Patent Number: 053295625
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of cutting and removing a nuclear reactor for dismantling and removing a used nuclear reactor. 2. Description of the Prior Art Generally, nuclear reactors reach a permanent operation termination period 30 to 40 years after the start of the operation. Even after a nuclear fuel is withdrawn, a nuclear reactor has residual radioactivity such as radioactivation products, and various regulations have been put in force to ensure safety management and disposal for decommissioning of such a nuclear reactor. Means for decommissioning a nuclear reactor comes in three types. First means closes a nuclear reactor facility and puts it under suitable management, second means executes work such as shielding of the nuclear reactor to tightly isolate radioactive materials form outside and third means dismantles and removes radioactive structures inside the reactor facility. Since only a limited environment for installation of the nuclear reactor is available, a new facility, too, is preferably set up inside the same facility as the existing facility and in this sense, the third means for dismantling and removing the used reactor is preferred among the means described above. Accordingly, the development of a method of dismantling and removing safely and efficiently the used reactor has been earnestly desired. As means for accomplishing the object described above, a method has been proposed which cuts vertically and horizontally the body base portion of the reactor, for example, by continuous core boring so as to cut off the base portion from a pressure vessel, and transports and preserves them under a sealed state in a large waste preservation warehouse. SUMMARY OF THE INVENTION The pressure vessel itself of the reactor, after the body base portion thereof has been removed, has a diameter of 5 to 8 m and a height of 12 to 18 m. The wall thickness of the pressure vessel made of an austenitic stainless steel is from about 150 to 200 mm. Since no method which divides the pressure vessel, being a large-scale facility, both horizontally and vertically has not yet been established, the pressure vessel has been preserved in the past in a large-scale sealed preservation warehouse. A technique which cuts such a pressure vessel in water by a remote-controlled plasma cutting method is now being developed but cutting of the structure inside the reactor by this method is extremely difficult, though this method can cut the pressure vessel and the outer peripheral portions. Nevertheless, the number of nuclear reactors the service life of which ends up shortly are acceleratingly increasing every year and hence, the development of a method which can dismantle and remove safely and efficiently the nuclear reactors has been strongly desired. As its dismantling technique, the Applicant of the present invention has developed a technique which pressure-feeds grouting material into a pressure vessel to integrally solidify them together, and then cuts them with a wire saw having diamond grains. When the pressure vessel is cut starting with its outer periphery, the technique described above can cut integrally all the structural members while preventing the scatter of radioactive appliances and base materials and can therefore carry out safely and reliably the dismantling operation. Moreover, since the grouting material isolates the internal materials having radioactivity, transportation and disposal can be carried out safely. In the technique described above, the outer periphery of the surface to be cut is made of a metal. Therefore, the development of a technique which can stabilize the initial cutting portion and can decrease wear of the grains of the wire saw during the cutting operation of the metal portions has been desired as a technique which further improves cutting efficiency. It is therefore a first object of the present invention to provide a method of safely and efficiently dismantling and removing a nuclear reactor, particularly its pressure vessel. It is a second object of the present invention to provide a method of cutting and removing a nuclear reactor which can stably cut the reactor when the reactor is cut with a wire saw, and can reduce the wear of the grains of the wire saw. To accomplish the first object, the method of cutting and removing the nuclear reactor according to the present invention pressure-feeds a grouting material into the pressure vessel and into peripheral members outside the pressure vessel so as to integrally solidify them together, and starts cutting with the outer periphery of the pressure vessel. When the cutting operation is carried out, the method of the invention uses a wire saw having diamond grains on the surface. Furthermore, when the cutting operation is carried out, the present invention executes double cutting by the use of a preceding cutting edge and a cutting groove width adjustment cutting edge, and in this way, the present invention can dismantle and remove safely and efficiently the pressure vessel, and the like, of the nuclear reactor. To accomplish the second object, the second means of the present invention comprises packing a cement grouting material or a synthetic resin material into the nuclear reactor, covering its outer periphery with a concrete, cutting the reactor into blocks starting with its outer periphery with a wire saw using diamond beads and carrying them out, and comprises packing a cement grouting material or a synthetic resin material into a nuclear reactor, cutting the outer portion of the reactor by a plasma, cutting then the inside of the reactor into blocks with a wire saw using diamond beads, and carrying them out, in order to cut and remove the nuclear reactor. When the nuclear reactor is cut with the wire saw, therefore, the present invention can stably cut the reactor, can reduce the wear of the grains of the wire saw and can improve cutting performance. A first means being constituted as described above, the present invention first pressure-feeds a grouting material into the pressure vessel and into the peripheral members outside the pressure vessel so as to integrate and solidify them together, then prevents radioactive appliances and materials from scattering when the cutting operation is started with the outer periphery of the pressure vessel, and makes it possible to carry out smoothly the cutting operation by using a wire saw, or the like. The present invention uses a wire saw having diamond grains on the surface and cuts integrally all the appliances and materials, conducts double cutting using a preceding cutting edge and a cutting width adjustment cutting edge, and keeps a predetermined cutting width by the cutting width adjustment cutting edge which compensates for a decrease in cutting groove width resulting from the wear of the preceding cutting edge. The second means being constituted as described above, the present invention first pressure-feeds a cement grouting material or synthetic resin material having high fluidity into the nuclear reactor before cutting of the reactor is started, and the cement grouting material or the synthetic resin material is packed even into fine spaces of internal appliances. Next, after the outer periphery of the reactor is covered with concrete, it is cut with the wire saw using the diamond beads. In this case, since the first cut portion is the concrete material, the cutting operation can be easily started with the wire saw using the diamond beads and when the cutting operation reaches the main body of the reactor made of a stainless steel, the wire saw can exhibit a stable cutting performance as it is guided by the cut concrete portion. Even when the sharpness of the cutting portion of the wire saw drops due to the wear of the surface of the bead material in the course of cutting of the stainless steel reactor, the concrete material which is cut from the metal portion by the wire saw functions as the grinding stone and polishes the wire saw. Therefore, the wire saw maintains its performance in cutting the stainless steel.