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We live in the atomic age where nuclear energy is employed in ways that are beneficial to mankind like generating power on a large scale, industrial uses, application in the field of cosmology and space explorations, etc. Being a low-cost energy source with almost zero carbon emissions, nuclear energy paves way for a promising future. However, there are other serious concerns associated with it as well, like, overloading the environment with toxic nuclear wastes, security threats, its non-renewability, and intensive usage of water. Nuclear energy is solely based on the nuclear fission reaction. The fission reaction has many complexities and a clear theoretical explanation of its pros and cons is yet to be clarified by experts. Various countries have started using technologies for their development based on this reliable and high-energy-density technique. Let’s discuss how a nuclear reactor is based on nuclear fission.
| Table of Contents |
Key Takeaways: Nuclear, Nuclear energy, Nuclear Fission, Nuclear Reactor, Atomic Bomb, Reactivity, Subcritical, Supercritical
A Brief History
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It all started in the year 1932 when the neutron, a constituent subatomic particle, was discovered by James Chadwick in England. Following this, physicists Otto Hahn and Strassman, in the year 1939, performed the famous nuclear experiment wherein they bombarded an isotope of Uranium with slow neutrons to produce transuranic elements and a very high amount of energy. Later, these transuranic elements were investigated and found to be radioisotopes of Uranium namely, Barium and Lanthanum, and some other elements in the periodic table. The most abundantly available Uranium-236 isotope was later understood to be playing a key role in this reaction. Fast neutrons were then used to bombard the Uranium 236 nucleus and it was discovered that the reaction produced energy exceeding 1 MeV and it was a self-sustaining chain reaction.
Nuclear Reactor Based on Nuclear Fission
Enrico Fermi and his associates, in the year 1942 carried out this nuclear reaction in a controlled environment, inside a nuclear reactor. This made way for the generation of new reactor models and gave birth to the Nuclear Power industry.
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Nuclear Fission- A Milestone Discovery
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Subdivision of heavy nucleus atoms like Uranium or Plutonium, by bombarding with neutrons, produces two light nuclei of relatively equal masses. This reaction is highly exothermic and produces radioactive isotopes with a large number of neutrons. This reaction is known as nuclear fission. The reaction can be a spontaneous one or could be induced by excitation of the atomic nucleus with certain particles like protons, deuterons, or even with electromagnetic radiation in the form of Gamma rays. The neutrons produced are capable of inducing fission reactions with closer fissionable nuclei and thus result in producing more energy and neutrons. The sequence hence keeps repeating itself thereby generating huge amounts of energy.
Nuclear Fission- A Milestone Discovery
When this reaction is carried out in a controlled environment like that of a nuclear reactor, the energy produced can be utilized to produce power on a commercial scale. However, an uncontrolled fission reaction can cause extensive damage to life and property, what we commonly know as an atomic bomb explosion.
Atomic Bomb Versus Nuclear Reactor
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We have so far understood that a nuclear fission reaction produces a huge amount of energy. Apart from this, recent technological advancements have enabled the application of nuclear energy in industries such as extraction of oil, desalination of water, hydrogen production, etc. Harnessing this energy and channeling it for the right use is a challenging task.
Atomic Bomb Versus Nuclear Reactor
Atomic bombs are nuclear weapons that employ massive energy from the fission reaction to produce explosions. Here the energy output is uncontrolled. Uranium atoms enriched in isotopes are bombarded with neutrons that cause the heavy atom to split into two lighter fragments, more neutrons and a high amount of energy is released. 20 % enrichment of Uranium is sufficient to make weapons-usable explosive material, however, weapon-grade is usually Uranium-235 enriched by 85%. ‘Little boy’ was the first Uranium bomb dropped to destroy the city of Hiroshima in 1945 which was 80% Uranium enriched material. The energy released per atom in a fission reaction is roughly about 180 MeV.
Nuclear reactors, on the other hand, work on the fundamental principle of fission reaction, where the overall reaction is controlled by certain parameters so that the massive energy release is targeted and harnessed appropriately. Let us understand this application of fission reaction a little better.
Nuclear Reactor
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Let us first look at a simple schematic to understand the components of a nuclear reactor that harnesses energy from fission reactions and produces power.
Major Components and Their Roles
- Reactor Vessel
The major reactor vessel is made typically from steel where the fission reaction is initiated.
- Fuel
This is the nucleus material that gets bombarded with neutrons for the fission reaction. Isotopes of Uranium like enriched Uranium Oxide in the form of cylindrical pellets encapsulated into metallic rods are used to start and simultaneously sustain the chain reaction.
- Control Rods
These are neutron absorbents which are the major controlling elements of the fission reaction. They are made from indium-cadmium or boron carbide and are critical factors in sustaining the fission reaction.
- Coolant
This is essentially water that passes through the reactor core and absorbs the excess heat and transfers this to the turbines.
- Turbines
The heat energy from the reaction is converted to mechanical energy with the help of the turbine paddles.
- Cooling Tower
Excess heat energy produced from the fission reaction is eliminated by converting or transferring it to the atmosphere.
- Containment
This acts as an envelope around the reactor vessel thereby separating the reactor from the surroundings.
Working Principle
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The reactor fitted with the control rods initiates the fission reaction and is capable of subsequently sustaining it. Unlike atomic bombs, there are constant checks enabled in nuclear reactors to prevent explosions and maintain the chain reaction. The neutrons are released to take care of carrying forward the chain reaction. Hence the neutron population inside the reactor is a critical factor that decides the extent of power production.
Working Principle
A constant neutron population results in a steady fission reaction inside the reactor and the reactor is said to be in the subcritical stage. When the neutron population slightly increases, the fission rate also increases, generating more power. The reactor is then said to be in the supercritical state. At the start of the fission reaction, the neutron population is zero and the control rods are slightly moved inside the reactor to enhance the reaction until nominal power is reached. Further, the rods are re-inserted to balance the growing neutron population as the reaction progresses. At this point, the reactor is said to be in the critical state. During the shutdown, the control rods are completely inserted so that the reaction is brought to a halt and the simultaneous chain reaction is suspended. This is the subcritical stage of the reactor.
Critical Factors Affecting Reactor Operation
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1. Reactivity
This is one of the most common, yet important factors, for the operation of a nuclear reactor. Reactivity, also known as the multiplication factor, is based on the neutron population that drives the chain reaction. Positive reactivity is indicative of the reactor operating at the supercritical condition as the neutron population increases exponentially along with a corresponding rise in power production. At critical conditions, i.e. at a steady state when the number of neutrons getting generated is equal to the number getting converted, the reactivity is zero. A negative reactivity (shut down mode) is when the reactor is in the subcritical stage, wherein there is an exponential decay in the neutron population. The operator needs to maintain the reactivity closer to unity to prevent explosion and stabilize the exponential power increase with increasing neutron population.
2. Adding or removing fuel
This is important to maintain a steady production of electricity and avoid overshooting.
3. Maintaining Neutron Ratio
As the reaction progresses, some neutrons are lost as part of leakages existing in the system and some neutrons stay in the system to facilitate the chain reaction. It is very important to maintain this ratio and an absorber element regulates the neutron population from exceeding the required value.
Things to Remember
- A nuclear fission reaction is the bombarding of a heavy atomic nucleus with a neutron to produce more neutrons and a very high amount of energy.
- A nuclear reactor is a classic application of a nuclear fission reaction taking place in a controlled manner.
- The major components of a nuclear reactor are the reactor vessel, fuel, control rod, coolant, turbines, cooling tower, and containment.
- Control rods play a prominent role in sustaining the fission reaction as they are the neutron absorbing elements.
- The neutron population in the reactor at any point in time is a critical factor based on which the reactor is operated.
- Reactivity, addition or removal of fuel, and the ratio of neutrons staying in and leaking out of the system, are the criticalities in the functioning of a nuclear reactor.
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Sample Questions
Ques: Explain the basic principle behind the working of the nuclear reactors? (4 marks)
Ans: Nuclear fission or bombarding of a heavy atomic nucleus into 2 light equal-sized fragments is the fundamental principle behind the working of a nuclear reactor. This reaction is highly exothermic and produces more neutrons that are capable of subsequently continuing the sequence of reactions. A nuclear reactor is a controlled application of harnessing the massive energy generated from the fission reaction. The kinetic energy gets converted to thermal energy as the fission reaction progresses because the reaction products undergo extreme deceleration. This thermal energy is then transferred to a coolant which is used to run turbines thereby converting thermal energy to mechanical energy.
Ques: How does a nuclear reactor produce electricity? (3 marks)
Ans: Nuclear reactors employ nuclear fission reactions for their basic working. Fuels like Uranium are used for initiating the reaction. Ceramic Uranium pellets are stacked end-to-end on metal fuel rods forming an assembly and are housed in the reactor core. The enormous thermal energy produced in the reactor is used to boil water into steam. This steam then spins the blades of the turbine thereby converting thermal energy into mechanical energy. The blades spin and drive the generators thereby producing electricity. The steam is also converted back into water using cooling towers which are part of the nuclear power plants. The water is then recycled again to produce steam
Ques: What controls the reaction in the nuclear reactor? (2 marks)
Ans: The control rods are responsible to control the reaction inside the core of the reactor. These are made from neutron absorbent materials like cadmium or barium. Moderators like heavy water, are used for slowing down the fast-moving neutrons before they hit the heavy atomic nucleus. An uncontrolled fission reaction leads to a massive explosion like the case of an atomic bomb.
Ques: What does the operator mean by saying ‘a critical reactor’? (2 marks)
Ans: When the number of neutrons that are produced from the fission reaction equals the amount that gets absorbed, the reactor is said to be critical. This is a steady-state of operation because there is a constant monitored supply and elimination of neutrons happening in the reactor core.
Ques: What is the containment around the reactor? (2 marks)
Ans: Containment around the reactor core is usually designed to contain the radioactive elements from escaping into the surrounding. The chamber is sealed and the air inside is maintained at a lower pressure compared to the surrounding air. This is to avoid escaping of harmful air during a leak. The air would hence move inwards rather than out of the containment.
Ques: List down the pros and cons of using nuclear energy? (3 marks)
Ans: Tabulated below are some advantages and disadvantages of using nuclear energy:
| Pros | Cons |
|---|---|
| A powerful energy source. | Nuclear waste generated is highly toxic and a burden to the environment. |
| Electricity produced is cheaper than conventional sources. | The initial set-up cost is very high. |
| Low on carbon emissions | Non-renewable source of energy |
Ques: What are fissile and fertile nuclides? How are they beneficial for a nuclear reaction? (3 marks)
Ans: Almost all heavy nucleus atoms are capable of undergoing fission reactions when they are bombarded with fast-moving neutrons. However, very few of the nuclides are capable to undergo fission readily when hit by slow (low-energy) neutrons. Such materials are called fissile nuclides. Uranium-233, Uranium 235, Plutonium-239, and Plutonium-241 are some of the most used fissile materials in the nuclear industry.
Some nucleotides are capable of transforming into fissile materials apart from the process of enriching Uranium. Such materials are called fertile materials. A classic example is Thorium-232, a natural Thorium isotope that can be used to produce Uranium-233 by the neutron capture process.
Ques: How is the large amount of heat produced by the nuclear reactors in power plants managed? (3 marks)
Ans: The components of the nuclear reactor are designed in such a way that there is appropriate heat channeling taking place within the system. Enormous heat is produced as soon as the fission reaction is initiated. Most of this is transferred to the fuel in the reactor core. A coolant, most commonly water, helps remove the excess heat to maintain the heat equilibrium in the system. Sophisticated coolant systems are kept in place for complex power plants because heat build-up within the system can fuel meltdown. However, in smaller research-based reactors that operate on low power, conduction and convection are used to balance the heat flow.
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