Patent Number: 
Section: claims

1. A fuel assembly configured to be positioned in a nuclear water reactor, wherein the fuel assembly comprises:an upstream end,a downstream end,a flow interspace between the upstream end and the downstream end,a plurality of fuel rods provided in the flow interspace between the upstream end and the downstream end, the flow interspace being configured to permit a flow of coolant through the fuel assembly along a flow direction from the upstream end to the downstream end in contact with the fuel rods,a filter device configured to catch debris particles in the flow of coolant,wherein the filter device comprises a first filter zone provided in the flow of coolant between the upstream end and the fuel rods, and a second filter zone,wherein the first filter zone comprises a plurality of passages arranged to guide at least a part of the flow of coolant to pass the first filter zone through the passages towards the downstream end,wherein the second filter zone comprises a plurality of passages arranged to guide at most a part of the flow of coolant to pass the second filter zone through the passages towards the downstream end,wherein the first filter zone has a first filtering efficiency and the second filter zone has a second filtering efficiency,wherein the second filtering efficiency is higher than the first filtering efficiency, andwherein the passages of the first filter zone and the second filter zone are formed by a plurality of sheets, which are arranged beside each other and oriented along the flow direction, and wherein adjacent sheets of the first filter zone are provided at a larger distance from each other than adjacent sheets of the second filter zone. 2. The fuel assembly according to claim 1, wherein the second filter zone is dimensioned to permit the second filter zone to be clogged and wherein the first filter zone is configured to secure a sufficient flow of coolant through the first filter zone even if no coolant passes the second filter zone. 3. The fuel assembly according to claim 1, wherein the first filter zone comprises an inlet end turned towards the upstream end, and an outlet end, and is arranged to guide said at least a part of the flow of coolant to pass the first filter zone through the passages from the inlet end to the outlet end, andwherein the second filter zone comprises an inlet end turned towards the upstream end, and an outlet end, and is arranged to guide said at most a part of the flow of coolant to pass the second filter zone through the passages from the inlet end to the outlet end. 4. The fuel assembly according to claim 1, wherein the first filter zone and the second filter zone are provided beside each other. 5. The fuel assembly according to claim 1, wherein each of the passages of the first filter zone defines a first flow area and each of the passages of the second filter zone defines a second flow area, which is smaller than the first filter area. 6. The fuel assembly according to claim 1, wherein each of the passages of the first filter zone defines a first passage length from an inlet end to an outlet end, and each of the passages of the second filter zone defines a second passage length from an inlet end to an outlet end, which second passage length is longer than the first passage length. 7. The fuel assembly according to claim 1, wherein the first filter zone has a first pressure loss coefficient ξ1 and the second filter zone a second pressure loss coefficient ξ2 and wherein the second pressure loss coefficient ξ2 is greater than the first pressure loss coefficient ξ1. 8. The fuel assembly according to claim 1, wherein the first filter zone has a first flow area A1 and the second filter zone a second flow area A2 and wherein the first flow area A1 is greater than the second flow area A2. 9. The fuel assembly according to claim 1, wherein the filter device comprises a magnetic member provided to create a magnetic field on at least some of the passages of the second filter zone to attract debris particles flowing through the passages of the second filter zone. 10. The fuel assembly according to claim 1, wherein the filter device is provided in the flow interspace to guide at most a minor part of the flow of coolant through the second filter zone. 11. The fuel assembly according to claim 10, wherein the filter device is provided in the flow interspace to guide at least a major part of the flow of coolant through the first filter zone. 12. The fuel assembly according to claim 1, wherein the filter device is provided in the flow interspace to guide the entire flow of coolant through the first filter zone. 13. The fuel assembly according to claim 12, wherein the second filter zone is provided at a distance from the first filter zone. 14. The fuel assembly according to claim 13, wherein the second filter zone is provided downstream the first filter zone. 15. The fuel assembly according to claim 13, wherein the second filter zone is provided upstream the first filter zone. 16. The fuel assembly according to claim 1, wherein each sheet comprises a first portion, which extends from the inlet end, a second portion, which extends from the outlet end, and a third portion, which extends between the first portion and the second portion, and wherein each sheet along the first portion has a wave-shape extending in a direction transversally to the flow direction and along the third portion has a wave-shape extending in the flow direction. 17. The fuel assembly according to claim 16, wherein each sheet along the second portion has a wave-shape in the direction transversally to the flow direction.