Nuclear Energy

In this article, we are going to study about the various kinds of nuclear energy, its pros and cons, etc.

Table of Contents

• What is Nuclear Energy?
• Advantages and Disadvantages of Nuclear Energy
• Types of Nuclear Energy

What is Nuclear Energy?

Nuclear Energy is a kind of a non-renewable energy, wherein energy is sourced from the nucleus of radioactive atoms. It is non-renewable because the radioactive material used in nuclear power plants is not renewable – it cannot be used again.

Advantages and Disadvantages of Nuclear Energy

Nuclear energy has gained prominence over the last few decades. Though this energy source has some pitfalls and risks involved with it. Before we get into the technicalities of nuclear energy, let’s consider both its advantages and disadvantages.

Advantages of Nuclear Energy

• It’s a source of clean energy. In fact, it is the second-largest source of low-carbon electricity in the world behind hydropower.
• Nuclear energy is a highly efficient source of energy. We can produce a lot more energy per unit mass of a radioactive material (it’s in the order of MeV), than we can produce from the same amount of fossil material such as coal or petroleum (which is in the order of eV). So, nuclear sources produce millions of times more energy than chemical sources. For example, while on burning 1 kg of coal we get 10⁷ J of energy, fission of 1 kg of uranium produces 10¹⁴ J. That is because nuclear reaction is much more exothermic than exothermic chemical reactions that underlie conventional energy sources like fossil fuels.

Disadvantages of Nuclear Energy

• Though nuclear energy production does not produce greenhouse gases, this process is not completely clean. A lot of radioactive waste is produced in this process, which poses an environment risk, and a health hazard of radiation for people working in the plant and people residing nearby. Such nuclear wastes need special care in the way they are stored and/or disposed.
• Nuclear power plants are susceptible to meltdown disasters. Such a disaster can render an entire city uninhabitable for decades. This is what happened in erstwhile Soviet Union in Chernobyl nuclear plant.

Note

Exposure to nuclear radiation can seriously affect animals, plants, and even materials (e.g. buildings). The health ill-effects of radiation are classified into two categories:

• Somatic effects: They lead to increased chance of deadly diseases, such as cancer. Needless to say, that these reduce the lifespan of people.

• Genetic effects: Exposure to radiation can even cause genetic modifications, which may have far reaching consequences. Most of such gene modifications are for the worse. Moreover, as genes are passed on to the next generations, radiation exposure effects multiple-generations.

Types of Nuclear Energy

Nuclear Fission

When a heavy nucleus is split into two or more lighter nuclei and some elementary particles, this process is called Nuclear Fission.

The heavy nucleus is generally a radioactive one, and scientists use slow-moving neutrons to split it. That is, slow-moving neutrons are the particles that start the nuclear fission reaction.

Now, if the process of nuclear fission itself produces such slow-moving neutrons as a product, they will further initiate the nuclear fission reaction in other heavy nuclei, which in turn will produce more slow-moving neutrons, and so on. So, a chain of fission reaction will start, called Nuclear Chain Reaction.

For example, when these slow-moving neutrons are used to split the uranium nucleus (\(U^{235}_{92}\)), we get to see the following reaction:

\(U^{235}_{92}\) + \(n^{1}_{0}\) → \({Ba}^{141}_{56}\) + \({Kr}^{92}_{36}\) + 3 \(n^{1}_{0}\) + Energy

As you can see, the uranium nucleus has split into \({Ba}^{141}_{56}\) and \({Kr}^{92}_{36}\) along with three neutrons. A lot of energy has also been produced.

The fragment products \({Ba}^{141}_{56}\) and \({Kr}^{92}_{36}\) are themselves radioactive nuclei. That is, they are not stable – they emit β particles in succession until stable end products are produced.

The slow-moving neutrons produced start the nuclear fission reaction in the nearby nuclei. So, a nuclear chain reaction starts.

Now, this nuclear fission chain reaction can be controlled or uncontrolled.

Controlled and Uncontrolled nuclear fission chain reactions

Fission Nuclear chain reactions are of two types: controlled chain reactions and uncontrolled chain reactions.

• Uncontrolled chain reactions: If the nuclear chain reaction is not controlled, i.e. the slow-moving neutrons are not stopped (i.e. absorbed) from splitting nearby nuclei, a lot of energy is released in a very small period of time. Atom bombs are based on this principle.

• Controlled chain reactions: If the nuclear chain reaction is controlled, i.e. some of the slow-moving neutrons are stopped (i.e. absorbed) from splitting nearby nuclei, energy is released in a very controlled manner. Fission nuclear reactors that produce electricity are based on this principle.
  

Note

Note that such reactions produce both fast- and slow-moving neutrons. Fast moving electrons have a higher probability of escaping the system than causing another fission reaction. So, even if no neutrons are absorbed, there’s no guarantee that a chain reaction will be caused. Only a possibility of chain reaction is there, as first suggested by Enrico Fermi.

Nuclear Fusion

When two lighter nuclei are combined to form one heavy nucleus, this process is called Nuclear Fusion. In this process a large amount of energy is released.

For two nuclei to combine or fuse together, they need to:

• overcome the coulomb repulsion between their positively charged particles (as both nucleus have positively charged protons). To overcome this coulomb barrier, they need to have enough energy. The strength of the coulomb barrier depends on the charges and radii of the two interacting nuclei.
• come close enough so that the short-range, and attractive, strong nuclear force can affect them.

For this purpose, we need very high temperatures (approximately 10⁷ K) and also high pressure (approximately 10⁶ atmosphere), at which the particles attain enough kinetic energy to overcome the coulomb repulsive behaviour.

As in this process fusion is achieved at very high temperature, we call this process thermonuclear fusion. At such high temperatures, we get the fourth state of matter – plasma, which is a mixture of positive ions and electrons.

This is what happens inside the stars, including our sun. That is, thermonuclear fusion is the source of energy of our sun. And stars are almost entirely made up of plasma. That’s why in our universe majority of the matter is said to be found in the plasma state, rather than in gaseous, liquid, or solid state.

Equation of a typical nuclear fusion reaction occurring in our sun has been depicted below:

\(H^{2}_{1}\) + \(H^{3}_{1}\) → \({He}^{4}_{2}\) + \(n^{1}_{0}\) + Energy (17.6 MeV)

But the fusion reaction in the sun (or any other star) is not so simple. It’s a multi-step process.

• In the first step, hydrogen is burned into helium (as depicted by the equation given above). This is what happens in the star in its early life.
• As the hydrogen in the core gets on depleting, producing more and more helium, the star core starts to cool. This causes it to collapse a bit under the force of its own gravity. This in turn increases the temperature of the core. If the temperature increases to about 10⁸ K, another type of fusion takes place - now helium nuclei start to burn into carbon.
• This process keeps on repeating, creating elements of higher mass number by the process of nuclear fusion. But this process stops once enough iron is produced in the core. As iron cannot be crushed, a star cannot produce an element heavier than iron.

Iron – the poison of a star!

Iron is often called the poison of a star - as production of iron in a star leads to its eventual death. All elements that are heavier than iron, say gold, silver, uranium, etc. are formed when a star dies, i.e. in a supernova explosion. They cannot be formed inside a living star. That’s why they are rarer in nature.

However, note that not all stars die in a supernova. How exactly a star will die depends on its size. It may explode in a supernova, become a black hole, a red giant, a neutron star, etc.

Life and Death of our Sun

Once our sun nears its death, i.e. once it has burnt all its hydrogen:

• its core will begin to cool, and hence will start to collapse under gravity. It will raise the core temperature of the sun and now helium will start to burn into higher elements.

• its outer layers will expand, i.e. the sun will convert into a red giant.

The present age of the sun is around 5 × 10⁹ years. The hydrogen fuel in its core is estimated to last for another 5 billion years.

Controlled and Uncontrolled nuclear fusion chain reactions

Just like nuclear fission reactions, nuclear fusion reactions are also of two types: controlled and uncontrolled.

• Uncontrolled thermonuclear fusion: Unlike fission reaction, fusion energy production is not based on a chain reaction. So, it’s easier to control it. Fusion will only continue till high temperatures and pressure are maintained. Hydrogen bombs, which are thousands of times more powerful than fission-based atomic bombs, are based on this principle. To start the fusion process in a hydrogen bomb, atom/fission bomb is used, which acts as a primer and generates the extremely high temperature and pressure required for the fusion process.

• Controlled thermonuclear fusion: As fusion reaction is not based on a chain reaction, it’s inherently much more controlled than a fission reaction. Fusion nuclear reactors or Thermonuclear fusion devices are based on this principle. Though this technology is still being developed – the present fusion nuclear reactors are still in the nature of prototypes.