Nuclear Power
8.4.1 Describe how neutrons produced in a fission reaction may be used to initiate further reactions (chain reaction).
8.4.2 Distinguish between controlled nuclear fission (power production) and uncontrolled nuclear fission (nuclear weapons).
If for every three neutrons released, only one reacted with Uranium, the chain reaction would progress at a constant rate. This would be a controlled or self-sustained chain reaction. If, however, all three neutrons triggered more fissions (as presented by the diagram to the right) and the amount of fissions increased exponentially, the chain reaction would be uncontrolled and energy would be released in explosive amounts. This is what happens in a nuclear weapon. Nuclear reactors contain special materials that both slow down and capture neutrons in order to make sure the reaction moves at a constant rate. When these materials fail, nuclear runaways can occur.
8.4.3 Describe what is meant by fuel enrichment.
In order to gather enough Uranium-235, samples of ordinary Uranium must be enriched. Not all Uranium is fissile (able to sustain a chain reaction), therefore the fissile isotopes of Uranium must occupy a certain percentage of the overall sample in order to drive nuclear fission; a high percentage of non-fissile Uranium would cause the reaction to stop. A sample of Uranium typically contains 99.2% Uranium-238 and 0.72% Uranium-235. Enriched Uranium contains at least 2-5% Uranium-235, although higher percentages are possible. Nuclear weapons require a sample that contains 99% Uranium-235. Uranium is enriched through the formation of hexafluoride followed by the separation of Uranium-238 and Uranium-235 in a gas centrifuge. In more detailed terms, a type of Uranium gas is put into a centrifuge -- a series of cylinders with a rotor inside of them. The mixture of isotopes is spun around at high rates (up to 70,000 rev/min). The heavier Uranium-238 requires a larger centripetal force to stay in the circular path and thus moves to the outside of the centrifuge, while the Uranium-235 requires a smaller force and collects together in the enter. The Uranium-238 (now called "depleted Uranium") can then be scraped off the edges while the remaining Uranium is spun a few more times. This process is repeated until the Uranium has the desired percentage of 3% Uranium-235. It requires very technical and challenging engineering, partly because of the tremendous forces involved.
The critical mass is the minimum amount of Uranium needed to sustain a controlled reaction. As the surface area to volume ratio of an object increases, the higher the probability of a neutron colliding with it, thus a greater percentage will cause fission. Uranium that contains 20% Uranium-235 has a critical mass of 400 kg.
The critical mass is the minimum amount of Uranium needed to sustain a controlled reaction. As the surface area to volume ratio of an object increases, the higher the probability of a neutron colliding with it, thus a greater percentage will cause fission. Uranium that contains 20% Uranium-235 has a critical mass of 400 kg.
8.4.5 Discuss the role of the moderator and the control rods in the production of controlled fission in a thermal fission reactor.
The thermal fission nuclear reactor or Magnox reactor utilizes nuclear fission to create energy.
- The control rods, held by electromagnets, take in neutrons in order to control the reaction and ensure that for every fission, one more fission is triggered at a constant, manageable rate. Control rods can be made of boron or cadmium steel, since these materials absorb neutrons without fissioning. When the reaction goes out of control, the control rods are released from their magnets in order to stop the fission reaction completely.
- Fast neutrons (neutrons at around 1-2meV) will end up bursting through the nucleus, so they need to be slowed down by the moderator. Moderators can come in the form of graphite, water, deuterium monoxide, beryllium and liquid sodium. These materials have small atoms with masses that correspond with the neutrons. The neutrons are slowed down from a speed of 10^6 ms-1 ("fast neutrons") to 10^4 ms-1. At this speed, they are known as thermal neutrons. Thermal neutrons have an energy of around 0.2MeV.
8.4.6 Discuss the role of the heat exchanger in a fission reactor.
The pump is a body of water that consistently flows throughout the system, collecting thermal energy from the reactor core. This water is used to heat up another body of water -- the heat exchanger. The heat exchanger is under high pressure; the heat from the pump causes the water within the heat exchanger to turn to steam. The steam pushes turbines, which generate electricity.
8.4.7 Describe how neutron capture by a nucleus of Uranium-238 results in the production of Plutonium-239.
Non-fissile Uranium can also be converted to a fissile material. Uranium-238, when bombarded with a neutron, turns into Uranium-239 through neutron capture (it absorbs the neutron). It then undergoes beta decay to turn into Neptunium. This element, still radioactive, undergoes another beta decay to turn into Plutonium-239. Plutonium-239 is a fissile element and can be used in the nuclear reactor. Plutonium-239 is attractive due to it being more easily fissile and producing slightly more energy than Uranium. Plutonium is made in a breeder reactor, a reactor that produces more fissile material than is consumed. Breeder reactors were particularly popular in the 1960s, however, as more Uranium reserves were found, Uranium enrichment became less costly.
8.4.8 Describe the importance of Plutonium-239 as a nuclear fuel.
Since Plutonium-239 has a half-life that is significantly higher than that of Uranium and Neptunium, it is easily stored. The short amount of time required to produce Plutonium also makes it an attractive source of fuel. Plutonium-239 can also be used in a nuclear weapon if it is pure enough.
Uranium enriched with 20% Plutonium-239 can be used in a fast breeder reactor. In this reactor, the fast neutrons released from the fission of Plutonium-239 are capable of triggering more fissions, therefore a moderator is not required. This saves spacing and costs. The overall process operates at a very high temperature that requires liquid sodium to cool it down. The liquid sodium can be contained with electromagnetic pumps.
Uranium enriched with 20% Plutonium-239 can be used in a fast breeder reactor. In this reactor, the fast neutrons released from the fission of Plutonium-239 are capable of triggering more fissions, therefore a moderator is not required. This saves spacing and costs. The overall process operates at a very high temperature that requires liquid sodium to cool it down. The liquid sodium can be contained with electromagnetic pumps.
8.4.9 Discuss safety issues and risks associated with the production of nuclear power.
Some problems associated with using nuclear power include:
- A thermal meltdown can occur when a component fails and the reactor core overheats. This usually occurs due to a lack of coolant or a power surge that overrides the coolant’s abilities. It can also be caused by failed control rods. A prime example would be the disaster at Chernobyl.
- Radioactive waste can be produced from nuclear power; it is generally costly to maintain. This waste can be either high-level or low-level, depending on its radiation per mass or volume. High-level waste is converted to a rock-like form and placed underground, while low-level waste is buried (20 ft) in shallow depths in soil. Incidents can occur with material that is not disposed of appropriately, leakages during transport, abandoned waste or stolen waste. Inappropriately handled waste can harm humans and other living organisms
- Mining uranium can cause toxic radon gas emissions, which decay to Radon-222, a carcinogen. Mining Uranium can also contaminate air, water and soil.
- Nuclear weaponry is possible. The Plutonium-239 generated in breeder reactors is very pure and Uranium enrichment is another process that can produce fuel for nuclear weapons.
8.4.10 Outline the problems associated with producing nuclear power using nuclear fusion.
Although nuclear fusion theoretically seems to be the cleanest and most efficient energy source, it is very difficult to manage with current-day science.
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8.4.11 Solve problems on the production of nuclear power.
Solar Power
8.4.12 Distinguish between a photovoltaic cell and a solar heating panel.
Photovoltaic Cell
Photovoltaic cells or solar cells create a direct conversion from light energy to electrical energy. Silicon, gallium and other semi-conducting materials are able to emit electrons when excited by photons. These electrons can only move in one direction, creating a potential difference.
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Solar Heating Panel
A solar panel uses solar heat to heat up water or air. Solar panels are usually connected to heat exchangers; the heated water or air can be provided to homes or used to produce steam and turn turbines. Most solar cells operate at less than 20% efficiency, with future hopes aiming at 40%.
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8.4.13 Outline the reasons for seasonal and regional variations in the solar power incident per unit area of the Earth's surface.
The power of radiation per unit area can be defined as intensity:
The intensity of the sun on the surface of the earth is approximately 1400W/m^2 -- also known as the solar constant. This value can vary by as much as 7%. The main reasons for variation in intensity include:
- Weather and climate will affect the intensity of the sun. Some areas are more cloudy than others.
- Due to the earth's spherical shape, the sunlight is more spread out near the poles because it is hitting the earth at an angle, as opposed to hitting the earth straight-on at the equator. There is also less atmosphere at the equator, allowing more sunlight to reach the earth. Hence, the intensity varies depending on the geographical latitude of the location.
- Due to the earth's rotation, all areas are not consistently exposed to sunlight. Areas that are experiencing 'nighttime' are not receiving a lot of the sun's power, therefore the time of the day or night will affect the solar constant.
- The angle of the surface to the horizontal at that location
8.4.14 Solve problems involving specific applications of photovoltaic cells and solar heating panels.
I=P/A
Hydroelectric Power
8.4.15 Distinguish between different hydroelectric schemes.
When water is able to freely flow downwards, the gravitational potential is converted to kinetic energy, which can be used to drive a turbine. In order for this process to work, it must be cyclical.
There are three different types of hydroelectric power:
There are three different types of hydroelectric power:
- Water storage in lakes
- Tidal water storage
- Pump storage
Water storage in lakes
Water storage in lakes is when water flowing down from mountains or from the sky (as rain) is stored in artificial lakes and run through dams, turning turbines and generating electricity. These lakes are called reservoirs. The processed water usually continues to flow into a river. Some of the water in the river evaporates due to convection currents and forms clouds, which then release either rain or snow, allowing more water to be stored in the reservoir. The largest dam in the world is the three gorges dam on the Yangtze river.
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Tidal water storage
Tidal water storage is when dams take advantage of the gravitational attraction between the earth and the moon. As the earth spins on its axis, this gravitational attraction causes some parts of the ocean to rise and others to fall. Oceans rise and fall approximately twice every day. Water is stored behind a dam through sluices at a high tide and then released at a low tide to generate electricity. The gravitational potential energy of the high tide is used to produce electrical energy by spinning turbines.
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8.4.16 Describe the main energy transformations that take place in hydroelectric schemes.
The main energy conversions are gravitational to rotational kinetic energy and kinetic to electrical energy (through electromagnetic induction). Energy is usually lost due to friction and heat.
8.4.17 Solve problems involving hydroelectric schemes.
The amount of energy available is directly proportional to the flow of the water and height at which the water falls.
Wind Power
8.4.18 Outline the basic features of a wind generator.
Radiation from the sun causes differences in temperature that result in changes in air density. These changes produce convection currents and differences in air pressure, which create wind -- a transfer of energy between areas of high pressure and low pressure. A wind generator converts the kinetic energy of wind to electrical energy.
The wind turbine is composed of a tower, rotating blades, a generator and a storage or grid system. When the blades of the wind turbine move, the axle rotates. This axle is connected to coil of wire in a generator -- this is called the armature. The armature is then rotated in a magnetic field and creates a current.
The wind turbine is composed of a tower, rotating blades, a generator and a storage or grid system. When the blades of the wind turbine move, the axle rotates. This axle is connected to coil of wire in a generator -- this is called the armature. The armature is then rotated in a magnetic field and creates a current.
The blades move in a vertical circle and are designed to be impacted by wind moving parallel to the earth's surface. The number of blades, their width and the angle to the wind are all carefully chosen to maximize the amount of power produced. Winds flowing close to the ground will lose energy due to friction from objects, so generators are usually located in open areas. Placing wind generators in shallow water has many advantages, but these are expensive to construct. The largest wind farms are located in the UK and Denmark
8.4.19 Determine the power that may be delivered by a wind generator, assuming that the wind kinetic energy is completely converted into mechanical kinetic energy, and explain why this is impossible.
Consider:
The mass can be defined as the volume of air hitting the blades at each given second. This is written as the volume of the air multiplied by its density. The volume can be redefined as the area multiplied by the velocity, since the amount of wind passing each second is equal to the length of the wind 'cylinder' hitting the blades:
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Since this is an energy per second value, it can be rewritten as a power value:
A common misconception is that all of the kinetic energy from the wind is transferred to the generator. This is impossible because if all of the wind were converted to electricity, there would be no wind left -- the wind would be completely absorbed by the generator. Other factors that make this impossible include the friction between the moving parts, the collisions of air molecules and degraded heat energy (resistive heating in wires).
8.4.20 Solve problems involving wind power.
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Wave Power
8.4.21 Describe the principle of operation of an oscillating water column (OWC) ocean-wave generator.
Oscillating wave column (OWC) generators can be moored into the ocean floor or built into cliffs. As a wave enters the chamber of the generator, the air inside the chamber is compressed and provides enough kinetic energy to spin a turbine which generates a potential difference. As the water drops, the air decompresses and flows in the opposite direction and an opposite potential difference is generated. The turbine is designed to continue to turn in the same direction regardless of the flow of the air. The best places for OWC generators are the north and south temperate zones, where the westerly winds are strongest in winter. There are OWC stations in England and Japan.
8.4.22 Determine the power per unit length of a wavefront, assuming a rectangular profile for the wave.
8.4.23 Solve problems involving wave power.
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