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Understanding Nuclear Fission: Process, Reactions, and Energy Output

Nuclear fission is the process wherein unstable, large mass nuclei split into smaller nuclei, releasing significant energy. This process typically occurs when a neutron interacts with a fissile isotope, such as U-235 or Pu-239. Fission not only produces neutrons that can perpetuate a chain reaction but also converts about 0.1% of mass into energy, as expressed in Einstein's E=mc². Controlled fission reactions are harnessed in nuclear power plants, while uncontrolled reactions result in nuclear weapons. This overview explores fission processes, critical mass, reactor types, and historical bombings.

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Understanding Nuclear Fission: Process, Reactions, and Energy Output

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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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