[en] Sixteen tests were conducted in the G.E. Quarter-scale Test Facility to study the effects of vent header flexibility on Mark I vent header impact loads caused by pool swell from a postulated loss of coolant accident. The first eight tests were conducted with a rigid vent header model; the remaining tests used a flexible model with ovalling stiffness approximately scaled for the Quarter-scale Facility. Both high- and low-impact conditions were studied. These conditions were produced by varying the drywell pressurization rate and the initial drywell-to-torus pressure difference (which controls the initial water level in the downcomers). Vent header loads were measured by a load cell and by eight flush-mounted pressure transducers distributed around the impact zone. The output from the pressure transducers provided details of the transient impact pressure distribution on the bottom quadrant of the vent header. Compared to the rigid vent header, the flexible vent header reduced the magnitude of the peak impact load and shifted the peak later in time. Local pressures generally were lower for the flexible cylinder