Patent Number: 052251543
Section: claims

1. A fuel assembly for a nuclear reactor, comprising a fuel cladding tube of three-layer structure having an outer surface layer to be in contact with reactor water of the nuclear reactor, which outer surface layer is made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy containing Nb, Sn and Mo consisting by weight of 0.5 to 2.2% Nb, 0.5 to 1.5% Sn, 0.1 to 0.8% Mo, and the balance Zr and incidental impurities, an inner surface layer to be in contact with nuclear fuel, which inner surface layer is made of pure zirconium, and an intermediate layer made of a high ductility alloy which is higher in ductility than the outer surface layer and which is higher in strength than the inner surface layer, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 2. A fuel assembly for a nuclear reactor, comprising a fuel spacer of three-layer structure having outer surface layers to be in contact with reactor water of the nuclear reactor, which outer surface layers are made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy containing Nb, Sn and Mo consisting by weight of 0.5 to 2.2% Nb, 0.5 to 1.5% Sn, 0.1 to 0.8% Mo, and the balance Zr and incidental impurities, and an intermediate layer adjacent to the outer surface layers, which intermediate layer is made of a high ductility alloy higher in ductility than the outer surface layers, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 3. A fuel assembly for a nuclear reactor, comprising a fuel channel box of three-layer structure having outer surface layers to be in contact with reactor water of the nuclear reactor, which outer surface layers are made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy containing Nb, Sn and Mo consisting by weight of 0.5 to 2.2% Nb, 0.5 to 1.5% Sn, 0.1 to 0.8% Mo, and the balance Zr and incidental impurities, and an intermediate layer adjacent to the outer surface layers, which intermediate layer is made of a high ductility alloy higher in ductility than the outer surface layers, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 4. A fuel assembly for nuclear reactor of any one of the claims 1 to 3, wherein the high ductility alloy forming the intermediate layer consists of a Zr-based alloy containing Sn, Fe and Ni. 5. A fuel assembly for nuclear reactor of the claim 4, wherein the high ductility alloy forming the intermediate layer consists by weight of 0.5 to 2.0% Sn, 0.05 to 0.4% Fe, 0.03 to 0.2% Ni, and the balance Zr and incidental impurities. 6. A fuel assembly for nuclear reactor of any one of the claims 1 to 3, wherein the high ductility alloy forming the intermediate layer is a Zr-based alloy containing Sn, Fe, Ni and Cr. 7. A fuel assembly for nuclear reactor of the claim 6, wherein the high ductility alloy forming the intermediate layer consists by weight of 0.5 to 2.0% Sn, 0.05 to 0.4% Fe, 0.03 to 0.2% Ni, 0.05 to 0.15% Cr, and the balance Zr and incidental impurities. 8. A fuel assembly for nuclear reactor of any one of the claims 1 to 3, wherein the high ductility alloy forming the intermediate layer is a Zr-based alloy containing Sn, Fe, Ni, Cr and Mo. 9. A fuel assembly for nuclear reactor of the claim 8, wherein the high ductility alloy forming the intermediate layer consists by weight of 0.5 to 2.0% Sn, 0.05 to 0.4% Fe, 0.03 to 0.2% Ni, 0.05 to 0.15% Cr, 0.01 to 0.8% Mo, and the balance Zr and incidental impurities. 10. A fuel assembly for nuclear reactor of any one of the claims 1 to 3, wherein the high ductility alloy forming the intermediate layer consists of Zr and incidental impurities. 11. A fuel assembly for nuclear reactor of any one of the claims 1 to 3, wherein the high ductility alloy forming the intermediate layer consists by weight of 0.1 to 0.7% Nb, and the balance Zr and incidental impurities. 12. A fuel assembly for nuclear reactor of any one of the claims 1 to 3, wherein the high ductility alloy forming the intermediate layer is a stainless steel containing carbon, the carbon being included in an amount, by weight, up to 0.08%, and the stainless steel also containing, by weight, 0-2.0% Mn, 0-1.00% Si, 16.0 to 20.0% Cr, 8.00 to 14.00% Ni, 0-3.00% Mo, and the balance Fe and incidental impurities. 13. A fuel assembly for nuclear reactor of any one of the claims 1 to 3, wherein the high ductility alloy forming the intermediate layer is a Cu-based alloy consisting by weight of 0-3.0% Pb, 0-6.0% Fe, 0-0.5% Zn, 0-11.0% Al, 0-2.0% Mn, 0-33.0%; Ni, and the balance Cu and incidental impurities. 14. A fuel assembly for nuclear reactor of claim 1, wherein in the fuel cladding tube the thickness of the outer surface layer to be in contact with the reactor water of the nuclear reactor is 10 to 30% of the total thickness of the cladding tube. 15. A method of producing a fuel assembly for nuclear reactor of any one of the claims 1 to 3, 14 and 28, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding when assembling the three-layer structure. 16. A method of producing a fuel assembly for nuclear reactor of claim 2, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding the end face of a tubular or plate-like member. 17. A method of producing the fuel assembly for nuclear reactor of any one of the claims 1 and 14, wherein the fuel cladding tube is produced by a process comprising the steps of: inserting a cylindrical billet of an alloy, which becomes an intermediate layer, into another cylindrical billet which becomes an outer surface layer in contact with the reactor water of the nuclear reactor; inserting into the intermediate layer billet a cylindrical billet which becomes an inner surface layer; integrating all of these billets by welding each end face thereof; extruding the integrated billets at a temperature of 500.degree. to 800.degree. C.; and repeating both cold working and annealing thereof by a plurality of times. 18. A method of producing the fuel assembly for nuclear reactor of claim 2, wherein the cylindrical fuel spacer is produced by a process comprising the steps of: inserting a cylindrical billet of an alloy, which becomes an intermediate layer, into another cylindrical billet which becomes an outer surface layer in contact with the reactor water of the nuclear reactor; inserting into the intermediate layer billet a cylindrical billet which becomes an inner surface layer and which is made of the same material as the outer surface layer; integrating all of the billets by welding each end face thereof; extruding the integrated billets at a temperature not less than 610.degree. C.; and repeating both cold working and annealing thereof by a plurality of times. 19. A method of producing the fuel assembly for nuclear reactor of claim 17, wherein the welding is effected under a pressure not more than 1.times.10.sup.-4 Torr. 20. A method of producing the fuel assembly for nuclear reactor of claim 17, wherein an intermediate annealing between adjacent two cold workings is effected at a temperature of 610.degree. to 750.degree. C. 21. A method of producing the fuel assembly for nuclear reactor of claim 17, wherein the fuel cladding tube is subjected to a heat treatment comprising the steps of heating the surface layer of or heating the total thickness of the cladding tube at a temperature of 800.degree. to 900.degree. C. after the final hot working but before the final cold working, and then quenching it. 22. A method of producing the fuel assembly for nuclear reactor of claim 3, wherein the fuel channel box is produced by a process comprising the steps of: inserting a plate of an alloy, which becomes an intermediate layer, between plate members which become outer surface layers in contact with the reactor water of the nuclear reactor; integrating them by welding each end face thereof; extruding the integrated members at a temperature of 500.degree. to 800.degree. C.; and repeating both cold working and annealing thereof by a plurality of times. 23. A method of producing the fuel assembly for nuclear reactor of any one of claims 22 and 46, wherein an intermediate annealing between adjacent two cold workings is effected at a temperature of 610.degree. to 750.degree. C. 24. A method of producing the fuel assembly for nuclear reactor of claim 22, wherein the fuel channel box is subjected to a heat treatment comprising the steps of heating it at a temperature of 800.degree. to 900.degree. C. after the final hot working but before the final cold working, and then quenching it. 25. A member for a fuel assembly for a nuclear reactor, comprising a three-layer structure having an outer surface layer of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy containing Nb, Sn and Mo consisting by weight of 0.5 to 2.3% Nb, 0.5 to 1.5% n, 0.1 to 0.8% Mo, and the balance Zr and incidental impurities, which outer surface layer is to be in contact with reactor water of the nuclear reactor, and an intermediate layer of a high ductility alloy which is higher in ductility than the Zr-based alloy containing Nb, Sn and Mo and which is higher in strength than pure Zr, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 26. A member for a fuel assembly of claim 25, wherein the three-layer structure further includes an inner surface layer, such that the inner surface layer and outer surface layer sandwich the intermediate layer, the inner surface layer being made of pure zirconium. 27. A fuel assembly for nuclear reactor of claim 3, wherein in the channel box a thickness of each of the outer surface layers is 20 to 35% of the total thickness of the channel box. 28. A member for a fuel assembly of claim 25, wherein the Zr-based alloy containing Nb, Sn and Mo has a tensile strength of at least 70 kgf/mm.sup.2 at room temperature. 29. A member for a fuel assembly of claim 25, wherein the high ductility alloy has a tensile strength of 45-60 kgf/mm.sup.2 at room temperature and an elongation rate of at least 25%. 30. A method of producing a fuel assembly for nuclear reactor of claim 5, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding when assembling the three-layer structure. 31. A method of producing a fuel assembly for nuclear reactor of claim 5, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding when assembling the three-layer structure. 32. A method of producing a fuel assembly for nuclear reactor of claim 6, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding when assembling the three-layer structure. 33. A method of producing a fuel assembly for nuclear reactor of claim 7, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding when assembling the three-layer structure. 34. A method of producing a fuel assembly for nuclear reactor of claim 8, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding when assembling the three-layer structure. 35. A method of producing a fuel assembly for nuclear reactor of claim 9, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding when assembling the three-layer structure. 36. A method of producing a fuel assembly for nuclear reactor of claim 10, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding when assembling the three-layer structure. 37. A method of producing a fuel assembly for nuclear reactor of claim 11 comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding when assembling the three-layer structure. 38. A method of producing a fuel assembly for nuclear reactor of claim 12, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding when assembling the three-layer structure. 39. A method of producing a fuel assembly for nuclear reactor of claim 13, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding when assembling the three-layer structure. 40. A method of producing a fuel assembly for nuclear reactor of claim 10, comprising the step of effecting a heat-treatment at a temperature of 400.degree. to 700.degree. C. after welding the end face of a tubular or plate-like member. 41. A method of producing the fuel assembly for nuclear reactor of claim 18, wherein the welding is effected under a pressure not more than 1.times.10.sup.-4 Torr. 42. A method of producing the fuel assembly for nuclear reactor of claim 18, wherein an intermediate annealing between adjacent two cold workings is effected at a temperature of 610.degree. to 750.degree. C. 43. A method of producing the fuel assembly for nuclear reactor of claim 18, wherein the cylindrical fuel spacer is subjected to a heat treatment comprising the steps of heating the surface layer of or heating the total thickness of the cylindrical fuel spacer at a temperature of 800.degree. to 900.degree. C. after the final hot working but before the final cold working, and then quenching it. 44. A method of producing the fuel assembly for nuclear reactor of claim 2, wherein the fuel spacer is produced by a process comprising the steps of: inserting a plate of an alloy, which becomes an intermediate layer, between plate members which become outer surface layers in contact with the reactor water of the nuclear reactor; integrating them by welding each end face thereof; extruding the integrated members at a temperature of 500.degree. to 800.degree. C.; and repeating both cold working and annealing thereof by a plurality of times. 45. A method of producing the fuel assembly for nuclear reactor of claim 44, wherein the fuel spacer is subjected to a heat treatment comprising the steps of heating it at a temperature of 800.degree. to 900.degree. C. after the final hot working but before the final cold working, and then quenching it. 46. A fuel assembly for a nuclear reactor, comprising a fuel cladding tube of three-layer structure having an outer surface layer to be in contact with reactor water of the nuclear reactor, which outer surface layer is made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy containing Nb, Sn and Mo consisting by weight of 0.5 to 2.2% Nb, 0.5 to 1.5% Sn, 0.1 to 0.8% Mo, and the balance Zr and incidental impurities, an inner surface layer to be in contact with nuclear fuel, which inner surface layer is made of pure zirconium, and an intermediate layer made of a high ductility alloy which is higher in ductility than the outer surface layer and which is higher in strength than the inner surface layer, wherein the high ductility alloy forming the intermediate layer consists by weight of 0.5 to 2.0% Sn, 0.05 to 0.4% Fe, 0.03 to 0.2% Ni, 0.05 to 0.15% Cr, 0.01 to 0.8% Mo, and the balance Zr and incidental impurities, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 47. A fuel assembly for nuclear reactor, comprising a fuel spacer of three-layer structure having outer surface layers to be in contact with reactor water of the nuclear reactor, which outer surface layers are made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy having properties that no nodular corrosion or corrosion of white color occurs when the alloy is exposed to steam of a pressure of 105 kgf/cm.sup.2 at 500.degree. C. for 24 hours, and having a tensile strength of at least 70 kgf/mm.sup.2 at room temperature, and an intermediate layer adjacent to the outer surface layers, which intermediate layer is made of a high ductility alloy higher in ductility than the outer surface layers, wherein the high ductility alloy forming the intermediate layers consists by weight of 0.5 to 2.0% Sn, 0.05 to 0.4% Fe, 0.03 to 0.2% Ni, 0.05 to 0.15% Cr, 0.01 to 0.8% Mo, and the balance Zr and incidental impurities, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 48. A fuel assembly for a nuclear reactor, comprising a fuel channel box of three-layer structure having outer surface layers to be in contact with reactor water of the nuclear reactor, which outer surface layers are made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy having properties that no nodular corrosion or corrosion of white color occurs when the alloy is exposed to steam of a pressure of 105 kgf/cm.sup.2 at 500.degree. C. for 24 hours, and having a tensile strength of at least 70 kgf/mm.sup.2 at room temperature, and an intermediate layer adjacent to the outer surface layers, which intermediate layer is made of a high ductility alloy higher in ductility than the outer surface layers, wherein the higher ductility alloy forming the intermediate layer consists by weight of 0.5 to 2.0% Sn, 0.05 to 0.4% Fe, 0.03 to 0.2% Ni, 0.05 to 0.15% cr, 0.01 to 0.8% Mo, and the balance Zr and incidental impurities, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 49. A fuel assembly for a nuclear reactor, comprising a fuel cladding tube of three-layer structure having an outer surface layer to be in contact with reactor water of a nuclear reactor, which outer surface layer is made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy containing Nb, Sn and Mo consisting by weight of 0.5 to 2.2% Nb, 0.5 to 1.5% Sn, 0.1 to 0.8% Mo, and the balance Zr and incidental impurities, an inner surface layer to be in contact with nuclear fuel, which inner surface layer is made of pure zirconium, and an intermediate layer made of a high ductility alloy, which is higher in ductility than the outer surface layer and which is higher in strength than the inner surface layer, wherein the high ductility alloy forming the intermediate layer is a stainless steel, containing carbon, the carbon being included in an amount, by weight, of not more than 0.08% C, and the stainless steel also containing, by weight, 0-2.0% Mn, 0-1.00% Si, 16.0 to 20.0% Cr, 8.00 to 14.00% Ni, 0-3.00% Mo, and the balance Fe and incidental impurities, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 50. A fuel assembly for a nuclear reactor, comprising a fuel spacer of three-layer structure having outer surface layers to be in contact with reactor water of the nuclear reactor, which outer surface layers are made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy having properties that no nodular corrosion or corrosion of white color occurs when the alloy is exposed to steam of a pressure of 105 kgf/cm.sup.2 at 500.degree. C. for 24 hours, and having a tensile strength of at least 70 kgf/mm.sup.2 at room temperature, and an intermediate layer adjacent to the outer surface layers, which intermediate layer is made of a high ductility alloy higher in ductility than the outer surface layers, wherein the high ductility alloy forming the intermediate layer is a stainless steel, containing carbon, the carbon being included in an amount, by weight, of not more than 0.08% C, and the stainless steel also containing, by weight, 0-2.0% Mn, 0-1.0% Si, 16.0 to 20.0% Cr, 8.00 to 14.00% Ni, 0-3.00% Mo, and the balance Fe and incidental impurities, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 51. A fuel assembly for a nuclear reactor, comprising a fuel channel box of three-layer structure having outer surface layers to be in contact with reactor water of the nuclear reactor, which outer surface layers are made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy having properties that no nodular corrosion or corrosion of white color occurs when the alloy is exposed to steam of a pressure of 105 kgf/cm.sup.2 at 500.degree. C. for 24 hours, and having a tensile strength of at least 70 kgf/mm.sup.2 at room temperature, and an intermediate layer adjacent to the outer surface layers, which intermediate layer is made of a high ductility alloy higher in ductility than the outer surface layers, wherein the high ductility alloy forming the intermediate layer is a stainless steel, containing carbon, the carbon being included in an amount, by weight, of not more than 0.08% C, and the stainless steel also containing, by weight, 0-2.0% Mn, 0-1.00% Si, 16.0 to 20.0% cr, 8.00 to 14.00% Ni, 0-3.00% Mo, and the balance Fe and incidental impurities, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 52. A fuel assembly for a nuclear reactor, comprising a fuel cladding tube of three-layer structure having an outer surface layer to be in contact with reactor water of the nuclear reactor, which outer surface layer is made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy containing Nb, Sn and Mo consisting by weight of 0.5 to 2.2% Nb, 0.5 to 1.5% Sn, 0.1 to 0.8% Mo, and the balance Zr and incidental impurities, an inner surface layer to be in contact with nuclear fuel, which inner surface layer is made of pure zirconium, and an intermediate layer made of a high ductility alloy, which is higher in ductility than the outer surface layer and which is higher in strength than the inner surface layer, wherein the high ductility alloy forming the intermediate layer is a Cu-based alloy consisting by weight of 0-3.0% Pb, 0-6.0% Fe, 0-0.5 Zn, 0-11.0% Al, 0-2.0% Mn, 0-33.0% Ni, and the balance of Cu and incidental impurities, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 53. A fuel assembly for a nuclear reactor, comprising a fuel spacer of three-layer structure having outer surface layers to be in contact with reactor water of the nuclear reactor, which outer surface layers are made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy having properties that no nodular corrosion or corrosion of white color occurs when the alloy is exposed to steam of a pressure of 105 kgf/cm.sup.2 at 500.degree. C. for 24 hours, and having a tensile strength of at least 70 kgf/mm.sup.2 at room temperature, and an intermediate layer adjacent to the outer surface layers, which intermediate layer is made of a high ductility alloy higher in ductility than the outer surface layers, wherein the high ductility alloy forming the intermediate layer is a Cu-based alloy consisting by weight of 0-3.0% Pb, 0-6.0% Fe, 0-0.5% Zn, 0-11.0% Al, 0-2.0% Mn, 0-33.0% Ni, and the balance Cu and incidental impurities, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 54. A fuel assembly for a nuclear reactor, comprising a fuel channel box of three-layer structure having outer surface layers to be in contact with reactor water of the nuclear reactor, which outer surface layers are made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy having properties that no nodular corrosion or corrosion of white color occurs when the alloy is exposed to steam of a pressure of 105 kgf/cm.sup.2 at 500.degree. C. for 24 hours, and having a tensile strength of at least 70 kgf/mm.sup.2 at room temperature, and an intermediate layer adjacent to the outer surface layers, which intermediate layer is made of a high ductility alloy higher in ductility than the outer surface layers, wherein the high ductility alloy forming the intermediate layer is a Cu-based alloy consisting by weight of 0-3.0% Pb, 0-6.0% Fe, 0-0.5% Zn, 0-11.0% Al, 0-2.0% Mn, 0-33.0% Ni, and the balance Cu and incidental impurities, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 55. A fuel assembly for a nuclear reactor, comprising a fuel cladding tube of three-layer structure having an outer surface layer to be in contact with reactor water of the nuclear reactor, which outer surface layer is made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy containing Nb, Sn and Mo consisting by weight of 0.5 to 2.2% Nb, 0.5 to 1.5% Sn, 0.1 to 0.8% Mo, and the balance Zr and incidental impurities, an inner surface layer to be in contact with nuclear fuel, which inner surface layer is made of pure zirconium, and an intermediate layer made of a high ductility alloy, which is higher in ductility than the outer surface layer and which is higher in strength than the inner surface layer, a thickness of the outer surface layer being 10% to 30% of the total thickness of the cladding tube, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 56. A fuel assembly for a nuclear reactor, comprising a fuel channel box of three-layer structure having outer surface layers to e in contact with reactor water of the nuclear reactor, which outer surface layers are made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy having properties that no nodular corrosion or corrosion of white color occurs when the alloy is exposed to steam of a pressure of 105 kgf/cm.sup.1 at 500.degree. C. for 24 hours, and having a tensile strength of at least 70 kgf/mm.sup.2 at room temperature, and an intermediate layer adjacent to the outer surface layers, which intermediate layer is made of a high ductility alloy higher in ductility than the outer surface layers, a thickness of each of the outer surface layers being 20 to 35% of the total thickness of the fuel channel box, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 57. A fuel assembly for a nuclear reactor, comprising a fuel spacer of three-layer structure having (a) outer surface layers to be in contact with reactor water of the nuclear reactor, which outer surface layers are made of a Zr-based alloy containing Nb, Sn and MO, the Zr-based alloy having properties that no nodular corrosion or corrosion of white color occurs when the alloy is exposed to steam of a pressure of 105 kgf/cm.sup.2 at 500.degree. C. for 24 hours, and having a tensile strength of at least 70 kgf/mm.sup.2 at room temperature, and (b) an intermediate layer adjacent to the outer surface layers, which intermediate layer is made of a high ductility alloy higher in ductility than the outer surface layers, the intermediate layer being made of a material selected from the group consisting of Zr-based alloy, stainless steel alloy and copper-based alloy, having a tensile strength of 45 to 60 kgf/mm.sup.2 at room temperature and an elongation rate of at least 25%, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 58. A fuel assembly for a nuclear reactor, comprising a fuel channel box of three-layer structure having (a) outer surface layers to be in contact with reactor water of the nuclear reactor, which outer surface layers are made of a Zr-based alloy containing Nb, Sn and Mo, the Zr-based alloy having properties that no nodular corrosion or corrosion of white color occurs when the alloy is exposed to steam of a pressure of 105 kgf/cm.sup.2 at 500.degree. C. for 24 hours, and having a tensile strength of at least 70 kgf/mm.sup.2 at room temperature, and (b) an intermediate layer adjacent to the outer surface layers, which intermediate layer is made of a high ductility alloy higher in ductility than the outer surface layers, the intermediate layer being made of a material selected from the group consisting of Zr-based alloy, stainless steel alloy and copper-based alloy, having a tensile strength of 45 to 60 kgf/mm.sup.2 at room temperature and an elongation rate of at least 25%, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 59. A member for a fuel assembly of claim 25, wherein the outer surface layer has a thickness that is 10% to 60% of the total thickness of the three-layer structure. 60. A member for a fuel assembly of claim 59, wherein the three-layer structure includes an inner surface layer, which has a thickness that is 5-20% of the total thickness of the three-layer structure. 61. A member for a fuel assembly for a nuclear reactor, comprising a three-layer structure having an outer surface layer of a Zr-based alloy containing Nb, Sn and Mo, which outer surface layer is to be in contact with reactor water of the nuclear reactor, and an intermediate layer of a high ductility alloy which is higher in ductility than the Zr-based alloy containing Nb, Sn and Mo and which is higher in strength than pure Zr, a thickness of the intermediate layer being 25%-85% of a total thickness of the three-layer structure. 62. A member for a fuel assembly of claim 61, wherein the intermediate layer is made of a material selected from the group consisting of Zr-based alloy, stainless steel and copper-based alloy. 63. A member for a fuel assembly of claim 62, wherein the outer surface layer has a thickness that is 10%-60% of the total thickness of the three-layer structure. 64. A member for a fuel assembly of claim 63, wherein the three-layer structure includes an inner surface layer, which has a thickness that is 5-20% of the total thickness of the three-layer structure. 65. A member for a fuel assembly of claim 64, said member being selected from the group consisting a fuel cladding tube, a fuel channel box and a fuel spacer. 66. A member for a fuel assembly of claim 64, wherein the three-layer structure includes an inner surface layer, made of Zr, the intermediate layer being higher in strength than the inner surface layer.