A nuclear reaction is a source of intense radiation apart from the heat generated in the exothermic process. Because of the risk, radiation shielding is required to prevent this harmful radiation from leaving the reactor and affecting the outside men and materials.
As you know when a nucleus gets split into two parts during the fission process it results in the production of large amounts of heat energy since the reaction is exothermic in nature. But this is not the only product of nuclear fuel "combustion" but there are several other by-products such as alpha rays, beta rays, gamma rays and of course the fast moving neutrons. The fast moving neutrons are controlled, moderation and reflected in order to contain them within the reactor core so that a sustained and controlled chain reaction takes place but what do you think happens to the other by-products? Just read on to find out why a shielding is one of the important components of nuclear power plant.
Yes, you guessed it right. These by-products in the form of different kinds of radiation would simply leak out into the atmosphere in the absence of proper arrangements to prevent this. Radiation leakage would be very harmful for the personnel working in the nuclear plant as well as the nearby flora and fauna.
This makes clear the case for having a proper shield so that these radiations get absorbed within the reactor without having a chance to escape into open air. This is done by using materials which are good absorbents of the same. Concrete and steel are very good at absorbing radiation and they are equally strong as well, hence used in forming the shielding material.
The question now arises that how much thickness of these materials should be used to prevent radiation from leaking out into the atmosphere? If you just compare it with the amount of thickness of typical steel plate required for preventing a powerful bullet from going across it you are in for a surprise. Although I am not a weapons expert but I know for sure that for stopping ordinary bullets a few mm of steel plate should be sufficient and a few cm of plate should be sufficient to stop even the most powerful of guns.
You might wonder that if such a thickness is required for a bullet which is quite bulky and dangerous, then only a couple of mm should be sufficient for humble intangible rays and neutrons but if you think so you are utterly wrong. A typical reactor core would require an inner lining which is of the order of nearly half a meter thickness of steel (don't gasp for breath).
The icing on the cake is that even this much thick steel is not considered entire safe. It is further reinforced by using a few meters of concrete to make it safer. This should give you an idea about how powerful these radiations are and their penetrating capability.
It is also interesting to note that the amount of radiation to which human beings could be exposed safely without causing any harm to the body is expressed in units of rad and rem which give the amount of absorbed radiation from different perspectives.