PURPOSE: To eliminate stress from locally being concentrated in part of a containment vessel body by transmitting force applied to shear lugs for constraining relative displacement between a containment vessel body and a containment vessel shell over the whole circumference of the containment vessel body through a ring reinforcement.

CONSTITUTION: An annular ring reinforcement 16 is fixed on the outside part of a containment vessel body 14 so that the annular ring reinforcement 16 having width through which workers can pass may be approximately concentrically positioned for the containment vessel body 14, and a stopper 20 is set on the inside part of a containment vessel shell 15 surrounding the ring reinforcement 16 and the containment vessel body 14 so that the ring reinforcement 16 may be surrounded. In addition, shear lugs 21 are fixed on the ring reinforcement 16 so that they may be engaged with each stopper 20.