A nuclear reactor vessel includes a shell (5) having a wall (9) traversed by at least one passage (11) having a central axis (C) and a valve (21) mounted in the passage (11), the valve (21) having a determined outer cross-section. The passage (11) has an inner end segment (23) that opens into the inner volume (7) and an outer end segment (25) that opens into an outer piping (13), the valve (21) being housed in the inner end segment (23). The valve (21) is capable of being extracted from the passage (11) through the inside of the vessel (1). The outer end segment (25) has at least one portion having an inner cross-section, perpendicular to the central axis (C), that is smaller than the outer cross-section of the valve (21), such that the valve (21) cannot be ejected out of the vessel (1).