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Too cheap to meter

Too cheap to meter. Today Electricity from nuclear fission Chapter 6 All power point images are only for the exclusive use of Phys3070/Envs3070 Spring term 2014. Premises.

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Too cheap to meter

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  1. Too cheap to meter Today Electricity from nuclear fission Chapter 6 All power point images are only for the exclusive use of Phys3070/Envs3070 Spring term 2014

  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. Outlinesee the newest posted syllabus • What is fission? Today • Uranium/ sources and enrichment Monday • Reactors/neutron budgets/ “ • Problems- Wednesday ionizing radiation weapons proliferation waste disposal financing/costs • Fusion very briefly Wednesday • Friday– The Canary in the Arctic

  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 today holds only 0.7% 235U, 99.3% 238U.

  5. Fusion  Fission 

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

  7. 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 released 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

  8. 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 11645Rh71 Check with A: 1+235=A +118+2x1 A=116 = N+Z

  9. Masses • 235 kg of 235U +1 kg of neutrons 116 kg of 116Rh and 118 kg of 118Ag and 2 kg of neutrons . • 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?

  10. Half life Radioactive decay is random, with a decay rate that is known. Half life= time for a sample to be half as ‘hot’, with half as many nuclei. Two half lives, ¼ as hot Three half lives 1/8 as hot….. Activity = initial activity *(1/2) N, with N the number of half lives. (1/2) time/half life

  11. Uranium half life • By what factor has the 235U present at the forming of the earth, 4.5 billion years ago, decayed away? T1/2=0.7 billion years • Count half lives=4.5/0.7=6.4 half lives. • At six half lives, (1/2)6=1/64=0.0156 • At seven half lives (1/2)7=1/128=0.0078 • So we have about 1% of the original 235U. • (found at 0.7% in ore)

  12. energies The stuff on the left is a bit more massive than the stuff on the right. E=mc2 Each fission releases about 200 MeV = 200 x 106eV *1.60 x 10-19 Joule/eV= 3.2 x 10-11 J from one fission So for 4 GWt of heat power 4x109J/sec =X fissions/sec * 3.2x10-11J/fission X=1.25 * 1020 fissions/sec. Each 235 grams of 235U hold 6.02*1023 atoms. 1.25*1020 atoms have mass= 235 * 1.25x1020/6.02x1023=49 x10-3 grams So you are consuming 49 mg of 235U fuel per second

  13. HW #8 • 1.(30 points) A proposed High Temperature Gas Cooled nuclear fission reactor would operate at a temperature of 800 deg C. This facility will be built by the sea, with cooling at 10 deg C. • A. (5) What is the best possible efficiency of this plant? • B.(10) In fact, this power source will perform at an average of 75% of that best efficiency. If the owners wish to sell 1 GWe, how much thermal power must be provided? • C.(5) How much thermal energy is generated in one day by this facility? • D.(10) Each nuclear fission releases 1.8*108 eV. How many 235U nuclei were fissioned during that day to make that much heat?

  14. HW #8 • 2. (10) The radioactive isotope 131I is particularly dangerous. The fission of 235U results in the formation of this isotope 3.6% of the time. If a nuclear power plant fissions 23 kg of 235U per day, how much 131I will be produced per day? In kg. • 3. (5) The half life of 131I is 8.04 days. How long need we wait after the fission reactions end for the 131I made in the last day to decrease to 1/32 of its hazard?

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