Nuclear Fission and Fusion
Section: Nuclear Physics | Syllabus: Cambridge AS Level Physics 9702
Nuclear Fission and Fusion Nuclear fission and nuclear fusion are processes involving changes to the nucleus that release energy. Key Concept In both fission and fusion, a small amount of mass is converted into energy.
You do not need to use numerical values for mass or energy changes at IGCSE level. Nuclear Fission Nuclear Fission The splitting of a heavy nucleus into two lighter nuclei. Key Points about Fission A heavy nucleus (e.g., uranium-235) splits into two smaller nuclei Additional neutrons are released Mass decreases - the total mass of the products is less than the original nucleus Energy is released - the "missing" mass has been converted to energy Example Nuclide Equation for Fission Example: Fission of Uranium-235 235 92 U + 1 0 n → 141 56 Ba + 92 36 Kr + 3 1 0 n + energy Uranium-235 absorbs a neutron and splits into barium-141, krypton-92, and 3 neutrons.
Energy is released. FIG: Nuclear fission process Neutron approaching U-235 nucleus, nucleus absorbing neutron and becoming unstable, nucleus splitting into two fragments, release of 2-3 neutrons and energy.
Nuclear Fusion Nuclear Fusion The joining of two light nuclei to form a heavier nucleus. Key Points about Fusion Two light nuclei (e.g., hydrogen isotopes) join together to form a heavier nucleus Mass decreases - the product nucleus has less mass than the sum of the original nuclei Energy is released - the "missing" mass has been converted to energy This is the process that powers stars, including our Sun Example Nuclide Equation for Fusion Example: Fusion of Hydrogen Isotopes 2 1 H + 3 1 H → 4 2 He + 1 0 n + energy Deuterium and tritium (hydrogen isotopes) fuse to form helium-4 and a neutron.
Energy is released. FIG: Nuclear fusion process Two small nuclei approaching at high speed, nuclei colliding and combining, formation of larger nucleus, release of particles and energy. Comparison: Fission vs Fusion Property Fission Fusion Process Splitting of a heavy nucleus Joining of light nuclei Nuclei involved Heavy nuclei (e.g., uranium) Light nuclei (e.g., hydrogen isotopes) Mass change Total mass decreases Total mass decreases Energy change Energy is released Energy is released Where it occurs Nuclear power stations Stars (e.g., the Sun) Exam Tip Remember: Fission = Splitting (both start with 'S' for 'Small pieces'), Fusion = Joining (think 'fusing together').
In both processes, mass decreases and energy is released.
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