Patent Number: 054935920
Section: claims

1. A fuel rod of a pressurized-water-cooled fuel assembly, comprising a fuel filling, a cladding tube enclosing said fuel filling and including a first thicker inner layer facing toward said fuel filling and being formed of a first zirconium alloy, and a second thinner outer layer being metallurgically bound to said inner layer and being formed of a second zirconium alloy, said two zirconium alloys each containing at least the metals tin, iron and chromium as alloying constituents, and: a) said first alloy containing 1-2% by weight of Sn, 0.05-0.25% by weight of Fe and 0.05-0.2% by weight of Cr;  b) said second alloy having a content of 0.5-1.3% by weight of Sn, 0.15-0.5% by weight of Fe and 0.05-0.4% by weight of Cr;  c) said second alloy having a total content of tin, iron and chromium of more than 1.0% by weight, a ratio of the content of tin in said second alloy to the content of tin in said first alloy being between 0.35 and 0.7; and  d) a ratio of the content of iron and chromium in said second alloy to the tin content of said first alloy being between 0.2 and 0.5.  a) said first alloy containing 1-2% by weight of Sn, 0.05-0.25% by weight of Fe and 0.05-0.2% by weight of Cr;  b) said second alloy having a content of 0.5-1.3% by weight of Sn, 0.15-0.5% by weight of Fe and 0.05-0.4% by weight of Cr;  c) said second alloy having a total content of tin, iron and chromium of more than 1.0% by weight, a ratio of content of tin in said second alloy to the content of tin in said first alloy being between 0.35 and 0.7; and  d) a ratio of the content of iron and chromium in said second alloy to the tin content of said first alloy being between 0.2 and 0.5. 2. The fuel rod according to claim 1, wherein each of said layers contains only further alloying constituents permitted for Zircaloy 2 and Zircaloy 4 in addition to Sn, Fe and Cr, said further alloying constituents having amounts being substantially within limits permitted for one of Zircaloy 2 and Zircaloy 4. 3. The fuel rod according to claim 1, wherein all of said constituents of said first layer have amounts being within limits permitted for one of Zircaloy 2 and Zircaloy 4. 4. The fuel rod according to claim 1, wherein at least said second alloy has a nickel content of less than about 0.007% by weight. 5. The fuel rod according to claim 1, wherein said cladding tube has a silicon content of more than 0.005% by weight and less than 0.02% by weight. 6. The fuel rod according to claim 1, wherein said cladding tube has a silicon content of more than 0.007% by weight and less than 0.012% by weight. 7. The fuel rod according to claim 1, wherein each of said two alloys of said cladding tube has a silicon content of more than 0.005 by weight and less than 0.02% by weight. 8. The fuel rod according to claim 1, wherein each of said two alloys of said cladding tube has a silicon content of more than 0.007% by weight and less than 0.012% by weight. 9. The fuel rod according to claim 1, wherein said cladding tube has an oxygen content of less than 0.2% by weight and more than 0.05% by weight. 10. The fuel rod according to claim 1, wherein said cladding tube has an oxygen content of less than about 0.16% by weight and more than 0.12% by weight. 11. The fuel rod according to claim 1, wherein the tin content of said first alloy is more than 1.2% by weight and less than 1.6% by weight. 12. The fuel rod according to claim 1, wherein the tin content of said first alloy is more than 1.4% by weight and less than 1.6% by weight. 13. The fuel rod according to claim 1, wherein the tin content of said second alloy is more than 0.7 and less than 1.1 by weight. 14. The fuel rod according to claim 1, wherein the tin content of said second alloy is more than 0.7 and less than 0.9% by weight. 15. The fuel rod according to claim 1, wherein the iron content of said first alloy is more than 0.1% by weight and less than 0.24% by weight. 16. The fuel rod according to claim 1, wherein the iron content of said first alloy is more than 0.18% by weight and less than 0.24% by weight. 17. The fuel rod according to claim 1, wherein the iron content of said second alloy is more than 0.18% by weight and less than 0.4% by weight. 18. The fuel rod according to claim 1, wherein the iron content of said second alloy is more than 0.24% by weight and less than 0.35% by weight. 19. The fuel rod according to claim 1, wherein the chromium content of said first alloy is more than 0.07 and less than 0.13% by weight. 20. The fuel rod according to claim 1, wherein the chromium content of said second alloy is more than 0.07% by weight. 21. The fuel rod according to claim 1, wherein the chromium content of said second alloy is more than 0.13% by weight. 22. The fuel rod according to claim 1, wherein the chromium content of said second alloy is less than 0.25% by weight. 23. The fuel rod according to claim 1, wherein the chromium content of said second alloy is less than 0.21% by weight. 24. The fuel rod according to claim 1, wherein the total content of chromium, iron and tin of said second alloy is between 1.1 and 1.5% by weight. 25. The fuel rod according to claim 1, wherein said first zirconium alloy has a content of (1.5.+-.0.1) % by weight of Sn; said second alloy has a content of (0.8.+-.0.1) % by weight of Sn; and each of said zirconium alloys has (0.21.+-.0.03) % by weight of Fe, (0.1.+-.0.03) % by weight of Cr, (0.14.+-.0.02) % by weight of O.sub.2, (0.01.+-.0.003) % by weight of Si and at most 0.007% by weight of Ni. 26. The fuel rod according to claim 1, wherein said first zirconium alloy has a content of (1.5.+-.0.1) % by weight of Sn, (0.21.+-.0.03) % by weight of Fe and (0.1.+-.0.03) % by weight of Cr; said second zirconium alloy has a content of (0.8.+-.0.1) % by weight of Sn, (0.28.+-.0.04) % by weight of Fe and (0.17.+-.0.04) % by weight of Cr; and each of said zirconium alloys has a content of (0.14.+-.0.02) % by weight of O, (0.01.+-.0.003) % by weight of Si and not more than 0.007% by weight of Ni. 27. A fuel assembly of a pressurized-water reactor, comprising fuel rods each including a fuel filling, a cladding tube enclosing said fuel filling and having a first thicker inner layer facing toward said fuel filling and being formed of a first zirconium alloy, and a second thinner outer layer being metallurgically bound to said inner layer and being formed of a second zirconium alloy, said two zirconium alloys each containing at least the metals tin, iron and chromium as alloying constituents, and: