A reaction vessel for making phlegmatized metal powder or alloy powder has a retort crucible with a heat-proof, coolable cover, a heatable reduction furnace into which the retort crucible may be inserted, and an inner crucible placed within the retort crucible and removable therefrom. At least one inlet is built into the coolable cover for adding a passivating gas. A heat-proof flange welded onto the retort crucible attaches the retort crucible to the heat-proof, coolable cover. A cooler welded onto a lower face of the heat-proof flange provides a cooling agent to the reaction vessel.