A container, usually made from stainless steel, designed to hold liquids or gases at a pressure higher or lower than the atmospheric pressure. In nuclear power plants, the reactor is usually a pressure vessel because the coolant operates at several times atmospheric pressure. For example, the typical pressure in the primary circuit of a PWR plant is 15 MPa, which is about 150 times atmospheric pressure. In thermonuclear fusion research, a pressure vessel is also used in which fusion plasma is generated. In this case, however, the pressure in the pressure vessel is very low, approaching the cosmic vacuum.