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Nuclear fission & fusion 07


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This powerpoint relates to Topic 4 Section 4 of the SACE in South Australia

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Nuclear fission & fusion 07

  2. 2. NUCLEAR FISSION <ul><li>Just prior to W.W.II, nuclear physicists began to investigate nuclear fission. </li></ul><ul><li>Enrico Fermi in Italy, Otto Hahn and Fritz Strassman in Germany experimented with neutrons being fired into stable nuclei. </li></ul><ul><li>They found that when the nucleus was very heavy ( 92 U 235 ), lighter nuclei were produced. </li></ul>
  3. 3. NUCLEAR FISSION <ul><li>This is called Nuclear Fission. </li></ul><ul><li>They had learned how to “split the Uranium atom”. </li></ul><ul><li>They found that huge amounts of energy were released due to the loss of mass. </li></ul><ul><li>This equated to 100 million times more energy than that found in chemical reactions. </li></ul>
  4. 4. NUCLEAR FISSION <ul><li>This is called induced fission as it only occurs when the nucleus is ‘prodded’. </li></ul><ul><li>Spontaneous fission occurs in some extremely unstable nuclei. The half-life for 92 U 238 if decaying by spontaneous fission, would be 10 16 years. </li></ul><ul><li>Spontaneous Fission </li></ul>
  5. 5. NUCLEAR FISSION <ul><li>Spontaneous Fission can be seen when Uranium naturally decays in the mineral mica (found in granite). </li></ul><ul><li>Scar marks can be seen in the granite from the energy released (kinetic energy in the fragments and the gamma ray released). </li></ul>
  6. 6. NUCLEAR FISSION <ul><li>Induced Fission can occur when a slow moving neutron (0.03eV = 20000 ms -1 ) strikes the 92 U 235 . </li></ul><ul><li>It is absorbed momentarily resulting in 92 U 236 </li></ul><ul><li>This is extremely unstable and decays into two approximately equal fragments in about 5 x 10 -8 sec. </li></ul>
  7. 7. NUCLEAR FISSION <ul><li>A slow thermal neutron is needed so that it may be captured by the nucleus. </li></ul><ul><li>The neutron gets close enough to the nucleus for the nuclear force to act and pull it in. </li></ul><ul><li>If it travelled too fast, it would collide with the Uranium nucleus, causing the ejection of an alpha particle. </li></ul>
  8. 8. NUCLEAR FISSION <ul><li>The reason that the neutron is pulled in is because it has no electrical charge, the nuclear force can therefore pull it into the Uranium nucleus. </li></ul><ul><li>A proton cannot be pulled in because of its positive electrical charge. It would be repelled by the nucleus. </li></ul>
  9. 9. NUCLEAR FISSION <ul><li>When the nucleus breaks up, a few neutrons are flung out ( 2 or possibly 3). </li></ul><ul><li>A typical reaction is: </li></ul><ul><li>Fission </li></ul><ul><li>Equation Below </li></ul>
  10. 10. NUCLEAR FISSION 235 U n 236 U 90 Kr 143 Ba n n n
  11. 11. EXAMPLE 1 <ul><li>Calculate the energy released when the following nuclear fission reaction occurs </li></ul><ul><li>1 0 n + 235 92 U  141 56 Ba+ 92 36 Kr + 3 1 0 n +  </li></ul><ul><li>Masses are </li></ul><ul><ul><li>0 n = 1.675 x 10 -27 kg </li></ul></ul><ul><ul><li>235 92 U = 3.9017 x 10 -25 kg </li></ul></ul><ul><ul><li>141 56 Ba = 2.28922 x 10 -25 kg </li></ul></ul><ul><ul><li>92 36 Kr = 1.57534 x 10 -25 kg </li></ul></ul>
  12. 12. EXAMPLE 1 SOLUTION <ul><li>Total mass of the reactants = 0. 01675 x 10 -25 + 3. 9017 x 10 -25 = 3.91845 x 10 -25 kg </li></ul><ul><li>Total mass of the products = 2.28922 x 10 -25 + 1.57534 x 10 -25 + 3(0.01675 X 10 -25 ) = 3.91481 x 10 -25 kg </li></ul><ul><li>Therefore the mass defect = 3.91845 X 10 -25 – </li></ul><ul><li>3.91481 X 10 -25 = 3.64 X 10 -28 Kg </li></ul>
  13. 13. EXAMPLE 1 SOLUTION <ul><li>E = mc 2 </li></ul><ul><li> = 3.64 X 10 -28 X (3 X 10 8 ) 2 </li></ul><ul><li> = 3.276 X 10 -11 J </li></ul><ul><li> = 2.05 X 10 8 eV </li></ul><ul><li> = 205 MeV of energy released (exothermic) </li></ul>
  14. 14. NUCLEAR FISSION <ul><li>How does Induced Nuclear Fission occur? </li></ul><ul><li>The neutron causes the Uranium-235 nucleus to distort its shape. This weakens the nuclear forces, allowing the repulsive coulombic charges to pull the neutron apart. </li></ul>
  15. 15. NUCLEAR FISSION <ul><li>Chance dictates the mass of each fission fragment. </li></ul><ul><li>This means that there are many different possible reactions resulting in a range of fission products being produced. </li></ul>
  16. 16. NUCLEAR FISSION <ul><li>The distribution of fission products with mass number is shown to the right: </li></ul><ul><li>Most of the mass numbers of the daughter products are around 145(Barium) and 95(Krypton). </li></ul>95 145
  17. 17. <ul><li> n + U  Rb + Cs + 2 n </li></ul><ul><li> Br + La + 3 n </li></ul><ul><li> Zn + Sm + 4 n </li></ul>NUCLEAR FISSION
  18. 18. <ul><li> These products decay further usually by  decay. </li></ul><ul><li> Rb  Sr + e t 1/2 = 2.8 min. </li></ul><ul><li> Sr  Y + e t 1/2 = 29 years </li></ul><ul><li> Y  Zr + e t 1/2 = 64 hours </li></ul>NUCLEAR FISSION
  19. 19. NUCLEAR FISSION <ul><li>When the nucleus splits, the Coulombic repulsion forces between the protons are great but are not only reduced by the emission of nucleons, but also minimised, and so much more energy is released. </li></ul><ul><li>Calculations indicate a loss of energy of as much as 200 MeV compared with 10 MeV for alpha decay. </li></ul>
  20. 20. NUCLEAR FISSION <ul><li>This energy is released in the form of kinetic energy of the product particles and the energy of the  ray photons. </li></ul>
  21. 21. NUCLEAR FISSION <ul><li>From the graph, the average binding energy per nucleon for U-235 is about 7.5 MeV, while the value for the daughter products krypton and barium nuclei is 8.5 MeV. </li></ul>
  22. 22. NUCLEAR FISSION <ul><li>Since there are about 235 nucleons involved in each fission and each is bound by an extra 1.0 MeV (8.5 - 7.5 MeV) after the fission, the energy released must be in the order of 235 MeV (as binding energy refers to the energy released by the nucleus). </li></ul>
  23. 23. NUCLEAR FISSION <ul><li>The energy released is some million times greater than the energy released from an equivalent mass of coal or petrol. </li></ul><ul><li>The burning of coal or petrol is a chemical process involving much less binding energy and so the smaller amount of energy is to be expected. </li></ul><ul><li>Nuclear Fission gives off gamma rays while chemical reactions give off visible light. </li></ul>
  24. 24. NUCLEAR FISSION <ul><li>DANGERS OF DAUGHTER PRODUCTS </li></ul><ul><li>After fission occurs, the daughters are radioactive and usually  emitters. </li></ul><ul><li>Looking a the proton/neutron ratio for the uranium nucleus, N/Z = 1.55. </li></ul><ul><li>As no protons or neutrons are destroyed, the ratio for the barium and krypton nucleus is the same. </li></ul>
  25. 25. NUCLEAR FISSION <ul><li>Stable nuclei for middle order mass elements have a ratio of 1.3. </li></ul><ul><li>This means there are too many neutrons present in the daughter nuclei which makes them radioactive. </li></ul><ul><li>The daughter product undergoes Beta minus decay. </li></ul>
  26. 26. NUCLEAR FISSION <ul><li>So one of the drawbacks of Nuclear Fission power plants are that the daughter products are radioactive (Beta minus emitters). </li></ul>
  27. 27. FISSION CHAIN REACTION <ul><li>FISSION CHAIN REACTION </li></ul><ul><li>This works because each fission produces extra neutrons. </li></ul><ul><li>They are free to collide with further nuclei that cause further fissions and so on. </li></ul><ul><li>Chain Reaction </li></ul>
  28. 28. FISSION CHAIN REACTION <ul><li>The uncontrolled chain reaction can be shown as a diagram seen to the right. </li></ul><ul><li>This chain reaction is initiated by a single slow neutron. </li></ul><ul><li>An average of 2.4 neutrons are produced by each reaction. </li></ul>
  29. 29. FISSION CHAIN REACTION <ul><li>The key is to have at least one neutron go on to make another successful fission reaction. </li></ul><ul><li>This can provide a continuous supply of energy. </li></ul><ul><li>MAJOR PROBLEM: A chain reaction cannot occur if the neutrons are moving too fast. They must be slowed down. </li></ul>
  30. 30. FISSION CHAIN REACTION <ul><li>MODERATORS - used in nuclear power plants to slow down the neutrons. </li></ul><ul><li>Moderators must have a small mass . </li></ul>Large Mass Moderator Neutron This would not work as the neutron would retain most of its kinetic energy.
  31. 31. FISSION CHAIN REACTION Small Mass Moderator Neutron This would slow down the neutron as it would give up some of its kinetic energy to the small mass moderator.
  32. 32. FISSION CHAIN REACTION <ul><li>A good example of a moderator is the Deuterium ( 2 H ) nucleus. </li></ul><ul><li>It is called “heavy hydrogen”. </li></ul><ul><li>It has a similar mass to neutrons. </li></ul><ul><li>We cannot use normal Hydrogen ( 1 H ) as it would simply absorb the neutron. </li></ul>
  33. 33. FISSION CHAIN REACTION <ul><li>Since each fission takes place over a very short time (less than 1 second) and many fissions take place, a large amount of energy is released in a short time - hence an explosion. </li></ul><ul><li>Although this happens, about 1% of the neutrons produced in nuclear fission are emitted after a delay of up to 10 seconds or more. </li></ul>
  34. 34. FISSION CHAIN REACTION <ul><li>If the uranium is too small or the wrong shape, too many neutrons will be lost and the reaction will not continue. </li></ul><ul><li>It has been determined for U-235 1 kg is needed to sustain a reaction. This mass is called the critical mass. </li></ul>
  35. 35. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Nuclear fission was first used as a power source in the mid 1950’s. </li></ul><ul><li>In Japan, over 50% of their electricity is generated from uranium. </li></ul><ul><li>The core is the region containing the uranium fuel where the fission chain reaction occurs. This core is enclosed by a thick steel ‘pressure vessel’. </li></ul>
  36. 36. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>The fuel rods are long thin metal tubes filled with pellets of uranium oxide containing a certain percentage of uranium 235. </li></ul><ul><li>Also in the tubes is helium gas to help with the heat transfer with the ends sealed with leak-tight caps. Hundreds of fuel rods are clustered together to form a fuel element . </li></ul>
  37. 37. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>The moderator in a pressurised water reactor is ordinary water under about 150 atmospheres of pressure. </li></ul><ul><li>The temperature of the water can be increased to about 325 o C without boiling. </li></ul><ul><li>The job of the moderator (water) is to slow the neutrons down to an energy of about 1 eV so the uranium can capture them. </li></ul>
  38. 38. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>This is achieved by collisions of the neutrons with the water. </li></ul><ul><li>Some energy is absorbed by the water, which slows the neutrons. </li></ul><ul><li>The best moderator will have a similar mass the neutron. </li></ul><ul><li>Water has the advantage of low mass (hence greater sharing of energy as a result of the collision) but it does absorb neutrons and so slows the rate of the chain reaction. </li></ul>
  39. 39. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Water is also plentiful and can be used to transport the heat produced in the reactor. </li></ul><ul><li>The water is also the primary coolant removing heat from the core to the heat exchanger. </li></ul><ul><li>Other types of reactors may have a different coolant to the moderator. </li></ul>
  41. 41. APPLICATION - NUCLEAR REACTORS AND POWER Pressurized Heavy Water(Moderator) Unpressurized water(heated and turns to steam).
  42. 42. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>The reaction must be slowed down so it can be controlled. </li></ul><ul><li>Using thin cadmium or boron rods, which absorb the neutrons that are produced by each fission, does this. </li></ul><ul><li>These control rods are positioned so that they can be inserted to slow or stop the reaction or withdrawn to increase the speed. </li></ul>
  43. 43. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>The energy created is in the form of heat and is carried away by pipes of unpressurised water . </li></ul><ul><li>This water is known as a secondary coolant. </li></ul><ul><li>The pressurised water is cooled to about 293 o C and returned to the core to be heated again. </li></ul><ul><li>It then turns water into steam in a heat exchanger to drive a turbine connected to an electric generator . </li></ul>
  44. 44. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Safety rods are also placed into the reactor so that the reactor can be shut down if necessary in a matter of seconds. </li></ul><ul><li>They are triggered automatically if the coolant pressure falls because of a pipe failure for example. </li></ul>
  45. 45. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Around the steel pressure vessel is a thick concrete shield. </li></ul><ul><li>This means that there are two layers of shielding. The building itself is also another layer of shielding. </li></ul><ul><li>It has an inner shell of steel and reinforced by concrete. </li></ul><ul><li>The building is designed to withstand the pressure of the entire primary coolant, should it be released due to a fracture in a coolant pipe. </li></ul>
  47. 48. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>A major problem is that the only isotope of uranium that is fissile (undergoes fission) is 235. </li></ul><ul><li>This only makes up 0.7% of the total uranium, the rest being uranium-238. </li></ul><ul><li>This means that using uranium-238 is not an option, as a chain reaction cannot be sustained. </li></ul><ul><li>As they are both chemically identical, they cannot be separated by conventional means. </li></ul>
  48. 49. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Nuclear Reactors </li></ul>
  49. 50. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Enrichment of uranium occurs so that the percentage of uranium-235 is increased to 2 or 3%. </li></ul><ul><li>This allows the reaction to keep its ‘critical mass’ (the mass required to keep the chain reaction going). </li></ul><ul><li>The method used to enrich the uranium relies on the different masses of the isotopes. This takes many stages and is very costly. </li></ul>
  50. 52. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>U 3 O 8 is then enriched by </li></ul><ul><ul><li>Turning it to a gas </li></ul></ul><ul><ul><li>Gas placed in room with tiny hole at high up at one end </li></ul></ul><ul><ul><li>Lighter 235 U has higher probability of diffusing through hole than 238 U </li></ul></ul><ul><ul><ul><li>Perhaps 1% higher </li></ul></ul></ul><ul><ul><li>Repeated through many (100 +) rooms </li></ul></ul><ul><ul><li>Concentration now high enough to use </li></ul></ul>
  51. 53. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>This is method Iran used to enrich their U </li></ul><ul><li>Alternative is much more efficient </li></ul><ul><ul><li>but much more expensive </li></ul></ul>
  52. 54. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Again start with gas </li></ul><ul><li>Put in the biggest and most expensive washing machine ever seen </li></ul><ul><li>Centrifuge </li></ul><ul><li>Heaver 238 U moves to outside and lighter 235 U can be extracted. </li></ul>
  53. 55. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Controlling a Nuclear Reactor </li></ul><ul><li>To maintain a constant power level from a reactor, each fission must produce, on average, one other fission. </li></ul><ul><li>As the average number of neutrons produced per fission is about 2.4, 1.4 neutrons must be absorbed per fission. </li></ul><ul><li>The fuel itself and the moderator absorb some neutrons. </li></ul>
  54. 56. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Inserting and removing the control rods control the other neutrons. </li></ul><ul><li>Commonly made of Boron or Cadmium </li></ul><ul><li>They are partially removed to start the reaction and when new, higher power levels are required. </li></ul><ul><li>Once the new power level is reached, the control rods are reinserted to return to the average of one neutron per fission. </li></ul>
  55. 57. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>As the fuel becomes depleted, the rods must also be partially removed to counter losses due to neutron absorbing fission products. </li></ul><ul><li>Water is also passed over the nuclear reaction core. </li></ul><ul><li>It serves to slow down the neutrons which is important because 235 U will not absorb very fast neutrons. </li></ul>
  56. 58. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>The reverse is also true, when the power level is to be decreased or stopped, control rods are inserted and more neutrons are absorbed to bring the average to below one per fission until the new power level is attained. </li></ul>
  57. 59. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Most of the neutrons produced appear almost immediately and are called prompt neutrons . </li></ul><ul><li>They take about 10 -4 seconds to slow down and cause another fission. </li></ul><ul><li>If all neutrons were produced this way, controlling the reaction would be very difficult. </li></ul>
  58. 60. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>The fuel used in reactors have about 0.5 to 1% delayed neutrons which are emitted after a time interval of less than one second to several minutes. </li></ul><ul><li>These neutrons are emitted from the decay of the products of the fission. </li></ul>
  59. 61. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>The time interval between fissions for these neutrons is long enough to allow control rod movements, taking between 10 and 20 seconds, to control the chain reaction. </li></ul>
  60. 62. APPLICATION - NUCLEAR REACTORS AND POWER <ul><ul><li>Advantages and Disadvantages of Nuclear Fission Power </li></ul></ul><ul><li>Advantages </li></ul><ul><li>After initial start up costs, the power is relatively cheap in large-scale production. The energy extracted is much greater than for the same amount of fossil fuels. </li></ul>
  61. 63. APPLICATION - NUCLEAR REACTORS AND POWER Energy Conversion: Typical Heat Values of Various Fuels (MJ = Megajoules), * natural U 500,000 MJ/kg Uranium* - in light water reactor 45-46 MJ/kg Crude Oil 39 MJ/m 3 Natural Gas 24-30 MJ/kg Black coal 13-20 MJ/kg Black coal (low quality) 9 MJ/kg Brown coal 16 MJ/kg Firewood
  62. 64. APPLICATION - NUCLEAR REACTORS AND POWER <ul><ul><ul><li> There are no greenhouse gas emissions unlike fossil fuels. </li></ul></ul></ul><ul><ul><ul><li>Waste generated by nuclear power plants are self contained </li></ul></ul></ul><ul><ul><ul><li>Typical 1000 MW coal plant emits </li></ul></ul></ul><ul><ul><ul><ul><li>100 000 t of sulfur dioxide </li></ul></ul></ul></ul><ul><ul><ul><ul><li>75 000 t of nitrogen oxides </li></ul></ul></ul></ul><ul><ul><ul><ul><li>5 000 t of fly ash </li></ul></ul></ul></ul>
  63. 65. APPLICATION - NUCLEAR REACTORS AND POWER <ul><ul><ul><li> As they can reprocess fuel, most of the original uranium can be used. Breeder reactors can produce their own fuel and so the lifetime of this fuel source is much greater than fossil fuels. </li></ul></ul></ul>
  64. 66. APPLICATION - NUCLEAR REACTORS AND POWER <ul><ul><ul><li>Predictions over loss of life over next 50 years for Europe: </li></ul></ul></ul><ul><ul><ul><li>Nuclear 1 000 deaths per year </li></ul></ul></ul><ul><ul><ul><li>Coal 10 000 deaths per year </li></ul></ul></ul>
  65. 67. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Other uses: </li></ul><ul><li>Using relatively small special-purpose nuclear reactors it has become possible to make a wide range of radioactive materials (radioisotopes) at low cost. For this reason the use of artificially produced radioisotopes has become widespread since the early 1950s, and there are now some 270 &quot;research&quot; reactors in 59 countries producing them. </li></ul>
  66. 68. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Radioisotopes </li></ul><ul><li>Medicine , radioisotopes are widely used for diagnosis and research. </li></ul><ul><li>Preservation of food </li></ul><ul><li>Growing of crops and breeding of livestock </li></ul><ul><li>I ndustrially </li></ul><ul><li>Smoke detectors . </li></ul><ul><li>Other reactors Over 200 small nuclear reactors power some 150 ships, mostly submarines, but ranging from icebreakers to aircraft carriers. </li></ul>
  67. 69. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Disadvantages </li></ul><ul><li>Expensive to establish power plants </li></ul><ul><li>Storage of waste products is difficult. The waste products have very long half-lives. </li></ul>
  68. 70. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Waste – Coal vs U </li></ul>
  69. 71. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>NUCLEAR WASTE </li></ul><ul><li>~27 tonnes of spent fuel per year, highly radioactive </li></ul><ul><li>reprocessed so that 97% of the 27 tonnes is recycled </li></ul><ul><li>remaining 3%, about 700 kg, is high-level radioactive waste which is potentially hazardous and needs to be isolated from the environment for a very long time. </li></ul>
  70. 72. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>COAL WASTE </li></ul><ul><li>7 million tonnes of carbon dioxide each year, plus perhaps 200,000 tonnes of sulfur dioxide which in many cases remains a major source of atmospheric pollution. </li></ul><ul><li>Other waste products are fly ash (typically 200,000 tonnes per year), containing toxic metals, including arsenic, cadmium and mercury, organic carcinogens and mutagens. </li></ul><ul><li>If not fully contained, these routine wastes can cause environmental and health damage even at great distances from the site of the power station. (Acid Rain) </li></ul>
  71. 73. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>NUCLEAR WASTE </li></ul><ul><li>About 25 tonnes of spent fuel is taken each year from a nuclear reactor. </li></ul><ul><li>Either all waste (in USA and Canada), </li></ul><ul><li>Reprocessed (as in Europe). </li></ul><ul><li>Whichever option is chosen, the spent fuel is first stored for several years under water in large cooling ponds at the reactor site. The concrete ponds and the water in them provide radiation protection, while removing the heat generated during radioactive decay. </li></ul>
  72. 74. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>REPROCESSING </li></ul><ul><li>Dissolved and separated chemically into uranium, plutonium and high-level waste solutions. </li></ul><ul><li>About 97% of the spent fuel can be recycled leaving only 3% as high-level waste. </li></ul>
  73. 75. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>REPROCESSING </li></ul><ul><li>The recyclable portion is mostly uranium depleted to less than 1% U-235, with some plutonium, which is most valuable. </li></ul><ul><li>About 230 kilograms of plutonium (1% of the spent fuel) is separated in reprocessing. This can be used in fresh mixed oxide (MOX) fuel (but not weapons, due its composition). </li></ul>
  74. 76. <ul><li>Low-level Waste </li></ul><ul><ul><li>is generated from hospitals, laboratories and industry, as well as the nuclear fuel cycle. </li></ul></ul><ul><li>It comprises paper, rags, tools, clothing, filters etc. </li></ul><ul><ul><li>which contain small amounts of mostly short-lived radioactivity. </li></ul></ul>Types of radioactive waste (radwaste)
  75. 77. <ul><ul><li>Intermediate-level Waste </li></ul></ul><ul><ul><ul><li>contains higher amounts of radioactivity and </li></ul></ul></ul><ul><ul><ul><li>may require special shielding. </li></ul></ul></ul><ul><ul><li>Typically comprises resins, chemical sludges and reactor components, </li></ul></ul><ul><ul><ul><li>as well as contaminated materials from reactor decommissioning. </li></ul></ul></ul>Types of radioactive waste (radwaste)
  76. 78. <ul><li>High-level Waste </li></ul><ul><ul><li>may be the spent fuel itself, </li></ul></ul><ul><ul><li>or the principal waste from reprocessing this. </li></ul></ul><ul><li>While only 3% of the volume of all radwaste, </li></ul><ul><ul><li>it holds 95% of the radioactivity. </li></ul></ul>Types of radioactive waste (radwaste)
  77. 79. Nuclear Waste The 3% of the spent fuel which is separated high-level wastes amounts to 700 kg per year and it needs to be isolated from the environment for a very long time. These liquid wastes are stored in stainless steel tanks inside concrete cells until they are solidified. The vitrified waste for one year would fill about twelve canisters, each 1.3m high and 0.4m diameter and holding 400 kg of glass.
  78. 80. Nuclear Waste The ultimate disposal of vitrified wastes, or of spent fuel assemblies without reprocessing, requires their isolation from the environment for long periods. The most favoured method is burial in dry, stable geological formations some 500 metres deep. Several countries are investigating sites that would be technically and publicly acceptable. The USA is pushing ahead with a repository site in Nevada (Yucca Mountain) for all the nation’s spent fuel.
  79. 81. Nuclear Waste Layers of protection The principal barriers are: Immobilise waste in an insoluble matrix, eg glass, Synroc (or leave them as uranium oxide fuel pellets - a ceramic) Seal inside a corrosion-resistant container In wet rock: surround containers with bentonite clay to inhibit groundwater movement Locate deep underground in a stable rock structure Site the repository in a remote location.
  80. 82. Nuclear Waste A more sophisticated method of immobilising high-level radioactive wastes has been developed in Australia. Called 'SYNROC' (synthetic rock), the radioactive wastes are incorporated in the crystal lattices of the naturally-stable minerals in a synthetic rock. In other words, copying what happens in nature.
  81. 83. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Disadvantages (cont) </li></ul><ul><li>Uranium enrichment plants (as most are in the USA), could be used to produce the highly enriched uranium used in nuclear weapons. </li></ul><ul><li>Accidents do happen and radioactive material can be released into the atmosphere such as in Chernobyl in the Ukraine. </li></ul><ul><li>Nuclear Power and the Environment </li></ul>
  82. 84. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Production of Radioisotopes </li></ul><ul><li>Radioisotopes can be produced in a reactor by either irradiating a target isotope with the neutrons present in the reactor or by extracting the required radioisotope if present amongst the fission fragments of the reaction. </li></ul>
  83. 85. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Both methods were used by the HI gh F lux A ustralian R eactor (HIFAR). </li></ul><ul><li>The HIFAR reactor operated from 1958 to 2006. </li></ul><ul><li>Main work was providing an intense source of neutrons </li></ul><ul><ul><li>for researchers studying physics and </li></ul></ul><ul><ul><li>the properties of various materials. </li></ul></ul>
  84. 86. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>In addition, it produced </li></ul><ul><ul><li>wide range of medical and industrial </li></ul></ul><ul><ul><li>radioisotopes for Australian hospitals and industry. </li></ul></ul><ul><li>Some of these isotopes were exported to </li></ul><ul><ul><li>nearby South-East Asian countries and </li></ul></ul><ul><ul><li>New Zealand </li></ul></ul>
  85. 87. APPLICATION - NUCLEAR REACTORS AND POWER Shut Down started in January 2007 it will take 10 years
  86. 88. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>HIFAR has been replaced by a more modern </li></ul><ul><ul><li>20 megawatt unit </li></ul></ul><ul><ul><li>called OPAL in 2007. </li></ul></ul><ul><ul><li>Open Pool Australian Light-water reactor </li></ul></ul><ul><li>It went critical for the first time on 23 rd December 2006. </li></ul>
  88. 90. APPLICATION - NUCLEAR REACTORS AND POWER <ul><li>Optional Notes on ‘Breeder Reactors’ and the ‘Uses for Radioisotopes’. </li></ul><ul><li>Catalyst: Nuclear China - ABC TV Science 22 Feb 07 </li></ul>
  89. 91. NUCLEAR FUSION <ul><li>NUCLEAR FUSION - the combining of light nuclei to form a large stable nuclei (such as Helium). </li></ul><ul><li>Energy is released when the average binding energy per nucleon of the reacting nuclei is less than the average binding energy per nucleon of the products. </li></ul>
  90. 92. NUCLEAR FUSION <ul><li>Looking at the graph shown at right, it was realised that energy could also be released by the fusion (joining together) of very light nuclei. </li></ul>
  91. 93. NUCLEAR FUSION <ul><li>Two deuterons could combine to produce a helium nucleus and release energy. </li></ul><ul><li>About 6.3 MeV of energy is released per nucleon per reaction (about 25 MeV is produced). </li></ul>
  93. 95. NUCLEAR FUSION <ul><li>- </li></ul><ul><li>Hydrogen Burning </li></ul><ul><li>A more typical reaction is: </li></ul><ul><li>Fusion </li></ul>
  94. 96. NUCLEAR FUSION <ul><li>Like in a Fission reaction, there is a LOSS OF MASS in a Fusion Reaction. </li></ul><ul><li>The reaction is EXOTHERMIC. </li></ul><ul><li>A Fusion reaction produces much more energy than fossil fuels (chemical reaction). </li></ul>
  95. 97. NUCLEAR FUSION <ul><li>The energy can be calculated as in all nuclear reactions </li></ul><ul><ul><ul><li>1. Calculate the total mass of the reactants. </li></ul></ul></ul><ul><ul><ul><li>2. Calculate total mass of the products. </li></ul></ul></ul><ul><ul><ul><li>3. Find the difference between the two to determine the mass defect. </li></ul></ul></ul><ul><ul><ul><li>4. Use E = mc 2 to determine the energy produced. </li></ul></ul></ul>
  96. 98. NUCLEAR FUSION <ul><li>WHY DON’T WE USE FUSION FOR OUR ENERGY SOURCE?? </li></ul><ul><li>This could be a key to unlimited energy. </li></ul><ul><li>The fusion reaction is more difficult to achieve because the reacting nuclei have to be brought together close enough for the strong nuclear force to act which requires overcoming their mutual repulsion. </li></ul>
  97. 99. NUCLEAR FUSION <ul><li>For this to happen, a large amount of energy needs to be added to the reactants before the reaction starts. This must be in the form of kinetic energy for the nuclei and is provided by heat. </li></ul><ul><li>The only induced fusion reactions have been in weapons. To initiate the reaction, temperatures of 10 to 100 million Kelvin is needed. </li></ul><ul><li>Helium Burning </li></ul>
  98. 100. NUCLEAR FUSION <ul><li>To do this in the laboratory, a new container is needed as no metal can cope with these temperatures. </li></ul><ul><li>Electromagnets, which produce magnetic fields, have been used to contain the ‘plasma’. </li></ul><ul><li>This plasma is the most common form of matter in the universe. </li></ul>
  99. 101. NUCLEAR FUSION <ul><li>Due to the high temperatures of our sun and other stars, they consist of plasma - ionised atoms. </li></ul><ul><li>They are ions due to the fact that the high temperatures provide enough energy for the electrons to escape. </li></ul><ul><li>The high density of the plasma within the star ensures that there will be plenty of collisions between the Hydrogen ions. </li></ul>
  100. 102. NUCLEAR FUSION <ul><li>The power source of all stars is nuclear fusion. </li></ul><ul><li>The core of the Sun contains a high proportion of hydrogen at high density and a temperature of some 100 million Kelvin. </li></ul>
  101. 103. NUCLEAR FUSION <ul><li>The hydrogen is converted to helium through a series of steps, each one releasing energy. </li></ul><ul><li>REACTION PATHWAY FOR SOLAR FUSION </li></ul><ul><li>Beside the stable helium produced, two protons are also produced to start the process again . </li></ul>
  102. 104. NUCLEAR FUSION <ul><li>Gravitation holds the plasma in the sun together due to its high mass. </li></ul><ul><li>The particles are very close together and so the numbers of collisions are high causing very high temperatures. </li></ul><ul><li>These temperatures are high enough to start the nuclear fusion process. </li></ul>
  103. 105. NUCLEAR FUSION <ul><li>Advantages and Disadvantages of Nuclear Fusion Power </li></ul><ul><li>Advantages </li></ul><ul><li>The fuel, (deuterium), is found in water in very large amounts. At today’s rate of energy consumption, should last for about 1 billion years. </li></ul>
  104. 106. NUCLEAR FUSION <ul><li>One litre of water has the fuel energy of 300 litres of petrol. </li></ul><ul><li>There are 30 litres of petrol in the average tank of a car. </li></ul><ul><li>One litre of water has the energy equivalent of 10 tanks of petrol. </li></ul>
  105. 107. NUCLEAR FUSION <ul><li>The waste products are generally not radioactive. </li></ul><ul><li> Most of the radioactive isotopes produced are very short lived </li></ul><ul><li>The product is helium gas, which is inert. It does not produce the greenhouse gases that fossil fuels do. </li></ul>
  106. 108. NUCLEAR FUSION <ul><li>3. The problems associated with fission power plant mishaps are almost non-existent. Although operating at very high temperatures, the energy of the plasma is very low and so the reactor temperature will not rise more than a few degrees. </li></ul>
  107. 109. NUCLEAR FUSION <ul><li>Disadvantage </li></ul><ul><li>Due to the very high temperatures, as mentioned previously, the process is difficult to maintain and contain. </li></ul><ul><li>Most industrial powers such as the USA, Europe and Japan are presently working to overcome the problem. </li></ul>
  108. 110. NUCLEAR FUSION <ul><li>To overcome the repulsion between </li></ul><ul><ul><li>two small nuclei the nuclei have to be </li></ul></ul><ul><ul><li>accelerated to ~ 10,000 times their normal </li></ul></ul><ul><ul><li>speeds meaning heating to 10 9 o C. </li></ul></ul><ul><li>If the nuclei hit the walls they are cooled </li></ul><ul><ul><li>and the reaction stops. </li></ul></ul>
  109. 111. NUCLEAR FUSION <ul><li>Now using magnets to confine the nuclei and </li></ul><ul><ul><li>prevent them from hitting the walls. </li></ul></ul><ul><li>S till a long way from producing energy. </li></ul><ul><li>Today reactions can be maintained for ¾ s </li></ul><ul><li>After ~ 1 second there would be a net evolution of energy. </li></ul><ul><li>Catalyst: Fusion - ABC TV Science 27 April 2006 </li></ul>
  110. 112. NUCLEAR FUSION <ul><li>Cold Fusion </li></ul><ul><ul><li>Fusion not involving high temperatures </li></ul></ul><ul><li>Is being investigated as a possible alternative </li></ul><ul><li>Cold Fusion </li></ul>
  111. 113. YEAR 12 SACE PHYSICS <ul><li>THE END! </li></ul>