During a postulated loss-of-coolant accident (LOCA) (complete severance of a primary coolant pipe) the upper barrel of the reactor internals in a pressurized water reactor is subjected to dynamic differential pressures. In case of a sudden hot-leg break, the initial disturbance is a compressive triangular pulse approximately uniformly distributed with the pressure rising to a peak of 250 psi in ∼ 0.010 sec and dropping to 0 in 0.020 sec. The possible response under this impulsive compressive pressure is dynamic instability (buckling) and/or large deflections of the upper core barrel. In the present paper, the dynamic response of the barrel under the actual triangular pulse is analyzed and, to obtain margins of safety, compared with the response to assumed more severe loading conditions. The response of the barrel to the pressure pulse consists initially in a uniform radial inward movement and results, therefore, in compressive hoop stresses (hoop response). Deviations of the barrel shape from the circular cross section (initial imperfections of the order of the manufacturing tolerances) result in circumferential bending moments and the excitation of higher shell modes (flexural response). For the actual triangular pulse the analysis shows that the dynamic effects are small and the occurring stresses and deflections are close to the values obtained by loading the shell statically with the pressure . For the step loads that are applied to investigate the margin of safety of the shell, the dynamic effects are no longer negligible and result in stresses above yield for p* = 350 psi and p* = 500 psi. However, for these loading cases, the maximum deflections remain on the order of magnitude of the initial imperfections and the barrel is therefore considered stable.