This patent document relates to systems, structures, devices, and fabrication processes for ceramic matrix composites suitable for use in a nuclear reactor environment and other applications requiring materials that can withstand high temperatures and/or highly corrosive environments. In one exemplary aspect, a method of joining and sealing ceramic structures is disclosed. The method comprises forming a joint of a ceramic structure and an end plug using a sealing material, wherein the end plug has a hole that goes through a top surface and a bottom surface of the end plug; filling the ceramic structure with a desired gas composition through the hole; heating a material into a molten form using a heat source; and directing the material into the hole, wherein the material solidifies to seal the end plug.