JPS646422B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a nuclear fuel assembly used in a nuclear power plant, and during the period of use of the fuel in the core,
This invention relates to a nuclear fuel assembly that reduces the probability of fuel leak even if foreign matter gets mixed into the coolant.

In a nuclear power plant, maintaining the integrity of the fuel during reactor operation is extremely important for safe plant operation.

As is well known, power generation using pressurized water reactors is
The heat generated by nuclear fission of the nuclear fuel material that is the fuel is transferred to the primary coolant, which then heats water, which is the secondary coolant, to generate steam, and the generated steam is used to generate electricity in a turbine. It drives the machine and generates electricity. For this reason, in the reactor core, many nuclear fuel assemblies are inserted in parallel with almost no gaps, and primary coolant is continuously supplied around the fuel rods, and the nuclear fuel pellets in the fuel cladding tubes are It is configured to transfer the generated heat to the primary coolant via the cladding tube.

As shown in FIG. 1, a conventional nuclear fuel assembly for a pressurized water reactor connects an upper nozzle 2 and a lower nozzle 3 with a plurality of control guide thimble 5.
A plurality of fuel rod support grids are fixed at intervals in the axial direction of the control rod guide thimble 5 to form a skeletal structure (referred to as a skeleton). is inserted. As shown in FIG. 2, the fuel rod 1 has a plurality of nuclear fuel pellets 17 stacked in an internal space (called a plenum) of a fuel cladding tube 9 whose both ends are sealed by an upper end plug 15 and a lower end plug 16. The upper end of the nuclear fuel pellet is inserted into the spring 18.
It is being held down. The space in which the spring 18 is housed is called a gas plenum, and the plenum of the fuel cladding tube containing the nuclear fuel pellets is filled with pressurized helium gas. The fuel rods 1 are inserted into lattice cells 19 of the fuel rod support grid shown in FIG. 4, and two dimples 1 are formed at intervals in the axial direction of the fuel rod support grid.
4 and the dimple, and is elastically supported by a grid spring 13 formed approximately in the center between the dimple and the dimple in the relationship shown in FIG. In such a fuel assembly, seven or eight fuel rod support grids are normally used, and in FIG. 1, one fuel rod support grid 6 related to the top of the nuclear fuel assembly is One fuel support grid 7 at the bottom of the grid is called a bottom support grid, and a plurality of fuel support grids 8 at the middle, excluding the top and bottom, are called intermediate support grids. Each of the plurality of fuel rod support grids is fixedly positioned on the control rod guide thimble 5. The control rod guide thimble 5 is a hollow tube with an open end 10 connected to the upper nozzle 2 so that a control rod can be inserted from the top, although not shown. Note that the upper nozzle 2 and the lower nozzle 3 are
Although shown partially cut away in the figure, each nozzle is provided with a number of holes 12 through which the primary coolant flows.

Although the core of the nuclear reactor is not shown, a plurality of nuclear fuel assemblies as described above are arranged in parallel so that they stand in a forest with almost no gaps, and the primary coolant is distributed between the nuclear fuel assemblies as shown by the arrows in the figure. It flows from the bottom to the top at a speed of about 5 meters per second.

However, in a nuclear fuel assembly configured in this way, if foreign matter such as chips gets mixed into the primary coolant, the foreign matter 2 will be removed as shown in Figure 3.
0 passes through the lower nozzle 3 and is most likely to catch on the fuel rod support part in the lowermost support grid 7, i.e. the grid spring 13 or the dimple 14. In this case, in a conventional nuclear fuel assembly, the fuel rod cladding Tube 9 is the lowest support grid 7
Since the foreign matter 20 extends lower than the fuel rod, there is a problem in that the foreign matter 20 causes hydraulic vibration due to the coolant in the supported part of the fuel rod, which damages the fuel cladding tube 9, and causes a fuel leak.

This invention has been made in view of the above circumstances, and it is an object of the present invention to provide a nuclear fuel assembly in which fuel leakage is less likely to occur even if foreign matter is mixed into the primary coolant.

Corresponding to this purpose, in the nuclear fuel assembly of the present invention, the upper nozzle and the lower nozzle are connected by a plurality of control rod guide thimbles, and the plurality of control rod guide thimbles include a plurality of control rod guide thimbles in the axial direction. fuel rod support grids are spaced and secured; and
In a nuclear fuel assembly in which a large number of fuel rods are inserted through the fuel rod support grid in parallel in the axial direction, the lower side of the fuel rod contact portion between the grid spring center P of the lowermost fuel rod support grid and the lower dimple. When the distance between the terminal ends Q is W, the expected irradiation growth amount of the fuel rod during its life is α, and the length of the lower end plug of the fuel rod excluding the chamfered portion C is L, the lower end plug of the fuel rod is L. In addition to satisfying the condition of ≧W+α, the lowermost fuel rod support grid is such that the center P of the grid spring is from the lowest end surface of the fuel pellet of the fuel rod in the assembled state before irradiation of the nuclear fuel assembly at least within the expected lifetime. It is characterized in that it is fixed to the control rod guide thimble at a lower position by the irradiation growth amount α.

Hereinafter, details of the present invention will be explained with reference to drawings showing embodiments.

FIG. 6 is a sectional view of a main part of a fuel rod used in the nuclear fuel assembly of the present invention. In the figure, 21 is a lower end plug, and the lower end plug 21 is longer than the lower end plug of a conventional fuel rod. In other words, in a nuclear fuel assembly for a pressurized water reactor, the fuel rod arrangement in the fuel rod support grid is 14 x 14, 15 x 15, or 17 x depending on the reactor type.
There are 17 different types of fuel rods, but in the conventional fuel rod shown in Figure 2, the total length S of the lower end plug 16 is about 21 mm to 23 mm (of which the length C' of the chamfered part is about 4 mm). 5 mm), whereas the lower end plug of the fuel rod used in the nuclear fuel assembly of the present invention is located between the center P of the grid spring 13 of the lowest fuel rod support grid 7 and the bottom immediately below it. The distance between the lower end Q of the side dimple 14 in contact with the fuel rod is W, the amount of fuel rod irradiation growth (elongation of the fuel rod due to neutron irradiation) is α, and the length of the lower end plug excluding the chamfered portion C. When the length is L, a lower end plug is used that satisfies the condition L≧W+α. The nuclear fuel assembly of the present invention uses a fuel rod equipped with the lower end plug, and the lowermost support grid 7 is attached to the lowermost end surface of the fuel pellet of the fuel rod, that is, the lowermost part inserted into the cladding tube 9. The end plug 21 is fixed to the control rod guide thimble at a position lower than the upper end surface of the cladding tube insertion portion by at least the irradiation growth amount α expected during the life of the fuel rod. The length L of the lower end plug excluding the chamfered portion C is preferably in the range of 25 mm to 40 mm, which is longer than the current 17 x 17 mm.
The distance between the grid spring center P of the fuel rod support grid of the nuclear fuel assembly in the array and the lower end Q of the lower dimple is approximately 15 mm, and the actual amount of irradiation growth α' of the fuel rods is the lifetime of the nuclear fuel assembly. This comes from the fact that it is 10 to 25 mm in the final stage. However, the specifications of the fuel rod support grid are not necessarily the same no matter what type of nuclear fuel assembly, so the length L of the lower end plug is determined by the specifications of the fuel rod support grid (in particular, the grid springs and dimples). Of course, the decision must be made depending on the distance from In this case, assuming that the total length of the fuel rod is approximately the same as the total length of a conventional fuel rod, the length of the gas plenum of the fuel rod is shortened by the length of the lower end plug.

Similar to FIG. 6, FIG. 7 shows a cross-section of the main part of the fuel rod of the nuclear fuel assembly of the present invention. The one in FIG. 7 shows a state at the end of its life after being irradiated with neutrons in a nuclear reactor. In the figure, reference numeral 1 denotes a fuel rod, and the fuel rod 1 receives neutron irradiation in the reactor core and grows downward by a distance α'. Since the fuel cladding tube 9 is fixed and positioned at 13, so even if fretting wear occurs due to hydraulic vibration of foreign matter, fuel leakage will not occur.

In addition, even in the current fuel rod assembly, there is a gap between the fuel rod 1 and the upper nozzle and the lower nozzle. For example, as shown in FIG.
The gap G between the upper surfaces is approximately 20 to 25 mm, and the elongation of the fuel rod due to irradiation growth tends to escape downward in the nuclear fuel assembly, and is absorbed in the gap G at the bottom of the fuel rod. Therefore, in the current situation where a method has been proposed for attaching the fuel rod support grid to the control rod guide thimble 5 so as to be able to slide in the axial direction without fixing the fuel rod support grid to the control rod guide thimble 5, it is possible to remove the lower end plug by using the above technique. The lowermost support grid 7 is positioned so that the length L is the shortest length that includes W, and the grid spring center P of the lowermost support grid is located at the lower end of the lowermost fuel pellet. Bottom support grid 7
can also be slidably attached to the control rod guide thimble without being rigidly attached.

According to the nuclear fuel rod assembly configured in this way,
For example, even if foreign matter mixed in the coolant gets caught in the grid spring of the lowest support grid or the fuel rod support part of the lower dimple and fretting wear occurs due to hydraulic vibration, fuel pellets will not be present in the support part. This prevents contamination of the primary coolant due to fuel leaks.
Note that there are many grid springs and dimples for elastically supporting the fuel rods in intermediate support grids and top support grids other than the bottom support grid, but the probability that foreign objects will be caught is
the lowest support grid is the highest (because the lowest support grid acts as a filter);
Therefore, the nuclear fuel assembly of the present invention can reduce the probability of fuel leakage.

Next, another embodiment will be described in which even if foreign matter is mixed into the primary coolant, the foreign matter is prevented from reaching the fuel rods. In FIG. 5, 23 is a filter. The filter 23 is, for example, a mesh or a punched sheet, and is fixed to the lower or upper surface of the lower nozzle 3 by means such as spot welding. Although the illustrated lower nozzle is provided with filters 23 on both its upper and lower surfaces, this is shown for convenience of explanation, and either one may be used. However, when using a filter, the conditions are almost the same as in the actual machine (flow rate of about 5
As a result of a coolant flow test using a simulator at
It was found that it was necessary to add numbers 4 and 25 to prevent the pressure loss from increasing.

According to this filter installation method, the purpose can be achieved without major design changes in the current nuclear fuel assembly design, so it is extremely simple and inexpensive. In addition, as mentioned above, as a measure to prevent fretting wear caused by fluid vibration of foreign objects without changing the design, in current nuclear fuel assemblies, at least the life span of the grid spring of the lowest support grid of the fuel rod is The range from the portion above the irradiation growth of the fuel rods expected within the center to the lower end of the fuel cladding tube, that is, the distance α corresponding to the downward displacement expected due to the irradiation growth of the fuel rods in Fig. 3 and the center of the grid spring. There is a method of applying ceramic coating to the range including the distance R from the fuel cladding tube 9 to the lower end of the fuel cladding tube 9. The ceramic coating is, for example,
This can be done by a known plasma spraying method, and in this case, the thickness of the coating layer (film) is
A thickness of about 50 to 100 microns is sufficient.

As explained above, according to the present invention, even if foreign matter gets mixed into the coolant during the period of use of the fuel in the core, the foreign matter will not reach the fuel rods, or even if it reaches the fuel rods, the foreign matter will not reach the fuel rods. It is possible to obtain a nuclear fuel rod assembly in which the probability of fuel leakage due to fuel leakage can be extremely minimized.

[Brief explanation of the drawing]

Fig. 1 is a side view showing a conventional nuclear fuel assembly, Fig. 2 is a longitudinal sectional view showing a conventional fuel rod, and Fig. 3 is a lower nozzle, a lowermost support grid, and a fuel rod in a conventional nuclear fuel assembly. Explanatory diagram of the relationship, 4th
FIG. 5 is a perspective view showing a part of the fuel rod support grid through which the fuel rods are inserted, FIG. 5 is a view showing an example in which a filter is provided in the lower nozzle, and FIGS. FIG. 6 is a sectional view of the essential parts of the lowermost support grid of the nuclear fuel rod assembly and the fuel rods. FIG. 6 is a diagram showing the positional relationship at the time of fabrication of the nuclear fuel rod assembly, and FIG. 7 is a diagram showing the positional relationship at the end of the life after irradiation. DESCRIPTION OF SYMBOLS 1... Fuel rod, 2... Upper nozzle, 3... Lower nozzle, 5... Control rod guide thimble, 7... Lowermost support grid, 9... Fuel cladding tube, 12... Coolant flow hole, 13 ...Grid Spring, 14
... Dimple, 16, 21 ... Lower end plug, 17
... Fuel pellets, 20 ... Foreign matter, 23 ... Filter, 24, 25 ... Taper.

Claims (1)

[Claims] 1. The upper nozzle and the lower nozzle are connected by a plurality of control rod guide thimbles, and the plurality of control rod guide thimbles have a plurality of fuel rod support grids arranged at intervals in the axial direction. In a nuclear fuel assembly in which a large number of fuel rods are inserted in parallel in the axial direction into the fuel rod support grid, the grid spring center P of the lowest fuel rod support grid and the lower side When the distance between the lower end Q of the fuel rod contact part of the dimple is W, the expected amount of irradiation growth of the fuel rod during its life is α, and the length of the fuel rod lower end plug excluding the chamfered part C is L. The lower end plug of the fuel rod satisfies the condition L≧W+α, and the lowermost fuel rod support grid is such that the center P of the grid spring is located at the top of the fuel pellet of the fuel rod in the assembled state before irradiation of the nuclear fuel assembly. A nuclear fuel assembly fixed to a control rod guide thimble at a position lower than the lower end surface by at least an amount of irradiation growth α expected during the life.