Patent Number: 061920967
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a position detector assembly for a control rod in a nuclear reactor, and in particular relates to a control rod position detector assembly employing a magnetostrictive wire which works in cooperation with a control rod drive unit to continuously monitor the longitudinal position of the control rod relative to the reactor core. 2. Description of the Related Art FIG. 8 is a cross-section of the construction of a pressurized water nuclear reactor. As shown in the figure, the output of the reactor is controlled by control rod drive units 5 disposed in an upper portion of a reactor vessel 6, which insert and extract control rods 4 into and out of the reactor core. The control rods are moved longitudinally by longitudinally moving drive shafts 3 connected to the control rods 4 within pressure housings 1. The positions of the drive shafts 3, that is, the positions of the control rods 4 relative to the reactor core, are detected by control rod position detector assemblies 7 comprising detector coils 2 disposed around the outer circumference of each of the pressure housings 1. FIG. 9 is a cross-section showing the relationship between the control rod drive unit 5 and the conventional control rod position detector assembly 7. As shown in the figure, the conventional control rod position detector assembly 7 comprises detector coils 2 mounted on the outside of the pressure housing 1 of the control rod drive unit 5. Detector coils 2 corresponding in number to the length of the control rod 4 when withdrawn from the reactor core, usually forty-two, are mounted with even spacing on a coil support pipe 8 on the outside of and coaxial to the pressure housing 1. In anticipation of events such as breakages in the wiring, the detector coils are divided into two systems consisting of an A system comprising the set of alternate detector coils 2a and a B system comprising the set of detector coils 2b. The spacing between adjacent detector coils is approximately 90 mm, or when each system is considered separately, 180 mm because of the alternation. At the same time, the drive shaft 3 of the control rod drive unit 5, which is the portion whose position is detected, is usually composed of a stainless magnetic material. As a result, the drive shaft 3 itself is magnetized because the magnetic field from the control rod drive unit 5 is strong. Since the temperature within the pressure housing 1 is approximately 300 degrees Celsius, limits on the working temperature of the insulating materials used in the detector coils 2 of the control rod position detector assembly 7 make cooling the coils compulsory. For that reason, air is supplied to the space 9 between the pressure housing 1 and the detector coil support pipe 8 on which the detector coils are mounted and the detector coils are air-cooled from within as shown in FIG. 9. Next, the method of detecting the position of a control rod 4 by means of the detector coils 2 of the control rod position detector assembly 7 will be explained. When the magnetic drive shaft 3 passes through the center of a detector coil 2, an electric potential is induced in the detector coil 2 and as a result the impedance in the detector coil 2 changes. Consequently, by detecting the changes in impedance in each of the detector coils 2, the position of the tip of the magnetic drive shaft 3 can be detected as it moves inside the pressure housing 1 of the control rod drive unit 5, and thus the position of the control rod 4 within the nuclear reactor can be ascertained. Also, in order to ensure the reliability of the reactor, it is necessary to measure the descent times (insertion times) of the control rods. The method of measuring the descent times of a control rod by means of the control rod position detector assembly 7 is to measure the insertion times from when a control rod starts to descend until it reaches a dashpot 10 (see FIG. 10) by means of the changes in electric potential (changes in velocity) in the generated electric currents which depend on the descent velocity of the magnetic drive shaft 3 as it passes through the detector coils 2. Thus, as shown in FIG. 11, when the descent velocity of the control rod 4 is fast, the electric potential of the electric current generated in an detector coils 2 rises, and when the velocity of the control rod 4 suddenly decreases, the electric potential suddenly decreases. In order to make use of such changes in the electric potential of the electric current generated in the detector coils 2, the dashpots in a fuel assembly which decelerate control rods 4 by means of fluid resistance are each disposed in a position approximately 85 percent of the fully inserted position. Consequently, the position of each control rod 4 can be precisely determined by a sudden decrease in electric potential at a position such as T1 shown in FIG. 11. Moreover, whether or not the control rod 4 has been completely inserted into the reactor core is determined, as shown in FIG. 11, by detecting the rebound waveform R up to the rest point T2, that is, the waveform of the rebounding of the drive shaft 3 due to shock absorbing springs mounted on the control rod clusters as the drive shaft 3 of the control rod 4 reaches the bottom end. However, since the position of the tip of the drive shaft 3 is detected by changes in impedance in the detector coils 2, signals indicating the position of the control rod 4 can only be obtained at the positions of the detector coils 2. In other words, the intervals at which the position of the control rod 4 can be detected, depend on the spacing at which the detector coils are mounted, which is approximately 90 mm. Generally, a control rod drive unit 5 drives a control rod 4 in steps, the length of each of these steps being approximately 16 mm. Consequently, one problem is that the physical position of the control rod 4 can only be confirmed at intervals corresponding to several drive steps. Furthermore, the detector coils 2 are divided into two systems, the A system and the B system, and when one system cannot be used because of circuit failure, the position can only be detected at the single system intervals of 180 mm, in other words, intervals approximately ten times the length of a drive step of the control rod 4, further reducing accuracy. Consequently, from the viewpoint of a protective system for the reactor, there is a need to consider the uncertainty of the position of the control rods when designing reactor cores. In addition, as explained above, measurement of the descent times of the control rods according to the control rod position detector assembly 7 involves measuring the insertion times from the commencement of descent until a dashpot is reached by means of the changes in electric potential in the electric currents which depend on the descent velocity of the magnetic drive shaft 3 as it passes through the detector coils 2, and it is well known that the descent times of a control rod cannot be accurately measured when the descent velocity is slow because the electric potential is low, making the commencement of descent, the position of the dashpot, and the fully inserted position unclear. Moreover, in a rare event such as the control rod 4 stopping during descent, it is impossible to confirm the rest position of the control rod 4 (fully inserted or partway), and therefore the accuracy and reliability of the detection of the position of the control rod is low. Similarly, when the rebound waveform used to determine whether the control rod 4 has been completely inserted is smaller than the descent velocity, the former is often unclear, and it is therefore difficult to detect whether the control rod 4 has been completely inserted into the reactor core. In addition, due to limits on the working temperature of the insulating materials used in the detector coils 2 of the control rod position detector assembly 7, the ambient temperature around the detector coils 2 must be vigorously cooled and it is necessary to ensure that design conditions are not exceeded, requiring that the volume of the cooling equipment for the control rod drive unit be made quite large so that it can handle the large amounts of heat given off by the housing 1 which is heated to temperatures as high as about 300 degrees Celsius. SUMMARY OF THE INVENTION The present invention aims to solve the above problems and a major object of the present invention is to provide a magnetostrictive wire control rod position detector assembly which will allow nuclear reactor output to be increased, control performance to be improved, and peripheral equipment to be rationalized by continuously and accurately detecting the position of a control rod in a reactor core. In order to achieve the above object, according to a major aspect of the present invention, a magnetostrictive wire control rod position detector assembly for detecting the position of a movable member within a cylindrical member comprises: a magnet or magnets mounted on a non-magnetic portion of the movable member which is free to move in the longitudinal direction on the inside of the cylindrical member and is at least partly composed of a non-magnetic material; a magnetostrictive wire detector longitudinally mounted on the outer circumference of the cylindrical member, which is provided in a predetermined place with a receiver which detects torsional waves; and a pulsed current generator circuit which supplies a pulsed current from the receiver end of the magnetostrictive wire detector to the magnetostrictive wire of the magnetostrictive wire detector. In such a construction, a rotational magnetic field is generated in the magnetostrictive wire by the pulsed current. When the rotational magnetic field approaches the magnetic field of the magnet or magnets mounted on the movable member, mutual interference between the magnetic fields generates torsional waves in the magnetostrictive wire. By measuring the propagation time of the torsional waves using the receiver which is disposed on a predetermined portion of the magnetostrictive wire, the physical position of the movable member can be accurately measured. It is preferable that the magnet or magnets be ring-shaped and that the magnetostrictive wire detectors be disposed plurally on the outer circumference of the cylindrical member. That way the magnetostrictive wire position detector assembly can be made to perform multiple functions. That is to say, the magnetostrictive wire position detector assembly will then be able to detect the position of the movable member precisely even if the movable member is inclined within the cylindrical member, and at the same time, the ability to detect the position of the movable member will then not be lost even if one of the magnetostrictive wire detectors malfunctions. In addition, a cylindrical support member should ideally be provided so as to seal closed the outer circumference of the cylindrical member and the magnetostrictive wire detectors with a predetermined spacing. As a result, a heat insulating effect will arise due to the layer of air existing in the space, reducing the radiation of heat from the cylindrical member. It is also preferable that a protective member formed from the same non-magnetic material as the non-magnetic portion of the drive shaft be mounted so as to hermetically seal the magnet or magnets against the non-magnetic portion. This will prevent oxidation of the magnet or magnets, ensuring that the strength of the magnetic field of the magnet or magnets remains constant and stabilizing the precision of the measurements as well as reducing maintenance costs. The cylindrical member may also be the pressure housing of the control rod drive unit, and the movable member may be the drive shaft connected to the control rod of the control rod drive unit, and the position of the drive shaft may be detected. The position within a reactor core of the control rod connected to the drive shaft can be accurately detected along the entire drive length from the fully inserted position to the fully withdrawn position by detecting the position of the drive shaft within the pressure housing. It is also desirable that the construction enable the detection of the control rod insertion times from the commencement of the descent of the control rod corresponding to the detected position of the control rod when the control rod is allowed to descend from the fully withdrawn position by measuring in advance a relationship between times and distances from the fully withdrawn position. By storing the descent times (insertion times) as high-precision digital data by means of the magnetostrictive wire detectors, the times taken by the control rod to reach the dashpot and the fully inserted position from the commencement of descent can be calculated from the data. For that reason, in the rare event that a control rod stops during descent, it will be possible to confirm the rest position of the control rod (fully inserted or partway) from the insertion times of the control rod, ensuring the reliability of the nuclear reactor.