Patent Number: 
Section: claims

1. A nuclear fuel assembly for a boiling water reactor comprising:a base,a head, anda bundle of full length fuel rods and partial length fuel rods, the bundle extending upwardly and longitudinally from the base to the head,at least one clamp for longitudinally retaining a lower plug of a partial length fuel rod with respect to the base, the clamp being an additional part fitted to the base, the clamp being assembled to the base by mechanical engagement of complementary assemblies, the clamp being embedded in a housing provided in the base such that the clamp is entirely below an upper surface of the base and entirely above a lower surface of the base, the housing consisting of a hole formed in the base, the clamp being comprised in the hole, wherein the clamp comprises elastic retaining tabs, spaced angularly about a longitudinal axis of the clamp and adapted to engage with a shoulder provided on the lower plug of the partial length fuel rod to prevent a disengagement of the partial length fuel rod from the base. 2. The nuclear fuel assembly as recited in claim 1 wherein the complementary assemblies comprise a first assembly and a second assembly, the first assembly being adapted to retain the clamp against an upward displacement of the clamp with respect to the base and the second assembly being adapted to retain the clamp against a downward displacement with respect to the base. 3. The nuclear fuel assembly as recited in claim 2 wherein the housing is a through-hole extending from an upper surface of the base to a lower surface of the base. 4. The nuclear fuel assembly as recited in claim 3 wherein the clamp allows the flow of coolant through the housing and along the lower end of the partial length fuel rod. 5. The nuclear fuel assembly as recited in claim 3 wherein the clamp comprises a support adapted to contact a portion of the lower plug of the partial length fuel rod in order to hold the lower plug of the partial length fuel rod at a distance from the lower surface of the base. 6. The nuclear fuel assembly as recited in claim 5 wherein the support comprises support tabs, spaced angularly about a longitudinal axis of the clamp, each support tab comprising a free end extending inwards in the direction of the longitudinal axis of the clamp, the free ends of the support tabs delimiting an opening for receiving the portion of the lower plug of the partial length fuel rod. 7. The nuclear fuel assembly as recited in claim 6 wherein the second assembly comprises a lower support surface provided in the housing, the support tabs being supported longitudinally on the lower support surface. 8. The nuclear fuel assembly as recited in claim 6 wherein the first assembly comprises an abutting surface provided in the housing, and elastic assembly tabs provided on the clamp and comprising free ends extending upwards, the abutting surface being adapted to engage with the free ends of the elastic assembly tabs,wherein the clamp comprises elastic retaining tabs, spaced angularly about a longitudinal axis of the clamp and adapted to engage with a shoulder provided on the lower plug of the partial length fuel rod to prevent a disengagement of the partial length fuel rod from the base,wherein the clamp comprises a body and wherein the support tabs extend inwards from a lower edge of the body, and the retaining tabs and assembly tabs extend upwards from an upper edge of the body. 9. The nuclear fuel assembly as recited in claim 8 wherein in a seated position, in which the support prevents the partial length fuel rod from moving further downwards, the clamp only contacts the lower plug of the partial length fuel rods by the support tabs and of the retaining tabs, a radial clearance existing between an inner surface of the clamp and the peripheral surface of the lower plug of the partial length fuel rod in the portion of the partial length fuel rod extending between the support tabs and the retaining tabs. 10. The nuclear fuel assembly as recited in claim 2 wherein the first assembly comprises an abutting surface provided in the housing, and elastic assembly tabs provided on the clamp and comprising free ends extending upwards, the abutting surface being adapted to engage with the free ends of the elastic assembly tabs. 11. The nuclear fuel assembly as recited in claim 1 wherein the complementary assemblies comprise a central flat base of the clamp, supported on a bottom of the housing and an abutting surface provided in the housing, the abutting surface being adapted to engage with upper free ends of the clamp. 12. The nuclear fuel assembly as recited in claim 11 wherein the clamp comprises a constriction, which is resiliently biased to engage with the lower plug of the partial length fuel rod and is adapted to cooperate with a shoulder provided on the lower plug of the partial length fuel rod. 13. The nuclear fuel assembly as recited in claim 1 wherein the housing is a through-hole extending from an upper surface of the base to a lower surface of the base. 14. The nuclear fuel assembly as recited in claim 13 wherein the clamp allows the flow of coolant through the housing and along the lower end of the partial length fuel rod. 15. A nuclear fuel assembly for a boiling water reactor comprising:a base,a head, anda bundle of full length fuel rods and partial length fuel rods, the bundle extending upwardly and longitudinally from the base to the head,at least one clamp for longitudinally retaining a lower plug of a partial length fuel rod with respect to the base, the clamp being an additional part fitted to the base, the clamp being assembled to the base by mechanical engagement of complementary assemblies, the clamp being embedded in a housing provided in the base such that the clamp is entirely below an upper surface of the base and entirely above a lower surface of the base, the housing consisting of a hole formed in the base, the clamp being comprised in the hole,wherein the complementary assemblies comprise a first assembly and a second assembly, the first assembly being adapted to retain the clamp against an upward displacement of the clamp with respect to the base and the second assembly being adapted to retain the clamp against a downward displacement with respect to the base,wherein the housing is a through-hole extending from an upper surface of the base to a lower surface of the base,wherein the clamp comprises a support adapted to contact a portion of the lower plug of the partial length fuel rod in order to hold the lower plug of the partial length fuel rod at a distance from the lower surface of the base,wherein the support comprises support tabs, spaced angularly about a longitudinal axis of the clamp, each support tab comprising a free end extending inwards in the direction of the longitudinal axis of the clamp, the free ends of the support tabs delimiting an opening for receiving the portion of the lower plug of the partial length fuel rod,wherein the first assembly comprises an abutting surface provided in the housing, and elastic assembly tabs provided on the clamp and comprising free ends extending upwards, the abutting surface being adapted to engage with the free ends of the elastic assembly tabs,wherein the clamp comprises elastic retaining tabs, spaced angularly about a longitudinal axis of the clamp and adapted to engage with a shoulder provided on the lower plug of the partial length fuel rod to prevent a disengagement of the partial length fuel rod from the base,wherein the clamp comprises a body and wherein the support tabs extend inwards from a lower edge of the body, and the retaining tabs and assembly tabs extend upwards from an upper edge of the body. 16. The nuclear fuel assembly as recited in claim 15 wherein in a seated position, in which the support prevents the partial length fuel rod from moving further downwards, the clamp only contacts the lower plug of the partial length fuel rods by the support tabs and of the retaining tabs, a radial clearance existing between an inner surface of the clamp and the peripheral surface of the lower plug of the partial length fuel rod in the portion of the partial length fuel rod extending between the support tabs and the retaining tabs. 17. The nuclear fuel assembly as recited in claim 15 wherein the clamp allows the flow of coolant through the housing and along the lower end of the partial length fuel rod. 18. The nuclear fuel assembly as recited in claim 15 wherein the second assembly comprises a lower support surface provided in the housing, the support tabs being supported longitudinally on the lower support surface.