1. Field of the Invention
The present invention relates to a guide thimble of a nuclear fuel assembly to which a dual-cooling nuclear fuel rod is applied for ensuring stability even at ultra-high combustion degree by reducing a center temperature and obtaining an economic effect by increasing power, and more particularly, to a guide thimble of a dual tube type structure for a nuclear fuel assembly, which is capable of ensuring an uniform flow inside a reactor core by preventing a flow split phenomenon between a nuclear fuel rod sub channel and a guide thimble sub channel, and minimizing the increase of a neutron absorption section to improve performance of a dual-cooling nuclear fuel rod.
2. Description of the Related Art
FIG. 1 is a schematic perspective view showing a conventional nuclear fuel assembly, FIG. 2 is a schematic plan sectional view showing the section of the conventional nuclear fuel assembly, and FIG. 3 is a schematic plan sectional view showing a nuclear fuel rod having a dual-cooling structure.
As shown in FIG. 1, the nuclear fuel assembly 100 includes a nuclear fuel rod 110, a guide thimble 140, a spacer grid assembly 150, an upper end fitting 120, and a lower end fitting 130.
As shown in FIG. 2, the nuclear fuel rod 110 includes a cylindrical uranium pellet within a zirconium-alloy cladding tube, and high-temperature heat is generated by a nuclear fission reaction of the uranium pellet.
Meanwhile, the guide thimble 140 is configured with a single tube that provides a moving path of a control rod 160 moving upward and downward in order to control a power of a nuclear reactor core and stop a nuclear fission reaction. The spacer grid assembly 150 is one of constituent parts of the nuclear reactor fuel assembly and functions to support the nuclear fuel rod 110 such that it is arranged at a predetermined position due to a friction force that is generated by a friction between the nuclear fuel rods and the spacers formed in each grid of the spacer grid assembly 150.
The spacer grid assembly 150 is generally made of zirconium alloy, and it includes nuclear fuel rod cells supporting the nuclear fuel rods 110, and guide thimble cells into which the guide thimbles 140 are inserted. An annular uranium dioxide pellet is inserted into the nuclear fuel rod 110. A coolant flows quickly from the lower portion to the upper portion of the reactor core in an axial direction through a sub channel 115, which is surrounded by four nuclear fuel rods 110, or a sub channel 115, which is surrounded by twelve nuclear fuel rods 110 and one guide thimble 140.
The sub channel 115 represents a space surrounded by the nuclear fuel rods 110 or a space surrounded by the nuclear fuel rods 110 and the guide thimble 140. Since the side of the sub channel 115 is opened, fluid can move freely to an adjacent passage.
The upper end fitting 120 and the lower end fitting 130 function to fix and support the nuclear fuel assembly 110 to upper and lower structures of the nuclear core. In particular, the upper end fitting 120 connects the upper portion of the reactor core to the nuclear fuel assembly 100 to prevent the shaking of the reactor core and the nuclear fuel assembly 100, and prevents the lifting due to the coolant flow. The lower end fitting 130 includes a filter (foreign particle filter) (not shown) for filtering foreign particles floating inside the reaction core, in addition to the function of supporting the nuclear fuel assembly 100 inwardly to the reaction core.
As shown in FIG. 1, the upper end fitting 120 includes an outer guide post 121, a press spring 122, a press plate 123, a passage plate 124. Four outer guide posts 121 are screwed to the guide thimble 140, and the passage plate 124 allows the coolant to properly flow upward within the reactor core.
In addition, the lower end fitting 130 includes a passage hole for proper flow of the coolant, a passage plate where a hole is perforated for connecting the guide thimble 140 to a measuring tube, and a leg for maintaining the position of the nuclear fuel. The lower end fitting 130 redistributes the coolant, supports the nuclear fuel assembly 100, and filters foreign particles.
Meanwhile, as shown in FIG. 3, instead of the conventional cylindrical nuclear fuel rod 110, a dual-cooling nuclear fuel rod 10 was developed in order to improve the cooling performance and the stability of the nuclear fuel and obtain high combustion degree and high power. The dual-cooling nuclear fuel rod 10 has an annular structure having an increased outer diameter, compared with the conventional nuclear fuel rod 110.
The dual-cooling nuclear fuel rod 10 having the annular structure includes an annular pellet 11, an inner cladding tube 12 provided in an inner periphery, and an outer cladding tube 13 provided in an outer periphery. The coolant can flow into the inside of the dual-cooling nuclear fuel rod 10 as well as the outside thereof, so that heat transfer is achieved in dual manner. Therefore, the surface temperature of the dual-cooling nuclear fuel rod 10 is kept low and the probability of the fuel damage due to the increase in the center temperature of the nuclear fuel is reduced. Thus, a safety margin of the dual-cooling nuclear fuel rod 10 is increased, thereby obtaining high combustion degree and high power.
However, if the guide thimble 140 having the single tube type structure used in the conventional nuclear fuel assembly 100 is adopted for compatibility with the existing reactor core structure (control rod and neutron source assembly), a flow split is generated by a pressure difference caused by a difference of a hydraulic diameter in the dual-cooling nuclear fuel rod (10) sub channel and the guide thimble (140) sub channel. Since the flow split phenomenon increases the flow in the guide thimble (140) sub channel, it is difficult to obtain the low-temperature high-power performance which is the greatest advantage of the dual-cooling structure.
To solve the problem, the hydraulic diameter in the guide thimble (140) sub channel must be similar to the hydraulic diameter in the dual-cooling nuclear fuel rod (10) sub channel. To this end, the thickness of the guide thimble 140 must be increased, or separate components such as a sleeve must be added to the conventional guide thimble 140 having the single tube type structure.
In the case of increasing the thickness of the conventional guide thimble 140 having the single tube type structure, the hydraulic diameter can be made similar to that of the dual-cooling nuclear fuel road 10, but the neutron absorption section absorbing neutrons is increased because the guide thimble 140 is made of zircaloy.
Furthermore, if the sleeve is inserted into the conventional guide thimble 140, the flow split can be reduced, but the increased section of the guide thimble 140 made of zircaloy increases the nuclear absorption section in a reaction degree region inside the reactor core. Therefore, the nuclear fission reaction is reduced, causing the reduction in the output of the reactor core. Moreover, the process of inserting the sleeve becomes complicated, causing the great reduction of productivity.