Patent Number: 
Section: claims

1. A method of operating a reactor (1) of a nuclear plant in which the reactor (1) comprises a reactor vessel (6) enclosing a core having a plurality of fuel elements (7) and a number of control rods (8),wherein each fuel element (7) includes a plurality of elongated fuel rods (9), which each has an upper end (9′) and a lower end (9″) and includes a cladding (10) and nuclear fuel in the form of fuel pellets (11) enclosed in an inner space (12) formed by the cladding,wherein the fuel pellets (11) are arranged in the inner space to leave a free volume in the inner space, wherein the free volume comprises an upper plenum (12′), containing no nuclear fuel and provided in the proximity of the upper end of the fuel rod, a lower plenum (12″), containing no nuclear fuel and provided in the proximity of the lower end of the fuel rod, and a pellet-cladding gap between the fuel pellets (11) and the cladding (10),wherein a reactor coolant, during operation of the reactor, is re-circulated as a coolant flow through the core in contact with the fuel rods (9) and is added to the reactor via a feed-water conduit (4) as feed-water having a normal feed-water temperature providing a sub-cooling of the reactor coolant, andwherein each of the control rods (8) is displaceable a control rod distance to be inserted into and extracted from a respective position between respective fuel elements in the core, the method including the following steps of operation:operating the reactor at a normal power and a normal sub-cooling during a normal state,monitoring the reactor for detecting a defect on the cladding of any of the fuel rods,upon detecting a defective fuel rod having said defect on the cladding thereof, changing the operation of the reactor to a particular state that causes an increase of the free volume at least in the defective fuel rod in which the defect is detected,operating the reactor at the particular state during a limited time period, andoperating, after said time period, the reactor at substantially the normal state. 2. A method according to claim 1, comprising providing the lower plenum (12″) to have a longitudinal length along the elongated fuel rod (9) and the upper plenum (12′) to have a longitudinal length along the elongated fuel rod (9), wherein the longitudinal length of the lower plenum (12″) is provided to be shorter than the longitudinal length of the upper plenum (12′). 3. A method according to claim 2 , wherein the longitudinal length of the lower plenum is provided to be less than 30% of the total longitudinal length of the upper plenum (12′) and the lower plenum (12″). 4. A method according to claim 1, wherein changing the operation of the reactor to the particular state comprises at least one of the following steps of operation:operating the reactor at a reduced power in relation to the normal power during the normal state, andoperating the reactor at an increased sub-cooling of the reactor coolant in relation to the normal sub-cooling during the normal state in order to achieve a larger temperature gradient over the fuel rod. 5. A method according to claim 4, wherein changing the operation of the reactor to the particular state comprises reducing the coolant flow of the reactor coolant through the core. 6. A method according to claim 4, wherein the added reactor coolant is preheated outside the reactor during the normal state by means of a preheating arrangement (14), and wherein said increased sub-cooling of the reactor coolant is obtained by reducing the preheating of the added reactor coolant. 7. A method according to claim 4, wherein said reduced power is obtained by displacing at least some of the control rods (8) into the core at least a part of the control rod distance. 8. A method according to claim 7, wherein substantially all control rods (8) are at least periodically displaced at least a part of the control rod distance during the particular state. 9. A method according to claim 4, wherein said reduced power is obtained by displacing successively different groups of the control rods (8) at least a part of the control rod distance, wherein each such group defines a respective specific part of the core. 10. A method according to claim 4, wherein the reactor is operated at the reduced power during the whole time period of the particular state. 11. A method according to claim 4, wherein substantially all control rods are displaced at least a part of the control rod distance during the whole time period of the particular state. 12. A method according to claim 1, wherein the particular state is initiated at least within 72 hours after the detection of a defect. 13. A method according to claim 1, wherein the particular state is initiated at least within 48 hours after the detection of a defect. 14. A method according to claim 1, wherein the particular state is initiated at least within 24 hours after the detection of a defect. 15. A method according to claim 1, wherein the particular state is initiated immediately after the detection of a defect. 16. A method according to claim 1, wherein the particular state involves that at least some of the control rods are alternately displaced in the core for obtaining an alternating increase and decrease of the power. 17. A method according to claim 1, wherein said monitoring includes continuous monitoring during the operation of the reactor. 18. A method according to claim 1, wherein the monitoring includes sensing of a radioactive activity in a gas flow from the reactor. 19. A method according to claim 1, wherein the fuel rod is provided to comprise a hydrogen absorbing element (21′) in at least one of the upper plenum (12′) and the lower plenum (12″). 20. A method according to claim 19, wherein the hydrogen absorbing element (21′, 21″) is provided to comprise a hydrogen absorbing body having a surface coated with a layer of a substance that is non-oxidizing and permeable to hydrogen. 21. A method according to claim 20, wherein the absorbing body is enclosed in an imaginary body having a substantially convex outer surface, and wherein a surface area of the absorbing body is greater than a surface area of the outer surface of the imaginary body. 22. A method according to claim 20, wherein the substance is provided to comprise at least one metal in the group consisting of palladium, rhodium, rhenium and alloys comprising one or more of these metals. 23. A method according to claim 20, wherein the absorbing body is provided to comprise at least one metal in the group consisting of zirconium, titanium, nickel and alloys comprising one or more of these metals. 24. A method according to claim 1, wherein the fuel rod (9) is provided to comprise a distance element (20′) provided in the upper plenum (12′) and/or a distance element (20″) provided in the lower plenum (12″). 25. A method according to claim 24, wherein at least one of the distance elements (20′, 20″) is provided to form the absorbing element. 26. A method according to claim 24, wherein at least one of the distance elements is provided to be deformable for permitting swelling of the fuel pellets.