Principle of Nuclear Reactor
Nuclear reactors are known to be a complete system in which several nuclear chain reactions take place, right from its initiation. The energy yield can be contained for several usage applications. In general, there are different types of nuclear reactors and could even mean a device that provides vast scope for research and development of radioactive isotopes. They are:
In Terms of General Usage
Nuclear Power Reactors
Nuclear Research Reactors
In Terms of Fuel Usage
Uranium fueled
Plutonium fueled
In Terms of Other Types
Pressurized Water Reactor
Boiling Water Reactor
Pressurized Water
Advanced Gas-Cooled Reactor
Light water Graphite-moderated Reactor
Spread across all corners of the world, it's the nuclear fission reactor that produces more than 11% of the world's gross electricity. It takes place because of the nuclear fission reaction in which the splitting of atoms in a nucleus takes place, which is otherwise bound together by the most potent force in nature. Therefore, elements like Uranium (the heaviest natural element) can be split apart on bombardment with a neutron and release a lot of energy because of the splitting.
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The nuclear reactors contain such fission reactions that release enough energy to heat the water to more than 271 degrees Celsius, which can help in the spinning of turbines and thus, generating electricity.
Working of a Nuclear Reactor
Driven by the nuclear fission reaction process, when a neutron is fired at an atom, it causes them to disintegrate into small, excited states that are continuously emitting neutrons, photons, and other subatomic particles. Such a vast release of energy causes these particles to trigger fission in its colliding atoms and set off a chain reaction by yielding more neutrons that also generates massive heat energy. When this generated heat is isolated from the system via heating water and produces steam that can further propel turbines, electricity production begins by the generator process.
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In an atom bomb, such chain reactions help in the natural fission of elements and create massive energetic explosions. However, in a nuclear reactor, such comprehensive processes require a detailed study on the precise temperature controls for slowing or accelerating reactions, which can be done via control rods. The control rods are made up of neutron-absorbing elements like silver and boron that can help in quick absorption of neutrons and, thus, maintaining and controlling the chain reactions that follow. As much as 85% of the total energy released from a fission reaction occurs early on the equation and can take place in the right amount of time. In comparison, the rest of the 15% of the energy is yielded via the radioactive decay of the element once the neutrons are emitted. Radioactive decay of an element is the process in which the atoms acquire a stable state, occurring over long time periods.
Chain Reaction
The nuclear fission reaction is a self-enriching process in which the rate of neutrons created is regulated can help the reactor stay on a critical scale. In the pre-starting of a nuclear core, the number of neutrons remains equivalent to zero. Still, as the reactor gets started with the removal of control rods from the core, the reactor goes into the supercritical scale, where the number of neutron populations increases rapidly over time, causing the energy generated to be improved. Once the threshold power scale is reached, the control rods are re-introduced to keep the neutrons in balance, taking the reactor in its steady-state operation. For a reactor to shut down completely, the control rods are entirely placed into the core, slowing down the fission and taking it back to the subcritical state where the rate of fission drops down zero.
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Reactivity
It is a widely used parameter that describes the measure of the reactor's state in terms of its undergone change in the core. The reactivity is generally represented as:
δk = (k − 1)/k, where δk = 0, when the reactor is in the critical, and positive, negative values in case the reactor is supercritical and subcritical, respectively.
These factors are controlled in three ways:
By regulating the absorber elements introduced
Addition or removal of fuel
Changing the ratio of neutrons that leak out and contain in a system.
In general, making changes in the neutron leakage is completely self-driven since the increase of power generated in the core causes the density of the coolant to drop and get boiled. Such a massive decrease can enhance the mass neutron leakage from the system and decrease the reactor's overall reactivity.
It takes an abrupt timing of one picosecond (which is 10-12 of a second) for a fission reaction to occur and be responded accordingly by the reactor.
Fissile Elements
There are only a handful of elements with heavy nuclides that can undergo fission reactions consistently via low-energy neutrons. Such elements are called fissile elements. Some of them are uranium-235, plutonium-239, and plutonium-241. Apart from the enriching of uranium-235, certain nucleotides can transform into fissile elements called fertile materials like thorium-232.
FAQs on Nuclear Reactor Based on Nuclear Fission
1. What is the basic principle of a nuclear reactor based on nuclear fission?
The basic principle of a nuclear reactor is to initiate and sustain a controlled nuclear chain reaction. When a heavy, fissile nucleus like Uranium-235 absorbs a slow-moving neutron, it splits into smaller nuclei. This process releases a large amount of energy and more neutrons, which in turn cause other U-235 nuclei to split. The reactor carefully manages this chain reaction to generate heat at a steady, controlled rate for electricity production.
2. What are the main components of a nuclear fission reactor and what is the function of each?
The main components of a nuclear fission reactor are:
- Core: This is where the nuclear fuel is housed and where the fission chain reaction occurs, generating immense heat.
- Moderator: A substance, typically heavy water or graphite, that slows down the fast neutrons produced during fission. Slower neutrons are more effective at causing further fission in U-235.
- Control Rods: Made of neutron-absorbing materials like boron or cadmium, these are inserted into or withdrawn from the core to control the rate of fission and thus the reactor's power output.
- Coolant: A fluid, such as water or liquid sodium, that transfers the heat from the core to a heat exchanger to produce steam.
- Containment Structure: A thick, reinforced concrete and steel shield enclosing the reactor to prevent the escape of radiation into the environment.
3. How is a controlled chain reaction in a nuclear reactor different from the uncontrolled reaction in an atom bomb?
The key difference is the control over the rate of fission. In a nuclear reactor, the reaction is controlled. Control rods absorb excess neutrons to ensure that, on average, only one neutron from each fission event causes another fission. This maintains a steady energy output (a critical state). In an atom bomb, the chain reaction is uncontrolled. It is designed to allow the number of fissions to multiply exponentially in a fraction of a second (a supercritical state), resulting in a massive, explosive release of energy.
4. Why is Uranium-235 commonly used as fuel in nuclear fission reactors instead of other elements?
Uranium-235 is used because it is a fissile material, meaning its nucleus can be split by absorbing slow-moving (thermal) neutrons, which are easier to control. This property is essential for sustaining a controlled chain reaction. While other heavy elements exist, many are either not fissile or require high-energy neutrons, making them less efficient for current reactor designs. U-235 is one of the few naturally occurring isotopes that meets this critical requirement for reliable power generation.
5. How do control rods and moderators work together to regulate the power output of a nuclear reactor?
They perform complementary functions to manage the chain reaction:
- Moderators (e.g., heavy water) do not control the power level directly but are essential for the reaction to occur efficiently. They slow down the fast neutrons produced by fission, making them more likely to be captured by other U-235 nuclei to continue the chain.
- Control Rods (e.g., boron) directly regulate the power. By inserting them into the core, they absorb neutrons, reducing the number available for fission and thus lowering the reactor's power. Withdrawing them increases the reaction rate and power output.
6. What is the key difference between nuclear fission and nuclear fusion, and why are current power reactors based on fission?
The fundamental difference is the nuclear process involved:
- Nuclear Fission is the splitting of a single heavy, unstable nucleus (like Uranium-235) into two or more lighter nuclei.
- Nuclear Fusion is the combining of two light nuclei (like hydrogen isotopes) to form a single heavier nucleus.
Current power reactors are based on fission because achieving a sustained, controlled fission reaction is technologically viable. In contrast, fusion releases more energy but requires recreating the extreme temperatures and pressures found inside the sun, which is incredibly difficult and energy-intensive to contain and sustain on Earth for commercial power generation.
7. What is the role of a coolant in a nuclear reactor, and what are some common examples?
The coolant's primary role is to transfer the immense heat generated by the fission reaction in the reactor core. This heat is carried away from the core and used in a heat exchanger to boil water, creating high-pressure steam. This steam then drives a turbine connected to a generator to produce electricity. The coolant is vital for both power generation and preventing the reactor core from overheating. Common examples include pressurised water, boiling water, and advanced coolants like liquid sodium or helium gas.
