Gastight case around a reactor and the circuit and auxiliary systems so that – even after an incident – no radioactive substances may escape into the atmosphere and environment. The containment is one of the barriers in a nuclear power plant that make it difficult for radioactive substances 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 pressurized water reactor 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 annular gap between containment and steel skin. The radioactive substances exiting the containment during normal operation enter the negative pressure zone and reach the vent stack via filters. During an incident, the air from the negative pressure zone is pumped back into the containment.