Patent Number: 
Section: description

The present invention relates to a safety valve drive system of a nuclear power plant, and particularly, to a drive system for an safety relief valve provided in a main steam system of a nuclear power plant to protect a reactor from applying pressure by opening the safety relief valve through the supply of a driving gas using a pilot valve if an accident or a transient state occurs. Safety relief valves provided for boiling-water reactor power plants and other types of nuclear power plants are equipments having steam relieving functions and safety functions and constituting a main steam system. The main steam system is comprised of a main steam pipeline, a safety relief valve, a steam flow restrictor, a main steam isolation valve, a main steam pipe drain system, and a feed water system. The functions of the main steam system include a steam supply from a reactor pressure vessel to a turbine, a pressure suppression of the reactor pressure vessel within a limit value in a transient state of a reactor, and steam releasing restriction from the reactor pressure vessel and a reactor containment vessel. The main steam system generally includes four main steam pipes for introducing steam generated within the reactor pressure vessel to the turbine. A plurality of safety valves is provided for each main steam pipe. Safety valves are provided for a main steam pipe in order to suppress reactor pressure to a value less than a specified value if, for some reason, an accident or the like occurs in the reactor or in the vicinity thereof. A safety valve has spring-operated safety functions and relief valve functions for forcibly opening the safety valve by an auxiliary actuator at a set pressure less than a blowout pressure. As the relief valve functions, the safety valve releases steam in the reactor pressure vessel to a pressure suppression pool by means of forced manual opening or automatic opening in response to a high relief valve pressure. Some safety valves are built in an auto-depressurization system to be enabled in case of a loss-of-coolant accident (LOCA). The safety valves are automatically forced to open by means of remote operation based on a high reactor containment vessel pressure signal or a low reactor water level signal, thereby depressurizing the reactor pressure vessel until cooling water injection by a low-pressure emergency core cooling system becomes possible. Conventional technology will be described hereunder with reference to FIGS. 4 to 7. FIG. 4 illustrates a reactor containment vessel of a boiling-water nuclear power plant, and FIG. 5 is an enlarged view of a pressure suppression pool illustrated in FIG. 4. As illustrated in FIGS. 4 and 5, a reactor pressure vessel 21 is installed within a reactor containment vessel 14, and a safety valve 5 is provided in a pipe of a main steam system 11. In the safety valve 5, there is provided a safety valve exhaust pipe 28 for introducing steam to a pressure suppression pool 10. Vent pipes 24 are provided in a wall of the pool, and a quencher 23 for facilitating steam condensation in the pressure suppression pool 10 is connected to the lower end of the safety valve exhaust pipe 28. Note that in FIG. 4, reference numerals 22a and 22b denote main steam isolation valves. FIG. 6 illustrates the configuration of a safety valve and FIG. 7 illustrates a safety valve drive system. As illustrated in FIGS. 6 and 7, accumulators 3 and 4 are provided conventionally to supply an operating gas from a high-pressure nitrogen gas supply system 1 (1a, 1b), in order to open the safety valve 5 if an accident or a transient state occurs. The safety valve 5 is a nitrogen- and spring-operated type and is mounted on a pipe stand provided for the main steam pipe of the reactor containment vessel. An outlet side of the valve is formed as a flange connected to an exhaust pipe. The safety valve 5 is designed to automatically open (safety functions) if a valve inlet pressure exceeds a spring load. A piston 18 disposed in an air cylinder 19 mounted on the valve main unit and a valve shaft 17 are coupled with each other by means of a pull-up lever 16. Thus, the valve is configured so as to be opened (relief valve functions) by supplying nitrogen into the air cylinder 19 using an external signal. Supply of nitrogen into the air cylinder 19 is performed by operating a controlling solenoid valve. Next, an operating logic of the safety valve 5 will be explained. As illustrated in FIG. 7, the safety valve 5 operates, in response to a simultaneous signal of a reactor water level “low” and a dry well pressure “high”, as the result of an auto-depressurization system actuating signal 9 being generated with an emergency core system pump enabled. If one of two solenoid valves for auto-depressurization functions 26a and 26b is opened in response to logic circuit output signals from the output signal cables 34b and 34c of logic circuits, a nitrogen gas is supplied from a high-pressure nitrogen gas supply system 1 or an accumulator 4, thereby forcing the safety valve 5 to open. Consequently, steam flows from the main steam system 11, in a direction shown by arrows S1 and S2, into the pressure suppression pool 10, thereby depressurizing the reactor pressure vessel. On the other hand, if the pressure of a reactor rises and a high relief valve pressure signal is generated by a pressure gauge 13 for relief valve functions, a signal for relief valve functions is generated through an output signal cable 34a of a logic circuit, thereby causing the safety valve 5 to operate. If one solenoid valve 25 having relief valve functions opens in response to an output signal of a logic circuit, a nitrogen gas is supplied from the high-pressure nitrogen gas supply system 1 or an accumulator 3, thereby forcing the safety valve 5 to open. Consequently, steam flows into the pressure suppression pool 10 in the same way as described above, thereby depressurizing the reactor pressure vessel. Further, in the figure, reference numeral 14 denotes a reactor containment vessel side and reference numeral 15 denotes a reactor building side. In addition, examples of conventional proposals of such a safety valve drive system described above include one described in Patent Document 1 (Japanese Patent Laid-Open No. 9-304584). As described above, the operating logic of a safety valve works in the manner that if at least one of three three-way solenoid valves operates, the safety valve is forced to open, thereby depressurizing the reactor pressure vessel. In addition, according to the current operating logic of a driving solenoid valve, there is a possibility that if a fire occurs, a cable short-circuits to another cable and a false signal is generated, thus causing the solenoid valve to open mistakenly. Moreover, if the safety valve opens due to the malfunction of the solenoid valve, the depressurization of the reactor pressure vessel or the outflow of reactor water occurs, thus causing the water level of a reactor to drop. In addition, it is conceivable that in current systems of safety valves, online maintenance becomes difficult to perform if a power source for driving an auto-depressurization system is lost. The present invention has been accomplished in view of such problems as described above, and an object thereof is to provide a system or facility capable of improving the operating logic of a safety valve, enhancing the reliability thereof, and making compatible with online maintenance in order to eliminate the possibility of occurrence of a loss-of-coolant accident caused by the malfunction of a solenoid valve resulting from cable short-circuiting due to a fire or the like. In order to achieve the above-mentioned object, the present invention provides a safety valve drive system in which a safety valve provided in a main steam system of a nuclear power plant is opened by supplying a driving gas by using a pilot valve at an occurrence of an accident or a transient state occurs, thereby protecting a reactor against pressure application, the safety valve drive system comprising: a safety valve drive unit, as safety valve actuating means, actuating in such a manner that the safety valve is opened in response to respective auto-depressurization system actuating signals for two or more segments among respective auto-depressurization system actuating signals for four segments, and is closed if an auto-depressurization system actuating signal for one or less segment among the auto-depressurization system actuating signals for the four segments is received; and cables connected to the safety valve drive unit and used to transfer the auto-depressurization system actuating signals for the four segments. In addition, there is also provided a safety valve drive system which comprises: a safety valve drive unit, as safety valve actuating means, actuating in such a manner that the safety valve is opened in response to respective relief valve actuating signals for two or more segments among respective relief valve actuating signals for three segments, and is closed if an auto-depressurization system actuating signal for one or less segment among the auto-depressurization system actuating signals for three segments is received; and cables connected to the safety valve drive unit and used to transfer the auto-depressurization system actuating signals for the three segments. Further, in the above-described safety valve drive system, the phrase “relief valve actuating signal” may alternatively be read as “auto-depressurization system actuating signal,” depending on a reactor type different in the configuration of a safety system. According to a safety valve drive system of the present invention of the characters mentioned above, it is possible to eliminate the possibility of occurrence of a loss-of-coolant accident caused by the malfunction of a solenoid valve resulting from cable short-circuiting due to a fire or the like, thereby improving the safety valve in the operating logic thereof, enhancing the reliability thereof, and making compatible with online maintenance. Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 3. It is further to be noted that although, in described embodiments, an explanation will be made of a case in which the present invention is applied to a boiling-water reactor, the present invention is also applicable to nuclear power plants other than a boiling-water reactor.  FIG. 3 is a configuration diagram illustrating a safety valve drive system in accordance with a third embodiment of the present invention, in which like reference numerals are added to elements or members corresponding to those of the first embodiment mentioned above with reference to FIG. 1, and duplicated explanations thereof are omitted hereunder. As illustrated in FIG. 3, in the present embodiment, an explanation will be made to the operating logic of the relief valve functions of a safety valve, the arrangement of an opening line, the change of the driving source of the safety valve, and a method of storing the driving source is also explained. A drive circuit 12 for relief valve functions of a safety valve 5 includes actuating signals 13 for relief valve functions composed of three segments and the circuits thereof, in which the respective segments have structures physically and electrically independent of one another. Consequently, the drive circuit has multiplicity and electrical and physical independency, so that the functions of the drive circuit are not hampered by a single failure of equipment. Further, this configuration may be also effective for the operating logic of an auto-depressurization system in a plant having a safety system divided into three segments.