TECHNICAL FIELD

The present invention relates generally to a system for separating and coupling a top nozzle of a nuclear fuel assembly and, more particularly, to a fastening structure of a top nozzle and a lock insert for improving a performance of disassembling and reassembling the top nozzle of a nuclear fuel assembly.

BACKGROUND ART

A nuclear reactor is a device made to be used for various purposes such as generating heat by artificially controlling a fission chain reaction of fissile material, producing radioisotopes and plutonium, or forming a radiation field.

More specifically, in typical light water reactors, enriched uranium that has a ratio of uranium-235 increased to 2-5% is used. In order for nuclear fuel to be used in nuclear reactors, uranium is processed into cylindrical pellets weighing about 5 g. Such pellets are charged into a zircaloy cladding tube, then one spring and helium gas are inserted into the cladding tube, and then a fuel rod is manufactured by welding an upper end cap of the cladding tube. The fuel rods finally form a nuclear fuel assembly and are burned by nuclear reactions in the reactor.

FIG. 1is a view showing a typical nuclear fuel assembly. With reference toFIG. 1, a nuclear fuel assembly1includes a skeleton, which is composed of a top nozzle2, a bottom nozzle4, spacer grids6, guide thimbles8, an instrumentation tube12, and the like, and fuel rods inserted into and supported by the spacer grids6.

The spacer grids6are arranged in the nuclear fuel assembly1in a number of about 10 to 13 from an upper side to a lower side and are welded with the guide thimbles8having a length of 4 m. Assembling the nuclear fuel assembly1is completed by charging the fuel rods into the skeleton and then attaching and fixing the top nozzle2and the bottom nozzle4. In other words, the nuclear fuel assembly1is arranged such that the fuel rods are maintained at a constant distance between the top nozzle2and the bottom nozzle4by the spacer grids6and the guide thimbles8as shown inFIG. 1.

FIG. 2is a view illustrating a typical top nozzle2. With reference toFIG. 2, the top nozzle2includes a flow channel plate24, and the flow channel plate24includes a instrumentation fixing hole26to which the instrumentation tube is fixed, guide holes22to which top nozzle is coupled, and flow channel holes28through which coolant flows.

During the operation of the reactor, hydraulic uplift force due to the flow of coolant may be applied to the nuclear fuel assembly1, thereby causing the assembly to be raised or vibrated. In addition, the nuclear fuel assembly may be subjected to an axial length change due to thermal expansion by an increase in temperature or due to irradiation growth and creep of the nuclear fuel cladding tube by long-term neutron irradiation. At this time, the top nozzle2is coupled to the guide thimbles to support a load of an axial direction, thereby functioning to maintain mechanical and structural stability of the nuclear fuel assembly.

On the other hand, the guide thimbles are usually coupled to the flow channel plate24together with the top nozzle insert to be firmly coupled to the flow channel plate24and are configured to be engaged into the top nozzle insert by a bulging process while coupling the top nozzle insert to the flow channel plate24.

FIG. 3is a view showing an above-mentioned fastening structure of the top nozzle and the top nozzle insert generally used. The fastening structure30of the top nozzle and the top nozzle insert is configured such that the top nozzle insert31is inserted into the guide hole22provided in the flow channel plate24at a side below the guide hole22, and in order to prevent the coupling of the top nozzle insert from being released, a lock tube33is inserted into an inner circumferential surface of the top nozzle insert at a side above the guide hole22, thereby being fastened.

However, when the top nozzle and the guide thimbles are separated from each other in a state of the above-described conventional configuration, it is not easy to dismantle and difficult to work remotely, because the top nozzle insert31and the lock tube33are provided being inserted into the guide hole22of the flow channel plate.

DISCLOSURE

Technical Problem

Accordingly, the present invention has been made to solve the above-described problems, and an object thereof is to provide a fastening structure of a top nozzle and a lock insert for improving a performance of disassembling and reassembling the top nozzle of a nuclear fuel assembly.

Technical Solution

In order to accomplish the above objective, there may be provided a system for separating and coupling a top nozzle of a nuclear fuel assembly according to the present invention, the system including: a lock insert configured to support the top nozzle of the nuclear fuel assembly by being coupled to a guide hole provided in a flow channel plate of the top nozzle, wherein the lock insert may include: a body in a hollow shape, and an insertion part provided on a top portion of the body and configured to be inserted into the guide hole, wherein a circumference of the insertion part may be variable in size, thereby being capable of being inserted into the guide hole.

The guide hole may include: a latching step provided at an upper portion of an inner circumferential surface of the guide hole and configured to fix the lock insert; and a latching groove provided at a predetermined portion of the inner circumferential surface of the guide hole and configured to fix the lock insert.

In addition, the insertion part may include: a first latching member fixed by being brought into contact with the latching step; and a second latching member fixed by being brought into contact with the latching groove.

The insertion part may be provided with at least one predetermined interval along a circumferential direction of the insertion portion, and the circumference of the insertion part may be variable in size as much as the predetermined interval.

The insertion part may be provided with at least one insertion member at a predetermined interval along a circumferential direction of the insertion portion, and the circumference of the insertion part may be variable in size as much as the predetermined interval.

Meanwhile, the system may further include: a separation part configured to separate the top nozzle and the lock insert from each other, wherein the separation part comprises a separation member configured to separate the lock insert from the guide hole and a fixing member configured to fix and lift the top nozzle.

In addition, the separation member may provide a predetermined accommodation space, and one side of the insertion part of the lock insert may be inserted into the accommodation space, whereby the circumference of the insertion part of the lock insert may be variable in size.

In addition, one end of the fixing member may be provided with a fixing latching member to be brought into contact with a bottom surface of the flow channel plate, thereby lifting the top nozzle.

Advantageous Effects

As described above, according to the present invention, disassembly and reassembly of the top nozzle of the nuclear fuel assembly and the lock insert are simplified, thereby simplifying and reducing the number of processes involved therein. Accordingly, the system is effective for maintenance and repair of the nuclear fuel assembly.

In addition, the number of parts is reduced to increase the handleability, so that it is easy to work remotely for repairing a nuclear fuel assembly.

In addition, it is possible to prevent the parts from being broken away from each other during the work for coupling and disassembling of the top nozzle and the guide thimble.

The effects of the present invention are not limited to those described above, and other effects not mentioned will be clearly recognized by those skilled in the art from the following description.

