Patent Number: 
Section: claims

1. A device for carrying out a leak test on a fuel rod capsule containing at least one fuel rod and test gas, the device comprising a test container which is designed for accommodating at least one fuel rod capsule and which is lowerable into a water-flooded pool of a nuclear plant, characterized in that a mass spectrometer is fluidically connected to the interior of the test container in such a way that the mass spectrometer may be supplied with a gas stream for detecting a concentration of the test gas that has diffused from the fuel rod capsule into the test container, and characterized in that the test container is evacuable in such a way that an internal pressure that is reduced compared to a hydrostatic pressure caused by water surrounding the test container is settable in the interior of the test container. 2. The device according to claim 1, characterized in that argon is provided as the test gas, and the mass spectrometer detects the concentration of the argon that has diffused from the fuel rod capsule. 3. The device according to claim 1, characterized in that the test container at a lower end has a connection device that connects to the interior of the pool. 4. The device according to claim 3, characterized in that the connection device in the pool is situated at a depth below the water surface, and an upper end of the test container is fluidically connectable to a first reservoir in which a purge gas is storable under pressure, the pressure being greater than the hydrostatic pressure of the surrounding water corresponding to the depth of the connection device. 5. The device according to claim 4, characterized in that the first reservoir is fluidically connectable to a sampling point of the mass spectrometer. 6. The device according to claim 4, characterized in that the sampling point of the mass spectrometer is fluidically connectable to a second reservoir in which test gas is storable under pressure. 7. The device according to claim 6, characterized in that at least one volume control valve is situated between the sampling point and the first reservoir, and/or between the sampling point and the second reservoir. 8. The device according to claim 3, characterized in that the interior of the test container at the upper end is fluidically connectable to a sampling point of the mass spectrometer in such a way that a gas column that is present in the test container is conveyable to the sampling point by introducing water via the connection device situated at the lower end of the test container. 9. The device according to claim 4, characterized in that a volumetric flow of the gas stream supplied to the sampling point of the mass spectrometer is changeable by means of an adjustment device, in particular at least one further volume control valve or a vacuum pump. 10. The device according to claim 9, characterized in that the adjustment device is connected to a pressure sensor for measuring the pressure at the sampling point. 11. The device according to claim 1, characterized in that the at least one fuel rod capsule is introducible into the test container via a closeable opening on the upper end. 12. The device according to claim 1, characterized in that components of the device, in particular lines and/or line sections, situated outside the pool have thermal insulation, at least in sections. 13. The device according to claim 12, characterized in that the components situated outside the pool are heatable, at least in sections, by means of a heating device. 14. A method for carrying out a leak test on a fuel rod capsule containing at least one fuel rod and test gas, using a device according to one of the preceding claims, wherein at least one fuel rod capsule is introduced into a test container that is lowered into a flooded pool of a nuclear plant, characterized in that a gas stream containing test gas that has diffused from the fuel rod capsule into the test container is supplied to a mass spectrometer, and a concentration of the test gas that has diffused from the fuel rod capsule into the test container is detected in the gas stream by means of the mass spectrometer in order to determine a leak rate, and characterized in that the at least one fuel rod capsule is introduced into the test container underwater, and the water that has penetrated into the test container is replaced by a purge gas. 15. The method according to claim 14, characterized in that the purge gas remains in the test container for a predefinable period of time before the gas stream containing the purge gas and the test gas that has diffused from the fuel rod capsule is supplied to the mass spectrometer. 16. The method according to claim 14, characterized in that the test container is flooded with water from the pool in order to generate the gas stream. 17. The method according to claim 14, characterized in that the gas stream is supplied to the mass spectrometer at a sampling point, and a volumetric flow of the gas stream is regulated in such a way that a constant pressure prevails at the sampling point. 18. The method according to claim 17, characterized in that a gas mixture containing a purge gas or test gas is supplied in a predefined mixing ratio to the sampling point in a controlled manner. 19. The method according to claim 14, characterized in that components of the device, in particular lines and/or line sections, that are situated outside the pool are heated, at least in sections. 20. The method according to claim 14, characterized in that an inert gas is used as the test gas. 21. The method according to claim 14, characterized in that argon is used as the test gas. 22. A device for carrying out a leak test on a fuel rod capsule containing at least one fuel rod and test gas, the device comprising a test container which is designed for accommodating at least one fuel rod capsule and which is lowerable into a water-flooded pool of a nuclear plant, characterized in that a mass spectrometer is fluidically connected to the interior of the test container in such a way that the mass spectrometer may be supplied with a gas stream for detecting a concentration of the test gas that has diffused from the fuel rod capsule into the test container, and characterized in that the test container at a lower end has a connection device that connects to the interior of the pool. 23. A device for carrying out a leak test on a fuel rod capsule containing at least one fuel rod and test gas, the device comprising a test container which is designed for accommodating at least one fuel rod capsule and which is lowerable into a water-flooded pool of a nuclear plant, characterized in that a mass spectrometer is fluidically connected to the interior of the test container in such a way that the mass spectrometer may be supplied with a gas stream for detecting a concentration of the test gas that has diffused from the fuel rod capsule into the test container, and characterized in that a volumetric flow of the gas stream supplied to a sampling point of the mass spectrometer is changeable by means of an adjustment device, in particular at least one further volume control valve or a vacuum pump. 24. A method for carrying out a leak test on a fuel rod capsule containing at least one fuel rod and test gas, using a device according to one of the preceding claims, wherein at least one fuel rod capsule is introduced into a test container that is lowered into a flooded pool of a nuclear plant, characterized in that a gas stream containing test gas that has diffused from the fuel rod capsule into the test container is supplied to a mass spectrometer, and a concentration of the test gas that has diffused from the fuel rod capsule into the test container is detected in the gas stream by means of the mass spectrometer in order to determine a leak rate, and characterized in that the test container is flooded with water from the pool in order to generate the gas stream. 25. A method for carrying out a leak test on a fuel rod capsule containing at least one fuel rod and test gas, using a device according to one of the preceding claims, wherein at least one fuel rod capsule is introduced into a test container that is lowered into a flooded pool of a nuclear plant, characterized in that a gas stream containing test gas that has diffused from the fuel rod capsule into the test container is supplied to a mass spectrometer, and a concentration of the test gas that has diffused from the fuel rod capsule into the test container is detected in the gas stream by means of the mass spectrometer in order to determine a leak rate, and characterized in that the gas stream is supplied to the mass spectrometer at a sampling point, and a volumetric flow of the gas stream is regulated in such a way that a constant pressure prevails at the sampling point. 26. A method for carrying out a leak test on a fuel rod capsule containing at least one fuel rod and test gas, using a device according to one of the preceding claims, wherein at least one fuel rod capsule is introduced into a test container that is lowered into a flooded pool of a nuclear plant, characterized in that a gas stream containing test gas that has diffused from the fuel rod capsule into the test container is supplied to a mass spectrometer, and a concentration of the test gas that has diffused from the fuel rod capsule into the test container is detected in the gas stream by means of the mass spectrometer in order to determine a leak rate, and characterized in that a gas mixture containing a purge gas or test gas is supplied in a predefined mixing ratio to a sampling point in a controlled manner.