Patent Number: 046817301
Section: summary

FIELD OF THE INVENTION The invention concerns the detection of leaking fuel elements in a nuclear reactor fuel assembly of the type comprising a bundle of fuel elements or "rods", held in a rigid structure including end pieces connected together by braces and grids spaced apart along the braces. Each element is formed by a metal tubular sheath closed by gas tight caps, containing a stack of nuclear fuel pellets surrounded by a dry atmosphere of gas under pressure, when the sheath is fluid tight. The pellets are held in contact by a spring compressed between the stack and an end cap and placed in a plenum occupied by the gas under pressure. BACKGROUND OF THE INVENTION It is necessary to check the fuel assemblies extracted from the core of the reactor. The conventionally used method consists in measuring the activity released by an assembly when it is plunged in a fluid. But with this method it is only possible to check that the whole assembly is tight. It does not allow to detect which element or elements of a defective assembly is or are leaking. Numerous methods for non-destructive testing of the fuel elements by using ultrasounds already exist. Among others, there is one (EP-A 0 115 231) which consists in the emission of a train of ultrasonic waves, fed into the sheath from one of its end parts, at a frequency selected so that they travel in the form of Lamb waves; the echoes are then detected and analyzed. Such detection is performed at frequencies whose range corresponds for one fraction to notable absorption by the water possibly contained in the sheath, and for another fraction to a reflection from the end of the spring, especially in case of pressurization. This solution is perfectly satisfactory when the waves are emitted and received by the transducer placed on the end face of the cap which closes the fuel element. But up to now it has only been possible to control all the elements using such a process when one of the end pieces of the assembly has been previously removed, because of the presence of the braces, of a diameter substantially greater than that of the fuel elements, and the differences of level of the caps in the assembly which have sejourned in the reactor. Unfortunately, the replacement of an end piece, in many assemblies, represents a delicate operation. To overcome the need of dismantling, devices have also been proposed (FR-A-2 341 183) allowing a transducer to be introduced into the bundle, transversely to the axis thereof, and to be applied against the side wall of the sheath of any one of the fuel elements. By advancing the transducer all the elements of the same network may be checked. The checking time may be further reduced by providing several transducers, carried by a comb shaped piece. But the use of transducers applied against the side wall of the sheath gives less favorable results than that of sensors applied against a cap and leaves unsolved the problem of the masking of some elements by braces; in addition, this use is independent of the position of the water in the element. SUMMARY OF THE INVENTION It is an object of the invention to provide a process and a device for detecting leaking elements in an irradiated assembly giving better answers than those known especially in transducer placed at the end of each element and having good contact therewith. To this end, the invention proposes a process of the above defined type, characterized in that, in the gap between one of the end pieces of an irradiated assembly and the fuel elements, a transducer is introduced through rod means movable transversaly to the elements, until the transducer is above the cap of an element; the rod means are moved in the longitudinal direction of the assembly so as to apply the transducer against the end face of the cap of the element to be checked before emitting a train of ultrasonic waves and detecting the echoes; the transducer is raised towards the end piece through rod means; the rod means are then removed for repeating the operations so as to check new elements. The rod means are advantageously introduced in a diagonal direction of the elements network, when this network is square, which is the most general case; it is in fact in this orientation that the presence of the braces is the least obstructive. To have access to all the elements, despite the presence of the braces, the test may be carried out in two or four steps, the assembly being rotated by 90.degree. about its longitudinal axis after each step. The invention also provides a device for implementing the above defined process. This device for ultrasonic detection of the leaking fuel elements in a nuclear reactor fuel assembly comprising a bundle of fuel elements held in a rigid structure including end pieces connected together by braces, comprising an ultrasonic transducer carried by rod means of dimensions such that it may be inserted in the assembly between an end piece and the caps of the elements, and comprising crossed movement maneuvering means for bringing the transducer above the different elements, is characterized in that it includes means for moving the rod means in the longitudinal direction of the elements, and in that the transducer is mounted on the rod means with at least one degree of freedom about an axis transversal to the elements so as to be applied flat against the end face of a cap.