Unit 5 - Nuclear Power and Other Energy Alternatives Flashcards
(41 cards)
What is the basic difference between the energy-producing reactions involved in burning fossil fuels and those important to nuclear energy?
The burning of fossil fuels involves chemical reactions in which energy is released as electrons encircling the nucleus of an atom are transferred or shared. Nuclear reactions change the bonds that hold an atom’s nucleus together, producing energy nearly a million times greater than that gained through chemical reactions.
What is radioactivity?
Radioactivity is the spontaneous decay of an element into one or more different elements, accompanied by release of energy in the form of waves or particles. Elements located near the lighter and heavier ends of the atomic number scale tend to have higher binding energies per proton (or neutron) within the nucleus. As a result, they release more energy when they decay.
What is the basic principle in the controlled fission of uranium−235 (235U)?
The only naturally occurring atom that is readily fissionable is 235U. When this atom is bombarded with neutrons, it may split into the isotopes of barium (141B) and krypton (92Kr), and may release other neutrons and energy in the process. The neutrons may collide with adjacent 235U atoms, thereby continuing the process and releasing more energy. If there is a sufficient supply of 235U present, a chain reaction will take place. Properly controlled, 235U can provide a continuous supply of energy.
Why must the uranium used in nuclear reactors be enriched? Describe the enrichment process.
Naturally occurring uranium is composed of 99.3 per cent 238U (nonfissionable) and 0.7 per cent 235U (fissionable), which must be separated in an enrichment process. This process is difficult and expensive because there is little chemical and mass difference between the two isotopes. The usual process involves gaseous diffusion, in which the uranium is converted to gaseous uranium hexafluoride (UF6) and passed through a porous membrane that separates the isotopes. Later, the UF6 is converted into a powder, compacted into pellets, and loaded into rods to be used in the reactor.
Describe the design and principle behind the use of fast breeder reactors.
If the uranium fuel is highly enriched, the core of a reactor can be small. In a fast breeder reactor, it is possible to dispense with the moderators and simply use the high velocity neutrons to maintain the chain reaction. If a blanket of nonfissionable 238U is wrapped around the reactor’s core, escaping neutrons from the 235U will interact with it to create plutonium−239 (239Pu), which is fissionable. In other words, fission in the core creates nuclear fuel in the blanket around it. In fact, it creates more fuel than it consumes.
What are the main dangers associated with a nuclear “accident”?
The main dangers associated with nuclear energy production are meltdown and vaporization of a reactor caused by the intense heat of an uncontrolled fission reaction. The major problem would be the release of radioactive gas into the environment. The most dangerous of these gases is radioactive iodine (131I), which has a half life of eight days and is readily absorbed by the human thyroid gland. Other effluents that could be released include radioactive xenon and krypton gases and solid isotopes of strontium (90S) and caesium (137Cs). Although these substances are less dangerous than iodine, they are still a serious threat. Nuclear explosion is not considered a potential danger of nuclear energy production.
Briefly describe the nuclear accident at Three Mile Island.
The problem at Three Mile Island began when the main feed water pump failed and three reserve pumps turned on automatically fifteen seconds later. In that short time, the primary system increased in heat and pressure enough to trigger the automatic reactor shutdown procedure and a pressure release valve. This response was normal; it was a safeguard built into the system. But unknown to the operators, the valves connected to the reserve pumps were closed, so the steam generators soon boiled dry. In addition, the pressure relief valve failed to reset properly, and was leaking. The problem with the relief valve was corrected, but the operators were misled into thinking that there was too much water in the primary system instead of too little, so the core lay uncovered for several hours. This greatly damaged the reactor, but fortunately there was little danger to the public beyond the release of some radioactive xenon (133X) into the environment.
Briefly describe the environmental effects of the Chernobyl nuclear accident.
At Chernobyl, two non-nuclear explosions blew off the roof of the reactor, scattering red-hot pieces of graphite and uranium oxide fuel over the immediate area. In the ten days it took to quench the fire, about ten per cent of the core material was dispersed into the atmosphere, eventually falling out over a large area of the Soviet Union and Europe, affecting crops and livestock. Close to the reactor, one person was killed in the explosion, twenty were severely irradiated (and subsequently died), and thousands were exposed to dangerous levels of radiation, dramatically increasing their risk of death from cancer.
What controls the solubility of uranium compounds in nature? Explain the role of groundwater in concentrating uranium in the Earth’s crust.
The most important ore mineral of uranium is uranitite (UO2), often called pitchblende. It exists in the uranous (U4+) state, which is highly insoluble but readily oxidized to the uranyl (U6+) state, which is soluble. Near-surface groundwater is oxidizing in the natural environment, and it both initiates the conversion and acts as the transporting agent for uranium ions. When this groundwater passes through the rocks or sediments that are reducing in the natural environment, the uranium converts back into the uranous state and is precipitated.
Describe the formation of uranium ores in roll-type deposits found in sandstone.
Groundwater carrying uranium in the uranyl state passes through porous and permeable sandstone until it reaches an area where the reducing conditions exist. These conditions are usually caused by decaying organic matter in an unaltered portion of sandstone. The contact zone is usually in the form of a C-shaped roll-front. Here, the uranium and other metals present are precipitated to form the ore (see Figure 6.15, p. 199 of the textbook).
Describe the techniques of gross count surveys, gamma-ray spectrometry, and radon measurements in searching for uranium deposits.
The simplest technique, the gross count survey, consists of scanning the ground for a higher-than-average radiation level, using low sensitivity Geiger counters or scintillation counters. A gross count survey only determines the level of radiation—not its nature. Gamma ray spectrometers are used in much the same way as Geiger or scintillation counters are used in gross count surveys. The main difference is that a gamma ray spectrometer can distinguish between the radiation intensities of different radioactive isotopes, providing information about the nature of the uranium source and the approximate amount of material present. The third uranium search technique measures the amount of radon gas, which is always given off during the decay of uranium. Radon gas measurements can be taken from the air over an area or from water from lakes or streams near the source.
What argument do American geologists use to support the claim that there will always be a plentiful supply of uranium for nuclear fission reactors?
The authors argue that the distribution of uranium in the Earth’s crust follows a log-normal abundance curve in which there is 300-fold increase in the amount of recoverable uranium for each 10-fold decrease in ore grade. This relationship means that as the demand for uranium increases and the price consequently rises, lower grade ores will become profitable to mine, and more of them will be available.
What are the main advantages of nuclear power generation compared to other forms of power generation?
While nuclear power as a method of generating electricity remains unpopular with many, it has several advantages over other methods of power generation. The main ones are noise-free working conditions at nuclear plants and the absence of toxic emissions at nuclear plants, such as acid mine drainage, acid rain, or greenhouse gases such as CO2 (unless there is an accident).
Describe, without using chemical formulae, the result of fusion between deuterium atoms.
Given the proper conditions, the fusion of two deuterium atoms can result in two possible products:
A. an atom of helium plus a neutron plus 3.2 million electron volts of energy.
B. an atom of tritium plus an atom of hydrogen plus 4 million electron volts of energy.
There is an equal chance of either of these two reactions occurring, and with enough deuterium atoms present, both will occur. A further reaction will occur between the tritium atom and another deuterium atom, creating an atom of helium plus a neutron plus 17.6 million electron volts of energy. This means that theoretically the combination of these reactions (initially involving as few as five deuterium atoms) will ultimately produce 24.8 million electron volts of energy, or 4.96 million each.
Describe the basic principles behind the magnetic confinement and inertial confinement systems proposed for containing heat in the fusion process.
Magnetic confinement involves the use of a very powerful magnetic field in a doughnut shape, to contain the plasma at temperatures that will allow a continuous fusion reaction to occur, that is, temperatures greater than 100 million degrees Celsius.
The inertial confinement method would use high energy beams fired into a central core of plasma from all directions to create shock wave compression and heat, and to confine the plasma at the same time.
Describe the advantages and disadvantages of each of the proposed fusion reactors: deuterium-deuterium, deuterium-tritium, and hydrogen-hydrogen.
Deuterium-deuterium fusion would have the great advantage of using seawater as a very cheap and plentiful fuel. The energy produced from one cubic kilometre of seawater would equal the energy from all remaining oil reserves in the world. Deuterium-tritium fusion would require much less stringent conditions, but the source resource, lithium, is quite rare, and its resources would be depleted in a few hundred years. Hydrogen-hydrogen fusion would be the best of all since hydrogen is the most widely available of the three resources, but the plasma would have to be heated to a much greater temperature for fusion to take place.
What would be the main advantage of nuclear fusion over nuclear fission?
Fusion would be a more economical and less hazardous method of creating energy than is fission. With the exception of the deuterium-tritium reaction, fusion fuel is virtually unlimited and very inexpensive. There is no problem of nuclear waste, as none is created in fusion. Only the reactor itself could be considered wasteful.
Differentiate between low-quality and high-quality solar energy.
Low-quality solar energy is basic sunlight as it arrives at the Earth’s surface. It can be used to produce low-temperature forms of energy. High-quality solar energy results from the concentration of normal sunlight from a number of collectors, or from a physical or chemical process that uses sunlight to produce electricity or a fuel such as hydrogen.
Describe the design and principle behind low-quality solar energy heating systems that use a solar collector system, a heat transfer fluid system, and a heat storage system.
The solar collector is usually a low albedo black panel with a glass panel over it to prevent heat loss by conduction and convection. The trapped air may be used as the heat transfer fluid or water pipes may be located in the collector panel. The fluid transfers the heat to a storage tank where it can be used in space-heating radiators or as a supply of hot, running water. Cold water is taken into the system, as required, to replace water loses.
Considering that these systems are inexpensive to operate and pollution free, why are relatively few in operation?
The initial cost of converting to a solar energy system is quite high, and it takes a long time to recoup costs. As long as conventional energy sources are relatively inexpensive, there is little incentive to make the change.
Describe the design and function of central tower solar collectors and parabolic reflectors for collecting high quality solar energy.
In central tower solar collectors, a number of heliostats, or reflecting mirrors, are grouped around a central collecting tower. Each heliostat is programmed to reflect the sun’s rays directly into the receiver and to track the sun’s path. The energy is then transferred to a storage tank from which it can be used. A parabolic reflector consists of a cylindrical reflector designed to focus solar radiation onto a collector located at the center of the reflector. This collector contains a heat transporting medium that can circulate the energy for use. Parabolic reflectors are suitable for smaller power plants.
Discuss the design and use of a photovoltaic cell.
In a photovoltaic cell, the solar radiation interacts directly with electrons in a semi-conductor to produce an electric current. As photovoltaic cells are both expensive to produce and relatively inefficient, they are not being used for large-scale power generation.
Why is the production of electricity from hydroelectric power more efficient than from fuel-powered generating stations?
In hydroelectric power, running water drives the turbines, which generate electricity. This production of electricity eliminates the step of creating mechanical energy to drive the turbines, which is necessary in fuel-powered generating stations.
Describe the pumped-water storage system.
At some hydroelectric power stations, the water that flows from the reservoir past the turbines to create electricity during peak demand hours is stored and pumped back to the reservoir at night, using excess energy available at that time.
