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Nuclear Power

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Title: Nuclear Power


1
Nuclear Power
  • Lecture 16
  • GLY 120

2
Nuclear Energy - Fission
  • Nuclear fission splitting an atom into smaller
    atoms and releasing heat and energy
  • Nuclear power plant operation - generates
    electricity in a manner similar to a coal-burning
    power plant
  • heating a fluid (usually water)
  • generates steam (directly or indirectly)
  • drives turbine blades connected to a power
    generator
  • Difference the method for heating the water
  • nuclear reactions vs. coal combustion

3
  • Two loops for water circulation and heat transfer

  • no exchange of radioactive material, just heat
  • Primary loop - core (no steam generated)
  • Pumped past water in a secondary loop (heat
    transferred in heat exchanger, steam generated).

4
  • Hot condenser water is then directly discharged
    into surface waters or cooling towers or ponds.
  • Water in the primary loop circulates back past
    the reactor core and gets reheated.

5
  • Isotopes elements with the same of protons,
    different of neutrons (and different atomic
    mass)
  • Example - Uranium (U) 92 protons in nucleus...

  • 238U (146 neutrons)
  • 235U (143 neutrons)
  • Radioactivity and radioactive decay (half-life)
  • Not all isotopes are stable
  • Radioactive isotopes undergo radioactive decay to
    eventually become a stable nucleus
  • Example - 238U eventually becomes stable 206Pb
    (lead)

6
  • Half-Life The time required for one half of a
    radioactive substance to decay into its daughter
    material.

7
  • Earth's internal heat - radioactive decay of
    elements in the core and mantle, such as U, give
    off a lot of energy in the form of HEAT.
  • Heat makes its way to Earths surface (volcanoes,
    hot springs) and this exchange of heat is the
    main driving force of plate tectonics.
  • Heat from a controlled nuclear reaction
    (encouraged decay) is also used to create power
    Nuclear Power

8
Nuclear Fuel
9
  • After enrichment, the uranium is formed into
    ceramic pellets which are then encased in metal
    fuel rods.
  • Bundles of rods are put into a stainless steel
    reactor vessel.
  • Fuel rods remain in the reactor core, undergoing
    fission, for an average of 3 years (235U content
    decreases from 3 to 1).
  • At below 1 the fuel is no longer efficient for
    use in the reactor

10
  • For nuclear fission
  • A neutron (non-charged particle) strikes the
    nucleus of a 235U atom
  • This causes it to split apart, releasing
  • heat
  • 2-3 other neutrons, and various kinds of fission
    products
  • Tin-131 (131Sn)
  • Molybdenum-103 (103Mo)
  • The released neutrons collide with other 235U
    nuclei, repeating the process in a chain reaction
    (self-supporting)

11
  • Usually, in nuclear reactors, operators control
    this chain reaction so it does not get out of
    control,
  • Only one (not 2 or 3) free neutron produced by
    each fission event goes on to split another
    nucleus.
  • Use
  • Control rods
  • Rods, composed of either cadmium or boron, are
    inserted into the core to capture neutrons
  • Control the chain reaction when the reaction
    accelerates too quickly.
  • Heavy water (2H2O) and graphite mixture
  • Used to slow neutrons and encourage more
    interaction with Uranium fuel
  • Critical mass the amount of U required to
    maintain a chain reaction in a nuclear reactor

12
  • Generally, completely out of control reactions
    would lead to an explosion (as in nuclear
    weapons)
  • Reactor materials are not concentrated enough to
    explode.
  • An out of control chain reaction would lead to
    overheating of the core, causing the fuel rods
    and vessel to melt
  • Meltdown supercritical mass - Chernobyl,
    Ukraine

13
  • Use of nuclear power
  • 104 reactors producing about 21 of our power
    needs in the U.S.
  • France 78 of power production is nuclear (59
    plants)
  • Japan 36 (55 plants)
  • Britain 26 (35 plants)

14
Change in attitudes
  • 1950s nuclear power was seen as the wave of
    the future
  • Clean, safe, limitless
  • Since 1979, new concerns
  • Plant safety
  • Radioactive waste disposal
  • Vast cost overruns in plant construction

15
  • Two major accidents have occurred at nuclear
    power plants
  • Three Mile Island, Pennsylvania (1979)
  • Mechanical and operator error prevented water
    from cooling the core
  • Allowed heat to build up and partial meltdown to
    occur.
  • Most radioactivity contained within the building,

  • small amount of water with low-level radiation
    leaked into the environment.
  • The building was highly contaminated but the leak
    probably did little damage to the environment
  • But, did damage the public perceptions of nuclear
    plants and their potential hazards.

16
  • Chernobyl, Ukraine, 1986
  • Site of the most serious nuclear accident in
    history April 26, 1986
  • Due to mistakes and problems with the cooling
    water system, the reactor core became
    supercritical during a test experiment
  • Allowed temperatures to rise above 3000 C,
    causing uranium fuel to completely melt.
  • A massive steam explosion occurred that
    (literally) blew the top off of the building.

17
  • The graphite surrounding the fuel rods ignited
    and burned for 10 days, sending radioactive gas
    into the atmosphere
  • The fire was eventually put out and the reactor
    core encased and buried in concrete.
  • The plant design has been criticized as being
    poor and there are other reactors of this same
    design in Chernobyl and elsewhere in the former
    Soviet Union

18
http//en.wikipedia.org/wiki/Chernobyl_accident
19
  • Lasting effects of Chernobyl
  • 30 people died immediately from radiation
    exposure
  • acute radiation syndrome
  • thousands died from exposure during clean-up
    operations
  • millions were exposed to high levels of radiation

20
  • The Soviet government handled the crisis very
    poorly and delayed notifying other countries
    about the radiation leak.
  • The accident occurred on Saturday
  • Swedish scientists detected the radiation on
    Monday.
  • The area was evacuated slowly and many people
    were put at risk unnecessarily.

Radioactive particles and gases
Dark red immediate downwind exposure after
Chernobyl
Light red one week later
21
Nuclear Power the Good and the Bad
  • Advantages of nuclear power
  • inexpensive (after plant is constructed)
  • few deaths per energy unit produced
  • no CO2 gas emitted
  • small amounts of solid waste generated

22
  • Disadvantages
  • Unresolved high level radioactive waste problem
  • what do we do with it, where do we store it, how
    do we transport it safely?
  • Yucca Mountain, Nevada
  • Requires a very large capital investment
    advanced technology
  • Poor technology, operation (human error), and
    plant design can lead to costly and deadly
    accidents (e.g., Chernobyl)
  • complex design (40,000 valves vs. 4000 in coal
    plants)

23
  • Breeder reactors (not in use in the US) produce
    plutonium from 238U (France, Japan)
  • Pu one of the most toxic substances known
  • Dangerous to produce due to chances of falling
    into the wrong hands!
  • Produces weapons-grade material
  • But, does reduce need for additional fuel

24
Is nuclear power a sustainable energy resource?
  • From a fuel standpoint, yes
  • From an environmental standpoint, no storage of
    waste, potential accidents
  • Society? Depends on public opinion
  • see France, Japan vs. Ukraine, U.S.

25
Storage
  • So far, there is no way to permanently dispose of
    high-level radioactive waste
  • Chemical reactions cannot destroy radioactive
    waste
  • radioactivity is a nuclear process
  • atomic nuclei are unaffected by chemical
    reactions
  • The only choice is to
  • store the waste in a place safe from
  • geological hazards (earthquakes, volcanic
    activity, landslides, creep, floods, and seeping
    water)
  • and human intervention
  • allow them to decay naturally (over 250,000 years)

26
  • Many tons of radioactive materials have
    accumulated over past decades and represent
    proven hazards to humans and future generations.
  • High-level nuclear waste needs to be isolated
    because high-energy radiation
  • Kills cells
  • Causes cancer and genetic mutations
  • Can cause rapid death in cases of high exposure
    levels.
  • Also, radioactive decay produces heat and can
    damage crystalline and metallic holding
    tanks/containers leaks!

27
  • Two choices
  • permanent disposal (injection into impermeable
    layers)
  • monitored, retrievable storage (isolated, safe
    storage, but able to pull out and move until
    technology finds a better solution)

28
1982 - Nuclear Waste Policy Act
the Department of Energy (DOE) was made
responsible for locating a fail-safe geological
repository for high-level radioactive waste.
Presently, the DOE maintains three high-level wa
ste "temporary" storage facilities in the US
West Valley, New York Savannah River, South Carol
ina Hanford, Washington.
29
  • Problems with temporary storage sites
  • In 1956, a tank leak was detected at the Hanford
    site
  • Since then, 750,000 gallons of high-level waste
    have leaked underground and costing an estimated
    57 billion to clean up.
  • Obviously a permanent disposal solution is
    required

30
Yucca Mountain, Nevada
  • Monitored retrievable storage facility of
    high-level radioactive waste
  • In 1987, the U.S. Congress directed the
    Department of Energy to study Yucca Mountain,
    Nevada, to determine whether it was a suitable
    location for the Nation's first deep geologic
    repository

31
  • Advantages
  • Far from large population centers (175 km from
    Las Vegas)
  • Dry, arid climate
  • Extremely deep water table

32
  • Problems
  • Geology
  • Area has been volcanically active in the past
  • Area is somewhat seismically active earthquakes
    are possible

33
  • Hydrology
  • The water table lies 550 meters below the surface
    of the site
  • Construction digging tunnels and caverns to a
    depth of about 300 meters
  • still 250 meters above the water table
  • Thus, radioactive waste would be isolated from
    both surface and ground waters.
  • Potential hydrologic problems
  • During earthquakes, increased pressure could
    cause elevation of the water table, causing
    direct interaction between hot decaying
    radioactive materials and water
  • could result in a steam explosion
  • Surface water, given sufficient time, could
    percolate through the volcanic rocks, interact
    with the waste, and eventually reach the water
    table, thereby polluting the groundwater
  • 9,000 to 80,000 years (assuming current
    conditions)

34
  • Legal and Political problems - N.I.M.B.Y.
  • The state of Nevada is against the development of
    the Yucca Mountain site.
  • The state contains ZERO nuclear reactors and only
    one Superfund site (also few representatives in
    congress)
  • BUT, the state contains a large amount of
    government-owned land and low population density
  • At one point, NV refused to issue air quality
    permits to operate drilling rigs at the
    repository
  • In 1995, lawmakers announced that storage at the
    site was not allowed until the year 2015
  • The DOE had anticipated beginning emplacement by
    2010
  • October 2003 conflict of interest charges
  • Governments former law firm won bid over other
    law firms
  • Further delays

35
While the politicians argue, 3000 tons of
high-level radioactive waste accumulate each year
(current total about 40,000 tons) in temporary
storage facilities. Other potential problems
Social Transportation National security
36
  • Transportation -
  • need to move high-level radioactive waste from
    temporary storage facilities (many in the Eastern
    U.S.) on a long journey (by train or semi)
  • travel through many states and past many people
    and communities
  • exposure concerns for drivers, passers-by
  • definite chance of possible accidents and spills
    (even small amounts)

37
  • Danger of terrorist activities
  • How can we tell if a country (Iran, North Korea)
    is just making nuclear fuel for power plants or
    weapons-grade material?
  • North Korea admitted to having nuclear weapons in
    February, 2005
  • Iran has been accused of trying to make nuclear
    weapons
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