[en] The purpose of this research was to gain improved understanding of the behavior of cylindrical reinforced-concrete containment vessels subjected to combined internal pressure and seismic loading. This research follows an experimental research program in which reinforced concrete panels were loaded with combined in-plane tension and shear. The panels were behavioral models of a segment of the containment vessel. The panel test results indicated that the cracking induced by internal pressure and seismic shear will cause a large reduction in containment stiffness. This implies that the containment may experience larger than anticipated displacements and liner stresses, and that there may be stress redistributions that could lead to overstress in regions of the vessel where current analyses predict small stresses. This research addresses these uncertainties