Nuclear Energy

Nuclear Fission Neutrons split a fissionable atom (U-235) which releases neutrons and other byproducts. New neutrons go on to break apart other isotopes. 4.shtml

How a Nuclear Fission Reactor Works U.S. uses light water reactors Core: contains 35,000 to 70,000 fuel rods packed with fuel pellets Fuel pellets contain enriched uranium oxide fuel which is 97% U-238 and 3% U-235 Control rods (usually made of B or Cd) move in and out of the spaces between the fuel rods and absorb neutrons to regulate fission rate and power

Moderator: slows neutrons emitted during fission to keep process going (usually water, graphite, or heavy water) Coolant: circulates through core. Removes heat from fuel rods and produces steam to turn turbine. Usually water. Containment vessel: surrounds reactor core. Has thick, strong walls. Water-filled pools or dry casks are used to store spent fuel.

Inside the Reactor Core

Fuel Rods in Pool of Water

Advantages of Nuclear Power Large fuel supply Low environmental impact from plant 1/6 the CO 2 of coal burning plants Moderate disruption of land Moderate water pollution Low risk of accidents (in developed nations) Low air pollution

Disadvantages of Using Nuclear Power Expensive Low energy yield Accidents Nuclear waste Terrorist attack Can spread knowledge and technology used for building weapons

Nuclear waste High level radioactive wastes give off large amounts of harmful ionizing radiation for a shor time and small amounts for a long time, Include: spent fuel rods and wastes from production of nuclear weapons Must be stored safely for thousands of years

Decommissioned Power Plant Has been retired b/c parts are worn down/out and reactor is highly contaminated with radiation Usually within 15 to 60 yrs. What can we do with them: –Dismantle and store waste in high level facility –Put up barrier around plant for 30 to 100 years and then dismantle: allow time for decay –Encase in tomb

Closed Nuclear Fuel Cycle Fissionable isotopes are removed from spent fuel assemblies and reused. The remainder must be stored safely for about 10,000 years. Rarely done because of high cost and potential of use in nuclear weapons.

Open Nuclear Fuel Cycle Isotopes not removed from fuel assembly. Must be buried in disposal facility for up to 240,000 years.

Storage Pond for containing spent fuel rods at nuclear facility

What to do with High Level Wastes? Scientists disagree Suggestions: –Bury deep underground –Shoot into space or sun –Bury under Antarctic ice sheet or Greenland ice cap –Dump into descending subduction zones in deep ocean –Bury in thick deposits of deep floor ocean mud which have been geologically stable for 65 million years –Change into less harmful or harmless isotopes

Yucca Mountain 1985, DOE proposed burying waste here 100 miles from Las Vegas Expected to open by 2010 and will be monitored for 300 years and sealed Concerns about safety of storage from water leaks, leaky storage containers, unstable geology, shipment of waste Approved in 2004

Where is Yucca Mountain?

Entrance to Yucca Mountain

Inside Yucca Mountain

Nuclear Waste Routes through Florida

Low Level Nuclear Waste Give off small amounts of ionizing radiation and must be safely stored for 100 to 500 years Include: tools, building materials, clothing, and other contaminated items Between 1940 and 1970: put into steel drums and dumped in ocean Today: put into steel drums and stored at 2 regional landfills. The NRC thinks that the waste could be dumped in landfills with ordinary trash

Low Level Waste

How did we start using nuclear power for electricity? 1) Atomic Energy Commission promised nuclear power would be cheaper than coal. 2) Government paid for the building of the first commercial reactors and promised no overruns 3) Price-Anderson Act would pay for accidents

Success? Predicted that nuclear power would provide 21% of the world’s commercial energy and 25% in U.S. Today only producing about 6% of world’s commercial energy. There would be at least 1800 plants in U.S. by In 2004, only 103. Major problems: costs, accidents, and neglecting the entire fuel cycle

Locations of Nuclear Power Plants

Turkey Point Facility south of Miami

Three Mile Island

What Happened? 4 AM March 28, 1979 near Harrisburg, PA Malfunction in the cooling system at the Three Mile Island Nuclear Generating Station Reactor core was slightly exposed releasing radioactive gases Most serious commercial nuclear accident in US history Paved way for reforms in nuclear power plant operations and regulations. Caused Americans question safety of nuclear power No new nuclear power plants have been built since

Chernobyl: Unit 4 was damaged

Chernobyl April 25, 1986: series of explosions in Unit 4 of the power plant Blew off roof of the reactor building and sent radioactive debris and dust into the atmosphere Caused by poor reactor design and human error Exact numbers of deaths and illness not known –30 people died from radiation exposure –2000 children developed thyroid cancer –8,000 to 15,000 premature deaths –100,000 people evacuated (many not for up to 10 days later)

Unit 4 after the explosion

Path of Radioactive Cloud

Abandoned Town

Sarcophagus was placed over Unit 4 Birds and rainwater have gotten in…What is getting out?

Breeder Reactors Generate more nuclear fuel than they use Convert U-238 into fissionable Pu-239 Would increase longevity of uranium reserves Produce Pu very slowly. –May take 100 to 200 years to provide enough fuel for other reactors Safety is concern: –Liquid Na is used as coolant (explosive in contact with water)

Half-Life Problems Example: How much of a radioactive material would be left after 3 half-lives if the original amount was 100 grams? Example: How many years would be required to reduce the radiation of a material containing 200 curies to 50 curies? Assume that the half-life is 25 years. Example: What is the half-life of a radioactive substance given 5 curies of the original 40 curies remain after a period of 15 years?