VisionIAS - Video Classroom Lecture

Science and Technology Class 01

INTRODUCTION

A brief overview of the previous class.

XPOSAT (09:14 AM)

It is an upcoming mission of ISRO, an astronomical observatory that will collect X-rays. There are many X-ray sources in the cosmos such as nebulae, and supernova among others.
This mission will determine X-ray sources by measuring the polarisation of X-rays. Polarisation is a property of electromagnetic waves such as light which signifies the vibration of the electric field in a specific way.

NUCLEAR TECHNOLOGY

Nuclear technology is the application of nuclear processes such as nuclear reactions, and nuclear decay among others.
It has a wide range of applications in energy production agriculture medicine, industry, space, and defense among others.
Atomic mass is the number of protons plus the number of neutrons.
Isotope- same atomic number but different atomic mass.
Isobars- same atomic mass but different atomic numbers.
N number of protons inside the nucleus.
Attractive force that exists inside the nucleus is a strong nuclear force.
E=mc^2
C is the speed of light in a vacuum (3*10^8 m/sec)

NUCLEAR TECHNOLOGY IN ENERGY PRODUCTION (10:11 AM)

There are mainly two processes to produce nuclear energy
1. Nuclear fission,
2. Nuclear fusion

NUCLEAR FISSION

In nuclear fission a heavy nucleus that is not very stable such as uranium 235 is bombarded with neutrons which makes the nucleus even more unstable and it disintegrates in two or more lighter nuclei. In the process, a huge amount of energy is released which can be calculated using Einstein's mass-energy equivalence equation :
E=mc^2
C is the speed of light in a vacuum (3*10^8 m/sec)
A nuclear fission reaction produces an excess of neutrons which can participate in more fission leading to a chain reaction, an uncontrolled chain reaction leads to an uncontrolled chain reaction (a nuclear bomb) however if we absorb the excess of neutrons, it can lead to a controlled chain reaction. This is the mechanism behind nuclear power plants which are used to produce electricity.
A nuclear reactor uses controlled fission which can be used to produce electricity. It has the following major components
1. Fissile material- which can undergo fission easily e.g Uranium 235, Uranium 233, Plutonium 239
2. Control rods- to absorb the excess of neutrons e.g boron, cadmium
3. Moderator- moderates the speed of neutrons, to slow down the speed of neutrons. slower neutrons are better at causing fission. e.g Water, heavy water
4. Coolant- they are regulating the temperature of the core reactor by removing the heat produced from fission. This heat is used to convert water into steam use that steam to move the turbine and to produce electricity. e.g. water, heavy water.
Often moderators and coolants are the same.
However there are coolants that are not good moderators such as liquid sodium.

TYPES OF REACTORS (11:00 AM)

1. Pressurized water reactor
2. Boiling water reactor ( not used in India)
3. Fast Breeder reactor

NUCLEAR FUEL CYCLE :

Mining -> Miling -> Enrichment -> Fabrication -> Fission at reactor -> Interim storage -> spent fuel reprocessing -> Final disposal
Spent fuel -> enrichment
Interim storage -> Final disposal
Uranium fuel cycle involves all the steps starting from mining and ending with nuclear waste management.
1. Mining- uranium is extracted from the earth's crust
2. Miling- uranium ore is purified using physical and chemical separation methods to recover natural uranium.
3. Enrichment- natural uranium is more than 99% of uranium 238 and less than 1% of uranium 235. In the enrichment process, using isotopic separation methods, the concentration of Uranium 235 is increased upto 3%-5% ( for nuclear/ atomic bombs more than 90% enrichment is required)
4. In fabrication, uranium is converted into spherical pellets embedded into a metallic tube which is called a fuel rod. Many fuel rods make an assembly. At one point in time, many assemblies are kept in the core of the reactor.
5. Fission at reactor- Large nuclear reactors do not require refueling for 2 to 3 years.
6. Interim storage- after fission nuclear waste is removed from the core and kept in a deep pool of water for a few years.
7. Spent fuel reprocessing- In this process, useful material such as enriched material can be recovered
8. Disposal- remaining waste can be disposed of in geological formations so that they remain there for hundreds of years.

INDIA'S THREE-STAGE NUCLEAR PROGRAM (11:38 AM)

First stage- Pressurised heavy water reactor (heavy water as moderator and coolant)
Second stage- Fast breeder reactor (liquid sodium as a coolant but no moderator)
Third stage- breeder reactor (thorium based)
It was envisaged by Dr. Homi Jahangir Bhabha to make India self-dependent on the nuclear fuel supply. India has uranium reserves but not as abundant as Thorium. The thorium reserve is mainly found in the western ghats, especially the Monazite sands of Kerala.
In the first stage enriched uranium will be used in PHWR while uranium 235 undergoes fission, Uranium 238 converts into plutonium by absorbing neutrons.
In the second stage a fast breeder reactor will be used. Plutonium 239 recovered from the first stage will be the main fissile material but thorium will also be used together which will convert into uranium 233.
Fast breeder reactor does not use a moderator even though fast neutrons are not good at fission, they readily convert thorium to uranium. In fact, the production of Uranium occurs faster than the fission of plutonium hence the name fast breeder.
In the third stage Uranium recovered from the second stage will remain the main fissile material while Thorium can be used together which can convert into uranium because of the ample abundance of thorium.
Third stage can continue for hundreds of years.
Most of our reactors are still in the first stage. The first operational fast breeder Reactor will be in Kalpakkam (Tamil Nadu).

The topic for the next class is radioactivity and the application of nuclear technology in various areas.