1. Field of the Invention
The present invention relates to a debris filtering device of a nuclear fuel assembly, and more particularly, to a bottom spacer grid that can filter debris flowed through a channel of a lower end fitting with coolant from an upper portion of the lower end fitting of the nuclear fuel assembly, and prevent uplift of fuel rods due to coolant simultaneously.
2. Description of the Related Art
A nuclear reactor is a device in which nuclear chain reactions of nuclear fission materials are artificially controlled to achieve several purposes such as generating heat, producing radioisotopes and plutonium, or forming radiation fields.
Generally, a light water reactor uses uranium enriched to about 2˜5% Uranium-235. The uranium is fabricated as a cylindrical pellet weighing about 5 g for to process as nuclear fuel used in a nuclear reactor. The pellets are bundled by hundreds together and inserted into a Zircaloy cladding tube in a vacuum, and an upper end cap is welded after insertion of spring and helium gas into the tube to manufacture a fuel rod. Finally, the fuel rod is burned through nuclear reaction in the nuclear reactor by forming a nuclear fuel assembly.
The nuclear fuel assembly and their elements are shown in FIGS. 1 to 3.
FIG. 1 is a schematic diagram illustrating a general nuclear fuel assembly, FIG. 2 is a plan view illustrating a spacer grid seen from above, and FIG. 3 is an exploded perspective view illustrating the spacer grid in detail.
Referring to FIG. 1, the nuclear fuel assembly includes a body formed with an upper end fitting 4, a lower end fitting 5, a guide tube 3 and a spacer grid 2, and a fuel rod 1 supported by a spring 6 (see FIG. 3) and a dimple 7 (see FIG. 3) inserted and formed inside the spacer grid 2.
To prevent scratch on the surface of the fuel rod 3 and damage to the spring 6 inside the spacer grid in the event of assembling a fuel assembly, the surface of the fuel rod 1 is painted with lacquer and the fuel rod 1 is inserted into a skeletal structure, then, the upper and lower end fittings 4 and 5 are attached and fixed thereto, finally, the assembling of the nuclear fuel assembly is completed. Then, after the assembly, a manufacturing process of the assembly is completed by checking intervals between fuel rods, distortion, total length, size, etc., after removal of the lacquer from the completed assembly.
Referring to FIGS. 2 and 3, the spacer grid 2 is formed in a grid shape by mutually engaging slots (not shown) on which each strip (a thin metal plate) is formed at regular intervals to divide space portions for insertion of each of a plurality of fuel rods 1. The spacer grid 4 is arranged in number of 10 to 13 up and down and welded to the guide tube 3 having a length of 4 m. The spring 6 and the dimple 7 are regularly formed in each space portion that is divided by the spacer grid 2. The spring 6 and the dimple 7 are contacted to the fuel rod 1 (see FIG. 1) to maintain the interval of the fuel rods 1 (see FIG. 1), arrange the fuel rods 1 at determined positions and fix the fuel rod 1 due to elasticity of the spring 6.
The completed nuclear fuel assembly emits heat due to nuclear fission inside the nuclear reactor and the heat generated from the nuclear fuel assembly is absorbed by coolant. The coolant circulates the nuclear reactor and tubes of the nuclear reactor by means of a nuclear reactor cooling pump and promotes heat transfer from the nuclear fuel assembly to the coolant.
Various kinds of foreign materials flowed into the nuclear reactor with the coolant may be mixed in the coolant in a circulation process of the coolant, including debris and fragments such as metal particles, chips and shavings in manufacturing, installation and repair of piping and cooling equipment. Additionally, micro-vibration between the fuel rod and the spacer grid may be generated by passing the debris through channels of the lower end fitting and cladding tubes of the fuel rod may be damaged.
Moreover, when the fuel rod is lifted by rapid increase of coolant flux during abnormal conditions, the fuel rod may be bent or damaged. Due to nuclear and thermal hydraulic incompatibility of the nuclear fuel assembly, it affects integrity of nuclear fuel assembly.
Methods for solving the above problems are a method for limiting a size of the channel 12 of the lower end fitting as shown in FIG. 4, a method for using a spacer grid for filtering debris (prior art 1) or a method for using a bottom spacer grid assembly for debris-filtering 15 (prior art 2) as shown in FIG. 5.
According to the prior art 1, it is comparatively simple and applicable, however, it promotes pressure drop due to decrease of channel size in comparison with improvement of debris filtering performance.
When coolant pressure drops, hydraulic uplift force increases. Thus, it needs higher pushing force of the spring of the upper end fitting. In addition, it accelerates fretting wear by increasing vibration of nuclear fuel rods or generating horizontal flow of coolant and it doesn't absorb heat from the fuel rods sufficiently.
Moreover, the bottom spacer grid enough to maintain spring force sufficiently must be mounted on the bottom of the nuclear fuel assembly to continuously fix fuel rods during combustion inside the reactor for preventing uplift of the fuel rod due to coolant.
In case of using the spacer grid assembly for debris filtering, it can reduce a size enough to pass debris by forming more complicated structures of dimples and arches to prevent inflow of debris or increasing a cross section. However, since the pressure drop of coolant is in proportion to cross section of the spacer grid assembly seen from an axial direction, thus it is possible to increase the cross-sectional size by making the structures of dimples and arches complicated unconditionally.
An example of using the debris filtering bottom spacer grid is U.S. Pat. No. 5,024,807, filed on Dec. 5, 1988, entitled “Debris Catching Spring Detent Spacer Grid” (prior art 2).
Referring to FIG. 5, the prior art 2 can support a fuel rod and prevent uplift of the fuel rod by inserting a cantilever spring 18 supporting an elongated bottom end cap 16 of the nuclear fuel rod in the lowest spacer grid 15 of nuclear fuel assemblies into a groove 17 of an outer circumferential surface of the bottom end cap 16. Additionally, the prior art 2 can prevent debris from being flowed into an effective section of the nuclear fuel assembly by protecting a debris filtering arch 19 from the fuel rods.
Moreover, debris is captured between the lower end fitting and the bottom spacer grid due to an interval from the lower end fitting 14.
However, the cantilever spring 18 of the prior art 2 reduces a necessary elastic section by restoring pushing volume of spring caused by an outer circumference of the elongated bottom end cap 16 at the depth of the groove 17, thus it can be restricted to obtain a desired spring characteristic. Additionally, the debris filtering arch 19 must be shaped with a thick strip plate enough to form a widely bent shape in a corner as illustrated in the view, and the pressure drop volume of the bottom spacer grid for debris filtering increases due to the thick plate.