Nuclear Reactors

In this article, we are going to study about various kinds of nuclear reactors. We will predominantly focus on the fission nuclear reactors, as they are the ones being actively being used for energy production. Though we will also touch upon fusion nuclear reactors that are still in the research and development phase.

Fission Nuclear Reactors or Atomic Reactors

Fission Nuclear Reactors use the concepts of nuclear fission and controlled chain reaction to generate electricity.

Parts of a Fission Nuclear Reactor

In a typical Fission Nuclear Reactor, you will find the following constituents.

Fuel

Of course, a nuclear reactor will need fissionable radioactive fuel for fission process to start. It is placed at the core of the reactor.

The most commonly used atomic reactor fuel are:

* Enriched Uranium: it has greater quantity of \({U}^{235}_{92}\) isotope, than the naturally occurring \({U}^{238}_{92}\) uranium isotope. ({U}^{238}_{92}\) isotope is non-fissionable.

* Plutonium, \({Pu}^{239}_{94}\).

Now, we know how energy is produced in an atomic reactor. But for us to make use of it, we need to control it, i.e. we need a controlled chain reaction. For this we use moderators and control rods in the reactor.

Control Rods

A fission reaction releases not just 1 neutron but around 2 to 3 neutrons. For example, fission of uranium nucleus is said to release 2.5 neutrons on an average - in some fission events 2 neutrons are produced, in some 3, etc.

These extra neutrons released can initiate further fission reactions in other radioactive nuclei. That in turn will produce still more neutrons. This can quickly get out of hand and lead to an uncontrolled chain reaction, which can produce explosive energy output, just like in a nuclear bomb. This is what happened in Chernobyl reactor in Ukraine in 1986. The chain reaction in the core of the reactor got out of hand and the whole core melted with an explosion.

However, if these released neutrons are controlled suitably, we can get a steady energy output. That’s why we use control rods. Controls rods are used to absorb extra neutrons and hence control the rate of fission chain reaction.

These control rods are made out of good neutron-absorbing materials such as cadmium or boron.

Moderators

Controlling fission chain reaction is a balancing act. That’s because if we let too many neutrons start too many fission reactions, we may end up with an overheated core and a catastrophe. However, if there are too little neutrons, the chain reaction may stop.

Nuclear fission reaction produces two kinds of neutrons: slow neutrons (thermal neutrons) and fast neutrons. It has been found experimentally that:

* slow neutrons are much more likely to cause fission in \({U}^{235}_{92}\) than fast neutrons.

* fast neutrons are much more likely to escape the system, than split another nucleus. Such neutrons can sustain a chain reaction only if a very large amount of fissionable material is used.

So, we need enough number of slow-moving neutrons in the system to enable the fission chain reaction to go on and produce energy for us constantly, even with limited amount of fissionable material. For this purpose, we use moderators.

As the name suggests, moderators slow down the fast-moving neutrons. This is done by elastic scattering of such neutrons with light nuclei of moderators. The most commonly used moderators in nuclear plants are water, heavy water (D₂O), beryllium oxide and graphite.

So, control rods are used to absorb excessive neutrons in the system, and thereby stop it from getting uncontrolled. Whereas, moderators are used to slow down fast-moving neutrons and thus increase the number of slow-moving neutrons in the system, and thereby avoiding stalling of the reactor.

Now we know how energy is produced and controlled in an atomic reactor. But how is this energy transported and used to generate electricity? – For this we use Coolant and Water.

Coolant and Water

Nuclear fission reaction constantly creates a lot of heat. This heat needs to be continuously removed:

• to keep the temperature of the core stable.
• to utilize this heat in the generation of electricity.

This is done by using a suitable coolant. The coolant keeps on removing the heat from the core of the nuclear power plant and transfers it to a working fluid, which in turn may produce steam. This steam in turn is used to run turbines and generate electricity.

The coolant used may be cold water, liquid oxygen, etc. If the core temperature is too high, engineers even use molten metal as a coolant (after all, heat/temperature is a relative phenomenon).

Types of Fission Nuclear Reactors

Thermal reactors

In thermal reactors, we generate electricity using the energy produced by the fission of \({U}^{235}_{92}\) by slow moving neutrons.

Breeder reactors

In breeder reactors, we generate electricity using the energy produced by the fission of \({Pu}^{239}\) or \({U}^{233}\) by slow moving neutrons.

The main characteristic of breeder reactors is that they produce more fissile material than they consume. Hence the name – they breed!

* \({Th}^{232}_{90}\) (Thorium) produces \({U}^{233}\) (Uranium). \({Th}^{232}_{90}\) is not fissionable, but \({U}^{233}\) is a very good nuclear fuel.

* \({U}^{238}\) (Uranium) produces \({Pu}^{239}\) (Plutonium). \({Pu}^{239}\) is a very useful nuclear fuel.

The core of a breeder reactor is much hotter than that of a thermal reactor. The temperature in the core may reach around 9000°C. Therefore, in such reactors we often employ molten metals as coolant.

Fusion Nuclear Reactors or Thermonuclear Fusion Device

Fusion nuclear reactors or Thermonuclear fusion devices are based on the principle of nuclear fusion. Though this technology is still being developed – the present fusion nuclear reactors are still in the nature of prototypes.

Such reactors try to replicate the natural thermonuclear fusion process happening within a star. If we can successfully tame this technology, humanity will get access to unlimited power.

Now, let’s have a look at some of the challenges that scientists are facing in developing this technology.

Challenges to the development of Nuclear Fusion Technology

The aim of a thermonuclear fusion device is to create a steady power source by heating the nuclear fuel to a temperature of around 10⁸ K. At such high temperatures, we get the fourth state of matter – plasma, which is a mixture of positive ions and electrons.

As you can imagine, no container in the world can withstand such high temperatures. Hence, the very first and basic challenge is to confine this plasma.

To confine this plasma, scientists have been using a torus-shaped machine called tokamak, and an alternating magnetic field of a very large magnitude (generated by mega ampere current).

The magnetic field repels the electrically charged plasma from the sides of the tokamak, and keeps it confined to the centre of the container. As the plasma does not touch the container, it cannot melt it.

Indian Atomic Energy programme

The main objectives of the Indian Atomic Energy programme are:

• generate electricity in a safe and reliable manner, so as to enable country’s socio-economic growth.
• become self-reliant in all aspects of nuclear technology.

Exploration of atomic minerals started in India in the early 1950s. It soon became clear that though our country has limited reserves of uranium, we have abundant reserves of thorium. Keeping this in mind, Indian nuclear scientists have devised and adopted a three-stage strategy of nuclear power generation.

Stage I of Indian Atomic Energy programme

In this stage, we used natural uranium as a fuel, with heavy water as moderator.

India is now almost self-sufficient with regards to almost all the complex technologies related to the first stage. For example:

• Now, India has become self-sufficient in heavy water production.
• Indian nuclear scientists now have a mastery on mineral exploration and mining of uranium, as well as on the process of fuel enrichment and reprocessing.
• We have learnt to meticulously design and build nuclear reactors, and operate them.

Pressurised Heavy Water Reactors (PHWRs) built at different cities in the country mark the accomplishment of the first stage of the programme.

Stage II of Indian Atomic Energy programme

In this second stage, we make use of fast breeder reactors.

The discharged fuel from the first stage reactors is reprocessed to obtain \({Pu}^{239}\) (Plutonium-239) . This serves as the fuel for the fast breeder reactors. These reactors not only generate power, but also generate/breed more fissile species (plutonium) than they consume.

They are called “fast”, because in these reactors fast-moving neutrons are utilized to sustain the chain reaction.

Stage III of Indian Atomic Energy programme

In this stage, scientists aim to use the fast breeder reactors to produce fissile Uranium-233 from Thorium-232, and build power reactors based on them. That is, this stage is based on the utilization of thorium.

As India has an abundance of Thorium, this third stage is most significant for us from the long-term perspective. Though India is still caught up in the second stage, a lot of research work is being done on the third stage too.