How much relevance does a particle having a radius of order 10-15m make in the life of an ordinary individual? Certainly, none. But for scientists, this tiny particle holds an immeasurable significance for the scientists and the field of science.
This particle is the nucleus of an atom. And today we will learn about some concepts and technologies based on the nucleus, these are nuclear fission and nuclear reactors.
History
It all started with the discovery of the nucleus in 1911 by Ernest Rutherford and the discovery of the neutron by James Chadwick in 1932. And after that, nuclear fission was discovered in 1938 by Otto Hahn and Fritz Strassman. It laid a foundation of concepts related to nuclear energy and nuclear bombs.
Nuclear Fission
Nuclear Fission is the process of splitting down a heavier nucleus into multiple smaller nuclei with an instant release of a huge amount of energy. The heavier nucleus is also called the parent nucleus and the smaller nuclei are also called daughter nuclei.
Most of the time, the parent nucleus splits into only two daughter nuclei. Splitting of a nucleus in more than two nuclei is very rare. After fission, the combined mass of these split fragments tends to be less than the mass of the parent nucleus. The mass difference makes up nuclear energy.
Some examples of nuclear fission,
1. Splitting of Uranium-235 into Barium-144 and Krypton-89,
(235,92)U + (1,0)n → (144,56)Ba + (89,36)Kr + 3(1,0)n
2. Splitting of Plutonium-239 into Xenon-137 and Zirconium-103,
(239,94)Pu + (1,0)n → (137,54)Xe + (103,40)Zr + 3(1,0)n
- Nuclear fission is a radioactive decay process. It generally produces free neutrons and gamma photons along with energy
- Nuclear fission may occur naturally or may occur by bombarding of various particles. such particles include neutrons, protons, deuterons, and alpha particles on the nucleus.
- The new neutrons released further trigger fission in the nearby fissionable nuclei causing a chain reaction.
- The chain reaction is the process of reactions in which the products formed by the previous reaction undergo further reactions by themselves.
- If the chain reaction control in a nuclear reactor, then it can be used for electricity production.
- If the chain reaction is left uncontrolled, then it can result in a massive explosion.
How does Nuclear Fission release energy?
- The Nucleus of an atom consists of nucleons. Neutrons and Protons are collectively nucleons.
- Mass number of an atom is equal to the number of nucleons.
- But the actual mass of a nucleus is somewhat less than the mass of nucleons.
- The difference between their mass is the mass defect. This mass defect is also the measure of the total binding energy of the nucleus.
- And the binding energy releases during nucleus formation.
- And the conversion of mass to energy follows Einstein’s equation, E = mc2.
- E is equal to the energy released, m is the mass and c is the velocity of light under vacuum.
Common Fissile Material
- The most common fissile materials are U-233, U-235, Plutonium-239, Plutonium-241.
- U-235 is present in natural Uranium but only 0.72%. Natural Uranium is not sufficient to sustain a chain reaction.
- Natural Uranium consists of 99.27% of U-238. U-238 can produce Pu-239.
- Thorium-232 can transmute itself to U-233.
Uranium Enrichment
- Enriched Uranium is necessary for most reactors.
- Natural Uranium comprises 0.7 U-235 and 99.2% of U-238.
- U-235 is fissionable but U-238 is not. That is why uranium enrichment is necessary.
- Uranium enrichment obtains by using centrifugal separators.
- Breeder reactors require 15-30% enrichment.
Nuclear Reactor
A Nuclear Reactor is a system that accommodates and controls nuclear chain reactions. Nuclear reactors are useful for various purposes such as electricity generation, marine propulsion, and production of radioisotopes for medical use. We will focus on electricity-generating reactors.
Basic components of a Nuclear Reactor:
1. Core: The core of a nuclear reactor is the place where all the nuclear reactions occur and the heat produced. It contains all the nuclear fuel in the form of nuclear fuel pins or fuel roads.
The Nuclear fuel pin is the smallest unit of the reactor which comes in the shape of thin roads of about 1 cm in diameter and contains some fissile material.
2. Moderator: Moderator is the medium that slows down the fast-moving neutrons. These neutrons become thermal neutrons. Thermal Neutrons can sustain a nuclear chain reaction. But when neutrons release after nuclear fission, they possess very high speed and energy.
This speed and energy can not trigger fission in other nuclei. Regular water is the most common moderator (in 75% of reactors) followed by graphite and heavy water.
3. Control Rods: Control rods control the rate of fission in the reactor core and made up of neutron absorbing material. When the fission rate increases, control rods push deeper into the reactor.
And when the fission rate decreases, the control rods pull back. Control rods can form up of elements like Boron, Silver, Indium, and Cadmium.
4. Coolant: Coolant circulates around the reactor core in multiple (usually 2-3) loops. The coolant absorbs the heat from the nuclear fission occurring in the core and transfers it to the turbine. The most commonly used coolant is water. Other used coolants are Carbon Dioxide, air, liquid Sodium, etc.
5. Containment: Containment is a very thick structure which covers the core and separates it from the environment. It forms up by high-density, steel-reinforced concrete. It contains harmful radiations within the system.
Types of Nuclear Reactors
There are many types of nuclear reactors based on various factors such as the use of moderator material, coolant material, technology, etc. One thing common between these reactors is that they all work on the basis of nuclear fission.
We can classify fission reactors into two classes based on the energy of bombarding neutrons, these are thermal reactors and fast neutron reactors.
Thermal Reactors
- Thermal Reactors use thermal or slowed down neutrons to induce nuclear fission.
- Almost all the power reactors are thermal reactors because they are comparatively easy to build and operate.
- Thermal Reactors can sustain chain reaction on enriched fissile material (2-5% enriched) due to slow-moving electrons. Slow-moving neutrons provide a high probability of fission.
Fast Neutron Reactors
- Fast Neutron Reactors use fast-moving neutrons to induce nuclear fission in the fuel.
- Only Breeder Reactors operate on fast neutrons and they are not much in use because they are relatively hard to build and operate.
- Sustainment of chain reaction requires Highly enriched fissile (at least 20% enriched) material.
- In a Fast Neutron Reactor (FNR), the fast neutrons sustain the chain reaction.
- In other reactors, slow neutrons are necessary. These moderators slow down the neutrons.
- Fast Neutron Reactors do not need a moderator.
- Liquid metal is the coolant of all the FNRs in use.
- Water is not a useful coolant because it also acts as a moderator.
- Enriched fuel is necessary for Fast Neutron Reactors which makes them non-feasible.
- The only advantage of Fast Neutron Reactors is that they release less toxic waste.
We can also classify the reactors on the basis of coolant and moderator used into two major types. These are:
1. Light Water Reactors (LWR)
2. Heavy Water Reactors(HWR)
Light Water Reactor (LWR)
Light Water Reactor uses normal or light water as both its coolant and moderator. It is the most common type of thermal neutron reactor. Light Water Reactor itself has three varieties that include- Pressurized Water Reactor (PWR), Boiling Water Reactor (BWR), and Supercritical Water Reactor (SCWR).
1. Pressurized Water Reactor (PWR)
- In Pressurized Water Reactor, water pumps under high pressure to the core of the reactor.
- The water gets heated there and then it flows down to a steam generator where steam generates.
- The generated steam flows down to a second system where a turbine is present which in turn spins and electricity generates.
- Coolant water is kept under high pressure so that it does not boil.
- It is the most commonly used nuclear reactor.
Advantages:
- Easier to operate due to more stability.
- In case of power failure, the control rods get fully immersed automatically as they are held by electromagnets, and because of no current, the control rods fall down.
- As the turbine cycle loop separates from the primary loop, the water in the secondary loop is not contaminated by radioactive materials.
Disadvantages:
- The coolant needs to be put under high pressure all the time.
- These reactors require stronger infrastructure to maintain the higher pressure which increases the construction cost.
- The necessity of enriched fuel (2-5%) also increases project costs.
2. Boiling Water Reactor (BWR)
- The Boiling Water Reactor is the second most used nuclear reactor after PWR.
- In BWR, there is a single system to take the heat from the reactor and generate steam for the turbine.
- The steam is directly generated by the boiling of water. Steam separators are present just above the reactor core where steam separates from the remaining water.
- The steam is later condensed and recycled.
Advantages:
- The reactor operates at a lower pressure as compared to the Pressurised Water Reactor.
- The Boiling Water Reactor operates at a lower nuclear fuel temperature.
- These reactors require a smaller number of components as there is no need for a pressurized vessel and steam generator.
- Due to smaller numbers of pipes, the pipes with smaller diameter, and the absence of steam generator tubes, there is a lower probability of rupture and core damage.
Disadvantages:
- Management of consumption of nuclear fuel requires complex calculations.
- A smaller number of components are required in the overall system but for the management of Nuclear fuel, more instrumentation is required inside the reactor core.
- Unlike PWR, the control rods insert from the bottom of the core which may prove to be catastrophic in case of power failure.
- Slightly lower thermal efficiency.
3. Supercritical Water Reactor (SCWR)
- This is still under the development phase.
- Supercritical of a substance is a state which exists at a temperature and pressure above its critical point, where distinguishable liquid and gaseous phase do not exist. At this stage, the substance can effuse from a solid substance like gas and can also dissolve a substance like a liquid.
- The reactor would operate under high pressure and temperature like a PWR and with a direct once-through cycle like a BWR
Advantages:
- SCWR is the simplest LWR in terms of design.
- The reactor requires less number of components due to absence of steam separators, dryers, and recirculation pipes and pumps.
- A smaller core and smaller containment require due to the excellent heat transfer property of supercritical water which allows high power density.
- A fast SCWR could be used as a breeder reactor.
Disadvantages:
- Achieving and maintaining supercritical water requires special procedures.
- Mechanical and thermal stress increases on some components due to higher pressure and higher temperature.
- Supercritical water needs to be extensively researched under radiation exposure.
Heavy Water Reactor (HWR)
- As the name suggests, a Heavy Water Reactor uses heavy water as both of its coolant and moderator. It is also known as PHWR (Pressurized Heavy Water Reactor)
- Conditions of coolant are nearly similar to a PWR, the heavy water is kept under pressure which allows it to be heated to a higher temperature without boiling.
- Heavy water absorbs fewer neutrons as compared to light water due to which enriched fuel can avoid.
- As of 2015, electricity production from HWRs accounted for only 6.5% of all the operating reactors in the world.
- Heavy Water Reactors are generally used in Indian, Pakistan, Canada, China, Argentina, Romania, and South Korea.
CANDU (Canadian Deuterium-Uranium) Reactor is a famous example of HWR.
Advantages:
- Enriched fuel is not required.
- In other designs, the moderator becomes much hotter as compared to HWR due to which resulting neutrons are less thermal in nature which reduces the efficiency of the reactor.
- Fission products are less dense due to unenriched fuel which significantly reduces heat generation and allows more compact storage.
Disadvantages:
- Heavy water isolation is costly.
- Fuel replacement is more frequent as the fuel is not enriched.
- An increase in fuel movement results in a higher volume of spent fuel.
- Neutron absorbed by deuterium results in tritium which is a radioactive isotope and often leaks in small quantities.
The above reactors are the major types of reactors throughout the world. There are many other designs possible. Many of those designs have either become obsolete or are not in use at a large scale.
These designs include Graphite moderated reactors such as RBMK, Molten Salt Reactors, Liquid metal Cooled Reactors, Gas-Cooled Reactors, etc.
Gas-Cooled Reactors
- BWR, LWR, HWR & PWR are inoperable at high temperatures. So, their thermal efficiency reduces with an increase in temperature.
- In Gas-Cooled Reactors, the coolant is a gas.
- The gas can be Helium or Carbon Dioxide.
- Mostly, the moderator is graphite and rarely it is heavy water.
- Another name for them is High-Temperature Gas-Cooled Reactors (HTGRs).
- An HTGR can provide thermal efficiency by up to 50%.
- An HTGR finds application in other fields such as water desalination, oil refineries, etc.
- Gas is not an efficient coolant, so an HTGR needs a highly efficient back-up coolant.
Fast Neutron Reactors
- In a Fast Neutron Reactor (FNR), the fast neutrons sustain the chain reaction.
- In other reactors, slow neutrons are necessary. These moderators slow down the neutrons.
- Fast Neutron Reactors do not need a moderator.
- Liquid metal is the coolant of all the FNRs in use.
- Water is not a useful coolant because it also acts as a moderator.
- Enriched fuel is necessary for Fast Neutron Reactors which makes them non-feasible.
- The only advantage of Fast Neutron Reactors is that they release less toxic waste.
Thorium Reactors
- It aims to achieve the Thorium fuel cycle.
- Thorium is more abundantly available than Uranium.
- Thorium is not a fissile material, but it can produce U-233. U-233 is a fissile material
- Both U-233 and Th-232 are fuel at the initial stage. After that, Th-232 can take care of the fuel cycle.
- Thorium reactors are smaller in size.
- Thorium reactors produce less radioactive waste and reduce the risk of a nuclear meltdown.
Also, another way to classify Nuclear Reactors based on their generation.
- Generation I: These reactors were the early prototype reactors for the purpose of research and noncommercial electricity production.
- Generation II: Most of the currently used Nuclear Reactors are of Generation II. These include PWR, BWR, CANDU, and RBMK.
- Generation III: Generation III reactors are the advanced versions of Generation II reactors with advanced fuel technology, superior thermal efficiency, and enhanced safety systems. But due to the continued popularity of Gen II reactors, very few Gen III reactors have been built.
- Generation IV: Gen IV nuclear reactors are still under research and development phase. They envisage being highly economical with enhanced safety and minimal waste.
Nuclear Reactors in India
Seven nuclear reactors are currently operating in India. They are-
1. Rawatbhata (Rajasthan)
2. Tarapur (Maharashtra)
3. Kudankulam (Tamil Nadu)
4. Kakrapar (Gujarat)
5. Kalpakkam (Tamil Nadu)
6. Narora (Uttar Pradesh)
7. Kaiga (Karnataka)
Reactors with Power Production Share
PWRs are the most popular reactors followed by BWRs, then by PHWRs, and so on. So, let us have a look at the percentage of share of electricity production of these reactors as per the report published by IAEA (International Atomic Energy Agency) in 2015.
- PWR: 68.3%
- BWR: 20.1%
- PHWR: 6.5%
- Others: 5.1%
Conclusion
In this article, we first learned about the concept of nuclear fission. After that, we studied Nuclear Reactors, basic components of nuclear reactors.
Along with that, we also learned about the different types of nuclear reactors based on different criteria, their advantages, disadvantages, and their percentage in nuclear power production. Lastly, we came to know that Pressurized Water Reactors are the most widely used nuclear reactors.
Also, there is a lot of scope of improvement in the field of nuclear power production which envisaged to achieve in coming technology reactors.