A nuclear reactor consists of various parts which carry out different functions related to heat generation by “burning” of nuclear fuel, but a housing is needed to contain all these parts and act as a covering for all these paraphernalia
Introduction
Just imagine if your beautiful body did not have the cover of the skin, and when you met any individual you could simply see through their various organs and into their "dirty" workings. This would certainly be not a very pleasing sight and would take out the very charm of human personality. This is not much different in the case of nuclear reactors as well. I cannot imagine going to a nuclear power plant just to find that the reactor core, fuel rods, control rods etc are all lying bare bones without any proper cover of enclosure. Hence the outside component of nuclear power plant is very important and is known as the reactor vessel.
Reactor Vessel
Vessels are often used to cook food, and though a nuclear reactor may not be cooking food directly for you, it certainly provides a source of an equally valuable food for the society: electrical energy. But apart from the cooking business there are a lot of functions which a nuclear reactor vessel has to perform and some of these are as follows.
It acts to enclose the various parts inside the reactor including the core, shield, reflector etc.
The coolant needs a passage to flow through the reactor so that it can be used to transfer the heat to the working fluid or the turbine directly, as the case may be, and this passage is provided by the reactor vessel.
To withstand the high pressure with exists inside the reactor and could be of the order of 200 kgf/cm2, to provide a safe working environment for all concerned.
Control of the nuclear reaction is absolutely necessary and this is done with the help of control rods. The reactor vessel provides a place to insert these control rods in the nuclear reactor and move them in or out of the reactor core depending on the requirements of power.
The Pressure Vessel
Although the reactor vessel has been compared to a cookery vessel in the common usage of the term, technically speaking it is more of a pressure vessel. There are legal implications associated with defining a pressure vessel and these vary with the country in which it is being used or manufactured. Different countries have different authorities which govern rules and regulations regarding pressure vessels and in the US this is done by the American Society of Mechanical Engineers Boiler and Pressure Vessel Code. The material used for the construction of a nuclear vessel is usually steel which would be expected as the material has to be very strong and resilient.
Pressure vessels of all kinds are subject to various tests to check for their strength against laid down standards which is very important to ensure safety of these vessels. This is moreso important in the case of nuclear reactor vessels which house source of intense raditaions and heat energy.
Hence we see that though a nuclear reactor vessel may not be performing any useful function direcly in the generation of electrical energy, it acts to hold together all major components of the power plant.
source:http://www.brighthub.com/