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 more so 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 directly in the generation of electrical energy, it acts to hold together all major components of the power plant.