Patent Number: 
Section: claims

1. A fuel rod having first and second closed ends, encapsulating a fissionable material axially along at least a portion of an interior volume thereof, and an exterior of the fuel rod comprising a cladding that extends substantially axially between the first and second closed ends, the cladding comprising:an external surface;a circumference;an axial length;a diameter; anda surface modification applied to at least a portion of the external surface, the surface modification comprising:a rib having a length, a height and a width, the length of the rib coils around the circumference of the external surface of the cladding;a sequence of continuous rib loops uniformly spaced along the axial length of the cladding formed by the length of the rib; anda pitch measured between adjacent rib loops in the sequence of continuous rib loops,wherein a ratio of the height of the rib to the diameter of the cladding is greater than or equal to about 0.0134 and less than or equal to about 0.0268,wherein a ratio of the height of the rib to the width of the rib is greater than or equal to about 0.8 and less than or equal to about 1.2, andwherein the pitch is greater than or equal to about nine times the height of the rib and less than or equal to about 12 times the height of the rib. 2. The fuel rod of claim 1, wherein each rib loop in the sequence of continuous rib loops is offset from a horizontal axis by an angle ranging from about 0 degrees to about 45 degrees. 3. The fuel rod of claim 1, wherein the surface modification is applied to only a portion of the external surface of the cladding. 4. A nuclear fuel assembly, comprising:a spaced array of a plurality of axially extending elongated tubular pressurized water reactor nuclear fuel elements, each having first and second closed ends, encapsulating a fissionable material axially along at least a portion of an interior volume thereof, and an exterior of at least one of the fuel elements comprising a cladding that extends substantially axially between the first and second closed ends, the cladding comprising:an external surface;a circumference;an axial length;a diameter; anda surface texture varying axially in a prescribed pattern along at least a portion of an axial length of the external surface of said cladding, the surface texture, comprising:a rib having a length, a height and a width, the length of the rib coils around the circumference of the external surface of the cladding;a sequence of continuous rib loops uniformly spaced along the axial length of the cladding formed by the length of the rib; anda pitch measured between adjacent rib loops in the sequence of continuous rib loops,wherein a ratio of the height of the rib to the diameter of the cladding is greater than or equal to about 0.0134 and less than or equal to about 0.0268,wherein a ratio of the height of the rib to the width of the rib is greater than or equal to about 0.8 and less than or equal to about 1.2, andwherein the pitch is greater than or equal to about nine times the height of the rib and less than or equal to about 12 times the height of the rib. 5. The nuclear fuel assembly of claim 4, wherein a plurality of axially spaced spacer grids surround and affix the fuel elements in the spaced array and wherein said surface texture of the cladding is located in a span between a consecutive pair of the spacer grids. 6. A method of enhancing heat transfer in a nuclear fuel core containing a plurality of fuel rods having first and second closed ends, encapsulating a fissionable material axially along at least a portion of an interior volume thereof, and an exterior of at least one of the fuel rods comprising a cladding having a diameter and an external surface that extends substantially axially between the first and second closed ends, comprising:modifying the external surface of the cladding for at least a portion of the plurality of fuel rods; andapplying to at least a portion of the external surface to be modified a roughness, comprising:a rib having a length, a height and a width, the length of the rib coils around a circumference of the external surface of the cladding;a sequence of continuous rib loops uniformly spaced along the axial length of the cladding formed by the length of the rib; anda pitch measured between adjacent rib loops in the sequence of continuous rib loops,wherein a ratio of the height of the rib to the diameter of the cladding is greater than or equal to about 0.0134 and less than or equal to about 0.0268,wherein a ratio of the height of the rib to the width of the rib is greater than or equal to about 0.8 and less than or equal to about 1.2, andwherein the pitch is greater than or equal to about nine times the height of the rib and less than or equal to about 12 times the height of the rib.