Nuclear Reactions PDF

WEEK ELEVEN; LESSON THREE ENERGY CHANGES IN NUCLEAR REACTIONS  LESSON OBJECTIVES By the end of these lessons, students should be able to: Differentiate between nuclear and chemical reactions Understand the different types of nuclear reactions Explain and perform c...

WEEK ELEVEN; LESSON THREE ENERGY CHANGES IN NUCLEAR REACTIONS  LESSON OBJECTIVES By the end of these lessons, students should be able to: Differentiate between nuclear and chemical reactions Understand the different types of nuclear reactions Explain and perform calculations involving mass defect and binding energy.  Nuclear Reactions Nuclear reactions differ markedly from chemical reactions in several ways: (1) Elements typically do change in a nuclear reaction. (2) Nuclear particles participate, but electrons do so much less often. (3) Nuclear reactions release so much energy that the mass does change. (4) Rates of nuclear reactions are not affected by temperature or catalysts.  Nuclear Reactions There are two types of nuclear reactions: Nuclear Fission and Nuclear Fusion Nuclear Fission: is the splitting of large nuclei into smaller nuclei of nearly equal parts with the release of energy. Spontaneous fission is rare and generally fission is induced by bombarding the heavy nucleus with neutrons. The U-235 nucleus is able to absorb the neutron to become (very briefly) U-236. The U-236 then undergoes fission to form two new nuclei called fission products. Bombarding the uranium-235 nucleus with a neutron leads to the formation of a uranium-236 nucleus, which very quickly undergoes fission. Fission products are formed, and neutrons are emitted. Note that gamma radiation is also emitted. Fission also produces neutrons and energy.  Nuclear Reactions 235 92U + 10n → 14054 Xe + 93 38 Sr + 3( 1 0𝑛 + energy A uranium-235 atom absorbs a neutron to become uranium-236 which then undergoes fission to form the products xenon-140 and strontium-93 with three neutrons. Features of Nuclear Fissions: 1. Fission products are radioactive and hence dangerous 2. More or excess neutrons are released 3. The fact that neutrons are also produced means that these neutrons can go on to induce further fissions. Once started, fission can become self-sustaining – this is called a chain reaction.  Nuclear Reactions 4. For fission to occur the neutrons must be going at the right speed – too fast and they will bounce off rather than be absorbed. The neutrons may need to be slowed down and are then referred to as thermal neutrons. Both the number and speed of the neutrons is crucial within a working reactor. 5. Excess neutrons produced in the reaction core are absorbed by boron rods. 6. Fast moving neutrons are slowed down by moderators such as graphite rods, deuterium or cadmium rods. 7. The energy released per fission is relatively large. It is 50 million times more energy than burning the equivalent amount of carbon.  Nuclear Reactions Nuclear Fusion: is the coming together of light nuclei to form a heavier one e.g. 2 2 2 1 1H + 1H → 1H + 0𝑛 + energy Fusion reaction is also known as thermonuclear reaction. Features of Nuclear Fusion: 1. It occurs mostly in the sun 2. It occurs at very high temperatures 3. Its products are not radioactive or dangerous 4. No material can withstand its high temperature without melting except when carried out under adiabatic demagnetization. 5. Enormous amount of energy is released, more than in fission 6. Formation of atomic bomb is based on fusion reactions  Advantages of Nuclear Fusion over Nuclear Fission 1. It is easily achieved by the lightest elements, so nuclear repulsion is easily overcome as the nuclei approach each other. 2. The raw material (hydrogen) is cheaply available in large quantities 3. Fusion products are not radioactive and hence less dangerous 4. It does not lead to chain reaction 5. There is no problem of disposal of waste products 6. There is no upper limit to the mass of hydrogen that can be used. In spite of these advantages, nuclear fusion is not used in generating power because of the very high temperature required to overcome the Coulomb’s repulsive force between the two light hydrogen nuclei as no material can withstand this very high temperature.  Binding energy The binding energy is the required to bring nucleons (protons and neutrons) together to form a nucleus or the energy to separate the nucleon (protons and neutrons). In nuclear reactions, the binding energy is equivalent to the mass defect, i.e. it is a measure of mass defect. The difference in mass is called the mass defect. Mass defect = Mass of separate nucleons - Mass of nucleus The binding energy is associated with the forces that bind the nucleus together.  Binding energy From E = mc2: Binding energy = mass defect × c2 Different nuclei have differing amounts of binding energy. In fission, a large nuclei is split into smaller parts. The total mass of these parts may be lower than the initial large nucleus. This difference in mass is due to the difference in binding energy between the nuclei and is released as energy.