The safe working of any gas-cooled reactor system is dependent on the virtually perfect containment of both fuel and coolant. In order to satisfy these containment requirements, much attention has been given to factors determining the safe working of welded thick-walled steel pressure vessels. In addition to the aspects common to other pressure circuits (stress, temperature, and variations of these) the reactor pressure vessel designer must also consider the effects of neutron bombardment. As a result of neutron irradiation, direct inspection of the reactor pressure vessel during service is not possible and maintenance cannot at present be contemplated. In view of this, and of the serious consequences of failure, somewhat more stringent precautions have to be taken than are common in conventional engineering practice.