Do you know what a nuclear reactor is?
Nuclear reactors are used in nuclear power plants for power generation and also in the propulsion of ships and submarines.
There are also reactors that produce isotopes for medical or industrial use, reactors for the production of military grade plutonium and reactors used exclusively for research.
In nuclear reactors, energy is released in the form of heat.
This heat can be converted into multiple forms of energy for use by society. In a typical nuclear power plant, this conversion simply involves producing water vapour that drives turbines that in turn drive electrical generators.
A nuclear reactor is defined as a facility capable of initiating, maintaining and controlling the chain fission reactions occurring in the reactor core, consisting of fuel, coolant, control elements, structural materials and moderator in the case of thermal nuclear reactors.
There are two ways to design a nuclear reactor: thermal or fast.
How does a nuclear reactor work?
In nuclear power plants, energy is produced by fission in a reactor. The heat released in the nuclear reaction transforms the liquid water into steam that drives the turbines that, in turn, drive the electric generators, and finally, provide electricity to the large populations.
Nuclear reactors are therefore one of the fundamental parts within a nuclear power plant, together with the steam generator, the turbine, and the condenser.
The most important parts of a nuclear reactor are the fuel, the reflector, the cooling system, the control system, and the radiation protection.
What is a nuclear reactor and what are its constituent elements?
A nuclear reactor is a facility capable of initiating, maintaining and controlling fission chain reactions, with the appropriate means of extracting the heat generated. A nuclear reactor consists of several elements, each of which plays an important role in the generation of heat.
These elements are
which consists of a fissile material, usually a compound of uranium, in which fission reactions take place, and is therefore the source of heat generation.
2. The moderator
who slows down the rapid neutrons produced in nuclear fission, turning them into slow or thermal neutrons. This element does not exist in so-called fast reactors. Water, graphite and heavy water are used as moderating materials.
3. The coolant
which extracts the heat generated by the reactor fuel. Liquid refrigerants such as light water and heavy water or gases such as carbon dioxide and helium are generally used.
4. The reflector
which reduces the escape of neutrons from the fuel area, and therefore provides more neutrons for the chain reaction. The materials used as reflectors are water, graphite and heavy water.
5. The control elements
which act as neutron absorbers, allow the neutron population, and therefore the reactivity of the reactor, to be controlled at all times, making it critical during operation and subcritical during shutdowns. The control elements are in the form of bars, although the absorber may also be diluted in the refrigerant.
6. The shielding
which prevents the escape of gamma radiation and neutrons from the reactor. The materials used as shielding are concrete, water and lead.
7. The reactor vessel
Houses the fuel, the control elements, and the moderator, allowing the indispensable passage of the refrigerant.
Difference between a thermal and a fast reactor
As we have said, there are two ways to design a nuclear reactor, either by slowing (moderating) the fast neutrons or by increasing the proportion of fissile atoms.
A moderator (light water, heavy water, graphite) is used to delay the neutrons and the resulting slow neutrons are called thermal, so that reactors based on this technique are known as Thermal Reactors, unlike those using fast neutrons, called Fast Reactors.
What does Critical Mass mean?
When building a reactor, it is necessary to have a critical mass of fuel, i.e. sufficient fissile material, in an optimum arrangement of the fuel and other nuking materials, to maintain the chain reaction.
The arrangement of the neutron absorbers and control rods allows criticality to be maintained in operation and subcriticity to be maintained during shutdown and start-up.
Nuclear reactor control
In order for a nuclear reactor to operate for a period of time, it must have an excess of reactivity over the critical value, to compensate for neutron losses that, due to various phenomena, tend to reduce them.
The reactor has to operate under criticality conditions, which means that the excess reactivity has to be kept at a zero value.
To control the reactivity in natural water nuclear reactors, several ways can be taken, which can act simultaneously and not simultaneously. The introduction of neutron absorbers into the core by means of control rods is a fast and efficient means of control.
Under certain circumstances, a neutron absorber such as boric acid can be dissolved in the moderator when it is liquid.
The design of the nuclear power plants is such that a failure in the equipment of the installation always acts in the sense of maximum reactor safety, inserting all the control rods into the core, instantly compensating for its reactivity and stopping the reactor.
Fast neutron reactors
Nuclear reactors can use fast neutrons with energies between 50-100 keV. These reactors typically have no moderators and can effectively produce 239 fissile Pu which can then be reused as fuel.
They can be cooled by helium or liquid sodium and can be made quite compact, for example, to be used in submarines.
An example of a nuclear power plant operating with a fast neutron reactor was SUPERPHENIX which operated in France from 1985 to 1997. Another similar, though not identical, breeder-type reactor (i.e. a reactor that produces its own fuel) in operation at Sverdlovsk in Russia since 1981.
Types of nuclear reactors
At the moment, nuclear reactors used to produce human consumable energy are fission reactors, although there is already some that experiment with nuclear fusion. Among the fission reactors we can distinguish:
- LWR – Light Water Reactors: The most common. They run on enriched uranium.
- FBR – Fast Breeder Reactors: They use plutonium as fuel.
- CANDU – Canada Deuterium Uranium: They use natural uranium as fuel.
- AGR – Advanced Gas-cooled Reactor: There are about 20 of them worldwide. They run on uranium and use gases as a refrigerant.
- ADS – Accelerator Driven System: Based on particle acceleration. Experimental phase.
- RBMK – Bolshoy Moshchnosty Kanalny Reactor: Type of reactor almost out of use, only about ten reactors remain worldwide. Chernobyl was one of them.
What was the first nuclear reactor in history
On November 16, 1942, under the bleachers of an abandoned football stadium, construction began on the first nuclear reactor in history. Enrico Fermi, the then Nobel Prize in physics, described it as a primitive pile of black bricks with wooden beams. He was right about that.
Chicago Pile-1 was a tower of perfectly ordered uranium tablets and graphite bricks. Uranium was the fuel and graphite was the nuclear moderator.
It had no cooling system and did not protect the workers from radiation. It was operated with cadmium and indium rods which, when introduced into the reactor, absorbed the free neutrons to prevent fission.
That first nuclear reactor was lifted brick by brick by people without special suits who took 12-hour shifts to finish it. It was ready on December 1st.
The next day, Fermi got it working. He was so confident in his calculations that he convinced the project manager, physicist Arthur Compton, that nothing would blow up.
This is how the first nuclear chain reaction in history took place in the middle of Chicago, the second most populous city in the United States.
On that December 2, 1942, a group of dignitaries gathered under the steps of Stegg Field to witness the demonstration.
A young scientist operated the control rods while Enrico Fermi monitored neutron activity. At 3.25 p.m., the reactor reached critical mass. Fermi turned it off 28 minutes later.
Buried in Chicago
In January, the Chicago Pile-1 was moved to the outskirts of the city, in an Argonne Forest reserve known as the Red Gate Woods. After adding a radiation shield, they renamed it Chicago Pile-2.
Shortly after she was buried right there. The U.S. government installed warnings carved in stone so that no one would even think of digging there. Chicago Pile-1 was part of the Manhattan Project.
Advantages of nuclear reactors
The main advantages of nuclear reactors are their ability to produce a large amount of energy in a short time. In addition, the process for energy production is not as polluting as other fossil fuel-based modes of energy production.
To produce this energy, nuclear reactors do not release greenhouse gases, or they do so to a much lesser extent than other industries, thus not contributing to worsening global warming and the greenhouse effect.
Disadvantages of nuclear reactors
The drawbacks to nuclear reactors and nuclear plants themselves are focused on safety and fear of accidents.
The waste produced by the nuclear fission process is highly radioactive and therefore a major threat to the health of any living being and therefore the safety conditions must be improved.
This waste must be stored in nuclear dumps, and there will come a time when it will be difficult to find the place for so much waste.
Another major fear is that nuclear reactors may be used for warfare, as was the case during World War II in Hiroshima and Nagasaki. Nuclear fission reactors could be used to create plutonium, with which to make atomic bombs.
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