You can't split an atom in an open fireplace or in a car engine. Nuclear power demands large and techically advanced equipment, and must be kept under strict control. Furthermore, the reactor must be surrounded with lots of protective materials that stops dangerous biproducts from escaping into the atmosphere. The main goal of a nuclear power plant is to provide the energy from nuclear fission as safe as possible.
The basic principle of nuclear power is to split an atom by hitting it with a neutron. When an Uranium atom is hit in this way, it splits into two halves. These two halves weigh less than the original atom. The difference in weight has been converted into pure energy. Albert Einstein showed in his equation E=mc2, that a small amount of matter transformed in such way gives huge amounts of energy. Also, when split, the atom releases many new neutrons which can hit new uranium atoms and so on. This is called a chain reaction. In a nuclear bomb this would be fine, but in a power plant we want to control the reaction and make sure that only a few of the new neutrons can hit another atom.
Uranium, the main fuel in the world's nuclear power plants, is a greyish metal. It was purified from uranium ore for the first time in 1841. Sweden has 80% of the uranium assets in western europe. The Uranium is purified by solving the ore in a set of different acids, and from the solution one can collect the uranium. The uranium assets in the world is calculated to be around 2 million tonnes. The annual need in all nuclear powerplants is about 30000 tonnes. As you perhaps know, the uranium found in nature consists of several isotopes. Only the isotope U-235 is used in nuclear powerplants. The various isotopes behave very differently during fission. That is why only U-235 is used. Natural uranium contains 0.7% U-235, the rest is mostly U-238. To get a higher percentage of U-235, the raw uranium goes through a process that is called enrichment. The aim is to raise the amount of U-235 to 3%. Today reactors use Uraniumdioxide as fuel, in the form of small pellets pressed into narrow pipes, fuel rods. They can be up to 3.7 m long. into so called fuel rods. The fuel rods are put together to fuel packs with up to 200 rods.
The reactor must work safely and smoothly. This can be achieved by controlling the flow of neutrons in the core. In most reactors you deliberately slow them down with a material called a "moderator". Slow neutrons split atoms better than fast ones, and keeps the process going in a less jerky manner. A moderator consists of substances with light atoms, like coal or water. If you want more accurate control, special control rodcs can be inserted into the core. They absorb neutrons and slows them down. If the control rods are pulled up again the fission process accelerates. The heat you get from the fission is lead away by a coolant, ordinarily gas or water. It passes the fuel pack and pick up the excess heat. It will now pass through a boiler or heat exchanger where it will deliver its heat to water which in turn will power the steam turbines.
All fuel needed for about a year is inserted into the reactor alla at once when it is started. At the end of the year there is still a lot of fissionable material left, but not enough to keep the process running at a reasonable speed. Before the process stops completely, the used fuel pack is removed a new one is put in. In one year, only 1.5 tonnes of uranium-235 is used for a 1000 MW reactor. 1000 MW is roughly the energy demanded by a fairly large city.
Fission products are collected inside the fuel rods and gets in the way of neutrons and dampens the process. The purpose of the purifying process is to recover the unused U-235, as it can be put in new fuel rods. But there is another just as important reason. Some of the U-238 has been transformed to plutonium-239 that is fissionable too. Therefore it is important to get the plutonium as well. The actual recovery process is a chemical process that is remote(styrd) behind thick concrete walls. The fuel rods are dissolved in sulphuric acid and are divided into three streams. One with uranium, one with plutonium and one with higly radiactive and toxic waste.