Method and apparatus for testing nuclear reactor fuel...

Induced nuclear reactions: processes – systems – and elements – Testing – sensing – measuring – or detecting a fission reactor... – Leak detection

Reexamination Certificate

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Details

C376S250000, C376S251000

Reexamination Certificate

active

06570949

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method and an apparatus for testing a plurality of fuel assemblies of a nuclear reactor, in particular a boiling water reactor, for fuel rod leaks while the fuel assemblies are resting under water on a working base, in particular while they are still inside a core assembly in a reactor pressure vessel. The fuel rods are heated in order to drive radioactive fission products out of defective fuel rods, and the radioactivity of the fission products that have been driven out is recorded in samples that are extracted from the environment of the fuel assemblies. The corresponding apparatus includes a hood, the opening of which faces downward and can be positioned over at least one of the resting fuel assemblies. At least one extractor hood with an extraction device leads out of the hood, and there is a detector device for analyzing the radioactivity of the extracted fission products.
In nuclear reactors, spent fuel rods have to be replaced with new fuel rods at regular intervals, and the new fuel rods are then redistributed through the core together with reusable fuel assemblies. Generally, the old fuel assemblies are lifted out of their working base, i.e. for example a lower core grid in the reactor pressure vessel, and are initially placed on a different working base, for example a storage rack in a dedicated water basin, in order subsequently to be returned to the reactor pressure vessel and inserted at the new position in the reactor core. The fuel assemblies are under water throughout the entire operation for radiation protection reasons, and to be transported they are held in a height-adjustable manner on the mast of a displaceable fuel handling machine.
Light water-cooled reactors have to be shut down, and for cost reasons operating pauses of this nature should be kept as short as possible.
However, irradiated fuel assemblies can only be reused if the cladding tubes of the fuel rods do not have any leaks through which radioactive fission products formed in the fuel as a result of the nuclear fission during the prior reactor operation could escape and unacceptably contaminate the cooling water of the reactor. As well as visual inspections of the fuel assemblies and testing of individual fuel rods by ultrasound or eddy-current probes, what is known as “sipping” is a conventional method of identifying the fuel assembly which contain a fuel rod with a leak. First, a pressure difference is generated between the internal pressure in the fuel rod and the external pressure in the surrounding water, in order to drive out the fission products formed in the fuel rod filling in the maximum possible quantities and then to analyze samples extracted from the environment of the fuel assembly. Detectors for radioactive radiation can be used for the analysis. Detectors of this type may, for example, be particularly sensitive to gaseous fission products, such as xenon 133 or krypton 85, in gaseous samples or to water-soluble fission products (e.g. iodine 131 or cesium 134) in water samples.
When testing for leaks in irradiated fuel assemblies, reliability of detection and speed are particularly important criteria.
For this purpose, “mast sipping” has been developed, in which the search for leaks is carried out while the fuel assemblies are hanging from the mast of the handling machine and are being transported between the two above-mentioned working bases. In the case of pressurized water handling machines, a fuel assembly, in order to laterally protect its fuel rods, is lifted into a centering bell, which is then introduced into the hollow mast of the handling machine. Since the fuel assembly is lifted several meters, the hydrostatic pressure in the surrounding water falls with respect to the internal pressure in the fuel rods, pressure equalization taking place at the leaks in fuel rods, which causes the radioactive fission products to be driven out of the defective fuel rod. Dry sipping is then possible, in which the escaping gas bubbles collect at the top of the centering bell and are extracted together with a purge gas which is introduced into the centering bell from below, displaces the cooling water and also entrains gaseous fission products which have been adsorbed on the outer surface of the fuel rods. The extracted gas can be analyzed on-line in a detector device with an electronic evaluation device, i.e. the radioactivity of the gaseous fission products which have been driven out is recorded while the fuel assembly is still hanging from the handling mast. It is possible to dispense with the introduction and extraction of the purge gas, in which case only water is extracted from the top of the fuel assemblies until virtually all the cooling water which was originally present in the fuel assembly or the centering bell has been exchanged (“dry sipping”). During the exchange of the water, it is also possible for gas bubbles that originally escaped to be dissolved or at least entrained by the flow of water and to be released again together with dissolved fission products by degassing the extracted water in a degassing device, in order for their radioactivity subsequently to be recorded in a detector device.
In the boiling-water reactor, the mast of the handling machine is simply a telescopic arm with a downwardly projecting gripper on which the fuel assembly is held outside the mast. In this case too, the above-mentioned mast sipping methods are possible if the gripper is disposed in a downwardly open hood that has been fitted over the top fitting of the fuel assembly. This is because boiling water fuel assemblies have a fuel assembly channel that laterally surrounds the fuel rods and during the sipping is responsible for the function of the centering bell in the hollow mast of the handling machine. Depending on the size of the core, the mast sipping requires 50 to 120 hours. Although the fuel assembly should if possible be tested while the fuel assemblies are being transported, the mast sipping requires additional time.
Another conventional way of saving time in boiling water reactors is to test a plurality of fuel assemblies simultaneously by a hood that is divided by side walls into individual cells for accommodating the individual fuel assemblies. The simultaneous testing of the fuel assemblies may take place as dry sipping. Although this only leads to a slight hydraulic pressure difference between the at-rest position in the core and the position in which the sipping is carried out, the expulsion of the fission gases is increased by the fact that so much gas is introduced into the hood which has been fitted over the fuel-assembly top fittings that the upper edge of the fuel assembly channel of each top fitting is positioned in a gas cushion which virtually suppresses the circulation of cooling water on the fuel rods. Therefore, the afterheat of the fuel heats the internal volume of the fuel rods and thermally generates a pressure difference that sufficiently reinforces the hydraulic pressure difference. The gas bubbles that escape through leaks and bubble upward combine with the gas cushion beneath the hood. After a predetermined heating time, the gas cushion can be extracted together with the collected gaseous fission products in order to be analyzed, for example in a laboratory, for the presence of typical fission products.
This dry sipping can also be carried out without the fuel assemblies having to be lifted so far out of the reactor core that their lower end would be accessible for the introduction of purge gas or without the fuel assemblies having to be moved in the reactor core at all, i.e. while they are on their standard working base (the lower core grid in the reactor pressure vessel or a storage rack). Although this eliminates the time required to raise the fuel assembly, and in particular the time required to reliably pick up all the fuel assemblies which are to be tested simultaneously, it is necessary, and this takes virtually the same amount of time, to position the extractor hood in a prec

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