Patent Number: 056384149
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the following, description will be given of an embodiment of the present invention in connection with the drawings. FIG. 1 represents the structure of a fuel assembly for a basic type fast reactor. FIG. 1 (a) is a general bird's-eye view of a fuel assembly, FIG. 1 (b) shows the fuel assembly, and FIG. 1 (c) is a cross-sectional view of the fuel assembly. In these figures, reference numeral (1) represents a fuel assembly, (11) represents a handling head, (12) an upper spacer pad, (13) an intermediate spacer pad, (14) a wrapper tube, (15) a wire spacer, (16) a lower spacer pad, (17) an entrance nozzle, (2) a fuel element, (21) an upper end plug, (22) a cladding tube, (23) a gas plenum, (24) an upper blanket fuel pellet, (25) a core fuel pellet, (26) a lower blanket fuel pellet, and (27) a lower end plug. In the fuel assembly (1), wrapping wires in spiral shape wound on fuel element (2) that closely fitted adjacent fuel elements are not brought into contact with each other. The upper end of the fuel assembly is the handling head (11) for suspending the fuel assembly. Each of the upper spacer pad (12), the intermediate spacer pad (13), and the lower spacer pad (16) is designed larger than that of the wrapper tube to prevent the wrapper tubes from being in contact with each other in the reactor. On the lower portion of the fuel assembly the entrance nozzle (17) is provided for the liquid sodium coolant. In the fuel element the upper blanket fuel pellet (24), the core fuel pellet (25) and the lower blanket fuel pellet (26) fill the cladding tube (22) as shown in FIG. 1 (b). The gas plenum (23) for accommodating the released fission products is provided above the upper blanket fuel pellet (24). Upper and lower ends are closed with the end plugs (21) and (27), and ends of the wire spacer (15) are welded to these end plugs. In general, fission products are accumulated in a burnt fuel element and most of them are radioactive substances, mainly radiating gamma rays. These radioactive substances collect in the gas plenum (23) as fissioned gas. This F.P gas can be utilized as a nuclide, which offers information on failure of the fuel element. Specifically, when the fuel element fails the F.P gas accumulated in the gas plenum (23) is released. As a result, the failed fuel element has a lower radiation intensity than a normal fuel element. In particular, such a difference in the radiation intensity becomes extremely marked at the gas plenum where the F.P gas is accumulated. Accordingly, it is possible to identify the failure by determining major nuclides in the F.P gas in the gas plenum, i.e. krypton (Kr), xenon (Xe) or iodine (I). The present invention is based on these findings. By obtaining a tomographic image of intensity distribution of the radiation emitted from the nuclides, using the ECT method, it is possible to identify the position of a failed fuel element, which may be present in the fuel assembly, and to display it on a cross-section of the fuel assembly. Unlike the methods used in the past, the failure can be detected quickly, accurately and economically without disassembling the fuel assembly. FIG. 2 shows a method for the identification of failed fuel elements according to the present invention. FIG. 3 represents a general configuration of a detecting system, and FIG. 4 represents positions of failed fuel elements detected by the method of the present invention. Explaining the arrangement of the detecting system referring to FIG. 3, a fuel assembly (1) is supported on a base (6) and is movable in the vertical direction and is also rotatable. A radiation detector 3 is provided which moves along a rail on a mobile base (4) and detects radiation from the fuel assembly (1). The radiation detector is driven and controlled by an ECT processing and drive control system (7) at a position shielded by a radiation shielding member (8). Thus, a CT image can be obtained. In FIG. 2, the fuel assembly (1) is moved in the vertical direction, and the gas plenum is set, for example, to a position where radiation can be detected by the radiation detector. The radiation detector (3) scans, with a predetermined spacing, a plane perpendicular to the axial direction of the fuel assembly, and radiation from a given direction is detected by a collimator (32) via a slit (31). The fuel assembly is rotated by one turn around the axis with given angular movements for each translated position of the radiation detector. Based on the radiation intensity data thus obtained, a tomographic image of radiation intensity distribution is obtained by means of the ECT processing and drive control system (7). Although the fuel assembly is rotated in the above, it is needless to say that the radiation detecter may be rotated instead. In FIG. 4 (a), there are four failed fuel elements marked with black circles in a fuel assembly. Some radiation from the gas plenum is from an activated nuclide in the 316 stainless steel used as the material for the wrapper tube and the fuel element. In the tomographic image of total radiation, only the outline of the fuel element from which the F.P gas has been released is displayed. In contrast, in a tomographic image using the F.P gas nuclide, only the information on the position of the gas plenum is given. Thus, radiation intensity is lower in the portion of the fuel element, from which F.P gas has been discharged, and the portion of the fuel element is displayed as an empty space as shown in FIG. 4 (c). As shown in FIGS. 4 (b) or (c), it is possible according to the present invention to use either of the tomographic images. As explained above, the high intensity fission products of the spent fuel element itself are used as the radiation source in the present invention, and the position of the radiation source is defined within the fuel element. As a result, the failed fuel element can be detected efficiently and accurately using emission computer tomography. This makes it possible to overcome inefficiency in the conventional method for identifying the failed fuel element in a fuel assembly, to provide timely action for operation and control of reactor core and to improve efficiency in nuclear reactor operation.