Patent Number: 061545147
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the invention is described hereinafter with reference to the attached drawings. First, the structure of a fuel assembly 10 will be described referring to FIG. 2. In FIG. 2, the fuel assembly 10 is shown in a state where it has been placed between a lower core plate "A" and an upper core plate "B" and an upper nozzle 11 of the before described construction and a lower nozzle 13 placed on the lower core plate "A" are connected to each other with a plurality of hollow guide tubes 15. The hollow guide tubes 15 are generally used to guide control rods (not shown), to which a plurality of control rod support grids 17 positioned with a distance between one another in the lengthwise direction, are secured. These support grids 17 have grid openings located in 15 lines and 15 rows and a plurality of fuel rods 19 are individually placed through the many openings which do not receive the hollow guide tubes 15 and are thereby supported by the support grid 17. In addition, the hold-down springs 20 are fitted on an upper surface of the upper nozzle 11 along two parallel side edges in a conventional manner. Referring to FIG. 1, the structure of the hold-down spring 20 will be further described in detail. In FIG. 1, the upper nozzle 11 has a generally rectangular nozzle plate 11a in which a number of coolant flow holes are distributively machined, and a box-shaped side wall 11b extending upward from the circumferential edge of the nozzle plate 11a. A base end of the hold-down spring 20 is fastened and secured with a fitting bolt 18 at a holding section 11c formed on an upper corner section of the side wall 11b. The hold-down spring 20 is composed of an upper spring 21 and a lower spring 23 and the upper spring 21 is a plate spring with a generally constant width and fabricated by being shaped from a blank of Inconel 718 (trade name) or a precipitation hardened nickel base alloy. Further, as clearly shown in FIG. 1a in particular, the upper plate spring 21 has a main body 21a, a bent portion 21b and a vertically oriented portion 21c. The lower plate spring 23 has a slot 23a extending widthwise at its distal end portion for receiving the vertically oriented portion 21c. Thus the upper and lower plate spring 21, 23 are combined with each other so as to form the upper hold-down spring 20 as shown in FIGS. 1 and 1a. As also shown in FIG. 1a, the upper plate spring 21 has an abutting ledge 21d which comes contact with an upper surface of the lower plate spring 23 after an initial deformation and thereafter the upper and lower plate spring 21, 23 work as an integral spring means. When the fuel assembly 10 equipped with the hold down spring 20 constructed in the above described manner is loaded in the core of a nuclear reactor in the state shown in FIG. 1, an initial deformation 20 is imparted to the hold-down spring. Since in such a cold state, the reactor coolant is forced to flow through the core by a coolant pump, the degree of the initial deformation is determined from its spring characteristics so as to withstand a floating force acting on the fuel assembly 10. Further, when the nuclear reactor reaches power operation and a "hot" condition, the thermal expansion of the inner core structure including the lower core plate "A" and the upper core plate "B" becomes relatively larger resulting in a decrease in the deformation of the spring. The relationship among the cold state spring force F.sub.C, the hot state spring force F.sub.H and the hot state spring stress .sigma..sub.H which correspond to such deformation and spring thickness t is illustrated in FIG. 3. In order to comprehensively show a state in which these relationships change with the number of spring plates n as a parameter, cases in which n equals 3 (conventional structure) and n equals 1 are also illustrated as references. In addition, in a graph of the cold state spring force F.sub.C and the hot state spring force F.sub.H, the lower and upper limit values are determined by the force needed to prevent the rise of the fuel assembly 10 and the limit permissible in view of the strength of the upper nozzle, respectively. As is evident from FIG. 3, the hold-down spring 20 (n=2) of the preferred embodiment according to the present invention satisfies the required conditions for the cold state spring force F.sub.C and the hot state spring force F.sub.H in the range where the thickness t of the spring is 4.6.about.5.2 mm, and the cold state spring stress .sigma..sub.H is also confined in a range where the sensitivity to stress corrosion cracking is small. Moreover, in the conventional structure (n=3, indicated by stars in the drawing), the thickness t of the plate spring is 4.0 mm which satisfies the required conditions of the cold state spring force F.sub.C and the hot state spring force F.sub.H, but the hot state spring stress .sigma..sub.H is relatively large. Though it is possible to decrease the number n of plate springs to 1 so as to further lower the hot state spring stress a .sigma..sub.H, such an option satisfies the requirements of the cold state spring force F.sub.C, but does not satisfy those of the hot state spring force F.sub.H. As a result, such an option is not suitable. Furthermore, in the above described preferred embodiment, since the upper plate spring 21 has the same plastic spring characteristics as the conventional spring, the hot state spring stress .sigma..sub.H will not vary significantly when the fuel assembly 10 grows under exposure to neutrons. As described above, according to the present invention, the suitable number of plate springs constituting the hold-down spring fitted onto the upper nozzle of the fuel assembly is established as two (2), and the thickness of the plate spring is set in a suitable range thereby lowering the hot state spring stress .sigma..sub.H and the sensitivity to stress corrosion cracking. As a result, the fuel assembly can be held down and kept at the predetermined suitable position during operation of the reactor and stress corrosion cracking can be prevented.