Nuclear Energy
Nuclear Energy Conversion of energy from nuclear bonds of U-235 isotope into thermal energy. Nuclear fission – the splitting apart of atomic nuclei releases nuclear energy Radioactive decay – when parent isotope (U-235) emits alpha, beta particle or gamma radiation 1 g of U-235 contains 2 – 3 million x the energy as 1g of coal
Nuclear Energy Uranium is mined from a ore of UO2 (rock) 99% of mined UO2 is U-238, not easily fissionable 1% of UO2 is U-235 but must be enriched to at least 3% to sustain chain reaction U-235 is the most easily fissionable isotope 1 ton of ore = 6.6 pounds of uranium 2001 - 78.9 million pounds Largest U deposits are in Australia
Mining Uranium
Chain reaction: Neutron strikes atom Splits atom into 2 or more parts Releases more neutrons, heat Additional neutron in turn, promote more fission reactions = chain reaction Large amounts of energy are released
Nuclear Energy Daughter products of U-235 fission are barium & krypton. Many other radioactive daughter products are released. Nuclear reactor will harness the kinetic energy from the 3 neutrons in motion which produces a self-sustaining chain reaction.
How Reactor Works Nuclear fuel stored in containment structure Fuel rods – contain pellets of (U-235) Hundreds of bundles of fuel rods in core Heat from fission heats water till steam is produced The rest of the process is exactly the same as coal power plants (steam turns turbines = generate electricity) Light water reactors are most common & the only kind used in US
Controlling Nuclear Fission For fission to begin, reactions in the must be slowed down by a moderator Moderator is usually water or graphite Excess neutrons produced must be absorbed by control rods to maintain fission reaction at desired rate.
Radioactive Waste Not a combustion reaction All CO2 comes from mining, transporting, processing Many radioactive isotopes are made during the fission process (waste) Ex. Ba-142 (4 months) , I-129 (15.7 million years) Human health risk – 10 half lives Storage (@ plants): Pools, above ground facilities & metal containers
Small amounts of radioactive gases Uranium fuel input (reactor core) Control rods Containment shell Heat exchanger Steam Turbine Generator Electric power Waste heat Hot coolant Useful energy 25%–30% Hot water output Pump Pump Coolant Pump Pump Cool water input Waste heat Figure 16.16 Science: light-water–moderated and –cooled nuclear power plant with a pressurized water reactor. QUESTION: How does this plant differ from the coal-burning plant in Figure 16-13? Moderator Coolant passage Shielding Pressure vessel Water Condenser Periodic removal and storage of radioactive wastes and spent fuel assemblies Periodic removal and storage of radioactive liquid wastes Water source (river, lake, ocean) Fig. 16-16, p. 372
Radioactive Waste Half-life for U-235 is 704 million years! Radiation is measured in a variety of units: Becquerel (Bq) – rate at which a sample of radioactive material decays. 1 Bq = decay of 1 atom/second Curie = 37 billion decays/second
Radioactive Waste Spent rods are stored in a water pool to cool Running out of pool storage
Radioactive Waste After spent fuel rods are cooled considerably, they are sometimes moved to dry-storage containers made of steel or concrete.
Radioactive Waste Long term storage?? 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. Proposed site in Nevada (Yucca Mountain) – near faults & groundwater; 2010 site was no longer funded. Alternative site not agreed upon.
Nuclear Accidents Chernobyl, Ukraine – (1986) explosion in reactor 3 Mile Island, PA (1979) – partial meltdown of core Fukushima, Japan (2011) – involving 6 units; EQ & tsunami cut off all emergency electricity required to cool core & waste storage.
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.
What Happened to Nuclear Power? Multi billion-dollar construction costs. Higher operation costs and more malfunctions than expected. Poor management. Public concerns about safety and stricter government safety regulations. Increased price of uranium Increased concerns of terrorist attacks
NUCLEAR ENERGY A 1,000 megawatt nuclear plant is refueled once a year vs a coal plant requires 80 rail cars a day. Figure 16-20
NUCLEAR ENERGY Currently there are 104 plants in US 70% of energy in France 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. Modern reactors are much more safe Aging reactors are subject to degredation and corrosion.
New and Safer Reactors Pebble bed modular reactor (PBMR) are smaller reactors that minimize the chances of runaway chain reactions. Figure 16-21
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.
Decommissioning of reactor Fuel assemblies Enrichment of UF6 Fuel fabrication (conversion of enriched UF6 to UO2 and fabrication of fuel assemblies) Temporary storage of spent fuel assemblies underwater or in dry casks Conversion of U3O8 to UF6 Uranium-235 as UF6 Plutonium-239 as PuO2 Spent fuel reprocessing Low-level radiation with long half-life Figure 16.18 Science: the nuclear fuel cycle. QUESTION: Are any parts of the nuclear fuel cycle within 27 kilometers (17 miles) of where you live or go to school? Geologic disposal of moderate & high-level radioactive wastes Open fuel cycle today “Closed” end fuel cycle Fig. 16-18, p. 373