NUCLEAR POWER

APES

2009

CHAPTER 21

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.

Natural Radiation?

• Natural sources of radiation…– Soil & rocks

– Water

– Air

– Cosmic rays

• 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

www.geology.fau.edu/course_info/fall02/ EVR3019/Nuclear_Waste.ppt

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

Yucca Mountain

www.geology.fau.edu/course_info/fall02/ EVR3019/Nuclear_Waste.ppt

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