A dual stage inflator has a divider plate that separates gas generant in the primary chamber from gas generant in the secondary chamber. Burning of the gas generant yields inflation gas for inflatingavehicle airbag. The gas generant in the primary chamber is ignited from a separate ignition train than the gas generant in the secondary chamber. The divider plate is capable of moving during the ope ration of the inflator. Before the inflator is activated, the divider plate rests against the inner surface of the lower housing. The divider plate prevents inflation gas from the primary chamber fromigniting the gas generant in the secondary chamber. When the internal pressure of the secondary chamber exceeds the internal pressure of the primary chamber, the divider plate is displaced in an upw ard direction thereby permitting inflation gas from the secondary chamber to flow around the divider plate and out of the inflator.