Patent Number: 050733346
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a self-actuated nuclear reactor shutdown system wherein a control rod suspended by a temperature sensitive electromagnet (TSEM) is automatically separated or detached from the TSEM and is inserted into a reactor core to effect an emergency shutdown of the reactor when the temperature of a coolant rises in an extraordinary manner. In the present invention, a temperature sensitive magnetic material (TSMM) is located in the wall of wrapper tubes adjacent to a control rod guide tube and constitutes a part of a magnetic circuit of the TSEM. Therefore, the response to the thermal transient can be improved and the stability in the actuation of the TSEM can also be expected. The reactor shutdown system of the present invention can be utilized for fast breeder reactors, light water reactors and the like. Nuclear reactors of various types are provided with some back-up control rods to be inserted into a reactor core when anything unusual occurs, in addition to control rods for power control, in order to enhance the reliability of the reactor shutdown. For a reactor shutdown system, it has been proposed that a TSEM in which a TSMM is provided as a part of the magnetic circuit thereof is installed inside the reactor core so as to implement the retention and separation of the control rod. As for the TSEM, a ferromagnetic material which has a suitable Curie point is employed to break the magnetic circuit by decreasing saturation flux density when the temperature of the TSEM is close to the Curie point. During a normal operation of the nuclear reactor, the control rod is suspended at the upper part of the control rod guide tube by the TSEM. When the temperature of the coolant flowing through a fuel assembly rises due to an extraordinary accident, the TSMM transforms from a ferromagnetic substance to a nonmagnetic one. Therefore, the magnetic circuit of the electromagnet is broken at the TSMM and the TSEM no longer exerts its holding force. Consequently, the control rod can be spontaneously unlatched from the TSEM and inserted into the reactor core and the reactor is shut down. It has been proposed in the prior art to provide some structure to introduce hot coolant from the fuel assembly to the TSEM such as, for example, a coolant introduction pipe, in order to prospectively obtain a quick response of the TSEM to thermal transient of the coolant. Such a prior art structure as described above, however, employs a complicated mechanism for introducing the coolant flowing through an adjacent fuel assembly into the control rod guide tube, and cannot provide high reliability. Besides, it is expected that a response time will be longer when the flow rate of the coolant decreases, because the flow rate of the coolant at a high temperature introduced into the control rod guide tube cannot be increased in comparison with the mass of the TSMM. Moreover, in the structure wherein the control rod guide tube is used as a part of the magnetic circuit, the vertical relative position between the electromagnet and the control rod guide tube must always be controlled with high precision, and this impairs the intrinsic feature of the self-actuated shutdown system which is to actuate without any external control. SUMMARY OF THE INVENTION An object of the present invention is to provide a self-actuated nuclear reactor shutdown system which improves the thermal transient response and reduces the influence of the flow of the coolant to thereby enhance the reliability of an actuation characteristic of the system. Another object of the present invention is to provide a self-actuated nuclear reactor shutdown system which dispenses with the control of the relative position between the electromagnet and the control rod guide tube and prevents the occurrence of a spurious actuation or a non-actuation of the control rod. The present invention provides an improvement in a self-actuated nuclear reactor shutdown system which comprises a control rod, a temperature sensitive electromagnet (TSEM) disposed above the control rod for causing the control rod to latch thereto and unlatch therefrom, and a control rod insertion portion around which a plurality of wrapper tubes each receiving a fuel assembly are arranged. According to the present invention, in order to accomplish the above-described objects, an upper part of a wall of each of the wrapper tubes arranged around the control rod insertion portion is made of a temperature sensitive magnetic material (TSMM) having a characteristic whereby the saturation flux density thereof will be reduced when there is an extraordinary rise in the temperature of a coolant flowing through the fuel assembly. The TSMM constitutes a part of a magnetic circuit of the electromagnet. It is also possible to accomplish the above-described objects by connecting an extension tube made of the TSMM to the upper end of the wrapper tube, so that the TSMM of the extension tube constitutes a part of the magnetic circuit of the electromagnet. The upper part of the wall of the wrapper tube or the extension tube made of the TSMM may have a length equal to the vertical stroke of the electromagnet. It is also possible that the TSMM is provided only in an area in which the electromagnet is set during a normal operation of a nuclear reactor and the part of the wall of the wrapper tube below the area is made of a ferromagnetic material extending over the range of the vertical stroke of the electromagnet. The electromagnet has an iron core and an armature capable of latching with an unlatching from the iron core, and a coil wound on the iron core. A nonmagnetic material is incorporated in a part of the iron core in the proximity of the TSMM. In the present invention, since the upper part of each wrapper tube arranged around the control rod insertion portion or the extension tube of the wrapper tube is made of the TSMM, the TSMM can be heated directly by the coolant at a high temperature coming out of the fuel assembly. Therefore, it is unnecessary to worry about uncertain factors such as the unstable flow of the coolant and the like, and extraordinary rise in the coolant temperature can be detected rapidly. The operation of the TSEM retaining the control rod is basically the same as that in the prior art. During normal operation of the reactor, the TSMM exhibits a ferromagnetic property, and the TSEM can support the control rod. In the case of an accident wherein the coolant temperature rises in an extraordinary manner in the reactor, the coolant at a high temperature coming out of the fuel assembly raises the temperature of the TSMM and therefore the saturation flux density of the material decreases. Consequently, the supporting force exerted by the TSEM is terminated. Therefore, the control rod is released and inserted into the core and thus the reactor is shut down.