Patent Number: 
Section: claims

1. A nuclear fuel rod for a nuclear plant, wherein the fuel rod comprises a cladding tube sealed by a first end plug at a first end and second end plug at a second end, a plurality of fuel pellets stacked on each other inside the cladding tube such that they form a column of pellets and a plenum spring arranged in a space in the fuel rod in order to, at least during transportation, hold the column of pellets with a spring force towards the second end of the cladding tube and wherein the plenum spring comprises a first length variable part arranged to abut towards the uppermost located fuel pellet in the column of pellets with an end portion and a second part arranged to allow engagement of the plenum spring against an inner surface of the cladding tube by a radially outwardly directed pressure, wherein the plenum spring comprises a third part arranged to allow releasing of the engagement of the second part of the plenum spring in the cladding tube during operation of the nuclear plant, wherein said third part allows such a releasing when the first length variable part of the plenum spring is compressed to be below a specific length, and wherein said third part comprises a stop portion arranged to obtain contact with the uppermost located fuel pellet in the column of pellets when the first part is compressed to said specific length. 2. The fuel rod according to claim 1, wherein the second part comprises a wire element having a helical extension with an outer diameter of such a size that said radially outwardly directed pressure towards the inner surface of the cladding tube is provided. 3. The fuel rod according to claim 2, wherein said third part comprises a wire element which comprises an extension of helical wire element of the second part. 4. The fuel rod according to claim 1, wherein said third part comprises a principally axial extension in the cladding tube in a direction towards the end portion. 5. The fuel rod according to claim 4, wherein the axial extension of said third part occurs substantially centrally in the cladding tube. 6. The fuel rod according to claim 1, wherein the first part comprises a wire element having a helical extension with an outer diameter which is less than the inner diameter of the cladding tube. 7. The fuel rod according to claim 1, wherein the plenum is formed by a wire element in one piece. 8. The fuel rod according to claim 1, wherein the plenum is manufactured of stainless steel. 9. The fuel rod according to claim 1, wherein said third part is coupled to said second part such that linear movement of said third part toward said first end plug reduces said radially outwardly directed pressure. 10. A nuclear fuel rod for a nuclear plant, wherein the fuel rod comprises a cladding tube sealed by the first end plug at a first end and a second end plug at a second end, plurality of fuel pellets stacked on each other inside the cladding tube such that they form a column of pellets and a plenum spring positioned within the tube between the first end plug and an uppermost fuel pellet in the column of pellets such that it applies a spring force to said fuel pellets toward the second end plug;the plenum spring including a first length-variable part abutting the uppermost fuel pellet, a second part initially in contact with and applying radially outward pressure to an inner surface of the cladding tube, and a third part including a stop portion positioned to contact the uppermost fuel pellet upon compression of the first spring part to a predetermined length;wherein said third part reduces the radially outward pressure upon compression of the first spring part below the predetermined length.