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Too cheap to meter

Too cheap to meter. Electricity from nuclear fission (and a bit about nuclear fusion) Chapter 6. Premises. Electricity is the most valuable form of energy, most directly connected to ‘quality of life’ Fossil fuels and CO 2 are a problem

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Too cheap to meter

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  1. Too cheap to meter Electricity from nuclear fission (and a bit about nuclear fusion) Chapter 6

  2. Premises • Electricity is the most valuable form of energy, most directly connected to ‘quality of life’ • Fossil fuels and CO2 are a problem • Energy from nuclear fission can be clean and effective, and has a good history—a known science and a known technology • But- there are problems-cost, proliferation of weapons, radioactive waste

  3. Outline • What is fission? • Uranium/Radioactivity • Reactors/neutron budgets/fuel • Problems- ionizing radiation weapons proliferation waste disposal financing/costs • Fusion • overview costs

  4. It all starts with 235U -the ‘interesting’ isotope of uranium— Q? Where did that 235U come from? A. From gravity- which drives nuclear reactions in stars which have exploded because they ran out of fuel—supernovae. These reactions made all heavy elements, and the ejecta condensed into new stars and planets. The isotope 238U has a half-life of 4.5 billion years, and 235U has a half-life of 0.7 billion years, so naturalU holds only 0.7% 235U, 99.3% 238U.

  5. Labels and equations 23592 U 143 AZ XX N with A=N+Z=atomic weight Neutrons and protons must balance on both sides of a reaction. XX—name/symbol of the element

  6. Why is 235U interesting? The ‘curve of binding energy’ shows us that the fission of a heavy nucleus into two lighter nuclei gives off energy. Most nuclei are solidly stuck on the edge of this curve, but 235U is very lightly stuck, and may be relased to roll ‘downhill’ by capturing a free neutron. That fission releases 2.4 neutrons, so a chain reaction may follow n + 23592U 143  236U *  FF1 +FF2+2.4 n

  7. A fission reaction 10n1 + 23592U143AZ XX N +11847Ag71+2 10n1 Protons: 0 +92 = Z +47 + 2 x 0 Z=45 (rhodium) Neutrons: 1 + 143= N + 71 + 2x1 N=144-73=71 Check with A: 1+235=A +118+2x1 A=116 = N+Z

  8. Masses • 235 kg of 235U 116 kg of 116Rh and 118 kg of 118Ag. • But—this is only one of many possible fission reactions. • 116Rh has a half life of 0.9 sec • 118Ag has a half life of 2.4 sec • What does that mean?

  9. Energies The stuff on the left is a bit more massive than the stuff on the right. E=mc2 Each fission releases 200 MeV = 200 x 106 x1.60 x 10-19 J= 3.2 x 10-11 J So for 4 GW of heat power 4x109 J/s =N fissions/sec x 3.2x10-11 J/fission N=1.25 x 1020 fissions/sec. Each 235 grams of 235U hold 6.02x1023 atoms. 1.25x1020 atoms have mass= 235 x 1.25x1020/6.02x1023=49 x10-3 grams

  10. Or-Plan B Make a new element that behaves just like 235U. n + 23892U146239U239Np23994 Pu145 Plutonium, an isotope with a 24,000 year half-life, so we can use it- fissions just like 235U,with extra neutrons So-once you have 235U to yield fission neutrons, you may breed from abundant 238U,yielding 239Pu, and you are in business for a long time.

  11. We now use an ‘open cycle’

  12. How to get 235U? Natural uranium is only 0.7% the good stuff. Efficiency requires higher enrichment---3.5%. The isotope 235U (0.7%) may be separated from naturalU by gaseous diffusion or centrifuges in large plants. The chemical compound used is UF6, a corrosive gas.

  13. Gas centrifuge farm

  14. Recap-- natU, marketed as “yellowcake”, U3O8, 0.7% 235U UF6, a gas used in enrichment LEU=Low Enriched Uranium, 3-5% 235U, for power reactors HEU=Highly Enriched Uranium >80% 235U, for bombs DU=Depleted Uranium, ~0.2% 235U 239Pu, made from 238U in reactors, for fuel or bombs

  15. Reactors Boiling water reactors (BWR) Heat  steam 2. Pressurized water reactor (PWR) Heat hot liquid steam (hot water or liquid metal)

  16. Neutron population dynamics Four things can happen to a neutron amid 235U #n Probability  Expected neutrons Fission 2.4 0.30 0.72 Scatter 1. 0.28 0.28 Capture 0 0.21 0. Escape 0 0.21 0. 1.00 1.00- a stable population But- each fission releases energy. ( with 2 net neutrons, 280=10 kilotons of TNT in a few microseconds)

  17. Homework #7 due Monday March 1 • A typical fission reaction for uranium is • 1on1 +23592U 14313153I78 +AZ XX N +3 10n1, • where I is the element iodine. • (2) What are the atomic weight A, the atomic • number Z and the neutron number N for the • element XX in the specific fission reaction above, • one of many possible. • (7) If we fission I kg of 235U completely in one year, • at a plant efficiency of 30%, how many electrical GW • can we sell?

  18. (4) The above specific process occurs with 2% probability. If we fission one kg of 235U completely, how many grams of 131I are produced? • (5) 131I has a half life of 8 days. If I start with one gram of it, how much is left after 32 days? • (2) Why is 131I a special concern?

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