NUCLEAR POWER APES 2009 CHAPTER 21 ISOTOPES Isotopes

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 Fissionreleasing 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/fall 02/ EVR 3019/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 underneathreduces 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/fall 02/ EVR 3019/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 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/esc 101/Chapter 14&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

Fukushima Accident March 11, 2011 Video Explains the Fukushima Accident

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

Use 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 www. bio. miami. edu/beck/esc 101/Chapter 14&15. ppt

NUCLEAR ENERGY • When isotopes of uranium and plutonium undergo controlled nuclear fission, the resulting heat produces steam that spins turbines to generate electricity. – The uranium oxide consists of about 97% nonfissionable uranium-238 and 3% fissionable uranium-235. – The concentration of uranium-235 is increased through an enrichment process.

Small amounts of radioactive gases Uranium fuel Control rods input (reactor Containment shell core) Heat exchanger Turbine Steam Generator Waste heat Hot coolant Hot water Pump output Pump Coolant Pump Moderator Shielding Coolant Pressure passage vessel Periodic removal and storage of radioactive wastes and spent fuel assemblies Water Periodic removal and storage of radioactive liquid wastes Pump Cool water input Electric power Useful energy 25%– 30% Waste heat Condenser Water source (river, lake, ocean) Fig. 16 -16, p. 372

NUCLEAR ENERGY • After three or four years in a reactor, spent fuel rods are removed and stored in a deep pool of water contained in a steel-lined concrete container. Figure 16 -17

NUCLEAR ENERGY • After spent fuel rods are cooled considerably, they are sometimes moved to dry-storage containers made of steel or concrete. Figure 16 -17

Decommissioning of reactor Fuel assemblies Enrichment of UF 6 Conversion of U 3 O 8 to UF 6 Reactor Fuel fabrication (conversion of enriched UF 6 to UO 2 and fabrication of fuel assemblies) Uranium-235 as UF 6 Plutonium-239 as Pu. O 2 Spent fuel reprocessing Temporary storage of spent fuel assemblies underwater or in dry casks Low-level radiation with long half-life Open fuel cycle today “Closed” end fuel cycle Geologic disposal of moderate & high-level radioactive wastes Fig. 16 -18, p. 373

What Happened to Nuclear Power? • After more than 50 years of development and enormous government subsidies, nuclear power has not lived up to its promise because: – Multi billion-dollar construction costs. – Higher operation costs and more malfunctions than expected. – Poor management. – Public concerns about safety and stricter government safety regulations.

Case Study: The Chernobyl Nuclear Power Plant Accident • The world’s worst nuclear power plant accident occurred in 1986 in Ukraine. • The disaster was caused by poor reactor design and human error. • By 2005, 56 people had died from radiation released. – 4, 000 more are expected from thyroid cancer and leukemia.

NUCLEAR ENERGY • In 1995, the World Bank said nuclear power is too costly and risky. • In 2006, it was found that several U. S. reactors were leaking radioactive tritium into groundwater. Figure 16 -19

Trade-Offs Conventional Nuclear Fuel Cycle Advantages Large fuel supply Low environmental impact (without accidents) Emits 1/6 as much CO 2 as coal Moderate land disruption and water pollution (without accidents) Moderate land use Low risk of accidents because of multiple safety systems (except for 15 Chernobyl-type reactors) Disadvantages Cannot compete economically without huge government subsidies Low net energy yield High environmental impact (with major accidents) Catastrophic accidents can happen (Chernobyl) No widely acceptable solution for long-term storage of radioactive wastes and decommissioning worn -out plants Subject to terrorist attacks Spreads knowledge and technology for building nuclear weapons Fig. 16 -19, p. 376

NUCLEAR ENERGY • A 1, 000 megawatt nuclear plant is refueled once a year, whereas a coal plant requires 80 rail cars a day. Figure 16 -20

Trade-Offs Coal vs. Nuclear Coal Nuclear Ample supply of uranium High net energy yield Low net energy yield Very high air pollution Low air pollution (mostly from fuel reprocessing) High CO 2 emissions Low CO 2 emissions (mostly from fuel reprocessing) High land disruption from surface mining Much lower land disruption from surface mining High land use Moderate land use Low cost (with huge subsidies) High cost (even with huge subsidies) Fig. 16 -20, p. 376

NUCLEAR ENERGY • Terrorists could attack nuclear power plants, especially poorly protected pools and casks that store spent nuclear fuel rods. • Terrorists could wrap explosives around small amounts of radioactive materials that are fairly easy to get, detonate such bombs, and contaminate large areas for decades.

NUCLEAR ENERGY • When a nuclear reactor reaches the end of its useful life, its highly radioactive materials must be kept from reaching the environment for thousands of years. • At least 228 large commercial reactors worldwide (20 in the U. S. ) are scheduled for retirement by 2012. – Many reactors are applying to extent their 40 year license to 60 years. – Aging reactors are subject to embrittlement and corrosion.

NUCLEAR ENERGY • Building more nuclear power plants will not lessen dependence on imported oil and will not reduce CO 2 emissions as much as other alternatives. – The nuclear fuel cycle contributes to CO 2 emissions. – Wind turbines, solar cells, geothermal energy, and hydrogen contributes much less to CO 2 emissions.

NUCLEAR ENERGY • Scientists disagree about the best methods for long-term storage of high-level radioactive waste: – Bury it deep underground. – Shoot it into space. – Bury it in the Antarctic ice sheet. – Bury it in the deep-ocean floor that is geologically stable. – Change it into harmless or less harmful isotopes.

New and Safer Reactors • Pebble bed modular reactor (PBMR) are smaller reactors that minimize the chances of runaway chain reactions. Figure 16 -21

Each pebble contains about 10, 000 uranium dioxide particles the size of a pencil point. Pebble detail Silicon carbide Pyrolytic carbon Porous buffer Uranium dioxide Graphite shell Helium Turbine Generator Pebble Core Reactor vessel Recuperator Water cooler Hot water output Cool water input Fig. 16 -21, p. 380

New and Safer Reactors • Some oppose the pebble reactor due to : – A crack in the reactor could release radioactivity. – The design has been rejected by UK and Germany for safety reasons. – Lack of containment shell would make it easier for terrorists to blow it up or steal radioactive material. – Creates higher amount of nuclear waste and increases waste storage expenses.

NUCLEAR ENERGY • Nuclear fusion is a nuclear change in which two isotopes are forced together. – No risk of meltdown or radioactive releases. – May also be used to breakdown toxic material. – Still in laboratory stages. • There is a disagreement over whether to phase out nuclear power or keep this option open in case other alternatives do not pan out.

How Would You Vote? To conduct an instant in-class survey using a classroom response system, access “Join. In Clicker Content” from the Power. Lecture main menu for Living in the Environment. • Should nuclear power be phased out in the country where you live over the next 20 to 30 years? – a. No. In many countries, there are no suitable energy alternatives to nuclear fission. – b. Yes. Nuclear fission is too expensive and produces large quantities of very dangerous radioactive wastes.