![]() If at least one surplus neutron can induce fission in another 235 92U nucleus and so on, then a self sustaining release of energy is possible. Discussion and demonstrations: Controlled chain reactions The two lighter fission fragments have a higher fractional neutron excess hence some are left over.) These left over neutrons are the vital key to unlock nuclear power using fission. The heaviest elements have the largest neutron excess to remain stable. Why are there some neutrons left over? (Relate this to the N- Z curve. What is required to balance the equation? (3 neutrons) Here is the nuclear equation for a typical fission process:Ġ 1n + 235 92U → 236 92U → 138 53I + 95 39Y + ? (Which engineering structures have existed for the last 600 years?) Think about the consequences if waste disposal has to be engineered to remain intact for many centuries. They form the high-level radioactive waste that cannot be simply disposed of it has to be stored somewhere for a minimum of 20 half lives.īy what factor will the activity fall after 20 half lives? ( ½ 20 is about 10 -6, or one-millionth)ġ37 55Cs has a half life of 30.23 years: 20 half lives is 605 yearsĩ0 38Sr has a half life of 28.1 years: 20 half lives is 562 years Because they have a relatively high fraction of neutrons, they are unstable, and decay with short half-lives. Most of the energy released is carried off in the high speeds of the fission fragments. Discussion: Fission products and radioactive waste Wobble the plate about and watch the drop split. An alternative is to grease a plate and put a large drop of water on it. As soon as the electric charge distribution departs from the spherical (pinch the balloon into a dumbbell like shape) the mutual coulomb repulsion between the two ends drives the fission process. After the absorption of the neutron, the nucleus of 238 92U wobbles. Show a water filled balloon – a good model for a nucleus. In many ways, nuclei behave like a drop of liquid. any two smaller nuclei that can be made from the 235 nucleons of the 235 92U.Įpisode 527-2: Nuclear fission (Word, 123 KB) Demonstration: The nucleus as a liquid drop It absorbs a neutron, then splits into fission fragments, i.e. How do the two common isotopes of uranium 235 92U and 238 92U differ? ( 238 92U has three more neutrons than 235 92U.) It is the 235 92U not the 238 92U that fissions. The two lighter elements are referred to as fission fragments. It breaks up into two large chunks – into two elements nearer to the middle of the periodic table – so-called induced fission. The behaviour of the heaviest natural element, uranium, is different. In the examples above, small parts are chipped off nuclei. Students can practise balancing equations.Įpisode 527-1: Isotope production (Word, 50 KB) Discussion: Induced fission Ask your students to complete the following nuclear equation that summarises the transmutation of nitrogen into oxygen:Ĭockroft and Walton were the first to split the atom, by bombarding lithium with protons from their accelerator.ġ 1H + 7 3Li → 8 4Be → 2 4 2He Student questions: Balancing equations Patrick Blackett carried out further experiments and showed that the bombarded nucleus had transmuted. Rutherford found that protons exist in the nucleus by bombarding nuclei with alpha particles. There is another way in which an element may be transmuted for example, the production of radioactive 14 6C used in radio-carbon dating in the atmosphere by the neutrons in cosmic rays. Write general equations for these processes. (Moves one place down the periodic table.) What about β - decay? (Moves one place up the periodic table.) Introduce the idea of β + decay. Using a Periodic Table, explain that α decay moves two places down the periodic table. What is the nucleus made of? (Protons and neutrons, collectively know as nucleons.) What two natural processes change one element into another? ( α and β decay). Start by rehearsing some assumed knowledge. Student questions: Fission calculations (20 minutes).Discussion: The possibility of fission (10 minutes).Discussion and demonstrations: Controlled chain reactions (15 minutes).Worked example: A fission reaction (10 minutes).Discussion: Fission products and radioactive waste (10 minutes).Demonstration: The nucleus as a liquid drop (10 minutes).Discussion: Induced fission (10 minutes).Student questions: Balancing equations (30 minutes).Discussion: Transmutation of elements (15 minutes).There are other decay processes, and there are other events that occur when a nucleus absorbs a particle and becomes unstable. Students need to move beyond the idea that nuclear changes are represented solely by alpha, beta and gamma decay. ![]()
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