A valve for a pressure bottle or vessel adapted to contain a dangerous fluid, such as a radioactive gas, can comprise two valve members in tandem which require actuation by separate operating members thereby eliminating the possibility for accidental or inadvertent opening. The main valve housing can be provided with a pressure equalization valve which opens when the bottle is immersed in a body of water at elevated water pressures to equalize the interior and exterior pressures while providing a water seal against escape of the bottle contents.