Patent Number: 
Section: claims

1. A method of measuring a relative magnitude of the burnout of a fuel element in a reactor, the method comprising the steps of sequentially:a) transferring a fuel element from the reactor to a measuring position,b) subjecting the transferred fuel element at the position to a neutron flux,c) measuring with a first detector the total γ radiation emitted by the transferred fuel element; andthereafterd) if the radiation measured by the first detector exceeds a predetermined first limit, returning the transferred fuel element back to the reactor ande) if the radiation measured by the first detector does not exceed the first limit, measuring with a second detector a magnitude of high energy γ radiation above 1 MeV emitted by the transferred fuel element; andthereafterf) only if the radiation measured by the second detector exceeds a predetermined second limit, returning the transferred fuel element back to the reactor, the transferred fuel element not being returned to the reactor if the radiation measured by the second detector is below the second limit. 2. The method according to claim 1 wherein the second detector determines a relative magnitude for the high energy γ radiation above 2 MeV emitted by the transferred fuel element. 3. The method according to claim 1 wherein the second detector operates with a count rate of at least 107/s. 4. The method according to claim 1 wherein a scintillation counter is used as the second detector. 5. The method according to claim 1, further comprisinga shield between the measurement position and the second detector. 6. The method according to claim 5 wherein the shield is a lead filter. 7. The method according to claim 1 in which the first detector detects the γ radiation of the fuel element in less than 2 seconds. 8. The method according to claim 1 in which the second detector detects the high energy γ radiation of the fuel element in less than 30 seconds. 9. The method according to claim 1, further comprising the step ofsurrounding the fuel element in the measurement position with water. 10. The method according to claim 1 wherein the first limit is so selected that a proportion of the fuel elements which are required for operating the reactor falls below this first limit. 11. The method according to claim 1 wherein the first limit is so selected that with a 1:10 mode of operating the reactor a maximum of 20% of the fuel elements lies below this first limit. 12. The method according to claim 1 wherein the second limit is so established that a proportion of all fuel elements which are measured but are required for operating the reactor falls below this second limit. 13. The method according to claim 11 in which the second limit is so established that in a 1:10 mode a maximum of 15% of all measured fuel elements lie below this second limiting. 14. A device for measuring a relative magnitude of the burnout of a fuel element in a reactor, the device comprising:means for transferring a fuel element from the reactor to a measuring position,means including a neutron source for subjecting the transferred fuel element to a neutron flux,means including a first detector for measuring the total γ radiation emitted by the transferred fuel element and for comparing the radiation measured by the first detector with a predetermined first limit;means for, if the radiation measured by the first detector exceeds a predetermined first limit, returning the transferred fuel element back to the reactor;means for, if the radiation measured by the first detector does not exceed the first limit, measuring with a second detector a magnitude for high energy γ radiation above 1 MeV emitted by the transferred fuel element and comparing the radiation measured by the second detector with a predetermined second limit; andmeans for, only if the radiation measured by the second detector exceeds the predetermined second limit, returning the transferred fuel element back to the reactor, the transferred fuel element not being returned to the reactor if the radiation measured by the second detector is below the second limit. 15. The device according to claim 14, further comprisinga shield between the measurement position and the second detector. 16. The device according to claim 15 wherein the shield is a lead filter. 17. The device according to claim 14, wherein a scintillation counter is the second detector. 18. The device according to claim 14 wherein the second detector has a counting rate of at least 107/s. 19. The device according to claim 14 wherein the measurement position is at least partly surrounded by water.