Download Fission, Fusion and Nuclear Energy - General Physics - Lecture Notes and more Study notes Physics in PDF only on Docsity! 1 Chapter 31: Fission, Fusion and Nuclear Energy Please remember to photocopy 4 pages onto one sheet by going A3→A4 and using back to back on the photocopier Nuclear Fission Nuclear Fission is the break-up of a large nucleus into two smaller nuclei with the release of energy (and neutrons)*. Natural Uranium is made up of two isotopes: 235U (0.7%), and 238U (99.3%). Only U-235 undergoes fission. This occurs if it is bombarded with fast-moving or slow-moving neutrons, but is more likely to occur if the neutrons are relatively slow moving. This reaction is represented as follows: 235U + 1 neutron 92Kr + 141Ba + 3 neutrons + K.E. * The neutrons produced are fast moving and may trigger further fission. E = mc2 The total mass on the left-hand side is greater than the total mass on the right- hand side. The mass which has disappeared has been converted (re-manifested) into the kinetic energy of the particles on the right (see note below). Atomic Mass Unit (a.m.u.) When we deal with energies on the atomic (quantum) level, the numbers are so small that instead of using Joules we use a much smaller unit called the electronVolt, where 1eV = 1.6 x 10-19 Joules. We do something similar with mass; we used to use the mass of a Hydrogen nucleus (which is a proton) as our basic unit, but in 1960 it was changed to 1/12th the mass of a carbon 12 atom (because it was easier to measure). This is now known as the unified atomic mass unit (u). You don’t need to know its value (you will be told in the question), but for what it’s worth 1 a.m.u. = 1.67 x 10-34 kg, which using E = mc2 is equivalent to 931MeV. Chain Reactions If the mass of the sample is above a certain critical mass, the process will become self-sustaining (as a result of the new neutrons colliding into more uranium atoms) and a chain reaction will occur. A chain reaction is a self-sustaining reaction where fission neutrons go on to produce further fission (giving more neutrons) etc. If the mass is below the critical mass the reaction will simply fizzle out*. This process also occurs in a nuclear reactor, but at a controlled rate*. Nuclear Reactors • The fuel is natural Uranium. • The moderator is Graphite or Heavy-Water This slows down the fast moving neutrons to enable further fission in 235U rather that being absorbed in 238U. • The control rods absorb neutrons; they look like sleeves. Lowering them over the fuel rods prevents the neutrons from one fuel rod reaching the next rod, and so they control the rate of the reaction. Lowering them completely causes the reaction to stop. 2 Environmental impact of fission reactors Positive: No CO2 emissions, no greenhouse gases. Negative: Radioactive waste, potential for major accidents. Nuclear Fusion* Nuclear Fusion is the combining of two small nuclei to form one large nucleus with the release of energy. Example (2H = Deuterium (Hydrogen with one neutron), 3H = Tritium) Note that this involves the combination of 2 positively charged particles. To overcome this Coulombic repulsion (remember Coulomb’s Law?) a large amount of energy must be supplied. Why is a fission reactor a more viable source of energy than a fusion reactor? 1. Easier to initiate reaction 2. Fission can be more easily controlled Advantage of Fusion over Fission: 1. Less radioactive waste. 2. Deuterium is readily available from the oceans. 3. No dangerous chain reactions. The energy we get from the sun comes from nuclear fusion reactions in the sun*. Leaving Cert Physics Syllabus Content Depth of Treatment Activities STS Nuclear Energy Principles of fission and fusion. Interpretation of nuclear reactions Fusion: source of Sun’s energy. Nuclear weapons. Mass as a form of energy. Mass-energy conservation in nuclear reactions. Appropriate calculations Mass transformed to other forms of energy in the Sun. E = mc2 Nuclear reactor (fuel, moderator, control rods, shielding, and heat exchanger). Audiovisual resource material Environmental impact of fission reactors. Development of fusion reactors. Disposal of nuclear waste.
