Nuclear power stations work in fossil fuel-burning stations, except that a "chain reaction" inside a nuclear reactor makes the heat instead.The reactor uses Uranium rods as fuel, and the heat is generated by nuclear fission: neutrons smash into the nucleus of the uranium atoms, which split roughly in half and release energy in the form of heat.Carbon dioxide gas or water is pumped through the reactor to take the heat away, this then heats water to make steam.The steam drives which drive generators.
Modern nuclear power stations use the same type of turbines and generators as conventional power stations. In Britain, nuclear power stations are often built on the coast, and use sea water for cooling the steam ready to be pumped round again. This means that they don't have the huge "cooling towers" seen at other power stations. The reactor is controlled with "control rods", made of boron, which absorb neutrons. When the rods are lowered into the reactor, they absorb more neutrons and the fission process slows down. To generate more power, the rods are raised and more neutrons can crash into uranium atoms.
Natural uranium is only 0.7% "uranium-235", which is the type of uranium that undergoes fission in this type of reactor. The rest is U-238, which just sits there getting in the way. Modern reactors use "enriched" uranium fuel, which has a higher proportion of U-235. The fuel arrives encased in metal tubes, which are lowered into the reactor whilst it's running, using a special crane sealed onto the top of the reactor. With an AGR or Magnox station, carbon dioxide gas is blown through the reactor to carry the heat away. Carbon dioxide is chosen because it is a very good coolant, able to carry a great deal of heat energy. It also helps to reduce any fire risk in the reactor (it's around 600 degrees Celsius in there) and it doesn't turn into anything nasty (well, nothing long-lived and nasty) when it's bombarded with neutrons You have to be very careful about the materials you use to build reactors - some materials will turn into horrible things in that environment.
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If a piece of metal in the reactor pressure vessel turns brittle and snaps, you're probably in trouble - once the reactor has been built and started you can't go in there to fix anything.. Uranium itself isn't particularly radioactive, so when the fuel rods arrive at the power station they can be handled using thin plastic gloves. A rod can last for several years before it needs replacing. It's when the "spent" fuel rods are taken out of the reactor that you need the full remote-control robot arms and Homer Simpson equipment Nuclear power stations are not atomic bombs waiting to go off, and are not prone to "meltdowns". There is a lot of U-238 in there slowing things down - you need a high concentration of U-235 to make a bomb. If the reactor gets too hot, the control rods are lowered in and it cools down. If that doesn't work, there are sets of emergency control rods that automatically drop in and shut the reactor down completely
With reactors in the UK, the computers will shut the reactor down automatically if things get out of hand (unless engineers intervene within a set time). At Chernobyl, in Ukraine, they did not have such a sophisticated system, indeed they over-rode the automatic systems they did have. When they got it wrong, the reactor overheated, melted and the excessive pressure blew out the containment system before they could stop it. Then, with the coolant gone, there was a serious fire. Many people lost their lives trying to sort out the mess. A quick web search will tell you more about this, including companies who operate tours of the site.
If something does go wrong in a really big way, much of the world could be affected - some radioactive dust (called "fallout") from the Chernobyl accident landed in the UK. That's travelled a long way. With AGR reactors (the most common type in Britain) there are additional safety systems, such as flooding the reactor with nitrogen and/or water to absorb all the neutrons - although the water option means that reactor can never be restarted. So should I worry? I think the answer is "so long as things are being done properly, I don't need to worry too much. The bit that does worry me is the small amount of high-level nuclear waste from power stations. Although there's not much of it, it's very, very dangerous and we have no way to deal with it apart from bury it and wait for a few thousand years.
There are many different opinions about nuclear power, and it strikes me that most of the people who protest about it don't have any idea what they're talking about. But please make up your own mind, find out as much as you can, and if someone tries to get you to believe their opinion ask yourself "what's in it for them?" Nuclear power costs about the same as coal, so it's not expensive to make. Does not produce smoke or carbon dioxide, so it does not contribute to the greenhouse effect. Produces huge amounts of energy from small amounts of fuel. Produces small amounts of waste.
Nuclear power is reliable
Although not much waste is produced, it is very, very dangerous. It must be sealed up and buried for many thousands of years to allow the radioactivity to die away. For all that time it must be kept safe from earthquakes, flooding, terrorists and everything else. This is difficult. Nuclear power is reliable, but a lot of money has to be spent on safety - if it does go wrong, a nuclear accident can be a major disaster. People are increasingly concerned about this - in the 1990's nuclear power was the fastest-growing source of power in much of the world. In 2005 it was the second slowest-growing.
Is it renewable?
Nuclear energy from Uranium is not renewable.
Once we've dug up all the Earth's uranium and used it,
there isn't any more.
We've used the Sun for drying clothes and food for thousands of years, but only recently have we been able to use it for generating power. The Sun is 150 million kilometres away, and amazingly powerful. Just the tiny fraction of the Sun's energy that hits the Earth (around a hundredth of a millionth of a percent) is enough to meet all our power needs many times over. In fact, every minute, enough energy arrives at the Earth to meet our demands for a whole year - if only we could harness it properly. Currently in the UK there are grants available to help you install solar power in your home.
How it works:
There are three main ways that we use the Sun's energy:
(really called "photovoltaic", "PV" or "photoelectric" cells)that convert light directly into electricity. In a sunny climate, you can get enough power to run a 100W light bulb from just one square metre of solar panel. This was originally developed in order to provide electricity for satellites, but these days many of us own calculators powered by solar cells. People are increasingly installing PV panels on their roofs. This costs thousands of pounds, but if you have a south-facing roof it can help with your electricity bills quite a bit, and the government pays you for any extra energy you produce and feed back into the National Grid (called the "feed-in tariff").
But what do solar panels cost?
How much might they generate for you?
What's the "payback time" until the money you've saved on bills is more than the cost of installation? Find out with the "solar calculator" a
Solar water heating, where heat from the Sun is used to heat water in glass panels on your roof. This means you don't need to use so much gas or electricity to heat your water at home. Water is pumped through pipes in the panel. The pipes are painted black, so they get hotter when the Sun shines on them. The water is pumped in at the bottom so that convection helps the flow of hot water out of the top.
This helps out your central heating system, and cuts your fuel bills. However, with the basic type of panel shown in the diagram you must drain the water out to stop the panels freezing in the winter. Some manufacturers have systems that do this automatically. Solar water heating is easily worthwhile in places like California and Australia, where you get lots of sunshine. Mind you, as technology improves it's becoming worthwhile in the UK. Here's a more advanced type of solar water heating panel. The suppliers claim that in the UK it can supply 90% of a typical home's hot water needs from April to November.
This "Thermomax" panel is made of a set of glass tubes. Each contains a metal plate with a blue-ish coating to help it absorb solar energy from IR to UV, so that even in diffuse sunlight you get a decent output. The air has been removed from the glass tubes to reduce heat loss, rather like a thermos flask. Up the back of the metal plate is a "heat pipe", which looks like a copper rod but contains a liquid that transfers heat very quickly to the top of the glass tube. A water pipe runs across the top of the whole thing and picks up the heat from the tubes. Advantages
Solar energy is free - it needs no fuel and produces no waste or pollution. In sunny countries, solar power can be used where there is no easy way to get electricity to a remote place. Handy for low-power uses such as solar powered garden lights and battery chargers, or for helping your home energy bills.
Doesn't work at night.
Very expensive to build solar power stations, although the cost is coming down as technology improves. In the meantime, solar cells cost a great deal compared to the amount of electricity they'll produce in their lifetime. Can be unreliable unless you're in a very sunny climate. In the United Kingdom, solar power isn't much use for high-power applications, as you need a large area of solar panels to get a decent amount of power. However, technology has now reached the point where it can make a big difference to your home fuel bills..
Is it renewable?
Solar power is renewable. The Sun will keep on shining anyway, so it makes sense to use it
As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and Earth, and the Moon fully or partially blocks ("occults") the Sun. This can happen only at new moon, when the Sun and the Moon are in conjunction as seen from Earth in an alignment referred to as syzygy. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses only part of the Sun is obscured. If the Moon were in a perfectly circular orbit, a little closer to the Earth, and in the same orbital plane, there would be total solar eclipses every single month. However, the Moon's orbit is inclined (tilted) at more than 5 degrees to Earth's orbit around the Sun (see ecliptic) so its shadow at new moon usually misses Earth. Earth's orbit is called the ecliptic plane as the Moon's orbit must cross this plane in order for an eclipse (both solar as well aslunar) to occur. In addition, the Moon's actual orbit is elliptical, often taking it far enough away from Earth that its apparent size is not large enough to block the Sun totally.
The orbital planes cross each year at a line of nodes resulting in at least two, and up to five, solar eclipses occurring each year; no more than two of which can be total eclipses. However, total solar eclipses are rare at any particular location because totality exists only along a narrow path on Earth's surface traced by the Moon's shadow or umbra. An eclipse is a natural phenomenon. Nevertheless, in some ancient and modern cultures, solar eclipses have been attributed to supernatural causes or regarded as bad omens. A total solar eclipse can be frightening to people who are unaware of its astronomical explanation, as the Sun seems to disappear during the day and the sky darkens in a matter of minutes.
Since looking directly at the Sun can lead to permanent eye damage or blindness, special eye protection or indirect viewing techniques are used when viewing a solar eclipse. It is technically safe to view only the total phase of a total solar eclipse with the unaided eye and without protection, however this is a dangerous practice as most people are not trained to recognize the phases of an eclipse which can p over two hours while the total phase can only last up to 7.5 minutes for any one location. People referred to as eclipse chasers or umbraphiles will travel to remote locations to observe or witness predicted central solar eclipses.
A total eclipse occurs when the dark silhouette of the Moon completely obscures the intensely bright light of the Sun, allowing the much fainter solar corona to be visible. During any one eclipse, totality occurs at best only in a narrow track on the surface of Earth. An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a very bright ring, or annulus, surrounding the dark disk of the Moon. A hybrid eclipse (also called annular/total eclipse) shifts between a total and annular eclipse. At certain points on the surface of Earth it appears as a total eclipse, whereas at other points it appears as annular. Hybrid eclipses are comparatively rare.
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