NUCLEAR POWER APES 2009 CHAPTER 21
Dec 25, 2015
ISOTOPES
• Isotopes- some atoms of the same element have different numbers of neutrons creating different mass numbers.
• EX: Uranium has 92 protons, & most uranium atoms contain 146 neutrons & have a mass number of 238… 92 + 146 = 238
• How many neutrons does U-235 have?
RADIOACTIVITY
• Some isotopes are unstable and decay slowly, emitting particles & energy.
• These are called radioactive atoms
• Radioactive atoms eventually become stable and stop decaying.
• Radiation can come from– Alpha particles– Beta particles– Gamma rays- used in radiation therapy for
cancer patients.
• When alpha or beta particles are given off, the mass # & atomic # change creating a different element.
• 2 radioactive isotopes of uranium are U-238 and
U-235 (know calculation!)– Both decay into stable form
of lead.
– The amount of time it takes for half of the atoms in a sample of a radioactive element to decay is called the isotope’s half-life.
– Half-lives can be a few seconds or billions of years.
– U-238 has half-life of 4.5 billion years.
– U-235 has half-life of 700 million years.
REACTIONS & REACTORS
• Nuclear Fission- releasing energy by splitting the nucleus of an atom apart.
• This energy can be used to create electricity.
STEPS OF NUCLEAR FISSION
– Neutron is fired into nucleus of U-235 atom.
– Nucleus splits, forming two daughter nuclei
– This reaction releases energy & several more neutrons.
– This continuous action of neutrons splitting atomic nuclei is called a chain reaction.
NUCLEAR REACTORS
• Nuclear fuel is usually 97% U-238 and 3%
U-235.• U-238 is not
fissionable so it is not part of the nuclear reaction (but can be used in plutonium reactors)
NUCLEAR REACTORS
• In the U.S., nuclear fission happens inside a nuclear reaction vessel– 20 m tall with walls that are
15-30 cm thick.
– Large shield surrounds the vessel to contain any stray radioactive particles
– The reactor is housed inside a concrete containment building.
NUCLEAR REACTORS
• Fuel rods are filled with pellets that contain the U-235. Positioned vertically in reactor so water can circulate betwn them.
NUCLEAR REACTORS
• Water is important because:– It absorbs heat & keeps
core from melting.
– It slows the movement of neutrons released during the chain reaction.
NUCLEAR REACTORS
• Speed of chain reactions is controlled by control rods made of cadmium, boron, etc. that absorb neutrons.– Raise control rods out of
reactor= absorb fewer neutrons, speed up reaction, hotter water.
– Lower control rods into reactor= absorb more neutrons, slow reaction, cool water
NUCLEAR REACTORS
– Hot water is passed to pipes where steam is created that turns turbines, creating electricity.
– Water cooling system & control rods regulate heat. If they fail, it would cause a “nuclear meltdown” at the core.
BREEDER REACTORS
• U-238 is most plentiful, but non-fissionable.
• Turn U-238 into plutonium-239 which is fissionable.
• It creates more fuel than you start with.
• Plutonium can be used to make atomic bombs as well as energy.
• Breeder reactors not used in U.S. because of potential threat of nuclear terrorism.
RADIATION & HEALTH
• Radiation is unhealthy– Fast dividing skin cells &
blood cells are particularly vulnerable
– Large doses cause skin burns, anemia, death, miscarriage
– Changes DNA leading to cancer & genetic mutations.
– Can be passed on to offspring
RADIOACTIVE WASTE
• HIGH LEVEL – Emit large amounts of
radiation– Very dangerous &
poisonous– Stored onsight in large
containment vessels stored in water
– Come from• Used uranium fuel rods• Control rods• Water used to cool &
control chain reactions
RADIOACTIVE WASTE• MEDIUM & LOW LEVEL
– Not as radioactive
– A lot more are produced vs. high level
– Pose a greater risk because they are more prevalent & not as obvious
• Clothing of nuclear power plant workers
• Tailings from uranium mines
• Hospital & laboratory waste
WASTE DISPOSAL
• Must be – stored in container that
will last tens of thousands of years.
– Stored in geologically stable area. No earthquakes!
– Stored deep underground
PROBLEMS WITH WASTE DISPOSAL
• Most high level wastes sit in storage tanks outside nuclear power & weapons plants. Some have begun to leak contaminating groundwater.
• Between 1940 & 1970, most medium & low level wastes were sealed in concrete & dropped into the ocean, exposing that environment to potential leaks. Now, it is put into landfills
PROBLEMS WITH WASTE DISPOSAL
• Send to Yucca Mountain in Nevada desert
• 160 miles from Las Vegas• Underground storage chamber• Cost $50 billion• All high level waste would
have to be containerized, and transported by train or truck to site across country
• Many people oppose because they do not want radioactive waste transported thru their cities.
Pros & Cons of Yucca Mountain
• Desert- very little rain, reduce chance of corrosion
• Secluded• Solid bedrock underneath-
reduces chance of aquifer contamination
• Has been geologically active- earthquakes
• Cracks from slight earthquakes could allow water into tunnels, not to mention endanger the integrity of the storage casks
• There is an aquifer underneath used for drinking & irrigating by desert population
NUCLEAR MELTDOWN
• Process by which nuclear chain reaction goes out of control & melts reactor core
• Releases huge amounts of radiation into environment.
Three Mile IslandThree Mile Island•March 29, 1979, a reactor near Harrisburg, PA lost coolant water because of mechanical and human errors and suffered a partial meltdown
•50,000 people evacuated & another 50,000 fled area
•Unknown amounts of radioactive materials released
•Partial cleanup & damages cost $1.2 billion
•Released radiation increased cancer rates.
www.bio.miami.edu/beck/esc101/Chapter14&15.ppt
CHERNOBYL
• Located in Ukraine• 1986 explosion killed 30 people
immediately• 116,000 had to leave homes
permanently• May cause 15,000 cases of
cancer.• 62,000 sq mi contaminated• Cost $358 billion• Chernobyl was old & lacked
safety equipment• Caused by human error
PROS OF NUCLEAR
POWER
• Use very little material to get a lot of energy.
• Does not produce much air pollution
CONS OF NUCLEAR
POWER
•Potential accidents
•Radioactive waste disposal expensive & difficult
•Safety equipment expensive
•High cost of building new plants
•Uranium is nonrenewable
www.bio.miami.edu/beck/esc101/Chapter14&15.ppt
Use of Nuclear EnergyUse of Nuclear Energy• U.S. phasing out• Some countries (France,
Japan) investing increasingly
• France 78% energy nuclear
• U.S. currently ~7% of energy nuclear
• No new U.S. power plants ordered since 1978
• North Korea is getting new plants from the US