fission and nuclear fusion. These are the very essential nuclear processes when it comes to the generation of energy. Fission is the process where the large atom splits up into two lighter atoms. For example - Plutonium. On the other hand, in fusion, two lighter nuclei collide and merge so as to form a heavy atom. It should be kept in mind that certain processes may appear like nuclear reactions but may not be the same. For example, nuclear scattering, radioactive decay, and spontaneous fission reactions.
Ques. What are the harmful effects of nuclear reactions? (3 marks)
Ans. Nuclear reactions, in turn, produce nuclear energy, which disturbs the ecosystem in different ways.
- They produce tonnes of radioactive waste. We are constantly exposed to minute particles of radioactivity from sources like cosmic rays. We are completely unaware that we end up inhaling this air around us. This leads to serious health issues.
- Nuclear power plants are not very reliable. These areas are prone to accidents. They release toxic radiation, for example, the Chernobyl incident. It witnessed the death of more than 10,000 people, due to being exposed to such radiation for a long time.
- Nuclear emissions pollute the environment to a greater extent. They affect the natural quality of the air as well. When coming upon contact with water, they also corrode aquatic life and cause soil erosion. Agriculture is heavily affected by this.
Ques. What is fusion? (2 marks)
Ans. In Nuclear Physics, nuclear fusion can be defined as the process of two separate and lighter nuclei merging together in order to form a heavier one. This nucleus thus framed, is heavier in mass than the two light ones. There is no chain reaction in this process or any form of nuclear waste. The nuclear fusion process releases huge amounts of energy.
Ques. Give some examples of nuclear fusion. (2 marks)
Ans. The Sun is the best example of nuclear fusion. Inside it, four hydrogen atoms, lighter in nature, come together, so as to merge in order to form helium.
Ques. What is the main point of difference between nuclear fission and nuclear fusion? (2 marks)
Ans. Nuclear fission is the process of splitting up the atomic nucleus into two or more parts, whereas on the other hand, in nuclear fusion, two lighter nuclei collide and merge so as to form a single heavy nucleus. Both of the processes witness release of lots of energy. In fission, nuclear waste is sure to be a byproduct of the process, but no waste is produced in fusion.
Ques. Can stars be called examples of nuclear fusion? (2 marks)
Ans. Yes, stars are definitely good examples of nuclear fusion. It is the process of nuclear fusion itself which pumps up the stars with tremendous heat and light energy. Many lighter nuclei merge together to make a heavier one. Stars are fueled by the mechanism of nuclear fusion in their cores. Inside the core, they convert hydrogen into helium.
Ques. Why do stable nuclei never have more protons than neutrons? (5 marks)
Ans. The reason is that protons, being charged particles, repel each other. This repulsion becomes so great in nuclei with more than 10 protons or so, that an excess of neutrons which produce only attractive forces, is required for stability.
Important point: As you get to heavier elements, with each new proton you add, there is a larger repulsive force. The nuclear force is attractive and stronger than the electrostatic force, but it has a finite range. So you need to add extra neutrons, which do not repel each other, to add extra attractive force. You eventually reach a point where the nucleus is just too big and tends to decay via alpha decay or spontaneous fission.
To view this in quantum mechanical terms, the proton potential well is not as deep as the neutron well due to the electrostatic repulsion. [Due to the Pauli exclusion principle, you only get two particles per level (spin up and spin down)]. If one well is filled higher than the other, you tend to get a beta decay to even them out. As the nuclei get larger, the neutron well gels deeper as compared to the proton well and you get more neutrons than protons.
Ques. State why in a nuclear reactor, moderators slow down the neutrons which come out in a fission process. And why does the moderator has light nuclei while heavy nuclei will not serve the purpose? (5 marks)
Ans. A moderator is a material used in a nuclear reactor to slow down the neutrons produced from fission. By slowing the neutrons down the probability of a neutron interacting with Uranium-235 nuclei is greatly increased thereby maintaining the chain reaction. Moderators are made from materials with light nuclei which do not absorb the neutrons but rather slow them down by a series of collisions.
The moderator only slows neutrons down in order to increase the interaction with Uranium nuclei. They do not give any protection if the reaction goes out of control. If a chain reaction is heading out of control the reactors needs to be able to reduce the concentration of neutrons. For this, the reactor uses control rods. Control rods are made from a material that has the ability to absorb neutrons. Cadmium and Boron are examples of suitable materials. By inserting. control rods between the fuel rods the chain reaction can be slowed dowp-or shut down. Withdrawing the control rods can restart or speed up the reaction.
In our given question, the moderator used have light nuclei (like proton). When protons undergo perfectly elastic collision with the neutron emitted their velocities are exchanged, i.e., neutrons come to rest and protons move with the velocity of neutrons.
Heavy nuclei will not serve the purpose because elastic collisions of neutrons with heavy nuclei will not slow them down.
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