Buxton & District U3A Science Discussion Group “Nuclear Fission: Nuclear Power & The Atom Bomb” - PowerPoint PPT Presentation

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Buxton & District U3A Science Discussion Group “Nuclear Fission: Nuclear Power & The Atom Bomb”

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  1. Buxton & District U3A Science Discussion Group“Nuclear Fission: Nuclear Power & The Atom Bomb” John Estruch17 May 2013

  2. What are we going to talk about • The basic physics • Uranium fission • Nuclear power generation • Atom bomb • Nuclear waste • Alternative fuels

  3. Quiz Time What are the following: Electron Atom Proton Proton Nucleus Neutron Electron Neutron Element Nucleus Isotope

  4. Elements and Isotopes Isotopes are variants of a particular element. All isotopes of a given element have the same number of protons, each isotope differs from the others in its number of neutrons. They have the same chemical properties but different physical properties. A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number (number of protons). Protium Deuterium Tritium 2 3 1 H H H 1 1 1 2H 3H 1H hydrogen-1 hydrogen-2 hydrogen-3

  5. Nuclear Fission • The protons and neutrons in a nucleus are held together by the Strong Nuclear Force which has a short range. • Without the Strong Nuclear Force the positively charged protons would be pushed apart by electrostatic repulsion. • If a nucleus has enough excess energy to deform its shape then the protons/neutrons may move far enough apart for the electrostatic repulsion to overcome the Strong Nuclear Force – the nucleus breaks into 2 or more pieces

  6. Energy stored in nuclei • The binding energy of a nucleus is the amount of energy needed to pull it apart. • If you rearrange the same number of protons and neutrons (nucleons) from nuclei with lower to higher binding energy the difference in energy is released from the nuclei • If a nucleus with about 240 nucleons fissions into 2 nuclei of about 120 nucleons each then: • The binding energy changes by about 1 MeV per nucleon • Therefore a total of 200-300MeV is released. MeV = million electron-volts 1eV = 1.6 x 10-19

  7. Nuclear energy v. chemical energy • Fission 1 atom of Uranium  approx 200 MeV • Burn 1 atom of Carbon  approx 1 eV • Uranium about 20x heavier than Carbon Fission 1 ton Uranium Burn 10,000,000 ton coal =

  8. What are we going to talk about • The basic physics • Uranium fission • Nuclear power generation • Atom bomb • Nuclear waste • Alternative fuels

  9. Uranium • Atomic number 92 (92 protons) • Naturally occurring Uranium is a mixture of isotopes: • 0.7% 235U (92 protons + 143 neutrons) • 99.3% 238U (92 protons + 146 neutrons) • 0.0055% 234U • 235U is the only “fissile” isotope occurring naturally in useable quantity on earth

  10. Uranium fission • Uranium can undergo “spontaneous” fission (only rarely – half life 7 x 10 8 years) • Also undergoes “induced” fission “Fission product” e.g. 90Rb 235U Neutrons (2.5 on ave.) neutron “Fission product” e.g. 143Cs

  11. Chain Reaction 235U 235U 235U 235U 235U neutron If at least 1 neutron from each fission goes on to cause another fission then we have a “chain reaction” 235U Some neutrons escape

  12. Critical Mass • If you have a small piece of 235U then lots of neutrons escape – no sustained chain reaction • If you get bigger piece then more neutrons will cause fission before they escape • When piece is just big enough so on average 1 neutron from each fission goes on to cause another fission - just get chain reaction – this is “critical mass” The nominal critical mass for a sphere of pure 235U is 52kg (17cm diameter)

  13. If only it were that simple!! • Natural Uranium is 0.7% 235U and 99.3% 238U • 238U is not fissile, it absorbs neutrons so tends to prevents chain reaction. • Fission produces “fast” neutrons • Fast neutrons more likely to be absorbed by 238U • Slow neutrons more likely to cause fission in 235U Any suggestions?

  14. Neutron Moderators 238U 235U Thermal neutrons 235U Moderator 235U Slow (or “thermal”) neutron 238U Fast neutrons

  15. Moderators

  16. The Heroes of Telemark(a small historical aside) • The 1965 film is a dramatisation based on a number of real Norwegian/British commando raids on the VermorkNorsk Hydro plant. • The plant was producing heavy water (D2O) which the Germans could use as a moderator in a Uranium reactor as part of a nuclear weapons programme.

  17. Enrichment Another way to increase number of neutrons causing 235U fission is to increase percentage of 235U i.e. “enrichment” IAEA / UN get very concerned about export of enrichment technology as a nuclear weapons proliferation issue. Can’t be separated chemically so use complex /expensive technology such as gas centrifuge cascade.

  18. What are we going to talk about • The basic physics • Uranium fission • Nuclear power generation • Atom bomb • Nuclear waste • Alternative fuels

  19. Nuclear reactor Containment Coolant Fuel Moderator

  20. Controlling the reactor Control Rods When the rods are inserted more neutrons are absorbed – power decreases When the rods are withdrawn fewer neutrons are absorbed – power increases • Control rods are made of a neutron absorbing material (e.g. cadmium ) • Pushing them in or out of reactor controls the flow of neutrons

  21. Chicago Pile 1 (CP-1)– The 1st Reactor • Team led by Enrico Fermi built CP-1 in rackets court under the stand of Chicago University football field. • Uranium pellets separated by graphite bricks “a pile of black blocks and wooden timbers” • Controls were rods coated in cadmium. • On 2 December 1942 the first sustained chain reaction was achieved. • No radiation shield, no coolant!

  22. Types of reactor

  23. Nuclear Power Plant • Pressurised water in reactor: • Acts as moderator • Is primary cooling circuit • Takes heat out of reactor and uses it to boil water in secondary circuit • Secondary circuit is just like coal, gas or oil power station • Steam turbine and generator is good old 19th century technology PWR electricity generation plant

  24. What are we going to talk about • The basic physics • Uranium fission • Nuclear power generation • Atom bomb • Nuclear waste • Alternative fuels

  25. Fission bomb (a.k.a. Atom bomb) Trigger mechanisms • A nuclear reactor without the controls • Want to release as much energy as possible as quickly as possible • Moderator cannot be used (too slow) • Requires highly enriched Uranium (>85% 235U) • The number of neutrons/fissions can double every 10-8sec • In theory several hundred tons of 235U could fission in 1/1,000,000 sec. • In practice there are only a few Kg and bomb blows itself apart before it all fissions • Hiroshima bomb only fissioned 1.3% of available 235U, “Gun method” – a sub-critical mass fired into another “Implosion method” – lots of sub-critical masses surrounded by high explosive

  26. History of nuclear bombs • Nuclear states • USA • USSR/Russia • UK • France • China • Apartheid era South Africa. • India • Pakistan • North Korea • Israel? • USA & USSR built bombs up to 50 megatons • “Little Boy” • Uranium bomb • Gun type mechanism • Detonated Hiroshima6 August 1945 • 16 kilotons TNT equivalent • “Fat Man” • Plutonium bomb • Implosion mechanism • Detonated Nagasaki 9 August 1945 • 21 kilotons

  27. What are we going to talk about • The basic physics • Uranium fission • Nuclear power generation • Atom bomb • Nuclear waste • Alternative fuels

  28. Source of nuclear waste Many fission products are highly radioactive Some neutrons do not cause fission but are absorbed by 238U and 235U to produce heavy (actinide) radioactive isotopes e.g. 234U, 237Np, 238Pu, 239Pu, 241Am 235U neutron

  29. Radioactive isotopes • When unstable nuclei “decay” by giving off ,  or  radiation •  and  change nucleus to a different element/isotope •  puts the isotope into a more stable state • So the higher the rate an isotope emits radiation the quicker it stops emitting the radiation. • This is measured by the “half-life” (how long it takes for half the nuclei of a particular isotope to decay) • Short Lived isotopes • When reactor shuts down radioactive isotopes continue to decay • Short lived isotopes decay quickly • High rate of decay generates lots of heat (5%-10% of reactor power) • The rate reduces rapidly reaching “cold shutdown” in a few days • Until cold shutdown, the reactor must be actively cooled. • Failure of cooling after shutdown caused all the problems at Fukushima

  30. Example isotopes in nuclear waste Medium-lived fission products Long-lived fission products Medium-lived actinides Long-lived actinides * Fissile

  31. How nasty (or useful) is the waste? It depends on the isotope:

  32. What to do with nuclear waste?

  33. What are we going to talk about • The basic physics • Uranium fission • Nuclear power generation • Atom bomb • Nuclear waste • Alternative fuels

  34. Plutonium • 239Pu and 241Pu are fissile • Does not occur naturally (except in minute amounts) • Pu is created in Uranium reactors (1n + 238U => 239U ==> 239Pu) • Not a neutron absorber • Can be used with fast neutrons – no need for a moderator – “fast reactor” 2  decays

  35. Thorium • Predominant isotope (232Th) is not fissile • 232Th is “fertile” (1n + 232Th => 233Th ==> 233U) • Lots of excitement in recent years that thorium will provide abundant, cheaper, cleaner power. 2  decays

  36. Thorium

  37. Questions?