Patent Number: 061920967
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Next, the preferred embodiment of the magnetostrictive wire control rod position detector assembly according to the present invention will be explained with reference to the attached drawings. In the drawings, use of the same numeral indicates identical or corresponding portions. In the following explanation, compositional elements which are similar throughout the figures will be given the same reference numeral. Moreover, in the following explanation, the terms "left", "right", "up", "down", etc., will be used for convenience, but these terms should not be interpreted to limit the invention in any way. FIG. 1 is a perspective view showing the overall construction of a magnetostrictive wire control rod position detector assembly 20 mounted at the upper end of a control rod drive unit 5, FIG. 2 is a vertical cross-section of part thereof, and FIG. 3 is an enlargement of part of FIG. 2. In these figures, the magnetostrictive wire control rod position detector assembly 20 comprises: permanent magnets 21 mounted on an upper end portion of a drive shaft 3 positioned within a pressure housing 1 of the control rod drive unit 5; and magnetostrictive wire detectors 22 secured to the outside of the pressure housing 1 by support structures 23. The magnetostrictive wire detectors 22 are also provided with receivers on the upper end portions thereof above a support member 25. Moreover, support cylinders 24 are disposed on the outside of the support structures coaxial to the pressure housing 1, and spaces 27 are defined between the support cylinders 24 and the magnetostrictive wire detectors 22. The magnetostrictive wire detectors 22, the lengths of which cover the drive stroke of a control rod 4, are disposed longitudinally along the pressure housing 1 and are secured in the circumferential direction by a plurality of annular support structures 23 so as to be in close contact with the outside of the pressure housing 1. The magnetostrictive wire detectors 22 can be placed in close contact with the pressure housing 1, whose internal temperature is approximately 300 degrees Celsius, because the detectors 22 are treated to make them heat-resistant to high-temperature specifications. The reason for placing the detectors 22 in close contact is to improve precision during detection of the position of the control rod. Furthermore, the mounting of one magnetostrictive wire detector 22 is sufficient from the viewpoint of measurement. However, in order to improve precision further, it is preferable to mount detectors 22 in sets of two. Moreover, when used as a protective system, four detectors should be mounted around the pressure housing 1 in sets of two. In this embodiment, four magnetostrictive wire detectors 22 are disposed with even spacing around the circumference of the pressure housing 1 as shown in the horizontal cross-section in FIG. 4 (a). Consequently, the magnetostrictive wire detectors 22 are able to detect the drive shaft 3 very precisely and accurately even if the drive shaft 3 is inclined within the pressure housing 1. This is desirable not only for a reactor control system, but also from the viewpoint of a reactor protection system, as will be explained in detail below. As shown in FIG. 3, the magnetostrictive wire detectors 22 comprise magnetostrictive wires 28 and protective tubes 29 consisting of stainless steel tubing with a diameter of about 10 mm. Moreover, the magnetostrictive wires 28 are constructed so as to prevent contact with the protective tubes 29 on the outside thereof by the method described below. As shown in FIGS. 1 and 2, the magnetostrictive wire detectors 22 comprise receivers 26 for detecting torsional waves (explained below) which propagate through the magnetostrictive wires 28 disposed at the upper end of the magnetostrictive wire detectors 22. FIG. 5 is a structural diagram showing part of one of these receivers 26. In this diagram, the receiver 26 comprises a casing 30 connected to the protective tube 29 of a magnetostrictive wire detector 22. A support member 31 is disposed within the casing 30. The magnetostrictive wire 28 is held at the center of a magnetostrictive wire holder 32 secured to the support member 31. The magnetostrictive wire 28 is prevented from contacting the protective tube 29 by being held in the center of the tube and by means of guide rings 33 disposed within the protective tube 29. Detector coils 34 are mounted on the support member 31 slightly lower than the middle of the casing 30 in the longitudinal direction so as to surround the magnetostrictive wire 28. Moreover, the signals detected by the detector coils 34 are sent to a control portion (not shown) by means of a cable 35. Furthermore, the upper end of the magnetostrictive wire 28, in other words, the end held by the magnetostrictive wire holder 32, is connected through the cable 35 to a pulsed current generator circuit or circuits 37. Furthermore, the magnetostrictive wire holder 32 is composed of a vibration absorbing material in order to prevent reflection of the torsional waves propagated in the magnetostrictive wire 28. Returning to FIG. 3, the permanent magnets 21 mounted on the upper end portion of the drive shaft 3 are formed into a ring shape. Furthermore, in order to apply the principle of position detection by means of a magnetostrictive wire (explained below) without major modifications to the conventional control rod drive unit 5, the permanent magnets 21 are mounted in a cylindrical formation longitudinally on the outer circumference of the drive shaft 3. That is to say that, whereas a conventional magnetostrictive wire detector 50 like that shown in FIG. 7 places a magnetostrictive wire 52 inside a ring-shaped permanent magnet 51, in this embodiment, there is a certain distance between the drive shaft 3 and the pressure housing 1, and furthermore, the permanent magnets 21 are relative to one side of the magnetostrictive wires 28 outside, and therefore the permanent magnets 21 are disposed in a cylindrical formation so that the magnetic field outside the pressure housing 1 is strong. Moreover, as shown in FIG. 3, the permanent magnets 21 comprise three magnets. However, using one integrated cylindrical permanent magnet strengthens the magnetic field and improves performance. Furthermore, when a magnetic jack control rod drive unit is used as the control rod drive unit 5, there is a possibility that the drive shaft 3 will vibrate during operation. As a result, there is a possibility that the drive shaft 3 may move radially and contact the inside of the pressure housing 1, damaging the permanent magnets 21. Consequently, the permanent magnets 21 are protected by mounting a protective tube 36 made of the same material as the drive shaft 3 onto the outside of the permanent magnets 21 after mounting the ring-shaped permanent magnets 21 onto the drive shaft 3. Furthermore, by sealing the permanent magnets 21 between the drive shaft 3 and the protective tube 36, oxidation of the permanent magnets 21 can be prevented. Moreover, magnetic material was used in the conventional drive shaft 3, but in the present embodiment, the upper end portion of the drive shaft 3 and the protective tube 36 are composed of a non-magnetic metal in order to make position detection signals from the permanent magnets 21 clear. Next, the method of detecting the position of a control rod 4 by means of the magnetostrictive wire control rod position detector assembly 20 will be explained using FIGS. 6 (a) to (c). As shown in FIG. 6 (a), in its initial state, the magnetostrictive wire 28 of the magnetostrictive wire detector 22 is positioned within the magnetic field M of the permanent magnets 21 mounted on the drive shaft 3. However, the magnetostrictive wire 28 is not twisted by the magnetic field M of the permanent magnets 21 alone and remains in its initial state. First, in this condition, a pulsed current 40 is imparted towards the magnetic field M in the longitudinal direction of the magnetostrictive wire 28 from the receiver 26 end by the pulsed current generator circuit 37. In so doing, a torsional magnetic field MP is generated around the magnetostrictive wire 28 in accordance with the right-hand rule, and the torsional magnetic field MP propagates along the magnetostrictive wire 28 towards the magnetic field M of the permanent magnets 21. Because the propagation velocity is extremely fast, this is taken as the "0" count for measurement purposes. Next, as shown in FIG. 6 (b), when the pulsed current 40 approaches the permanent magnets 21 mounted on the drive shaft 3, that is to say, when the torsional magnetic field MP of the pulsed current 40 interferes with the magnetic field M of the permanent magnets 21, the magnetostrictive wire 28 is instantaneously twisted. As a result, torsional waves 41 are generated along the magnetostrictive wire 28 in both directions from the point at which the two magnetic fields interfere and twisting occurs. Then, as shown in FIG. 6 (c), the propagation time from the imparting of the pulsed current 40 to the arrival of the torsional wave 41 is measured by the receiver 26 disposed at the upper end of the magnetostrictive wire detector 22. By calibrating the measured propagation time against the longitudinal propagation velocity in the magnetostrictive wire 28, it is possible to calculate the absolute value of the distance from the receiver 26 to the permanent magnets mounted on the upper end portion of the drive shaft 3. Using this method, it is possible to precisely and accurately measure the physical position of the control rod 4 to within the length of one control rod drive step (approximately 16 mm). As a result, it is possible to check for the presence of discrepancies in the position of the control rod 4. Furthermore, it is also possible to reduce the design margins introduced into core designs to compensate for uncertainties in the positions of the control rods, leading to merits such as increased output. Next, the measurement principle shown in FIGS. 6 (a) to (c) can be used to measure the descent times (insertion times) of a control rod 4. That is to say, by generating pulsed currents 40 at predetermined time intervals and continuously measuring the physical position of the control rod 4 by means of the magnetostrictive wire detectors 22, the changes in the position of the control rod 4 corresponding to the commencement of descent of the drive shaft 3 until the drive shaft 3 stops can be accurately measured. As a result, the descent times (insertion times) can be stored as high-precision digital data. Using these data, the times taken for the control rod 4 to reach the position of the dashpot, the fully inserted position, or any intermediate position from the commencement of descent can be accurately calculated. For that reason, the precision of measurement of the descent times of the control rod 4 can be improved, ensuring reliability of the reactor. Furthermore, since the changes in descent velocity of the control rod 4 and the confirmed positions of the control rod 4 are stored as digital data, even in the rare event that the control rod 4 stops during descent, it is possible to ascertain the rest position of the control rod 4 (fully inserted or partway) accurately. Consequently, as the accuracy and reliability of the detection of the position of the control rod 4 are improved, it is also possible to determine easily whether the control rod 4 has been completely inserted or not. Because the longitudinal position of the control rod 4 can be continuously ascertained with high precision and the lowest inserted position (fully inserted position) of the control rod can be accurately detected in this manner, it is possible to reduce the design margins introduced into core designs to compensate for uncertainties in the positions of the control rods, and the design merits of the control system enable increased safety margins, increases in output, etc., to be considered in the operation of the reactor. In this embodiment, as explained above, four magnetostrictive wire detectors 22 are disposed with even spacing around the circumference of the pressure housing 1 (see FIG. 4 (a)). Here, one system is composed of a pair of magnetostrictive wire detectors 22 disposed in positions 180 degrees opposite to each other, and two such systems are installed. A plurality of magnetostrictive wire detectors 22 are mounted in this manner so that the position of the drive shaft 3 can be precisely and accurately detected even if the drive shaft 3 is inclined within the pressure housing 1, as shown in FIG. 4 (b). For example, as shown in this figure, if the drive shaft 3 is positioned centrally within the pressure housing 1 (see center line 3a), the magnetic field M1 of the permanent magnets 21 is equal on the left and right, but when the drive shaft 3 is inclined to the right (see center line 3b), the magnetic field M2 of the permanent magnets 21 is stronger on the right, and when the drive shaft 3 is inclined to the left (see center line 3c), the magnetic field M3 of the permanent magnets 21 is stronger on the left. Consequently, if a magnetostrictive wire detector 22 were mounted only on the right, when the drive shaft 3 was inclined to the left the magnetic field M3 of the permanent magnets 21 would be weak on the right and there would therefore be a risk that the torsional magnetic field of the pulsed current propagated in the magnetostrictive wire detector 22 would interfere in a different position and precision would deteriorate. However, when a pair of magnetostrictive wire detectors 22 are disposed in positions 180 degrees opposite to each other, the detection function is multiplied, and even if the drive shaft 3 is inclined within the pressure housing 1, the precision of detection of the position of the drive shaft 3 can be increased by averaging the positional data detected by the pair of magnetostrictive wire detectors 22. Nevertheless, even if only one magnetostrictive wire detector 22 is mounted and the drive shaft is inclined to the maximum, the position of the drive shaft 3 can still be measured precisely to within a drive step (16 mm). Furthermore, by installing a plurality of systems of pairs of magnetostrictive wire detectors 22, even if one detector fails, the position of the drive shaft 3 can be detected by the other detectors, enabling the invention to be applied not only to reactor control systems, but also to reactor protection systems. Also, by incorporating more precise position signals for control rods 4 into reactor protection systems, operability can be improved, the number of thermocouples, etc., can be reduced, output can be increased further, etc., contributing significantly to plant rationalization. Furthermore, in this embodiment, as explained above, support cylinders 24 are disposed coaxial to the pressure housing 1 on the outside of the support structures 23 which seal and secure the magnetostrictive wire detectors 22 to the pressure housing 1. A layer of air exists in the space 27 sealed by the magnetostrictive wire detectors 22, the support cylinders 24 and the support member 25. Consequently, a heat insulating effect is generated by the layer of air (see FIGS. 1 and 2). As a result, a large amount of the radiant heat from the high-temperature interior (approximately 300 degrees Celsius) of the pressure housing 1 can be shut out, enabling the volume of the cooling equipment for the control rod drive unit to be reduced. In this embodiment, the permanent magnets 21 mounted on the drive shaft 3 are ring-shaped, but the permanent magnets are not limited to this shape, and half-rings or flat bars can also be mounted. Furthermore, the permanent magnets 21 are mounted on an upper end portion of the drive shaft 3, but the permanent magnets 21 can be mounted on any position on the drive shaft 3, provided that the position of the control rod can be detected from the fully withdrawn position to the fully inserted position. In addition, in this embodiment, only the upper end portion of the drive shaft 3 is composed of non-magnetic material, but the entire body thereof can be composed of non-magnetic material. As explained above, in this embodiment, the magnetostrictive wire control rod position detector assembly 20 can be constructed simply by mounting magnetostrictive wire detectors 22 and receivers 26 on the outside of the pressure housing 1 of the control rod drive unit 5 and mounting permanent magnets 21 on an upper portion of the drive shaft 3, enabling the existing control rod drive unit to be used without modification. The embodiment of the present invention presently considered to be ideal and other alternative embodiments have been explained in detail with reference to the drawings, but the present invention should not be limited to these embodiments, and additional applications and modifications of any sort to the magnetostrictive wire control rod position detector assembly can be easily conceived and realized by those skilled in the art without deviating from the spirit and scope of the present invention.