Selective plugging of broken fibers in tubesheet-hollow fiber assemblies

There is disclosed a method of selectively plugging broken fibers included in a bundle of hollow fibers having open ends terminating in or protruding from one face of a tubesheet and closed ends or loops depending from the other face of the tubesheet. The open ends are covered with a curable and flowable sealant and a pressure differential is utilized to draw the sealant into the broken fibers. The resistance to compression of the gases in the unbroken fibers is such that the sealant does not move into them a substantial distance. The sealant is diluted and another pressure differential employed to expel the sealant from the good fibers. The diluted sealant is removed and the plugs in the broken fibers are cured. This method is of particular value in preparing tubesheet-fiber assemblies for use in high temperature alkali metal-sulfur battery cells, wherein the hollow fibers function as an electrolyte separator, but also has utility for remedying leaks in devices employing hollow fibers as separatory membranes.