Abstract: A method for providing a leak-tight metal enclosure to a fuel matrix penetrated by coolant channels, wherein the mutually contacting surfaces of said metal enclosure and said fuel matrix are metallurgically bonded, comprising placing a metal cladding about the lateral surface of said fuel matrix; disposing metal coolant tubes within said coolant channels; placing a perforated header plate having tubular extensions at each end of the fuel matrix from which the coolant tube ends protrude, said coolant tubes passing through said perforated header plate and said tubular extensions and terminating even with the ends of said extensions; welding, under vacuum, said cladding to said header plates, and the ends of said coolant tubes to the ends of said tubular extensions; exposing the assembly comprising the fuel matrix and enclosure to a gas at high temperature and pressure; and machining said header plates to provide a finished fuel element.