NUCLEAR REACTOR

TABLE OF CONTENTS

Sl.	CONTENTS
	Two Words.......
1.	Nuclear reactor
2.	Construction of nuclear reactor
3.	Working of nuclear reactor
4.	Energy produced in nuclear reactor
5.	Types of nuclear reactor
6.	Advantages of nuclear reactor
7.	Disadvantages of nuclear reactors
8.
9.

1. NUCLEAR REACTOR

Nuclear reactor is a device that produces a vast amount of energy from a small amount of fuel. The fuels that are used in the nuclear reactor are nuclear fuel, for instance, U²³⁵ (Uranium 235), Polonium, etc. The nuclear reactors are sometimes called an atomic reactor or an atomic pile. It generates energy mainly in the form of heat by means of a process known as nuclear fission. Nuclear fission is defined as the splitting of the nuclei of atoms of uranium or plutonium, etc.

Nuclear reactors provide heat to power stations, large ships, and submarines. The heat usually used to boil water which produces the high pressure steam that drives steam turbines. Generating steam for revolving the turbine, as we now that, is the oldest and traditional method that is being used for years in our country for energy production. No, other economic method has yet been discovered that can convert nuclear energy directly into other forms of energy.

Pictorial representation of fission

Inside of a nuclear reactor

2. CONSTRUCTION OF A NUCLEAR REACTOR

A nuclear reactor consists of various parts. These include:

(i) The core;

(ii) The moderator;

(iii) Control rods;

(iv) Coolant or moderator; and

(v) The pressure vessel.

Besides these, it also has a biological shield and a safety system to protect reactor operators and technicians, as well as the general public.

Descriptions of each part of the nuclear reactor are given below:

(i) The core - The core is the central part of a reactor. It consists of the nuclear fuel. The fission process occurs in the core.

(ii) The moderator – The moderator is a material used in many reactors to increase the probability of fission and thus promotes a chain reaction. Most moderators consist of graphite, water, or heavy water (D₂O). Such a substance slows down the neutrons released by U-235 atoms during fission. By doing so, the moderator enables other U-235 atoms to capture the neutrons more readily and to split in turn. If a moderator did not reduce the neutrons’ speed, many neutrons would be absorbed by U-238 atoms, which do not take part in fission.

(iii) The control rods – The control rods regulate the rate of a chain reaction. They are made of Boron, Cadmium, or some other element that can absorb neutrons without being changed by them. After loading the core with fuel, the operator of the reactor partially withdraws the control rods so that they absorb a relatively small number of neutrons. This action allows a chain reaction to become self-sustaining. The operator then inserts the rods partway into the core to absorb enough neutrons to slow the reaction and prevent an explosion. If the operator wishes to increase the power level of the reactor, the control rods are partly withdrawn to free more neutrons and speed up the reaction.

(iv) The coolant – Te coolant carries the intense heat produced by fission out of a reactor. It makes the heat available to other systems of a nuclear power plant for the generation of electricity. At the same time, the coolant controls the temperature of the reactor core and prevents it from overheating. Various substances, including gases, liquids, and liquid metals, can be used as a coolant.

Reactors can be classified according to their coolant and the way they use it. For example, a gas-cooled reactor uses carbon dioxide or helium to transport heat to a steam generator outside the core of the reactor. The other two types of reactor use water as a coolant. In boiling-water reactor, the heat of the core causes the water to boil and turn to steam. While in the pressurized-water, the water absorbs heat but does not boil because it is kept under high pressure. The hot water is transferred from the reactor to separate heat exchange that produces steam for a turbine.

(v) The pressure vessel – The pressure vessel holds the core of most reactors. It also contains the circulation channels for the reactor’s coolant. The walls of the vessels are designed to withstand the high pressure generated by a chain reaction. In most cases, the vessels are lined with thick steel slabs to reduce the flow of radiation from the core. Nuclear fission releases large amount of penetrating neutrons and gamma rays.

(vi) The biological shield – It consists of tick concrete blocks that surround the pressure vessel. The shield protects people from radiation exposure. Concrete absorbs the gamma rays and neutrons that escape from the pressure vessel. Special instruments monitor the radiation level around the shield continuously to make sure no leaks occur.

3. WORKING OF A NUCLEAR REACTOR

In a nuclear reactor the atom of Uranium 235 (U²³⁵) undergoes fission reaction, when a neutron is bombarded on it and breaks into the atoms of Barium (Ba¹⁴¹) and Krypton (Kr⁹²). Three neutrons are also produced in the fission reaction. In a nuclear reactor, controlled reaction is preferred. In this type of reaction, two molecules are absorbed by the cadmium rod, used in the reactor. One neutron is again bombarded on the Uranium atom. This process is continued. A large amount of energy is released during fission reaction. This energy is used to heat the water in order to produce steam. The steam generated is thrown on the turbine with very high pressure. The turbine rotates and the current is produced.

In the nuclear reactor, fuel assemblies in the reactor’s core hold individual fuel rods that contain atoms of nuclear fuel. As the fuel nuclei split, they produce energy and release neutrons. Other fuel nuclei split when struck by the neutrons, and thus the reaction continues.

The control rods limit the number of fuel nuclei that can split. Materials in the rod absorb neutrons. When fully inserted in the core, the rods absorb so many neutrons that few nuclei can spit. To start the reactor, some rods are partly withdrawn.

Controlled chain reaction is necessary in the reactor, because if uncontrolled chain reaction starts, it will explore an atom bomb. The concept of uncontrolled chain reaction is applied in formation of atom bomb where no neutron is absorbed and struck on the Uranium atom at the same time and produces enormous amount of energy.

In a nuclear reactor, the water that is once used can also be used again and again. For example in a pressurized water reactor, water is heated under high pressure. The pressure allows the water to heat past its normal boiling point without actually boiling. Heat from this water boils water in a steam generator, which produces steam. The water from the reactor is pumped back to the reactor for re-use. After the steam has operated the plant’s turbine, it is sent to a steam condenser, which changes it back to water for re-use in the system generator.

In another type of reactor, called fast reactor, no moderator is used. It uses fast neutrons to bring about fission. It uses a mixture of Plutonium and Uranium oxide as fuel. When operating, Uranium is converted into Plutonium, which can be extracted and used later as fuel. It is also called the fast breeder because it produces more Plutonium than it consumes. Heat is removed from the reactor by a coolant of liquid sodium.

Steam supply system in nuclear power plant.

4. ENERGY PRODUCED IN NUCLEAR REACTOR

Mainly, Uranium (U²⁹⁵) is preferred as the fuel of the nuclear reactor. But, it is present in fewer amounts on the earth. So, another isotope of Uranium (U²⁹⁸) is now used as the fuel. In addition to it Polonium (Po²¹⁰), Plutonium (P²⁴⁴) and Thorium (Th²³²) are used. When Plutonium or Polonium is used as the fuel, they are first converted into an isotope of Uranium. Plutonium undergoes alpha decay (α- decay) to form an isotope of Uranium.

To estimate the produced in a nuclear reactor, there is a formula given by Albert Einstein,

E = ΔmC²

Where, E = Total energy produced,

Δm = Mass defect or mass consumed during reaction,

C = Velocity of light in vacuum.

Let us assume that 0.001 g of Uranium is consumed then the energy produced can be calculated by above equation. We know that the velocity of light in vacuum is 3 x 10⁸ ms^-1. On putting the value of Δm = 0.001 x10^-3 Kg and C in the equation, we have,

E = 0.001 x 10^-3 x (3 x10⁸)² Kg m²s^-2

Or, E = 9 x 10¹⁶ x 1 x 10^-6 joule

Or, E = 9 x 10¹⁰ J

We can now easily predict that if such a large amount of energy is released from such a small amount of Uranium, then how much energy we can obtain from 1 Kg of Uranium. It’s just about 9 x 10¹³ J, an enormous amount of energy. In comparison to the energy produced from coal, it is much greater. We can obtain such a large amount of energy by burning about 14 Lacs tonnes of coal!

5. TYPES OF NUCLEAR REACTOR

There are many types of nuclear reactor. Some of them are included below.

- Pressurized water reactor (PWR).

- Boiling water reactor (BWR).

- Pressurized Heavy water reactor (PHWR).

- High power Channel reactor (HPCH)

- Gas cooled reactor (GCR)

- Advanced gas cooled reactor (AGR)

- Liquid metal fast breeder reactor (LMFBR)

- Aqueous homogeneous reactor (AHR)

Slight different techniques are used in different reactors.

6. ADVANTAGES OF NUCLEAR REACTOR

The advantages of using a nuclear reactor are listed below:

- A large amount of energy can be produced by using very small amount of fuel.

- Water once used can be used again and again. No wastage of water.

- More water can be converted into vapour in much less time.

- No wastage of organic fuels such a coal or oil.

- Helps in the conservation of world supply of coal and oils.

- No smoke or pollutant gases are given off in the atmosphere.

7. DISADVANTAGES OF NUCLEAR REACTOR

Along with the advantages, there are a lot of disadvantages of a nuclear reactor. These disadvantages are listed below:

- A large amount of radioactive substances are required as fuel.

- Its waste materials are also radioactive, which are very harmful to living organisms.

- These substances are disposed in sea, which leads to water pollution.

- These all need much care and attention while one is handling it.

- Sometimes a reactor becomes critical and it may explode.

- It produces much excess heat in comparison to hydral and thermal power plants. Due to this the condition of thermal pollution is generated. This may kill those organisms which live at lower temperature.

- Leakage of shield tanks, which contain the nuclear wastage and buried underground, can also take place leading to serious radiation to all living organisms.

Natural Nuclear reactor

A natural nuclear reactor can occur under certain circumistances that mimic the conditions in a constructed reactor . fifteen natural nuclear reactor have so far been found in 3 separate ore deposits at OKLO mine in GABAN WEST AFRICA .

It was first discovered by french physicist Francis Perrin and all the reactors are collectively known as Okalo Fossil Reactor .self sustaining nuclear fission reactions took place in these reactors approximately 1.5 billion years ago and ran for about few hundred thousand years

Such reactors can no longer form on earth because radioactive decay this immense time span has reduced the proportion of U-235 in naturally occurring uranium to below the amount required to sustain a chain reaction .

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