A rupture of the primary piping in the helium-cooled and graphite-moderated high-temperature gas-cooled reactor (HTGR) represents a design-basis event that should not result in significant safety consequences. In such a loss-of-coolant event, the reactor would be shut down inherently, and the decay heat would be removed passively with the ultimate reactor temperature rise being less than the design limit. Still, an important concern for reactor safety continues to be graphite oxidation damage to the fuel and core should a major air ingress take place through the breached primary pressure boundary. Two major cases of air ingress are studied. The first case results from the rupture of a control rod or refuel access standpipe atop the reactor pressure vessel (RPV). To rule out the possibility of such a standpipe rupture, a design change is proposed in the vessel top structure. The feasibility of the modified vessel local structure is evaluated. The second case of air ingress results from the rupture of one or more main coolant pipes on the lower body of the RPV. Experiment and analysis are performed to understand the multiphased air ingress phenomena in the depressurized reactor. Accordingly, a new passive mechanism of sustained counter air diffusion is proposed and shown to be effective in preventing major air ingress through natural circulation in the reactor. The results of the present study are expected to enhance the HTGR safety and economics.