2What is the idea behind nuclear energy? Well… the secret lies in Albert Einstein’s equation:Imagine a cat… The cat is floating in space and suddenly emits a burst of energy in all directions. Where did this energy come from? Since energy is emitted in all directions, the velocity of the cat remains the same. Similarly, the potential energy of the cat has not changed, as it is not in a field. So where does the energy come from?Do you realize the implications of this?? This means that the cat’s mass is decreasing and the lost mass of the cat is being converted into energy. In fact, if the cat’s mass falls by even one gram, the energy given out by the cat will be:Wow!!So what is the basis for nuclear energy? Answer: Converting mass into energy
3Converting mass into energy Mass is converted into energy. (Note that the equation implies that the reverse is also possible: Energy can also be converted into mass.)NucleusThe atom:ProtonNeutronElectron(Charged and light)(Charged and heavy)(Uncharged and heavy)2) Usually only small amounts of mass is lost at a time (1 gram is a very big amount!). This is because the mass must be lost from the nuclei in atoms, which are very small. (And thus the term – “nuclear energy”).However, even a small mass can be converted to a large amount of energy.3) There are three main ways that energy can be released due to a fall in mass: Fission, Fusion and Radioactive decay. We will explore these later on.
4FissionFission is when a large, heavy nucleus absorbs a neutron to decay into lighter, more stable nuclei accompanied by the release of large amounts of energy.For example: Uranium-235 absorbs a slow moving neutron and decays into Barium and Krypton through the following reaction:Extension:Just remember that in fission reactions, the reactant and products have the same total number of protons and neutrons.Therefore, equating mass numbers:And equating atomic numbers:
5Different products from fission Barium and Krypton are not the only products that can arise from fission of uranium. From the fission of uranium nucleus, you can obtain nuclei of Xenon and Strontium or Lanthanum and Bromine also.XenonStrontiumLanthanumBromineQuestion: In these examples, you might notice that two neutrons are produced instead of 3. Can you try equating mass numbers and find out why?Clue: (Refer to the “extension” on the previous slide)
6Binding Energy (Extension) Although this is not in your syllabus, understanding binding energy will significantly improve your understanding of nuclear physics.Binding energy is the energy required to pull the constituents of a nucleus apart.In the first slide, we made things simple by considering a cat. But the nucleus is very similar:In order to pull apart the protons and neutrons in the nucleus, work needs to be done.Work done => energy is given to the protons and neutrons.But neither the velocity nor the potential energy of the protons change (Just like the cat).So where has the energy gone? This time you know the answer:But what does this mean? Well – since the energy given has been converted to mass, It means that the mass of particles when they are apart is greater than the mass when they are in a nucleus! This means that the mass of the nucleus is less than the sum of masses of protons and neutrons!Question: If the number of protons and neutrons in a fission reaction remain the same, how can the mass of the products is less than the mass of the reactants?Answer: The binding energy of the products’ nuclei is greater than that of the reactant nucleus (As the products are more stable). Therefore, the mass of the products is less than the mass of reactants reactants and energy is given out. Please refer to the previous slides on fission and then think about the effect.
7Fission Chain reaction A self sustaining reaction forming a reaction chain is called a chain reaction.1) Only one Neutron is required to trigger a fission reaction.2) But three neutrons are produced.3) The 3 produced neutrons react with 3 other nuclei, causing them to split (further fission).4) 9 neutrons are now available for reaction. The rate of reaction has rapidly increased (There has been a multiplying effect). The process continues.
8Uncontrolled fission!!An uncontrolled fission reaction is a chain reaction that is allowed to proceed naturally without further interference.In a nuclear bomb, the explosion is produced by uncontrolled fission.Question: Can you now explain why uncontrolled fission reaction is so violent (Releasing large amounts of energy with very fast rate)?Nuclear bomb pictureAnswer: The rate of reaction will keep increasing, as more neutrons are produced from each reaction than consumed. Thus, very soon, the rate will be extremely high, and a large number of nuclei will undergo fission. As they do so, mass is converted to energy. Since a very large amount of energy is released from even a small amount of mass, a large quantity of energy is released very quickly.
9Controlled fissionTo generate electricity, we require a steady source of energy. As can be seen from the picture of the nuclear bomb, an uncontrolled reaction will release the energy quickly and not steadily.For the purposes of generating electricity, we need to control the rate of reaction to keep it at a steady rate. This is difficult since every fission of a nucleus produces more than one neutron so the reaction tends to increase in rate.In practice the reaction is controlled by absorbing some of the neutrons.1) 2 Neutrons absorbed2) Only one neutron available for fission3) 2 Neutrons absorbedFission of one nucleus triggers fission of one nucleus. Rate of reaction is maintained (controlled).
10Critical size TOO SMALL !! Fission reactions are sustained by the production of neutrons which react further with nuclei.However, if the size of the sample of Uranium (or other fuel) is too small, the neutrons will leave the sample before they can slow down enough to trigger fission (Only slow neutrons can trigger fission). As a result, a sustained fission reaction will not occur.The minimum size of fissionable material required to sustain a fission reaction is called “critical size”TOO SMALL !!Neutrons escape before they can slow down enough to trigger fission!Question: Why doesn’t a nuclear bomb explode before it hits the ground?!?Answer: When the bomb is dropped, it has two separate compartments of fissionable material, each smaller than the critical size. Therefore, the reaction inside the bomb will not be sustained.When the bomb hits the ground, the two compartments are merged, and the sum of the sizes are bigger than the critical size, so the reaction will take place.
11The nuclear power plant (reactor) A nuclear reactor is a device which controls the nuclear fission chain reaction to harness nuclear energy. A nuclear power plant is a reactor that generates electricity.Energy released by nuclear energy heats water, converting it to superheated steam.The steam rises, turning a turbine, which contains magnets and a coil. As the magnets turn, a current is induced in the coil. This is the principle of an electric generator.Kaiga Atomic Power Station, Karnataka
12Ingredients of the nuclear power plant (Part 1) Fuel: Enriched Uranium-235 or Plutonium-239.Enrichment: Uranium exists as a mixture of U-235 and U-238. While the fuel is U-235, 99.3% of it is U-238, which is in fact a neutron absorber (which will prevent a chain reaction). For a fission reaction to be sustained, the percentage of U-235 must be increased. This is called fuel enrichment.Fuel Assembly: Fuel in the form of pellets is placed in long tubes of Steel or Aluminium.Control rods: It has been mentioned in “Controlled Fission” that in order to maintain a steady reaction, neutrons need to be absorbed. Controls rods, made of Boron or Cadmium, absorb neutrons to control the reaction.They can be moved up or down (see diagram in “The reactor”) and thus can absorb different amounts of neutrons. If they are moved fully down, almost all neutrons will be absorbed and the reaction will stop. Therefore, they can be used to shut down the reactor.
13Ingredients of the nuclear power plant (Part 2) Moderator: In order for a neutron to be absorbed by a nucleus, it must be moving slow enough. Fast moving neutrons will simply pass straight through the sample without triggering a reaction!In order to slow down the neutrons, a moderator (e.g. – Graphite) is used. When the neutrons collide with the nuclei of the moderator, they lose energy and slow down.Coolant: This removes the heat generated and transfers the heat to convert water into steam in the heat exchanger. Usually, the coolant is water, although other coolants such as liquid sodium, molten salts, hydrocarbon liquids and heavy water (deuterium oxide).Containment building: Prevents harmful radiation from being released to surroundings
14Nuclear FusionNuclear Fusion is the process that powers the sun! It is another mechanism (different from fission), where mass is converted to energy.Fusion reactions release even more energy than fission reactions, as more mass is lost!In fact, the fusion of even 1 gram of hydrogen gives 6 ✕ 1011 J of energy energy equivalent to combustion of 350 million tons of coal.Nuclear fusion is a nuclear reaction in which two or more light nuclei combine to form a heavier nucleus releasing very large amount of energy.Two light nucleiA heavier nucleusReleasing very large amounts of energyImportant: The sum of masses of the heavy nucleus and the proton is less than the sum of masses of the two light nuclei, which is the reason why energy is released (E = mc2).
15Fusion reactionsFusion reactions you will be studying involve isotopes of hydrogen. An isotope is an alternative form of the same element with the same number of protons but different numbers of neutrons. (Same atomic number, but different mass numbers)Regular hydrogen contains 1 proton and zero neutrons.Deuterium is an isotope of hydrogen with twice the mass of hydrogen. It contains 1 proton and 1 neutron.Tritium is an isotope of hydrogen with thrice the mass of hydrogen. It contains 1 proton and 2 neutrons.There are two fusion reactions which you need to be familiar with:2 deuterium nuclei can fuse to form a tritium nucleus and a neutron, releasing energy.2) A deuterium nucleus can combine with a tritium nucleus to form a helium nucleus and a neutron, releasing energy.Tritium nucleus ProtonDeuterium nucleus Helium nucleus
16A fusion reactor?A fusion reaction is not as simple to stimulate as a fission reaction. This is because, for nuclei to fuse, they must overcome the strong electric repulsion.++Repulsive forceDirection of velocityNucleusTo overcome the repulsive force, the nuclei must be provided with a very high kinetic energy and velocity.Since temperature is a measure of average kinetic energy, an alternate approach is to make temperature very high (as is in the sun). This is called a thermonuclear reaction.However, once fusion is initiated, enough energy is produced to maintain the temperature and sustain the reaction.Have humans made a working fusion reactor? Although scientists have been able to achieve the conditions necessary for fusion, they have not been able to make a sustained reaction so far. However, in the future, fusion reactors are a possibility.Uncontrolled fusion has been achieved in the form of the hydrogen bomb. Here, a fission reaction provides the energy necessary to trigger the bigger fusion reaction.
17Radiation Hazards (Part 1) There are many types of radiation. α-radiation, β-radiation and γ-radiation are the ones generally produced during nuclear reactions.X-rays, Ultraviolet radiation, and other electromagnetic radiation can also be harmful, but are not as harmful as the other three.High energy radiation, such as α-radiation, β-radiation, γ-radiation and X-rays can ionize atoms (or strip the electrons from the atoms by providing them with energy). This process damages DNA, and can lead to genetic disorder and cancer.Least ionizingMost ionizingVisible lightUVX-rayγβαWhile α-radiation and β-radiation have the most energy and ionizing power, they are not that penetrable. While alpha radiation can be stopped by a piece of paper, beta radiation can be stopped by the skin. However, gamma and X-rays are more penetrable, and are sometimes considered most dangerous due to the high penetrability. Though not penetrable, alpha and beta radiation may enter the body through water/food.Least penetrableMost penetrableαβγ, X-rays,
18Radiation Hazards (Part 2) Although Ultraviolet radiation (UV), visible light, and microwaves have less energy and less ionizing capability, they can also be harmful.Visible light and microwaves in the form of a high power laser can inflict damage.Ultraviolet radiation can cause skin burns, premature cataract (eyesight becomes blurred), and even skin cancer.Intense radiations may weaken metals and other structural materials.Radioactive radiation from nuclear installments and nuclear explosions(α-radiation, β-radiation, γ-radiation) are the most dangerous. Sometimes, they can contaminate the environment (e.g. – Alpha and beta radiation may contaminate into the ground water).The greatest danger from radioactive radiations is the genetic damage leading to defective offspring.