Fundamentals of Nuclear Technology

Nuclear Technology

It refers to the technology involved in the nuclear reactions of the atomic nuclei. Nuclear technologies are used in nuclear reactors, nuclear weapons based on nuclear fission and fusion, nuclear medicines, smoke detectors, gun sights etc.

Nuclear Energy:

  • It is the energy released by the change the nucleus of an atom caused by either Nuclear Fission or Fusion.
  • Atom is the fundamental building block of matter. The nucleus of an atom is a congregation consisting of protons and neutrons. Protons carry a positive charge, while neutrons do not have any electrical charge.
  • Nuclear technology is used for changing the nucleus of some specific elements such as uranium etc, which releases nuclear energy.
  • Nuclear energy is produced naturally by nuclear fusion in the sun and other stars and release heat, light and radioactive radiations.
  • Nuclear energy is also released by Nuclear Fission naturally of decay of elements such as uranium etc. It is also produced through man-made operations in the nuclear power plants and nuclear weapons.

Evolution of Nuclear Technology

  • Henri Becquerel in 1896, while investigating phosphorescence in the uranium salts discovered the phenomenon of radioactivity. Later on, Henri Becquerel, Pierre Curie and Marie Curie investigated the phenomenon of radioactivity and discovered that materials having radioactivity produce three distinct penetrating rays called alpha rays, beta rays and gamma rays.
  • Later on, it was discovered that radiation produced by radioactive decay was ionizing radiation which could pose long term hazards.
  • Afterward, it became clear that nuclear energy, either radiation from the sun caused by thermonuclear fusion reactions or the radioactive decay of uranium by fission reactions within the earth is the ultimate source of most terrestrial energy.

Types of Radiation

  • Unstable atoms go spontaneous transformation to form more stable product atoms and release radioactive radiation and energy. The substances which release such kind of radioactive radiations are said to be Radioactive and the process is called as radioactive decay.
  • Radioactive substances are naturally radioactive or can be made Radioactive by bombarding them with nuclear particles. Apart from alpha rays, beta rays, and gamma rays; neutrons are the fourth kind of particulate radiation.
  • Alpha rays consist of helium nuclei i.e two neutrons and two protons. Beta rays consist of high speed electrons originating from the nucleus of an atom. Gamma rays are electromagnetic radiation consisting of packets of photons transmitted as waves.

Nuclear Fission

  • Nucleus fission refers to the process of splitting of the nucleus of an atom into two or more smaller parts (lighter nuclei) and the release of neutrons and energy in the form of heat, kinetic energy and electromagnetic radiation.
  • Nuclear fission of heavy elements is an exothermic reaction and produces free neutrons, Gamma photons and a large amount of energy.
  • Nuclear fission can occur naturally in substances having heavy nuclei which become unstable and undergo spontaneous fission. It can occur by man-made process by bombarding nuclei with neutrons.
  • Nuclear fission reaction releases fast moving neutrons. ‘K’ is the number of neutrons released per nucleus. If the value of 'K’ is greater than 1 then the reaction releases more neutrons than it absorbs and hence called as self-sustaining chain reaction.
  • Nuclear fission was first used by humans for developing atomic bomb on the eve of World War 2 in the Manhattan Project. Later on, in 1951, the first nuclear fission power plant was developed at the Experimental Breeder Reactor No. 1, in Arco, Idaho for producing electricity.
  • Different nuclear fuels used in nuclear fuel reactions are U-235, Pu-239, U-233, Th-232 etc.

Fission of Uranium Atom:

  • Most of the naturally occurring Uranium is in the form of its isotope U-238 (around 99%) and the remaining in the form of U-235. Only U-235 has the capability to undergo nuclear fission easily to any great extent.
  • Enriched Uranium is used in nuclear power applications whose concentration varies from Low Enriched Uranium (LEU) having 3 to 5% enrichment to higher levels.

When U-235 absorbs a slow moving neutron, nuclear fission reaction takes place.

235U + 1 neutron = 89Kr + 144Ba + 3 neutrons + Energy

The products of nuclear fission of U-235 can vary.

  • Additional neutrons released during this reaction can cause other uranium atoms to split further releasing even more neutrons leading to a chain reaction which continues until all the fuel is spent.
  • In this reaction, the energy released is due to the changes in binding energy after the nuclear fission reaction. The total mass reduces by a very small amount during this reaction and gets converted into energy. This energy can be calculated using the Einstein's equation E=MC2.

Nuclear Fusion

  • Nuclear fusion refers to the reaction in which two or more lighter nuclei join together to form one or more heavier products. The products of fusion reactions are usually different atomic nuclei and sub-atomic particles such as neutrons and protons.
  • Change in mass takes place between the reactants and products which is manifested as either the release of energy or absorption of energy.
  • Nuclear fusion takes place in in the sun of our solar system and other active stars of the universe. Nuclear fusion is opposite to nuclear fission where the heavy nucleus splits apart. A large amount of heat and pressure is required for the nuclear fusion reaction.
  • Nuclear fusion process producing nuclei lighter than iron-56 or nickel-62 are exothermic reactions and releases energy. Nuclear fusion producing nuclei heavier than these elements are endothermic reactions and absorb energy.
  • In Sun, nuclear fusion takes place in which hydrogen nuclei combine together to produce helium. This releases a large amount of energy due to changes in binding energy and conversion of mass into energy according to Einstein formula E= MC2.
  • On earth, nuclear fusion reaction has been achieved to produce a hydrogen bomb which is much more destructive than atomic bombs based on nuclear fission.

Nuclear Power Reactors

  • Nuclear power reactor is a device used for initiating and controlling a self sustained Nuclear chain reaction. It produces and controls the energy released from fission of nuclear fuels such as Uranium.
  • The energy produced in the reactor is used for heating the steam and producing electricity. Nuclear reactors are used for nuclear research, producing electricity at nuclear power plants, producing propulsion in ships.

A nuclear reactor has the following components

  • Nuclear fuel: It is the element used in nuclear power plants for producing energy for powering the turbines. Usually enriched uranium-235 or plutonium-239 is used as fuel in nuclear reactors. The major fuel elements are uranium dioxide, uranium nitride, uranium carbide, plutonium, Thorium etc. The pellets of Uranium oxides are arranged in tubes forming fuel rods which are arranged in fuel assemblies inside the nuclear reactor core.
  • Moderator: moderators are used for reducing the speed of fast neutrons released from fission reaction and making them capable of sustaining a nuclear chain reaction. Usually, water, solid graphite and heavy water are used as a moderator in nuclear reactors.
  • Control rods: these are a neutron absorbing materials used for controlling the number of neutrons which can cause further fission of nuclear fuel. Control rods are like brakes which absorb neutrons so that fewer neutrons are available to cause a fission reaction. Boron, silver, cadmium, Indium other materials used as control rods. These are inserted or withdrawn from the core for controlling the rate of reaction and changing the power output.
  • Coolant: it is a liquid or gas which circulates through the core and transfers the heat produced by fission reaction to an external boiler and turbine for producing electricity. Light water, molten sodium, lead, lead-bismuth mixture, and molten salt etc are used as a coolant in nuclear reactors. The moderator acts as a coolant in light water reactors.
  • Steam generator: it is the part of the cooling system where steam is generated by using the heat from the reactor.
  • Shield and Containment system: Nuclear Fission reaction inside the reactor produces a neutron and other radiation. The radioactive decay of products formed after Nuclear Fission produces radioactive radiation. Thus a containment structure is built around the reactor core that keeps the radioactive gases and liquids inside the reactor and protects those outsides from the effects of radiation. Usually, containment system consists of big, heavy structure which is several feet thick and made of concrete and steel structure.

Types of Nuclear Reactors

Boiling water reactor (BWR): in boiling water reactors, the reactor core heats water passing through it and converts it into steam which is used for driving a steam turbine for generating electricity. The exhausted steam coming out of the turbine is condensed into water in the condenser. This condensed water is again pumped to the reactor core and the cycle repeats.

Pressurized water reactor (PWR): in pressurized water reactors, pressurized water is used in primary coolant loop for carrying heat to the steam generator. The reactor core heats this pressurized water which does not boil due to high pressure and high boiling point. This water gets heated and reaches a higher temperature and then it is used for exchanging heat with a secondary low pressure water system which gets converted into steam which is used for driving a steam turbine for generating electricity.

Heavy water reactors: These reactors uses heavy water i.e. Deuterium oxide (D2O) as its coolant and moderator. Heavy water reactors use un-enriched Uranium (U-235) as fuel. Normal water when used as a moderator can absorb some neutrons and can post hurdle in sustaining the chain reaction. On other hands, heavy water already has one extra neutron so it does not absorb neutrons as readily as light water. Thus, heavy water reactors can work without having expensive uranium enrichment facilities. However, the cost of heavy water is higher than light water.

Gas cooled reactor: in gas cooled reactor, gas is used as a coolant which drives the gas turbine for generating electricity. These reactors have the advantage of higher thermal efficiency of around 50% and can operate at very high temperatures. Light water, heavy water, boiling water, and pressurized water reactor have lower thermal efficiency and these cannot work at very high temperatures.

Fast reactors: fast reactors use fast moving neutrons for sustaining a fission reaction. These reactors don't use any neutron moderator for slowing down the fast moving neutrons. However, these reactors used highly enriched uranium as fuel which is costlier than low enriched Uranium. Production of electricity from fast reactors is costly but, the waste produced by these reactors is of lower radiotoxicity and has reduced lifetime.

Fast Breeder reactors: breeder reactors are those which are capable of generating more fuel than it consumes. These have higher neutron economy for breeding fissile fuel from materials like U-238 and Th-232. Breeder reactors use fast moving neutrons for producing energy.

Usually, in these reactors, 25 to 30% of Pu-239 is mixed with U-238 and are exposed to fast moving neutrons. Fission of Pu-239 producers about 20 times more power than natural uranium reactors. Also, U-238 in the fuel absorbs some fast moving neutrons and converts itself into Pu-239.

Around 1.1 kg of plutonium is produced after the reaction for every 1 kg of plutonium used as fuel. As these reactors produce more plutonium fuel than it consumes these are known as fast breeder reactors. These reactors are capable of utilising 70-75% of Uranium and produce less radioactive waste. Usually, sodium is used as a coolant in these reactors.

Thorium reactors: Th-232 is not a fissile material, but when it absorbs a neutron, it converts itself to U-233 which is a fissile material. Thorium-232 can be used multiple times for producing electricity and can create endless cycles of fuel availability. Th-232 is more abundant in nature and produces less radioactive waste than uranium based reactors.

Applications of Nuclear Technology

  • Nuclear technology is used for producing electricity.
  • It is also used in different industries for different purposes such as in the manufacture of plastics, sterilization of disposable products etc.
  • Production of nuclear weapons based on nuclear fission and nuclear fusion.
  • Nuclear technology is also used in medical science for different purposes such as radiotherapy for treatment of malignant tumors.
  • Nuclear Technology also has application in agriculture for controlling pests and insects etc.

Advantages of Nuclear Energy

  • Nuclear energy does not emit greenhouse gases like CO2 etc., and does do not cause global warming and climate change.
  • Nuclear power, unlike fossil fuels, do not release chemical pollutants or solid pollutants in the atmosphere.
  • Nuclear fuel releases more energy per unit fuel compared to any fossil fuel found on earth.
  • Unlike solar power or wind power, nuclear energy does not require very large area per unit energy produced for set up.
  • The maintenance cost of nuclear power plants is cheaper once it becomes operative after construction.

Disadvantages of Nuclear Energy

  • the construction cost of nuclear power plants is very expensive.
  • The waste produced by nuclear power plants remains radioactive for thousands of years.
  • Since nuclear power uses hazardous radioactive materials, setting up nuclear power plants require certain government approvals which is a difficult process.
  • Nuclear power plants can pose the threat of nuclear accident which can cause severe damage to life and property. The Chernobyl nuclear disaster of 1986 in USSR is an example.

Nuclear Energy around the world

  • Nuclear power contributes around 14% of the world's total electricity requirements.
  • USA ranks first in the production of nuclear power and produces around one-third of the world's total nuclear energy.
  • The second largest producer of nuclear energy is France. France meets about 72% of its energy needs through nuclear power.
  • About 31 countries in the world have either developed or are involved in developing nuclear energy. More than 450 nuclear reactors have been built and several others are under construction.

Nuclear Fusion based power plants

  • Nuclear fusion based power plant is presently not in operation for meeting the energy needs. However, some nuclear fusion based power plants are under construction around the world.
  • The International Thermonuclear Experimental Reactor (ITER) is constructing the world's first nuclear fusion based power plant. ITER is a partnership of 35 countries to develop nuclear fusion technology which will benefit all the participating countries. India is a participant in this research.
  • ITER will be the first fusion device which will produce net energy and would maintain fusion for longer periods of time. The machine is being designed for producing 500 megawatts of Fusion Power.
  • MIT has received millions of funding for building fusion based power plant in the next 15 years.
  • A private UK-based nuclear fusion company ‘Tokamak Energy’ is also conducting research for building nuclear fusion based power plant. The company is on its quest to be the first in the world to produce commercial electricity from nuclear fusion by as early as 2030.

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