Neutron moderators
The moderator is a substance used in nuclear reactors, which slows down the fast neutrons released in the fission. The low atomic mass of the moderator assures the neutron’s major loss of energy during just a few clashes. So the most suitable moderating substance is hydrogen used in the form of water (in boiling- and pressurized water reactors). Its disadvantage is that it absorbs the neutrons easily, becoming deuterium, that is why enriched uranium should be used. Another solution could be the deuterium or heavy water, which, however, is extremely expensive because on the one hand it is rare, on the other it has to be very clean (99,75%). They use heavy water in the Canadian CANDU reactor. The second best substance that can be used is coal, in the form of graphite (in gas-cooled reactors). Other alternatives are the beryllium and some carbon hydrides, but only in experimental reactors.
Fuel elements
Fuel elements are usually uranium-dioxide balls filled into thin pipes. Hundreds of such pipes are placed commonly in vertical bunches, the optimal geometric positioning. They are not exactly next to each other so that in the spaces left between them the coolant can flow easily.
Once placed in the reactor, the fuel elements can remain on their positions for even three years without using up the whole uranium. At the same time, however, the by-products start to accumulate. One part of them are gases, such as krypton, the other is solid, like caesium and plutonium. Before the fuel elements would completely corrode because of the water, they are lifted out and moved to a special reactor, where, during the reprocessing, the uranium and plutonium are separated from waste materials.
Cooling mediums
In a nuclear reactor the heat produced by the fissionable material is taken over by the streaming coolant or gas. The cooling medium passes through a heat exchanger, where its heat energy is transmitted to the water which boils, and the steam produced drives the turbines. 
In boiling- and pressurized water reactors the cooling medium is water. In the latter equipment the pressure is controlled, so that the water cannot boil. In some gas-cooled reactors carbon-dioxide (in Magnox type reactors) is used, while in high temperature reactors helium is the medium.
Reactor types
Boiling Water Reactor-BWR
The boiling water reactor is a light water reactor. There are many similarities between this and pressurized water reactors. The difference is that in this case the steam is not produced with heat exchangers, but inside the reactor core.
The produced steam is transported directly to the turbines. The coolant contains radioactive nuclei so it must be isolated from the outside world. This raises the upkeeps, compared to pressurized water reactors, but the higher efficiency and the simpler construction should counteract the price.
 
There are two ways to control the efficiency of the reactor:
            - by pulling out or pushing in control rods. The control rods are very good neutron absorbers (moderators), so they can easily stop the reaction.
            - by changing the water’s circulation speed, so the water would flow faster through the reactor core and less steam bubbles would be produced. This way more neutrons slow down and the number of fissured neutrons raises. If there are more bubbles in the water, fewer neutrons slow down and therefore the fissured nuclei number is higher.

Advantages:

1. Simpler construction, there is no heat exchanger
2. Higher efficiency
3. It adapts itself easily to the fluctuation of the daily or weekly energy need

Disadvantages:

1. The reactor vessel is bigger so the costs are higher
2. The turbines get radioactive contamination