[en] Three-dimensional, coupled, fluid-structure calculations were performed for the dynamics of the core support barrel in an HDR reactor vessel during blowdown. For these calculations we coupled the three-dimensional, two-fluid code K-FIX to the three-dimensional elastic shell code FLX. The results demonstrate the importance of the coupled interaction. We investigated the utility of a two-dimensional fluid description and obtained reasonably good results for displacements and stresses. The influence of a massive ring attached to the core barrel bottom is shown to be predominantly local. Maximum deflection of the core barrel bottom is reduced by the ring because its stiffness is sufficient to preclude any cross-sectional distortion. Without the ring, the total deflection of the core barrel bottom is shown to be more from distortion than displacement of the mass center. A nodalization study indicates the degree of circumferential and axial resolution required for accurate results. Fine axial resolution is needed near the top, built-in boundary of the core barrel to determine stresses accurately, but is less necessary for displacements