Lecture on Nuclear Energy

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Nuclear Energy

• What so special about nuclear energy?
• Fission process, energy balance
• History
• Natural nuclear reactor
• Nuclear reactor
• Waste Management
• Social and Health impact

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Nuclear Electricity to Armament in the news
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Nuclear Energy in the Financial News
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Why Nuclear Energy?
Conventional energy source comes from breaking or
making chemical bonds C O2 ? CO2 and/or H2
½O2 ? H2O Energy released eV/bond (energy
of a chemical bond) Release of green-house
gas During nuclear reactions mass is not
conserved missing mass is converted into energy
according to Einsteins equation E m c2
Energy released MeV/event no release of
green-house gas 1 Mev 1000000 eV 1 eV
1.6019 x 10-19 J
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Energy density of various sources
Need 2.4 MJ to bring 1 L water to a boil and
vaporize it
M(mega) million (x106), G(giga) billion (x109)
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Some facts associated with Nuclear Energy

• - Huge potential due to energy concentration
• - Geological origin, ie, not renewed (on
  geological time scale)
• - All other energy sources are due to solar
  activity
• - Output from Nuclear Energy is heat needs to be
  converted to
• Mechanical
• Electrical
• Not suitable for light transportation
• Waste problem
• Security Problem

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Quick history

• - 1896 Henri Becquerel discovers radioactivity
  (from Uranium)
• - Rutherford, Soddy, Marie and Pierre Curie main
  names associated with elucidation of
  radioactive phenomena
• - 1905 Einstein Emc2
• - 1911 Atomic model of matter (Rutherford)
• - 1919 First nuclear reaction in lab (4He 14N ?
  17O 1H)
• - 1932 Chadwick discovers the neutron
• 1934 First artificial radioactive element in lab
• 4He 27Al ? 30P n (30P 2.5 min t1/2)
• - 1938 Hahn and Strassman discover
  neutron-induced fission of U
• 1942 Fermi first sustained nuclear reactor
• 1940 1945 Development of Atomic bomb
• - 1955 First nuclear powered submarine (Nautilus)
• 1956 First civilian nuclear power station
  (Britain)
• 1979 Three Mile Island core melt-down (no
  contamination)
• - 1986 Explosion and fire at Chernobyl (large
  contamination)

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The fission process

• Various mode for radioactive decay
• ? emission (He nuclei) 5 MeV/event
• ? emission (electron ve or ve) 1 MeV/event
• ? emission (electromagnetic radiation, ie
  photons) 1 MeV/event
• Fission heavy nuclides may split into two
  fragments
• 235U slow n ? 2 fragments 2 fast n
• During process 200 Mev/event relased as kinetic
  energy of fragments
• If each released neutron can trigger another
  fission event, then potential for a chain
  reaction

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Conditions for sustained chain reaction

• Neutrons released during fission are fast
  neutrons
• Need to slow down these neutrons
• Some neutrons escape
• Some neutrons react with other things
• For chain reaction, at least one neutron from
  each fission event must induce another fission
  event.
• Define the multiplication factor
• If klt1 subcritical ? reaction dies
• If kgt1 supercritical ? reaction diverges
  (explosion)
• If k1 critical ? sustained reaction
• k depends on geometry, surrounding, temperature,
  fission product, etc.

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Fission in Nature Fossil Nuclear Reactor

• 1972, Oklo uranium mine in Gabon (Africa)
• Found area where 235U is 0.25 (normal 0.7) ?
  depleted U
• Ore has very low level of mineral with high
  neutron absorption probability (low in Vanadium,
  Boron, )
• Presence in ore of excess trapped Kr and Xe
  (fission products)
• Evidence of heated rock (no volcanoes around)
• - Geological composition of site
• Right conditions for sustained fission chain
  reaction about 2M years ago

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Nuclear Energy Production Ore to Wastes
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Nuclear Fuel need fissile material
Ky 1000 years, My million years, Gy billion
years Pu plutonium, Th thorium
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Nuclear Fuel natural U or 235-enriched U?

• Natural Uranium ? need to economize neutrons to
  sustain chain reaction, ie, requires heavy water
  (low probability of neutron capture - CANDU
  reactor)
• If uranium enriched to 3 - 7 in 235 isotope
  much easier to sustain chain reaction ? can use
  regular water
• If enriched to gt 80 ? bomb material critical
  mass lt 1 kg
• Fuel as UO2 in zirconium tubes (fuel cladding)

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Moderator to slow down neutrons

• Need some material which can slow down neutrons
  but with minimum neutron capture
• Most efficient are low atomic weight element H,
  D, Be, C
• Better if same material can transport heat away
  from core
• H2O (light water reactor) cheap, but
  significant neutron capture ? need enriched U
• D2O (heavy water reactor) very small neutron
  capture with D ? can use natural U, but need to
  prepare enough D2O
• Be, good but very toxic metal
• Carbon, good but combustible (Chernobyl)
• H (hydrogen), D (deuterium - heavy isotope of
  hydrogen),Be (beryllium), C (carbon)

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Coolant to extract heat from core

• Need to remove heat from core and make use of
  this heat
• Desired properties of heat extractor
• Can be circulated (gas, liquid)
• Low neutron capture (to economize neutron and to
  prevent too much radioactivation)
• Chemically stable
• High thermal conductivity
• H2O, D2O, He, liquid Sodium
• Then heat is exchanged to a steam boiler to power
  turbines

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Control Material
Required for adjusting system in order to keep
k1, ? control the number of neutrons available
for fission Need material high neutron capture
probability Best as rods of alloys containing
Cadmium, Boron, Indium (high neutron absorption),
which can be inserted into the core For safety,
want k to decrease with temperature (negative
temperature coefficient)
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Everything put together
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Improvements recycle spent fuel
Normal reaction 235U slow n ? fission fragments
2(fast)n Parasite reaction 238U fast n ?
239U ? 239Np (neptunium) ? 239Pu (fissile) Pu/U
fuel cycle Process used fuel to extract 239Pu
to be mixed with U (or to make bomb!) Other
parasite reaction 232Th fast n ? 233Th ? 233Pa
(protactinium) ? 233U (fissile) Th/U/Pu
cycle Lace natural Uranium with Thorium to
extract 239Pu and 233U to be re-injected into
natural Uranium.
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Plutonium/Uranium cycle
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Thorium/Plutonium cycle
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Phoenix reactors

• Reactors may be optimized to generate more fuel
  than they burn!
• Still technological problems
• Need to reprocess very radioactive material
• Plutonium very toxic elements
• Big security issue (bomb grade material)

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Waste Treatment

• What to do with highly radioactive wastes?
• Prevent dispersion
• Shield
• Present solution (temporary)
• Stored in pools next to site
• Long term
• Store (burry) in deep stable geological formation
• Treatment
• Chemically processed
• Vitrified
• Packed in special canisters
• Stored in disaffected mines (can be retrieved)

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Security and Social aspect
Population exposure due to radiation
release Normal operation, radioactive releases
less than burning fossil fuel (coal, oil contain
traces of 238U, 232Th, 222Ra) Mine
tailings Serious environmental concerns Radiation
release, Waste handling These can be controlled
very well, but serious security concerns
(accidents, terrorism)
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In summary

• Energy content ? cant beat it
• Room to improve efficiency
• Clean ? electricity
• Not for light transportation
• Wastes ? manageable
• Security ? ??
• Public acceptance ? ??