How Nuclear Reactor works?

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-Ujjal lamichhane
Msc Physics 2nd year

Background and History:

The nuclear power plant stands on the border between humanity’s greatest hopes and its deepest fear for the future. Atomic energy offers a clean energy alternative that frees us from the shackles of fossil fuel dependence. At the present age, the dependence of human beings on nuclear power increases day by day. As a June 01, 2015, there are 438 operating nuclear power reactors spread across the planet in 31 different countries with an installed electric net capacity of about 379GW. Not merely this, 67 plants with an installed capacity of 65GW in 16 different countries are under construction.
But our concern here is what happens inside a nuclear power plant to bring such marvel and misery into being? Let us imagine a volt of electricity back through the wall socket, all the way through miles of the power lines to the nuclear reactor that generated it. We would encounter the generator that produces the spark and the turbine that turns it. Next, we would find the jet of steams that turns the turbine and finally the radioactive Uranium bundle that heats water into steams. Here we go on exploring the phenomenon what exactly happens in the Reactor that produces the electricity.

Nuclear Fission

Despite all the cosmic energy that the word ‘Nuclear’ involves power plants that depend on atomic energy. Nuclear plants depend on the heat that occurs during nuclear fission when one atom splits into two and releases energy. Nuclear fission happens naturally every day. But, we now talk about the chain reactions that we initiated, maintained and controlled inside the nuclear reactor where the desired level of energy can be obtained. Firstly, let us discuss about the fuels required for the initiation of chain reactions. Generally, Uranium-235 (U-235) is used as the fuel for the nuclear reaction.

Why is U-235 chosen?

Uranium is a common element on Earth and has existed since the planet formed. While there are several varieties of Uranium, U-235 is one of the most important productions of the both nuclear power and nuclear weapons. U-235 decays naturally by alpha (α) – radiation. It throws of α – particles, or two neutrons and two protons bound together. It is also one of the few elements that can undergo induced fission.
Also, the decay of a single U-235 atom releases approximately 200 MeV (million electron volts). That may not seem like much, but there are lots of Uranium atoms in a pound (0.45 Kg) of Uranium. So many, in fact, that a pound of highly enriched Uranium as used to power a nuclear submarine is equal to about a million gallons of gasoline. 3% enrichment of U-235 is sufficient for nuclear power plants.
Now, Let us discuss what exactly happens inside the Nuclear power plant.

Inside the Reactor:

In order to turn nuclear fission into electrical energy, nuclear power plant operators have to control the energy given off by the enriched Uranium and allow it to heat water into steam. Enriched Uranium typically is formed into inch-long (2.5cm) pallets, each with approximately the same diameter as the dime. Next, the pallets are arranged into long rods, and the rods are collected together into bundles. The bundles are submerged in water inside a pressure vessel. The water acts as a coolant. Left to its own devices, the uranium would eventually overheat and melts.
To prevent overheating, control rods are made of the material that absorbs neutrons, are inserted into the uranium bundle using a mechanism that can raise or lower them. Raising and lowering the control rods help to control the rate of the nuclear reaction. When the control rods are lifted out of the uranium bundle, its core get more heated and lowered the rods to reduce the heat. The control rods can also be lowered completely into the uranium bundle to shut the reactor down in case of accident or even to change the fuel. The uranium bundle acts as an extremely high- energy source of heat. It heats the water and turns it to steam. The steam drives a turbine, which spin a generator to produce power.

In some nuclear plants, the steam from the reactor goes through a secondary intermediate heat exchanger to convert another loop of water to steam, which drives the turbine. The advantage to this design is that the radioactive water/steam never contacts the turbine. Also, in some reactors, the coolant fluid in contact with the reactor core is gas (carbon-dioxide) or liquid metal (sodium, potassium); these types of reactors allow the core to be operated as higher temperatures.
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Physical Construction:

The radiations from the radioactive elements are much more hazardous to human health. It is therefore, while constructing the nuclear power plant, the prevention of the leakage of radioactive radiations should be strictly controlled. Not only the leakage but also the control measures should be adopted for the all sorts of possible accidents and calamities. Extra precautions are mandatory while constructing the power plant.
A concrete linear typically houses the reactor’s pressure vessel and acts as a radiation shield. That linear, in turn, is housed within a much larger steel containment vessel. This vessel contains the reactor core, as well as the equipment plant workers use to refuel and maintain the reactor. The steel containment vessel serves as a barrier to prevent any radioactive gases or fluids from the plant.
An outer concrete building serves as the final layer, protecting the steel containment vessel. This concrete structure is designed to be strong enough to survive the kind of massive damage that might result from earthquakes or crashing jet airliner. These secondary containment structures are necessary to prevent the escape of radiation/radioactive steam in the event of an accident.

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Pros and Cons of Nuclear power:

Why are nuclear power generated? What’s nuclear power’s highest advantage? What is its drawback? These questioned are to be answered before any inventions. Nuclear powers have got its both advantages and drawbacks. The pros and cons of the nuclear reactor are summarized below:
• It doesn’t depend on fossil fuels and isn’t affected by fluctuating oil and gas prices and availability.
• With nuclear power plants, CO2 emissions are minimal in comparison to coal and natural gas power plant.
• Nuclear fission produces roughly a million times more energy per unit weight than fossil fuel alternatives.
• It offers clean energy alternatives to any other sources of energy.
• Once the fuel is spent, it cannot be just thrown in the city dump.
• Even transportation of nuclear fuel to and from plants posses a contamination risks.
• The radioactive wastes are always harmful to living beings.
• Nuclear plants can be the sources of more destruction in case of any accidents happen to burst the core.

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