Patent Number: 046876243
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 2 and 3 show a loop-type liquid metal cooled fast breeder reactor incorporating double rotating plugs, offset-arm type fuel handling mechanism and a ramp-type refueling system. The illustrated reactor is substantially similar to the conventional loop-type liquid metal cooled fast breeder reactor except for the structure of the fuel handling mechanism. A reactor core 8 composed of a plurality of core assemblies 9 is positioned at the center of the reactor vessel 1, and an upper core structure 5 is located above the reactor core 8. Within the reactor vessel 1 is filled liquid metal up to a liquid surface 12, and the vessel 1 is sealed by a shield plug device. The shield plug device consists of an outermost fixed plug 2, a large rotating plug 3 concentrically and inwardly positioned relative to the fixed plug 2 and a small rotating plug 4 eccentrically positioned relative to the large rotating plug 3. As illustrated in FIGS. 4A and 4B, the fuel handling mechanism according to the invention does not have a cylindrical housing, which is essential and indispensable to the conventional prior art technique, and comprises a fuel handling body 6 which is exposed and not surrounded by any housing. An upper portion of the fuel handling body 6 is rotatably secured to the small rotating plug 4 by means of a rotational driving device 16 and a flange 15 so that the body 6 is extended downwardly from a through-hole of the small rotating plug 4 into the reactor vessel 1. The lower end portion of the fuel handling body 6 is anchored by an aseismatic support 7 which is outwardly extended from the outer surface of the upper core structure 5. The rotational driving device 16 mounted on the small rotating plug 4 is constructed with a thrust bearing, an electric motor, etc. and rotationally drives the fuel handling body 6. As illustrated in FIGS. 5 and 6, the aseismatic support 7 has a securing ring 20 fixed to the outer surface of the upper core structure 5, a holder 21 fixed to the fuel handling body 6 and provided with three projections 21a, 21b, 21c outwardly extended in three radial directions, and three rolling elements 22 rotatably mounted to the respective ends of the projections of the holder 21. These rolling elements 22 are rotatably contacted with the inner surface of the securing ring 20, so that the holder 21 is smoothly rotatable clockwise and counterclockwise as shown with an arrow in FIG. 5 within the securing ring 20 by the aid of the lubricating effect of the liquid metal coolant. A fender 14 is preferably provided on the outer surface of the upper core structure 5 slightly higher than the position at which the securing ring 20 is fixed, so that it functions to prevent the core assemblies 9 during handling from colliding against the upper core structure. The fender 14 may be froemd integral with the securing ring 20 by extending upward the connecting part of the latter. An operation of the improved fuel handling mechanism will be described. In order to take the core assemblies 9 in the reactor core 8 out of the reactor, a gripper 13 disposed within the fuel handling body 6 is moved to a desired position above the reactor core 8 by rotating the large rotating plug 3, the small rotating plug 4 and the fuel handling body 6, as illustrated in FIG. 4A. Then, the selected core assembly 9 is gripped by the gripper 13 and lifted upward, as illustrated in FIG. 4B. Thereafter, the large rotating plug 3, the small rotating plug 4 and the fuel handling body 6 are rotated to move the gripper 13 to the position right above a loading and unloading housing 10 disposed in the vessel 1, and the gripper is lowered to unload the core assembly. The unloaded fuel assembly is taken out of the reactor through a ramp 11. When the core assemblies are loaded into or unloaded from the core 8, the selected core assembly 9 is held by the gripper 13 and vertically lifted or lowered along the fuel handling body 6 by means of a gripper lifting device 17. At the time of the loading/unloading, it is necessary to lower the gripper 13 to the top 18 of the core assemblies and completely pull out the gripped core assembly from the core 8. In case of the liquid metal cooled fast breeder reactor on the scale of 1000 MWe, a stroke of vertical movement of the gripper needs approximately 5 m in length, and accordingly the fuel handling body 6 is required to have a length of about 10 m from its lower end to the upper surface of the shield plug device. The aseismatic support 7 can improve the positioning accuracy of the gripper 13 of such an elongated and thin fuel handling body 6, and prevent the fuel handling body 6 from being buckled at the time of earthquakes. Therefore, provision of the aseismatic support 7 can exclude a housing which has been essential to the conventional technique, and permits a closely adjacent installation of the fuel handling body 6 relative to the upper core structure. When an earthquake occurs during the loading/unloading of the core assembly, the aforementioned fender 14 and prevent the suspended core assembly from colliding with the upper core structure. Although the present invention has been described with reference to the loop-type liquid metal cooled fast breeder reactor having the double rotating plugs, it will be readily contemplated by the skilled in the art that the present invention can be applied to fast breeder reactors having a single rotating plug or triple rotating plugs, and to a tank-type fast breeder reactor. According to the present invention, a cylindrical housing which was essential to the conventional fuel handling mechanism is not necessary, and the weight can be decreased by approximately 10 ton. Besides, both the fuel handling mechanism and the upper core structure can be made closer to each other, and the rotating plug can be reduced in diameter by approximately 10%. Thus, the fixed plug and the reactor vessel can be made smaller in diameter, and weight of these members can be reduced considerably. In addition, the present invention does not require provision of a thermal insulator, which has been essential to maintain a reliable structure of the housing of the conventional fuel handling mechanism. Accordingly, the total weight of the reactor structure can be considerably decreased in the present invention.