Patent Number: 050733346
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1A, a control rod 10 having therein a neutron absorber is suspended by an electromagnet (TSEM) 14 located above a control rod guide tube 12. Around the control rod guide tube 12 are disposed wrapper tubes 18 accommodating fuel assemblies 16, respectively (See FIG. 2). A coolant (not shown) flows upwardly through the fuel assemblies 16 in the wrapper tubes 18. The principal part of the electromagnet 14 comprises, as shown in FIG. 3, a combination of an iron core 20 and an armature 22 capable of vertically latching to and unlatching from the iron core 20, and a coil 24 wound around the central leg part 20a of the iron core 20. The outer ferromagnetic member of the iron core 20 is divided into two portions 20b, 20c which are magnetically isolated from each other by a nonmagnetic material 26 interposed therebetween. The iron core 20 is connected to a drive shaft 28 on the upper side, while the armature 22 is connected to the control rod 10 on the lower side. The main feature of the present invention resides in that the upper part (at least a part facing the abovedescribed nonmagnetic material 26) of each of the wrapper tubes 18 of the adjacent fuel assemblies is made of a TSMM 30 and constitutes a part of the magnetic circuit of the electromagnet 14. This TSMM 30 has a magnetic characteristic whereby the saturation flux density thereof is reduced when the coolant temperature rises extraordinarily. At a normal operation temperature, the temperature of the TSMM 30 above the adjacent fuel assemblies 16 is below its Curie-point, and the magnetic circuit shown by a broken line a in FIG. 3 is formed. In this way, the control rod 10 is retained and suspended by the electromagnet 14. When the coolant temperature rises due to the occurrence of any abnormality in the fuel assemblies 16 or the like, the temperature of the wrapper tube 18 at a part above the fuel assembly 16 rises rapidly, and the magnetic circuit is broken at the time when the temperature exceeds the Curie-point of the TSMM 30 constituting a part of the wrapper tube 18. Then the magnetic force supporting the armature 22 is reduced, and the control rod 10 drops to be inserted into the core of the reactor as shown by a phantom line in FIG. 1A. Thus the reactor is shut down. A resetting or initial setting of the control rod 10 is conducted in the following manner. As shown in FIG. 1B, the main part of the electromagnet moves to the upper end of the control rod 10 located in the bottom position, and at this position the armature 22 is latched magnetically to retain the control rod 10. In order to enable the implementation of this operation, the TSMM 30 is provided over the range of the vertical stroke of the electromagnet 14 in the present embodiment. In other words, the length of the TSMM 30 is equal to the stroke of the control rod 10 (a length of a fuel-charged part in the wrapper tube 18). The electromagnet 14 and the control rod 10 joined in one body are pulled up by the drive shaft 28 and are set in a predetermined position. Although the control rod guide tube 12 cannot be installed within the range of the vertical stroke of the electromagnet 14, a short one can be employed as shown in FIGS. 1A, 1B. Such an installation is easily accommodated for, by making some modifications in a handling device for refueling or the like, despite the control rod guide tube 12 being shorter than a conventional one. The shape of the TSMM and the position of installation thereof can be modified variously. Examples of such modifications are shown in FIGS. 4 and 5. Since the basic concept is the same as that of the above-mentioned embodiment, the same reference numerals are used to designate corresponding components for simplicity and detailed descriptions thereof are omitted. FIG. 4 shows an example wherein an extension tube 40 made of the TSMM having a length equal to the stroke of vertical movement of the electromagnet 14 is attached to the upper end of a wrapper tube 42. According to this structure, the wrapper tube 42 and control rod guide tube 44 can be conventional, and so this structure is easy to put into practice. However, the time required for the arrival of the high temperature coolant at the TSMM becomes longer by an amount corresponding to the additional length of the extension tube 40. FIG. 5 shows another example wherein an extension tube 46 made of the TSMM is fitted to the upper end of the wrapper tube 48 to cover only the set position of the electromagnet 14 during normal operation. The upper part of the wrapper tube 48 located in the range of the vertical stroke of the electromagnet 14 is made of a ferromagnetic material 50, such as Cr-Mo steel or ferrite stainless steel, for instance, which is inexpensive and easy to manufacture. While the preferred embodiments of the present invention are described in the foregoing, the present invention is not limited only to these embodiments. For example, the upper part of the wall of the wrapper tube itself may be made of the TSMM and the ferromagnetic material. In any case, the TSMM has to only be provided at an area constituting part of the magnetic circuit in the wall (at least on the electromagnet side) of the adjacent wrapper tubes. The present invention enables a very quick response to an extraordinary rise in the temperature of the coolant, because the wrapper tube itself accommodating the fuel assembly or the extension tube of the wrapper tube is made of the TSMM. Further, since the flow of the coolant in the wrapper tube is stable, the problems presented by the conventional structure in which insufficient coolant may be introduced into the control rod guide tube can be avoided. Namely, the present invention can avoid the problems in which the coolant to be introduced into the control rod guide tube is unstable and the introduced coolant is insufficient with respect to the mass of the TSMM. As a result, a response to accidents, as well as the reliability of an actuation characteristic, are improved. Moreover, it is unnecessary to control the relative position between the electromagnet and the control rod guide tube, since the area of TSMM of the wrapper tube can be made large enough. Therefore, spurious actuation or non-actuation due to positional slippage are eliminated, and the reliability is improved in this respect as well.