Patent Number: 
Section: claims

1. A final, ready to use, spacer grid configured to separate and hold nuclear fuel rods in a nuclear reactor of the boiling water reactor type in predetermined positions relative to each other, wherein the final spacer grid comprises:i) a spacer grid structure made of an alloy that has been formed and assembled such that it constitutes a spacer grid, andii) an outer oxide coating on the surface of the spacer grid structure, said outer oxide coating comprises a first inner oxide layer of a first composition and a second outer oxide layer of a second composition different from the first composition, wherein the first inner oxide layer mainly contains Cr2O3 and the second outer oxide layer mainly contains NiFe2O4;wherein said alloy is a Ni base alloy that consists of the following:Element% by weightNi>50.0Cr14.0-21.0Fe12.0-23.0Ti1.50-3.0 Al0.40-1.50Co0.0001-0.010 C0.001-0.050N0.001-0.030Nb0.001-1.50 Ta0.001-0.030Si0.01-0.50Mn0.01-1.0 S0.001-0.020P0.001-0.050Cu0.01-0.50Mo + W0.001-1.0 the total amount of one or more elements chosen from the  0-1.0group consisting of all elements except for the elementsreferred to in the table above. 2. A final spacer grid according to claim 1, wherein the amount of Fe in said alloy is 15.0-19.0% by weight. 3. A final spacer grid according to claim 1, wherein the amount of Co in said alloy is <0.0050% by weight. 4. A final spacer grid according to claim 1, wherein said alloy in the final spacer grid comprises a substantial amount of γ′ secondary phase particles such that the final spacer grid has a sufficient mechanical strength. 5. A final spacer grid according to claim 4, wherein the mole fraction of γ′ secondary phase particles in said alloy in the final spacer grid is 5-25%. 6. A final spacer grid according to claim 1, wherein said outer oxide coating has a thickness of 50-1000 nm. 7. A final spacer grid according to claim 1, wherein the first inner oxide layer has a thickness of 50-200 nm and the second outer oxide layer has a thickness of 20-80 nm. 8. A final spacer grid according to claim 1, wherein said alloy is a Ni base alloy that consists of the following:Element% by weightNi>60.0Cr14.0-17.0Fe15.0-19.0Ti1.750-2.750Al0.40-1.0 Co0.0001-0.0050C0.001-0.050N0.001-0.030Nb0.70-1.20Ta0.001-0.030Si0.01-0.50Mn0.01-1.0 S0.001-0.010P0.001-0.020Cu0.01-0.50Mo + W0.001-0.20 the total amount of one or more elements chosen from the  0-0.50group consisting of all elements except for the elementsreferred to in the table above. 9. A method of manufacturing the final, ready to use, spacer grid comprising the steps of:providing a Ni base alloy that consists of the following:Element% by weightNi>50.0Cr14.0-21.0Fe12.0-23.0Ti1.50-3.0 Al0.50-1.50Co0.0001-0.010 C0.001-0.050N0.001-0.030Nb0.001-1.50 Ta0.001-0.030Si0.01-0.50Mn0.01-1.0 S0.001-0.020P0.001-0.050Cu0.01-0.50Mo + W0.001-1.0 the total amount of one or more elements chosen from the  0-1.0group consisting of all elements except for the elementsreferred to in the table above.forming and assembling the alloy such that said spacer grid structure is obtained, andheat treating the spacer grid structure at a temperature of 650-750° C. for 5-23 hours, the heat treatment being performed in an oxidizing atmosphere, wherein the heat treatment is such that an outer oxide coating is formed on the surface of the spacer grid structure comprising a first inner oxide layer of a first composition and a second outer oxide layer of a second composition different from the first composition such that the first inner oxide layer mainly contains Cr2O3 and the second outer oxide layer mainly contains NiFe2O4, thereby obtaining the final, ready to use, spacer grid. 10. A method of manufacturing the final spacer grid according claim 9, wherein said oxidizing atmosphere comprises aqueous vapour and air. 11. A method of manufacturing the final spacer grid according to claim 9, wherein said heat treatment of the spacer grid structure is such that γ′ secondary phase particles are formed in said alloy, thereby obtaining improved mechanical properties of the final spacer grid. 12. A method of manufacturing the final spacer grid according to claim 11, wherein the mole fraction of γ′ secondary phase particles in said alloy in the final spacer grid is 5-25%. 13. A method of manufacturing the final spacer grid according to claim 9, wherein the first inner oxide layer has a thickness of 50-200 nm and the second outer oxide layer has a thickness of 20-80 nm.