Nuclear Power William Goodnow Sean Burger Brodin Jentz Dan Milavitz
Technology developed in the 1940s during World War II Used worldwide since the 1950s Commercial plants use controlled, non-explosive fission reactions to generate heat. The heat is used to create steam and turn turbines which generate electricity. Nuclear Power
Nuclear fission is the reaction used in nuclear power plants to generate heat. Fission is the splitting of the nucleus of an atom into smaller parts. Fission of heavy elements is exothermic. Produces free neutrons, radiation, large amounts of kinetic energy in the fragments (producing heat). Nuclear Fission
Chain Reaction The three main isotopes used in nuclear reactors as nuclear fuel: Uranium-233 Uranium-235 Plutonium-239 These isotopes are used because it is possible to create a chain reaction with them. Chain reaction: a self-sustaining process in which an atom is split by a free neutron and then produces additional free neutrons as it breaks apart. Nuclear reactors utilize controlled, self-sustaining chain reactions that produce heat at a constant rate.
Chain Reaction
Controlled Versus Uncontrolled Nuclear Fission Controlled Uncontrolled Used for power Regulated by control rods No explosions Chain reactions limited Both Utilize the splitting of atoms Can use the same fuel Used for bombs No control rods Explosion or meltdown Chain reaction not limited
Fuel Enrichment Uranium ore consists of approximately 0.7% uranium- 235 (isotope used in nuclear power plants). Uranium-238 makes up 99.3% of ore Plants refine the uranium ore into nuclear fuel. This process is known as fuel enrichment. Three methods of enrichment: Gaseous diffusion Gas centrifuge enrichment Laser separation Gaseous diffusion is currently the only method being used to enrich uranium in the United States.
Uranium Hexafluoride Both gas diffusion enrichment processes and gas centrifuges use uranium hexafluoride (UF 6 ) gas to enrich uranium. Uranium hexafluoride is used because it contains only one uranium atom and fluorine has only one stable isotope. Uranium Hexafluoride is made through the following process: Uranium is mined as U 3 O 8 and dissolved in nitric acid. Pure uranyl nitrate (UO 2 (NO 3 ) 2 ) is extracted from the resulting solution and reacted with ammonia. The resulting ammonium diuranate ((NH 4 ) 2 U 2 O 7 ) is reduced with hydrogen to make uranium (IV) oxide (UO 2 ). The uranium (IV) oxide is reacted with hydrofluoric acid to make uranium tetrafluoride (UF 4 ), which is oxidized with fluorine to make uranium hexafluoride.
Gaseous Diffusion Uranium hexafluoride gas is pumped through filters that allow 234 U and 235 U to diffuse through faster than 238 U. 238 U atoms are larger than 234 U and 235 U. Gas passes through hundreds of filters until uranium hexafluoride molecules containing 235 U make up 5% of the gas. The enriched gas is removed from the pipes and allowed to condense and solidify.
Gas Centrifuge Enrichment Uranium hexafluoride gas is pumped into a centrifuge. The heavier 238 U molecules drift to the outside of the centrifuge. Slightly enriched gas is transferred to another centrifuge. The depleted gas remains in the first centrifuge. This method of enrichment is much more efficient than gas diffusion. Less uranium is needed, less radioactive waste produced, same amount of enriched uranium is produced.
Enrichment Processes Gaseous Diffusion Gas Centrifuge
Production of Plutonium-239 Plutonium-239 is the primary nuclear fuel used in nuclear weapons. Must be manufactured as the half life is only 24,200 years. Few natural occurrences. Production of plutonium-239: Atom of uranium-238 exposed to neutron radiation. Nucleus captures neutron, becomes uranium-239. Undergoes two beta decays: Emits one electron and one anti-neutron during first decay to become neptunium-239, then emits another electron and antineutron to become plutonium-239
Plutonium-239
Plutomium-239 as Nuclear Fuel Plutonium-239 is one of the three primary fissile materials used as nuclear fuel. Only fuel capable of being used in nuclear weapons. Manufacturing produces a variety of grades. Grade of plutonium-239 based on the percentage of plutonium- 240 contained within the sample. Uranium-238 tends to take on two neutrons occasionally, resulting in some particles degrading into plutonium-240. Grades of plutinium-239: Reactor grade (18%+), fuel grade (7-18%), weapons grade (less than 7%), supergrade (2-3%).
Nuclear Process to Generate Electricity is generated in the nuclear process through the production of heat. A neutron strikes a uranium atom causing it to split. The fragments produced have less mass than the original atom. The mass is then converted into energy. Given by the equation E=MC^2 Pressurized water reactor shown at right. Electricity
Nuclear Reactors The energy produced is in the form of heat. The heat is then used to boil water and create steam. The steam pressure from the expansion is then used to turn a turbine. Pressurized Water Reactor: Highly pressurized water is pumped to the reactor core. Pressure prevents it from boiling within the reactor. Water is then pumped into a steam generator where the heat is transferred to a secondary system, where steam is generated. Boiling Water Reactor: Water is pumped directly into reactor chamber. Water is converted to steam within the same fluid-flow system. More complex design.
Nuclear Reactor Types
What is a Heat Exchanger? A heat exchanger, or "Steam Engine," transfers the heat from the liquid that cycles through the core, to the water/steam that powers the turbine.
Why is Heat Exchange Important? Keeps process efficient: transfers heat from high pressure system to low pressure system. Keeps radioactive water within containment dome. Prevents damage to turbine.
What is Cooling and Why is it Nuclear reactors need a continuous supply of highly purified water, which is most easily obtained by recycling the water within the system. Coolant is used to turn the steam back to water via condensation. Coolant is pumped through pipes that come in contact with the steam, which cools it, returning it to a liquid state. Important?
Why is Cooling Important? Reactor continuously produces heat. The core will melt down" if left unattended. Cooling removes heat from the system. Regulates temperature. Failure of the cooling system may result in a meltdown. Failure of cooling systems is currently affecting Japanese plants after the destruction caused by the earthquake and tsunami. Power plants are heat engines. They are more efficient when the temperature difference is greater.
Coolant Options The universally used coolant is water. Plants that are located by lakes, rivers, or the ocean pump water from that source to cool the reactor core. Nuclear plants that are not located near bodies of water utilize the iconic cooling towers that are now synonymous with nuclear power.
Moderators and Control Rods Moderators: Usually consist of heavy water or graphite. Located within the core of a nuclear reactor. Purpose is to slow the speed of neutrons to a point at which they can be absorbed by the fuel and cause further nuclear fission reactions. Nuclear reactors contain a slightly supercritical amount of fuel. This means that if the reactor were to be left unchecked, it would continually increase in temperature and melt down. Control Rods: Made of neutron absorbing materials such as boron, cadmium, or hafnium. Safety control mechanism that can be inserted or removed from the core. Used to slow down or speed up reactions.
Moderators and Control Rods Reactor Core
Heat/energy is lost in several locations: Most of the energy consumed in a nuclear reactor is used to heat water. Some is used to sustain reaction. Energy is lost in heat exchanger as it passes from one medium to another. Steam is condensed into water to dissipate excess heat. Energy Loss
Simple Sankey Diagram A: energy output of the core B: Energy used to start further reaction C: Energy lost in heating reaction D: Energy lost in heating environment E: Energy lost in friction of turbine F: Energy output in electricity
Safety Issues and Risks
Risk #1: The disasters of Chernobyl and Three Mile Island have given rise to widespread fear of nuclear meltdowns. This fear is now not as relevant considering the improvements that have been made in safety mechanisms and emergency cooling systems. Risk of a meltdown is almost nonexistent under normal conditions.
Risk #2: Terrorism/Sabotage Dirty bombs and suitcase nuclear weapons are unlikely. Sabotage of nuclear facilities is a risk that the U.S. is now taking seriously. Damaging or destroying a reactor is unrealistic. Waste is contained in large and practically impenetrable containers stored and protected in secure facilities. Several concrete and steel mechanized doors, massive wet storage pool. To destroy dry casks, something with the power of a tomahawk missile or a nuclear bomb would be needed. U.S. military runs drills to keep security tight.
Risk #3: Waste Storage The biggest risk associated with nuclear power is disposal of waste produced by the nuclear process. Waste is an unavoidable part of the nuclear process. Will last for ten thousand years. Current storage methods only temporary.
Risk #3: Waste Storage There are two methods of storage currently used in the U.S.: Pool storage (preferred method) Dry cask storage Both of these types of storage are considered temporary. Not as safe as many people, scientists included, would like.
Risk #3: Waste Storage The risk of radiation exposure is miniscule. Fear is based on the destructive power of the atomic bombs and of the harmful effects of radiation. Impossible for a nuclear plant to produce nuclear explosion. Nuclear power has an impressive safety record. The only deaths in the history of nuclear power resulted from Chernobyl (38 deaths). This was a result of poor Soviet-era engineering and human error. By contrast, the National Resources Defense Counsel estimates that 64,000 people are dying prematurely each year from particles released by coal and gas/oil power plants.
Why Is Fusion Difficult? The energy required to overcome the magnetic force of hydrogen atoms is 150 million Kelvin. 150million Kelvin = highly energized particles. Highly energized particles are difficult to contain, which has been the biggest obstacle to overcome. Stars have fusion reactions; they use gravity to contain the reaction. Obviously, this is impossible on Earth. Creating inertia by utilizing a laser will work but only for a fraction of a second. Current systems use intense magnetic fields to control, and further heat, the plasma.
Nuclear Fusion
Problems
Problem To produce a given amount of electrical energy, is the amount of coal burned in a coal-burning power plant greater than, less than, or equal to the amount of 235 U consumed in a nuclear power plant? Explain.
Solution The amount of coal burned is much greater than 235 U consumed. This is because coal burning power plants use chemical reactions to generate heat, whereas nuclear power plants use nuclear reactions. Nuclear reactions produce millions of times more energy than chemical reactions.
Problem Assume a release of 173 MeV per fission reaction. Determine the minimum mass of 235 U needed to supply the annual energy needs of the United States (The amount of energy consumed in the United States each year is 8.4 x 10 19 joules ).
Solution 1.2 Gigagrams (1.2x10 6 kilograms) of 235 U Work: 173 MeV = 1.602176x10-13 J/MeV x 173 MeV = 2.77176258x10-11 J 8.4x10 19 J / (2.77176258x10-11 J) = number of 235 U atoms = 3.03055964x10 30 atoms 235 amu x 3.03055964x10 30 atoms = 7.12181515x10 32 amu 7.12181515x10 32 amu x 1.66053886x10-21 kg/amu = 1.18260508x10 6 kg 1.2 Gg
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