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Splitting The Atom

Splitting The Atom. Nuclear Fission. Fission. Large mass nuclei split into two or more smaller mass nuclei Preferably mass numbers closer to 56 Neutrons also emitted Fission can occur when unstable large mass atom captures a neutron can happen spontaneously. Fission is Exothermic.

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Splitting The Atom

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  1. Splitting The Atom Nuclear Fission

  2. Fission • Large mass nuclei split into two or more smaller mass nuclei • Preferably mass numbers closer to 56 • Neutrons also emitted • Fission can occur when • unstable large mass atom captures a neutron • can happen spontaneously

  3. Fission is Exothermic • Masses resulting nuclei < original mass • ~0.1% less • “Missing mass” converted into energy • E = mc2 • Large quantities of energy are released because the products are higher up the Binding Energy Curve

  4. The Fission Process

  5. Energy Released By A Fission • U + n --> fission + 2 or 3 n + 200 MeV1MeV (million electron volts) = 1.609 x 10-13 j • This corresponds to 3.2 x 10-11 j • Production of one molecule of CO2 in fossil fuel combustion only generates 4 ev or 6.5 x 10-19 j of energy • This is 50,000,000 times more energy

  6. Fissile Nuclei • Not all nuclei are capable of absorbing a neutron and then undergoing a fission reaction • U-235 and Pu-239 do undergo induced fission reactions • U-238 does not directly undergo an induced fission reaction

  7. The Fission of U-235

  8. Nuclear Chain Reactions • The process in which neutrons released in one fission reaction causes at least one additional nucleus to undergo fission. • The number of fissions doubles generation if each neutron releases 2 more neutrons • In 10 generations there will be 1024 fissions and in 80 generations about 6 x 1023 (a mole)

  9. Nuclear Chain Reaction

  10. Critical Mass • If the amount of fissile material is small, many of the neutrons will not strike another nucleus and the chain reaction will stop • The critical mass is the amount of fissile material necessary for a chain reaction to become self-sustaining.

  11. Nuclear Chain Reactions • An uncontrolled chain reaction is used in nuclear weapons • A controlled chain reaction can be used for nuclear power generation

  12. Uncontrolled Chain Reactions The Atomic Bomb

  13. Little Boy Bomb • The atomic bomb dropped on Hiroshima on August 6, 1945 • Little boy was a U-235 gun-type bomb • Between 80,000 and 140,000 people were killed instantly

  14. The Gun-Type Bomb • The two subcritical masses of U-235 are brought together with an explosive charge creating a sample that exceeds the critical mass. • The initiator introduces a burst of neutrons causing the chain reaction to begin

  15. Fat Man • A plutonium implosion-type bomb • Dropped on Nagasaki on August 9, 1945 • 74,000 were killed and 75,000 severely injured

  16. Plutonium Implosion-Type Bomb • Explosive charges compress a sphere of plutonium quickly to a density sufficient to exceed the critical mass

  17. Controlled Nuclear Fission • Maintaining a sustained, controlled reaction requires that only one of the neutrons produced in the fission be allowed to strike another uranium nucleus

  18. Controlled Nuclear Fission • If the ratio of produced neutrons to used neutrons is less than one, the reaction will not be sustained. • If this ratio is greater than one, the reaction will become uncontrolled resulting in an explosion. • A neutron-absorbing material such as graphite can be used to control the chain reaction.

  19. From Steam To Electricity • Different fuels can be used to generate the heat energy needed to produce the steam • Combustion of fossil fuels • Nuclear fission • Nuclear fusion

  20. Types of Fission Reactors • Light Water Reactors (LWR) • Pressurized-light water reactors (PWR) • Boiling water reactors (BWR) • Breeder reactors

  21. Light Water Reactors • Most popular reactors in U.S. • Use normal water as a coolant and moderator

  22. Pressurized Water Reactor • The PWR has 3 separate cooling systems. • Only 1 should have radioactivity • the Reactor Coolant System

  23. Inside Containment Structure • Fuel Rods contain U (3-5% enriched in U-235) or Pu in alloy or oxide form • Control rods (Cd or graphite) absorb neutrons. Rods can be raised or lowered to change rate of reaction

  24. U-235 Enrichment • Naturally occurring uranium is 99.3% non fissile U-238 & 0.7% fissile U-235 • In light water reactors, this amount of U-235 is not sufficient to sustain the chain reaction • The uranium is processed to increase the amount of U-235 in the mixture

  25. Graham’s Law of Diffusion & Effusion • Diffusion- rate at which two gases mix • Effusion- rate at which a gas escapes through a pinhole into a vacuum • In both cases the rate is inversely proportional to the square root of the MW of the gas • Rate  1/(MW)0.5 • For 2 gases the rate of effusion of the lighter is: Rate = (MWheavy/MWlight)0.5

  26. U-235 Enrichment • Rate U-235/238= (352/349)0.5= 1.004 • The enrichment after n diffusion barriers is (1.004)n • Minimum amount of U-235 needed in LWR is 2.1% (3x more concentrated than in natural form) • (1.004)263=3 or 263 diffusion stages needed • Enrichment is an expensive process • Large amount of energy is needed to push the UF6 through so many barriers

  27. Inside Containment Structure • Coolant performs 2 functions • keeps reactor core from getting too hot • transfers heat which drives turbines

  28. Water as Coolant • Light Water Reactor (LWR) • uses ordinary water • needs enriched uranium fuel • common in U.S. • 80% of world’s reactors • Heavy Water Reactor (HWR) • uses D2O • can use natural uranium • common in Canada and Great Britain • 10% of world’s reactors

  29. Water As Coolant • Pressurized Water Reactors • uses a heat exchanger • keeps water that passes the reactor core in a closed loop • steam in turbines never touches fuel rods • Boiling Water Reactors • no heat exchanger • water from reactor core goes to turbines • simpler design/greater contamination risk

  30. PWR vs. BWR

  31. The Moderator • The moderator is necessary to slow down neutrons (probability of causing a fission is increased with slow moving neutrons) • Light water will capture some neutrons so enriched enriched fuel is needed • Heavy water captures far fewer neutrons so don’t need enriched fuel

  32. Breeder Reactors • Generate more fissionable material than they consume • Fuel U-238, U-235 & P-239 • No moderator is used • Fast neutrons are captured by non fissionable U-238 to produce U-239 • U-239 decays to fissile Pu-239 • Coolant is liquid sodium metal • None in U.S. • France, Great Britain, Russia

  33. Breeder Reactor Processes

  34. Breeder Reactors • Advantages • creates fissionable material by transforming U-238 into Pu-239 • Fuel less costly

  35. Breeder Reactors • Disadvantages • no moderator (if something goes wrong, it will happen quicker) • liquid sodium coolant is extremely corrosive and dangerous • Plutonium has a critical mass 50% less than uranium • more widely used for weapons • more actively sought by terrorists • Fuel rods require periodic reprocessing to remove contaminants resulting from nuclear reactions (cost consideration)

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