JPS6352352B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a fuel assembly capable of capturing and retaining radioactive corrosion products contained in the coolant of a nuclear reactor in a reactor core. [Technical background of the invention and its problems] In fast breeder reactors, an alkali liquid metal represented by liquid sodium is generally used as a coolant. After being heated in the reactor core inside the reactor vessel, such liquid metal coolant is led to the primary cooling system installed outside the vessel, and then returned to the reactor vessel, where it is circulated. do. By the way, in the case of fast breeder reactors, the cladding tubes and core structures of the nuclear fuel elements are usually made of stainless steel, but when these constituent materials are irradiated with neutrons, they are Iron, cobalt, etc. cause a nuclear reaction, producing manganese-54, cobalt-
Large amounts of radionuclides such as cobalt-60 and cobalt-58 are produced. This material is corroded by the alkali liquid metal used as the coolant and released into the coolant. At this time, the aforementioned radionuclides are also released into the coolant, and so-called radioactive corrosion products are mixed into the coolant. The radioactive corrosion products mixed into the coolant are carried to the primary cooling system according to the flow of the coolant, and are deposited on the walls of the piping of primary cooling system equipment such as intermediate heat exchangers. In this way, the radioactivity of radioactive corrosion products deposited on the walls of the primary cooling system is absorbed by pumps, heat exchangers,
It may interfere with maintenance and repair work of equipment such as valves, flowmeters, etc., and the piping connected to these equipment.
In particular, manganese-54, cobalt-60, cobalt-
58 etc. is produced in large quantities and has a long half-life, so its impact is large. Therefore, in order to eliminate such problems,
Recently, a radioactive corrosion product capture device has been installed inside the reactor vessel that utilizes the property of nickel to efficiently capture radionuclides such as manganese-54 and cobalt-60 in high-temperature liquid metal sodium. It is considered to be installed. This radioactive corrosion product capture device has a plurality of elements containing capture materials such as nickel placed opposite the coolant outlet of the reactor core, that is, above the core, and directs the coolant flowing out from the core to the capture materials. By contacting them, radionuclides are captured. However, in a conventional nuclear reactor configured such that a capture device is provided above the reactor core, it is necessary to secure an installation space for the capture device above the core. Normally, in order to efficiently capture radioactive corrosion products, it is necessary to secure a height of about 30 cm or more in the installation space. For this reason, such a nuclear reactor has the problem of increasing the size of the reactor main vessel and increasing the construction cost of the plant. In order to solve this problem, it has been proposed to install the above-mentioned capture device inside the fuel assembly that constitutes the reactor core. That is, a fuel assembly is generally constructed by accommodating a plurality of fuel rods inside a mantle having a regular hexagonal cross-sectional shape. Therefore, if the capture device is installed in the space above the fuel rods inside the outer mantle, the surplus space can be used effectively, and as a result, it is possible to avoid increasing the size of the reactor main vessel. However, in nuclear reactors using such fuel assemblies, although it is possible to capture radioactive corrosion products released from the inside of the mantle into the coolant, the outside of the mantle corrodes and leaks into the coolant. There was a problem in that the radioactive corrosion products released into the air could not be captured, resulting in a decrease in overall capture efficiency. [Object of the Invention] The present invention was made based on the above circumstances, and its purpose is to improve the capture efficiency of radioactive corrosion products without increasing the size of the reactor main vessel. The object of the present invention is to provide a fuel assembly that can contribute to reducing the construction cost of a nuclear reactor plant. [Summary of the Invention] The present invention includes a mantle that is implanted in the core of a nuclear reactor and forms a coolant flow path along the inner and outer surfaces, and a plurality of fuel elements housed inside the mantle. In the fuel assembly, first and second radionuclide capture bodies (hereinafter simply referred to as "capture bodies") are provided inside and on the outer peripheral surface of the mantle, respectively. The first capture body is installed in the downstream region of the fuel element inside the mantle. The second capture body is composed of a plurality of axially extending protrusions that protrude circumferentially from the outer circumferential surface of the mantle body toward the downstream region, and at least the surface thereof is made of nickel. ing. [Effects of the Invention] A plurality of fuel assemblies are usually installed close to each other in a nuclear reactor core. The coolant is
In addition to the axial flow inside these fuel assemblies,
The flow also flows in the axial direction through the gap between the fuel assemblies. According to the present invention, radioactive corrosion products mixed into the coolant flowing through the fuel assembly are captured by the first capture body. On the other hand, radioactive corrosion products mixed into the coolant flowing outside the fuel assembly are captured by the second capture body in the following manner. That is, the cross-sectional area of the coolant flow path formed between the fuel assemblies is narrowed due to the presence of the plurality of ridges provided at the downstream portion of the fuel assemblies. Therefore, in the areas where these protrusions exist,
The flow velocity of the coolant increases, and the so-called boundary layer formed on the surface in contact with the coolant becomes thinner, making it easier for radionuclides mixed in the coolant to diffuse within the boundary layer. In addition, in this part, the surface area of the surface that comes into contact with the coolant can be increased due to the presence of the above-mentioned protrusions, so that a large amount of radionuclides that have diffused within the boundary layer come into contact with the surface of this part. Become.
Since at least the surface of this part is made of highly active nickel, radionuclides are efficiently captured by this nickel. As described above, according to the fuel assembly according to the present invention, radionuclides can be captured with high efficiency without particularly increasing the size of the fuel assembly, reactor main vessel, etc. Therefore, it is possible to contribute to reducing the construction cost of a nuclear reactor plant. [Embodiment of the Invention] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a fuel assembly A according to this embodiment, and numeral 1 in the figure is a mantle. This mantle 1 is composed of a wrapper tube 2 and an entrance nozzle 3. The wrapper tube 2 consists of a tube with a regular hexagonal cross section, and a handling head 4 is located at the top in the figure.
It has become a form of development. A plurality of, for example, hemispherical spacers 5 are provided at predetermined positions on the outer peripheral surface of the wrapper tube to ensure gaps between the fuel assemblies A. Entrance nozzle 3
is a bottomed cylindrical tube whose diameter gradually decreases toward the bottom in the figure, and a plurality of orifice holes 6 for introducing cooling are bored in the side wall of the tube. A plurality of fuel elements 7 are absorbed inside the mantle 1 and supported by a fuel element support mechanism (not shown). Further, inside the mantle 1 and above the fuel element 7, a first capture body 8 made of, for example, nickel mesh is arranged. On the other hand, a second
A capture body 9 is constructed. This second capture body 9 includes a plurality of axially extending protrusions 10 provided circumferentially on the outer circumferential surface of the wrapper tube 2, and a wrapper tube exposed on the surface of the protrusions 10 and between the protrusions. 2 and a nickel plating layer (not shown) formed on the outer surface. The height of the protrusion 10 is set to be several mm lower than the height of the spacer 5.
Moreover, the cross-sectional shape thereof is formed to be rectangular. Therefore, the fuel assembly A configured in this way
In a nuclear reactor using a nuclear reactor, the function of capturing radionuclides is demonstrated as follows. That is, as shown in FIG.
The coolant flowing through the internal flow path P is introduced into the inside of the jacket body 1 from the orifice hole 6 shown in FIG.
The fuel element 7 is cooled by moving in the direction of the arrow in the figure. At this time, the radionuclides released into the coolant are captured by the first capture body 8 provided in the downstream region of the flow path. On the other hand, the coolant that has not flowed into the fuel assembly A flows into the gap Q that is formed when a large number of nuclear assemblies A are installed in the reactor core. Radioactive corrosion products such as manganese-54, cobalt-60, and cobalt-58 are mixed into the coolant flowing through the gap Q due to corrosion of the outer surface of the lapper tube 2. Furthermore, the coolant moves in the direction of the arrow in FIG. 1 and comes into contact with the second capture body 9. In this case, the captured body 9
The surface of the tube is covered with a nickel plating layer, and the cross-sectional area of the flow path at this portion is small and the area of contact with the coolant is large, so as described above, it exhibits an efficient trapping function. In this way, according to this embodiment, the fuel assembly A
Since it is possible to exhibit an efficient trapping function without making it particularly large, the above-mentioned effects can be fully achieved. Note that the present invention is not limited to the above embodiments. For example, in the above embodiment, the entire second capture body 2 is plated with nickel, but the protrusions 10 may be made of nickel. Moreover, the cross-sectional shape of the protrusion 10 may be formed into a trapezoid or a semicircle.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the structure of a fuel assembly according to an embodiment of the present invention, and FIG. 2 is a plan view illustrating a state in which a plurality of the same fuel assemblies are installed in a reactor core. 1 ... Outer body, 2... Rapper tube, 3... Entrance nozzle, 4... Handling head, 5...
...Spacer, 6... Orifice hole, 7... Fuel element, 8... First capture body, 9 ... Second capture body,
10... Projection, A... Fuel assembly, P... Coolant flow path, Q... Gap.

Claims (1)

[Claims] 1. In a fuel assembly comprising a mantle that is implanted in the core of a nuclear reactor and forms a coolant flow path along the inner and outer surfaces, and a plurality of fuel elements housed inside the mantle, the mantle a first radionuclide trap installed inside the body in a downstream region of the fuel element; and a plurality of axially extending protrusions protruding circumferentially in the downstream region on the outer peripheral surface of the mantle. and a second radionuclide trapping body made of nickel and having at least its surface made of nickel.