Abstract: A microporous plug is provided which serves to seal a vessel so as to prevent the bulk flow of gases through an outlet. The pores of said microporous plug are sized so as to permit the transport of gases to and from the vessel interior by Knudsen diffusion. In preferred embodiments, the pores of the plug are sized so as to release helium from the interior of a vessel and restrict the transport of carbon dioxide into the vessel from the surrounding atmosphere by Knudsen diffusion. Such embodiments are particularly useful on Mars where the helium is transported into an environment of higher pressure. In other embodiments, the microporous plug is used in conjunction with an absorber positioned within the vessel to react with atmospheric gases that pass through the plug, such as carbon dioxide. Also provided are radioisotope thermoelectric generators that incorporate a microporous plug and optionally an absorber.