Download
1 / 46
E N D
Advantages of Nuclear Power • Nuclear electricity is reliable and relatively cheap (with an average generating cost of 2.9 cents per kW/h) once the reactor is in place and operating. • Large reserves of Uranium in United States - Fuel for nuclear power plants will not run out for tens of thousands of years • Nuclear power plants contribute no greenhouse gasses and few atmospheric pollutants
Disadvantages of Nuclear Power • Uranium is ultimately a nonrenewable resource. • Nuclear power plants are extremely costly to build. • The slight possibility that nuclear power plants can have catastrophic failures. • Large environmental impact during the mining and processing stages of uranium are numerous. • Nuclear waste (Spent nuclear fuel) is extremely hazardous and must be stored safely for thousands of years.
Comparing Uranium to Coal *1 kg of uranium-235 will generate as much energy as 3,000 tons of coal without CO2 emissions
Coal Power Plant Environmental Concerns
Nuclear Power Plant Environmental Concerns
Nuclear Power in the U.S. • The U.S. has 104 nuclear power plants (more than any other country) • Combined, these reactors produce about 100 billion watts of electricity (20% of the total)
Nuclear Power in the U.S. • No new reactors have been completed in the U.S. since 1979. • Why??? – High Operating Costs – Liability Problems – Wastes – Health Concerns – Terrorism – Public Misconceptions
Illinois Nuclear Power • Nuclear power typically accounts for nearly one-half of Illinois electricity generation and more than one-tenth of all the nuclear power generated in the United States. • With 11 operating reactors at six nuclear power plants, Illinois ranks first among the States in nuclear generation. • The growth of the Illinois nuclear industry is due largely to State government initiatives, which began encouraging nuclear power development in the 1950s
Illinois Nuclear Power • The origin of all of the commercial and military nuclear industries in the world can be traced back to December 2, 1942 at the University of Chicago. • On that day, a team of scientists under Dr. Enrico Fermi initiated the first controlled nuclear chain reaction.
Illinois Nuclear Power • There are 6 operating nuclear power plants in Illinois: Braidwood, Byron, Clinton, Dresden, LaSalle, and Quad Cities. • With the sole exception of the single-unit Clinton plant, each of these facilities has two reactors. • The two reactors at Braidwood and both reactors at Byron are pressurized light water reactors (PWR). • The reactors at the other facilities (Clinton, Dresden, LaSalle, and Quad Cities are boiling water reactors (BWR).
Fission and Fusion • Fission: A heavy nucleus is split into smaller nuclei, releasing energy • Fusion: Two light nuclei fuse into a heavy nucleus, releasing energy – Fusion generates much more energy than fission – Fusion of hydrogen into helium is what provides power to the sun (and thus to the Earth)
Energy from Fission • When a neutron strikes a uranium-235 nucleus, it splits into two nuclei, releasing energy • Example: n + 235U -> 93Kr + 141Ba + 2n • One neutron goes in, two neutrons come out
Chain Reactions • If at least one neutron from each fission strikes another nucleus, a chain reaction will result • An assembly of uranium-235 that sustains a chain reaction is critical
Power Plants and Bombs • Natural uranium contains 99.3% uranium-238 and 0.7% uranium-235 • Only uranium-235 can create fission chain reactions – Nuclear power plants use uranium that has been enriched to 3% uranium-235 – Nuclear bombs use uranium that has been enriched to 95% uranium-235 • 3% enriched uranium can never explode!
Nuclear Bombs • Since neutrons initiate fission, and each fission creates more neutrons, there is potential for a chain reaction
Nuclear Bombs • Have to have enough fissile material around to intercept liberated neutrons • Critical mass for 235U is about 15 kg, for 239Pu it’s about 5 kg • Bomb is dirt-simple: separate two sub- critical masses and just put them next to each other when you want them to explode! – difficulty is in enriching natural uranium to mostly 235U
Mining and Milling • Uranium is usually mined by either surface (open cut) or underground mining techniques, depending on the depth at which the ore body is found. • From these, the mined uranium ore is sent to a mill which is usually located close to the mine.
Mining and Milling • At the mill the ore is crushed and ground to a fine slurry which is leached in sulfuric acid to allow the separation of uranium from the waste rock. • It is then recovered from solution and precipitated as uranium oxide (U308) concentrate. – Sometimes this is known as "yellowcake“
Conversion • Because uranium needs to be in the form of a gas before it can be enriched, the U308 is converted into the gas uranium hexafluoride (UF6) at a conversion plant.
Enriching • Need to enrich uranium to at least 3% U-235 for a power plant • Gaseous Diffusion Method – UF6 (hexafluoride) gas heated – U-238 is heavier than U-235 – Hexafluoride Gas can be separated into two streams • Low velocity U-238 • High Velocity U-235 • Centrifuge Method – Gas spun in centrifuge – Lighter U-235 will separate from heavier U-238
Fuel Conversion • Enriched Uranium transported to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and pressed into small pellets. • These pellets are inserted into thin tubes, usually of a zirconium alloy (zircalloy) or stainless steel, to form fuel rods. • The rods are then sealed and assembled in clusters to form fuel assemblies for use in the core of the nuclear reactor.
Fuel Packaging in the Core • Rods contain uranium enriched to ~3% 235U • Need roughly 100 tons per year for a 1 GW plant • Uranium stays in three years, 1/3 of the fuel rods are cycled annually
The Reactor Core • The reactor core consists of fuel rods and control rods – Fuel rods contain enriched uranium – Control rods are inserted between the fuel rods to absorb neutrons and slow the chain reaction • Control rods are made of cadmium or boron, which absorb neutrons effectively
Moderators • Neutrons produced during fission in the core are moving too fast to cause a chain reaction – Note: This is not an issue with a bomb, where fissile uranium is so tightly packed that fast moving neutrons can still do the job. • A moderator is required to slow down the neutrons • In Nuclear Power Plants water (light or heavy) or graphite acts as the moderator – Graphite increases efficiency but can be unstable (Chernobyl)
Light vs. Heavy Water • 99.99% of water molecules contain normal hydrogen (i.e. with a single proton in the nucleus) • Water can be specially prepared so that the molecules contain deuterium (i.e. hydrogen with a proton and a neutron in the nucleus) • Normal water is called light water while water containing deuterium is called heavy water • Heavy water is a much better moderator but is very expensive to make
Boiling Water Reactor (BWR) • Heat generated in the core is used to generated steam through a heat exchanger • The steam runs a turbine just like a normal power plant
Pressurized Water Reactor (PWR)
Pressurized Water Reactor (PWR) • Water in the core heated top 315°C but is not turned into steam due to high pressure in the primary loop. • Heat exchanger used to transfer heat into secondary loop where water is turned to steam to power turbine. • Steam used to power turbine never comes directly in contact with radioactive materials.
Spent Nuclear Storage • Fuel rods must be replaced every 3 years. • Spent Fuel Rods are temporarily stored in special ponds which are usually located at the reactor site. • The water in the ponds serves the dual purpose of acting as a barrier against radiation and dispersing the heat from the spent fuel.
Spent Fuel Rods Radiation • Spent fuel assemblies taken from the reactor core are highly radioactive • Contain plutonium-239, which is radioactive. • Forms from the reaction of U-238 and neutrons in the core
Fate of Spent Fuel There are two alternatives for spent fuel: 2. Reprocessing to recover the usable portion of it 3. Long-term storage and final disposal without reprocessing.
Uranium Reprocessing • Spent fuel still contains approximately 96% of its original uranium, of which the fissionable U-235 content has been reduced to less than 1%. • About 3% of spent fuel comprises waste products and the remaining 1% is plutonium produced while the fuel was in the reactor • Reprocessing extracts useable fissile U-238 and Plutonium from the spent fuel rods
Uranium Reprocessing • Most of the spent fuel can be reprocessed but isn’t in the U.S. • Federal law prohibits commercial reprocessing because it will produce plutonium (which can be used both as a fuel and in constructing bombs)
Nuclear Waste Disposal • In the U.S., no high-level nuclear waste is ever disposed of--it sits in specially designed pools resembling large swimming pools (water cools the fuel and acts as a radiation shield) or in specially designed dry storage containers. • Spent nuclear fuel must be isolated for thousands of years • After 10,000 years of radioactive decay, according to EPA standards, the spent nuclear fuel will no longer pose a threat to public health and
Yucca Mountain • The United States Department of Energy's long range plan is for this spent fuel to be stored deep in the earth in a geologic repository, at Yucca Mountain, Nevada.
Concerns about Yucca Mountain • Possible health risks to those living near Yucca Mountain and transportation routes • Eventual corrosion of the metal barrels containing the waste • Located in an earthquake region and contains many interconnected faults and fractures – These could move groundwater and any escaping radioactive material through the repository to the aquifer below and then to the outside environment
Reactor Safety • These are the only major accidents (Three Mile Island and Chernobyl) to have occurred in more than 12,700 cumulative reactor-years of commercial operation in 32 countries. • The risks from nuclear power plants, are minimal compared with other commonly accepted risks
Breeder Reactor • In a fission reactor, both uranium-235 and plutonium-239 can be used as nuclear fuel • Most of the uranium in a typical reactor is uranium-238, which cannot be used as nuclear fuel • When a high-energy neutron collides with the nucleus of uranium-238, plutonium-239 is sometimes produced • Could a nuclear reactor create more fuel while it generates energy? Yes, this is a breeder reactor
Breeder Reactor • To convert uranium-238 to plutonium-239, you need fast neutrons • Therefore, water (which acts as a moderator) cannot be used as the coolant • Liquid sodium or high pressure gasses are used instead Uranium-238 only captures fast (high-energy) neutrons
The Core of a Breeder Reactor • A breeder reactor core consists of a normal uranium-235 reactor (moderated by graphite rods) surrounded by a jacket of uranium-238 • The uranium-238 will be converted to plutonium-239 by neutrons escaping from the core Orange: uranium-238 Green: enriched uranium-235 Blue: graphite rods
A Typical Breeder Reactor • Because liquid sodium becomes radioactive, it is separated from the rest of the power plant using a heat exchanger • High pressure gas-cooled reactors are also being researched
Comments on Breeder Reactors • Breeder reactors must be tightly controlled, because plutonium-239 can be used to build atomic weapons • Liquid sodium is extremely reactive (and dangerous) thus high-pressure gasses are preferred as the coolant • Only France and Japan are actively pursuing breeder reactor technology • Breeder reactors are Illegal in the united States due to safety and terrorist concerns.
