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