The present invention relates to protection of nuclear fuel assemblies and, more particularly, to a method of and an arrangement for protecting a fuel assembly such as to avoid damaging and scratching of the fuel rod surfaces during assembly and transportation of the fuel assembly.
Different types of fuel assemblies are used in different types of nuclear reactors such as boiling water reactors (referred to as "BWR", hereinunder), pressurized water reactors (PWR) and so forth. Generally, however, a fuel assembly is an elongated member in which a plurality of elongated fuel rods are arranged and held in parallel with one another by spacer assemblies.
FIGS. 1 to 3 show, by way of example, a fuel assembly for use in a BWR having a square cross-section with fuel rods arranged in a square pattern. Known also is a fuel assembly having a circular cross-section in which fuel rods are arranged along concentric circles. This type of fuel assembly is suited for use in heavy-water moderated reactors and gas-cooled reactors.
The spacer assembly for holding the fuel rods in parallel can have various forms such as rectangular grids and circular grids. The fuel rods are held in the cells of the spacer assembly by means of holders such as a spring-clip type holder and lantern-ring type holder.
The fuel assembly shown in FIG. 1 will be explained in detail. This fuel assembly has a channel box 1 accomodating a plurality of fuel rods 4 and water rods 5 which are supported at their upper and lower ends by an upper tie plate 2 and a lower tie plate 3. A plurality of spacer assemblies 6 are disposed in the channel box 1 in the axial direction of the latter, such as to hold the fuel rods 4 in parallel and to suppress any lateral vibration of the same, while preserving passages of the coolant.
As will be seen from FIGS. 2 and 3, the spacer 6 has a square frame 7 in which are disposed bars 8, 9 and dividers 10, 11 in a grid-like form thus defining a plurality of cells 12. A lantern spring 13 is provided at each point of intersection of the bars 8 and 9. Each lantern spring 13 has leaf springs which project outwardly from all sides thereof. The leaf springs 13a resiliently press the fuel rod 4 in each cell 12 against S bends 10a, 11a formed on the dividers 10, 11 and dimples 14 projecting inwardly from the walls of the frame 7.
For the assembly of the fuel assembly, the lower tie plate 3 and the spacer assemblies 6 are fixed at suitable positions and the fuel rods 4 and the water rods 5 are inserted into the cells 12 of the spacer assemblies 6, such that the lower ends of the rods 4, 6 are fixed in the lower tie plate 3. Then, the upper tie plate 2 is mounted on the upper end of the frame 7 thus fixing the fuel rods 4 and the water rods 5.
During the assembly of the fuel assembly, the surfaces of the fuel rods tend to be damaged because they slide on the surfaces of the leaf springs 13a of the lantern spring 13 and the S-bends 10a and 11a. The same problem is encountered not only by the spacer assembly shown in FIG. 2 but also by various other types of spacer assemblies.
Hitherto, various methods have been proposed to protect the surfaces of the fuel rods. For instance, it has been proposed to tie the leaf springs 13a and S-bends 10a, 11a by cords against their resiliency thus preserving a large area in each cell 12 for receiving the fuel rod. It has been also proposed to keep the fuel rod below the ice point such as to form a coating layer of ice or frost on the fuel rod surface. According to still another method, the fuel rods are beforehand painted with a lacquer and, after the assembly, the lacquer is washed away.
The protection method relying upon the ice or frost coating layer necessitates pre-cooling of the fuel rod down below the ice point. In addition, this method is not adoptable for the fuel rod protection during withdrawal which often becomes necessary when, for example, the defective fuel rod is renewed, because the formation of the ice or frost coating layer on the fuel rod situated in the fuel assembly is almost impossible. The protection method using the cords is quite troublesome and takes a long time. The use of a lubricant may impair the performance of the fuel assembly due to deposition of impurities on the fuel rod. In order to completely remove the lubricant, it is necessary to wash it away at a cost of time and labour.
Thus, the conventional fuel rod protection methods are generally laborious and hinders the assembling.
The damaging of the fuel rods in the spacer assembly takes place not only during the assembly but also during transportation. Namely, during transportation of the completed fuel assembly, the fuel rods which are held in pressure contact with the S-bends 10a and 11a by means of the resilient pressure members such as the leaf springs are liable to be fretted due to vibration of the fuel assembly. To avoid this problem, it has been proposed to use flexible plastic liner 21 of a length almost equal to that of the fuel rod. Namely, as shown in FIG. 4, a liner 21 is placed at each side of the fuel rod in each cell, so as to suppress the vibration of the fuel rod and to prevent fretting of the fuel rod surface by the spacer assembly. However, the fuel assemblies 4a adjacent the walls of the frame 7 are likely to be damaged due to vibration, because these rods are pressed onto the dimples 14 by the leaf springs 13.
Japanese Patent Laid-Open No. 4609/1976 discloses a rod-like jig having a plurality of tabs designed for pressing the springs 13. On the other hand, Japanese Patent Laid-Open No. 36282/1977 and Japanese Patent Laid-Open No. 16089/1979 disclose jigs which are designed to be inserted into cells 12 such as to press the springs 13.
These known jigs are intended to retract the springs 13 in advance to the insertion of the fuel rods, thereby smoothing the movement of the fuel rods during insertion. However, these jigs cannot prevent the damaging of the fuel rods perfectly because, although the springs are retracted beforehand to become inactive, the fuel rods slide on the S-bends 10a, 11a and the dimple 14 when they pass through the cells 12. In some cases, the spring 13 is adversely affected by an excessive depressing by the jigs.
Japanese Patent Publication No. 11244/1978 discloses a fuel rod protection method in which a stainless steel protecting tube of about 0.1 mm thick is fitted around the fuel rod, and the fuel rod is inserted into the cell 12 together with this protecting tube. However, insertion and withdrawal of the fuel rod into and out of such a thin protecting pipe are troublesome. In addition, the fabrication and handling of such thin-walled long tube are quite difficult.