Gastight case around a System used to initiate, maintain and control a fission chai... and the circuit and auxiliary systems so that – even after an Sequence of events, on occurrence of which plant operation o... – no Radioactive substances within the meaning of the Atomic Ener... may escape into the atmosphere and environment. The containment is one of the barriers in a Thermal power plant, primarily used for electricity generati... that make it difficult for Radioactive substances within the meaning of the Atomic Ener... to escape into the environment. It surrounds the nuclear part of the plant and is designed so that in case of serious malfunctions it collects the exiting steam without failing itself. The containment of a Power reactor in which the heat is dissipated from the core ... is e.g. a steel ball with a diameter of approx. 50 m and a wall thickness of 30 mm. It includes rapidly closing valves in the pipings leading out of the containment and personal and material locks. The case is enclosed by an up to 2 m thick reinforced concrete dome to protect against external impacts. The inner wall of the dome is lined with a gas-tight steel skin. Negative pressure exists in the Clearance between the two parts of a double containment kept... between containment and steel skin. The Radioactive substances within the meaning of the Atomic Ener... exiting the containment during normal operation enter the negative pressure zone and reach the vent stack via filters. During an Sequence of events, on occurrence of which plant operation o..., the air from the negative pressure zone is pumped back into the containment.