Patent Number: 
Section: claims

1. A water reactor fuel cladding tube for a pressurized water reactor, said cladding tube comprising an outer layer of a first zirconium based alloy and having directly metallurgically bonded thereto an inner layer of a second zirconium based alloy, wherein the cladding tube does not have any further zirconium based alloy layer, wherein the inner layer is adapted to protect the cladding tube against stress corrosion cracking, wherein the second zirconium based alloy comprises tin as an alloying material, and wherein each one of the zirconium based alloys comprises at least 96 percent by weight zirconium, wherein the main alloying materials of the first zirconium based alloy are niobium, iron and tin with the content of any additional substances being below 0.05 percent by weight, wherein the first zirconium based alloy comprises oxygen;the first zirconium based alloy comprising 0.6-1.2 percent by weight niobium, 0.6-1.2 percent by weight tin, and 0.1-0.3 percent by weight iron;wherein the main alloying materials of the second zirconium based alloy are tin and iron and wherein the content of any additional substances is below 0.05 percent by weight, and wherein the second zirconium based alloy comprises 0.1-1 percent by weight tin and 0.02-0.3 percent by weight iron; andwherein the thickness of the inner layer is 5-40% of the thickness of the reactor fuel cladding tube. 2. A water reactor fuel cladding tube according to claim 1, wherein the first zirconium based alloy comprises 500-2000 ppm by weight oxygen. 3. A water reactor fuel cladding tube according to claim 1, wherein the inner layer is partially re-crystallized. 4. A water reactor fuel cladding tube according to claim 1, wherein the inner layer is fully re-crystallized. 5. A water reactor fuel cladding tube according to claim 1, wherein the outer layer is partially re-crystallized. 6. A water reactor fuel cladding tube according to claim 5, wherein the degree of re-crystallization in the outer layer is 45 percent −90 percent. 7. A water reactor fuel cladding tube according to claim 6, wherein the degree of re-crystallisation in the outer layer is 50 percent −70 percent. 8. A water reactor fuel cladding tube according to claim 1, which has been manufactured by co-extrusion of a first tube of the first zirconium based alloy and a second tube of the second zirconium based alloy. 9. A water reactor fuel cladding tube according to claim 1, wherein the thickness of the inner layer is 5-15% of the thickness of the reactor fuel cladding tube. 10. A water reactor fuel rod comprising a water reactor fuel cladding tube according to claim 1 and fuel pellets enclosed by the water reactor fuel cladding tube. 11. A water reactor fuel assembly comprising at least two fuel rods according to claim 10.