The Casing for the steam turbine is under the same pressure as the exhaust of the final stage. This is ~650psi for the High Pressure turbine and ~13psi of vacuum for the Low pressure turbine. It is necessary for the shaft to penetrate the casing, which creates an opportunity for steam to escape, or for air to leak in. This is prevented through a combination of labyrinth seals, 4 pounds of supply steam, and steam capture and condensation.