Patent Number: 
Section: claims

1. A method of making a fuel channel for a fuel assembly for a nuclear power boiling water reactor, the fuel channel defining a length direction which in use corresponds substantially to the vertical direction, the method comprising:providing at least one first sheet of a Zr-based material, said at least one first sheet having a first thickness, wherein said first thickness is constant throughout each first sheet,providing at least one second sheet of a Zr-based material, said at least one second sheet having a second thickness, wherein said second thickness is constant throughout each second sheet and said second thickness is less than said first thickness,assembling different elements which together form the fuel channel, which elements comprise said at least one first sheet and said at least one second sheet, wherein a fuel channel is formed and said at least one first sheet forms a lower part of the fuel channel and said at least one second sheet forms a higher part of the fuel channel and said lower part is joined with said higher part in that said at least one first sheet is joined with said at least one second sheet, wherein the formed fuel channel has a first cross sectional inner area in the higher part, the first cross sectional inner area is larger than a second cross sectional inner area in the lower part,wherein a joint is located between said lower part and said higher part wherein the lower part constitutes 20-75% of the length of the fuel channel. 2. A method according to claim 1, wherein said higher part constitutes 20-75% of the length of the fuel channel. 3. A method according to claim 1, wherein said second thickness is 40-85% of said first thickness. 4. A method according to claim 1, wherein said first thickness is 2.00-3.50 mm. 5. A method according to claim 1, wherein the joint between said lower part and said higher part is formed by one of welding or soldering. 6. A method according to claim 1, wherein the formed fuel channel has an outer cross sectional area in the higher part, which is at least substantially the same as an outer cross sectional area in the lower part. 7. A method according to claim 1, wherein the formed fuel channel has an outer cross sectional area in the higher part, which is less than the outer cross sectional area in the lower part. 8. A method according to claim 1, wherein said at least one first sheet is joined with said at least one second sheet when the sheets are flat, after which the joined sheets are configured and arranged to form the fuel channel, which surrounds an inner space. 9. A fuel channel for a fuel assembly for a nuclear power boiling water reactor, the fuel channel defining a length direction which in use corresponds substantially to the vertical direction, wherein the fuel channel comprises:at least one first sheet of a Zr-based material, said at least one first sheet having a first thickness, wherein said first thickness is constant throughout each first sheet;at least one second sheet of a Zr-based material, said at least one second sheet having a second thickness, wherein said second thickness is constant throughout each second sheet and said second thickness is less than said first thickness; andsaid at least one first sheet and said at least one second sheet being shaped to form said fuel channel, wherein said at least one first sheet forms a lower part of the fuel channel and said at least one second sheet forms a higher part of the fuel channel, said at least one first sheet joined with said at least one second sheet to form a joint arranged where said at least one first sheet has been joined with said at least one second sheet, wherein said joint also forms a joint between said lower part and said higher part,wherein the fuel channel has a first cross sectional inner area in the higher part, the first cross sectional inner area is larger than a second cross sectional inner area in the lower part,wherein the joint is located between said lower part and said higher part wherein the lower part constitutes 20-75% of the length of the fuel channel. 10. A fuel channel according to claim 9, wherein said higher part constitutes 20-75% of the length of the fuel channel. 11. A fuel channel according to claim 9, wherein said second thickness is 40-85% of said first thickness. 12. A fuel channel according to claim 9, wherein said first thickness is 2.00-3.50 mm. 13. A fuel channel according to claim 9, wherein the joint between said lower part and said higher part is one of a weld joint or soldering joint. 14. A fuel channel according to claim 9, wherein the fuel channel has an outer cross sectional area in the higher part, which is at least substantially the same as the outer cross sectional area in the lower part. 15. A fuel channel according to claim 9, wherein the fuel channel has an outer cross sectional area in the higher part, which is less than the outer cross sectional area in the lower part. 16. A fuel assembly for a nuclear power boiling water reactor, comprising:a plurality of fuel rods arranged substantially parallel to each other, said fuel rods comprising nuclear fuel material,a plurality of spacer grids arranged to hold the fuel rods at predetermined positions relative to each other, anda fuel channel according to claim 9, arranged to surround said fuel rods and said spacer grids.