JP2583763B2 - Nuclear fuel assembly - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a nuclear fuel assembly, and more particularly to the above general fuel assembly which is unlikely to cause a fuel leak caused by foreign matter mixed in a primary coolant of a nuclear reactor. .

(Prior Art) At present, the fuel assembly (6) basically comprises a number of parallel fuel rods (4) supported by a support grid (3) as shown in FIG. And a control rod guide tube (2) connected to the nozzle (1). The primary coolant enters from around the core to remove heat, and the Fuel rods from the bottom of the (4)
It is a mechanism that flows and circulates upward through the space.

In this case, the lower nozzle (5) usually has a lower plate and legs, and a number of water holes for circulating the coolant in the plate are perforated in a required arrangement.

Meanwhile, in the nuclear fuel assembly as described above, when the primary coolant is introduced from below, foreign substances such as small metal pieces may be mixed into the coolant.

If such foreign matter is mixed in the coolant, the foreign matter (10) passes through the lower nozzle water flow hole (7) as shown in FIG. 8) Dimple or support lattice strap (11)
Trapped at the bottom of The trapped foreign matter (10) causes hydraulic vibration due to the coolant, causing fretting wear between the fuel rod (4) and the cladding tube as shown in FIG. And may cause a fuel leak.

Therefore, there is an idea to prevent the above point by additionally providing a trapping grid or a supporting grid at the lowermost part (see Japanese Patent Application Laid-Open No. 61-31993).
This becomes a resistance to the flow of the coolant, so that the flow of the coolant becomes difficult, and the following problems occur. (A) When the already loaded fuel (referred to as conventional fuel) has not added the support grid, Since the conventional fuel has a small resistance to the flow of the coolant, the coolant is easy to flow as compared with a fuel having a support grid added (referred to as a new fuel). Since the flow rate of the coolant is constant throughout the furnace, the amount of coolant flowing through the fresh fuel is reduced, and the cooling effect is reduced, thereby adversely affecting the performance of the fuel.

Further, a phenomenon occurs in which the coolant flows from the adjacent conventional fuel downstream of the coolant of the added support nucleon, and a flow in a lateral direction with respect to the fuel rod occurs. This has a negative effect on the fuel rod vibration, and the supporting effect causes a large water vibration when the force holding the fuel rod is small, and as a result, fretting wear occurs between the fuel rod and the support grid, resulting in fuel leakage. It is possible to reach.

(B) When all loaded fuels are new fuels All the fuels have high resistance to the flow of the coolant. Since the ability to flow the coolant into the reactor (pumping ability) is determined, the flow rate of the coolant is reduced. This impairs the cooling effect of the fuel and adversely affects the fuel behavior.

In addition to the above problems, there is a problem that the pressure loss of the aggregate increases, and the foreign matter escapes from the side, so that the foreign matter has not yet been completely trapped.

(Problems to be Solved by the Invention) In view of the above situation, the present invention considers the configuration and the position of the lowermost support grid of the nuclear fuel assembly, so that no additional support grid is required. The position where the hydraulic vibration is caused is specified to a specific position, thereby preventing the fuel leakage due to the foreign matter.

(Means for Solving the Problems) That is, a feature of the present invention is that, in the fuel assembly, the lowermost support lattice position is lowered, and dimples for trapping foreign substances or foreign substances having the same effect are provided below the straps. The trap vanes or vanes are directed toward the fuel rods, and a gap is provided between the fuel rods so as to protrude therefrom, and the dimple or the center point of the effect structure, that is, the portion closest to the fuel rods, and the lower nozzle When the distance from the upper surface is L and the length of the lower end plug of the fuel rod is l, the lowermost support lattice is arranged and mounted at a position where l> L.

(Operation) In the nuclear fuel assembly of the present invention configured as described above, since the position of the lowermost support grid is lowered, it is not necessary to add a special grid or support grid for preventing foreign matter inflow, and therefore, the pressure loss is reduced. Even if foreign matter enters the coolant without causing the foreign matter, the foreign matter will be trapped at the lower end plug without fretting wear of the cladding tube as before, and the foreign matter will be cooled. Even if hydraulic vibration is caused by the material, no fuel leak occurs and damage to the fuel rod is avoided.

(Examples) Hereinafter, specific examples of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a view showing a configuration in the vicinity of a lower nozzle (5) which is a main part of a nuclear fuel assembly according to the present invention, and FIGS. 2 to 4 are modifications thereof.

In these figures, the reference numerals are common, (4) indicates a fuel rod, (5) indicates a lower nozzle, (8) indicates a soft stop of a support grid, (9) indicates a hard stop, and (11) indicates a strap. Of the support grids (3), the lowermost support grid (3) is arranged close to and close to the lower nozzle (5) as shown in FIG. Foreign matter that has passed through (see Fig. 6) (10)
Are trapped at the lower end of the support grid hardstop (9), softstop (8) or strap (11) as shown in FIG.

Since the trapped foreign matter (10) causes hydraulic vibration due to the coolant and damages the cladding tube, the position where the hydraulic vibration is caused is set to the position of the lower end plug (12) of the fuel rod. Under the lattice soft stop (8), dimples (1
3) is provided.

The dimple (13) is formed by processing the strap in advance at the time of assembling. In order to facilitate the insertion of the fuel rod, the dimple (13) has a slight distance of about 0.5 to 1.0 mm between the fuel rod (4). It is protruding with a gap.

Also, from the above process, the lowermost support grid position is the lower end plug (1
2) When the length is 1 and the distance from the center point of the support grid hard stop (9) and the dimple (13), that is, the portion closest to the fuel rod to the upper surface of the lower nozzle (5) is L, It is effective to attach it to a position satisfying the condition of> L, and such an arrangement is illustrated in the example shown in FIG.

In the example shown in FIG. 1, the support lattice hard stop (9) also serves as a foreign matter trapping dimple on one side, but it may be provided separately.

FIG. 2 shows such an example, in which the dimples (1) are also provided completely below the hard stops (9) of the support grid.
3) is provided.

FIG. 3 shows an example in which the lower end of the support lattice strap (11) is bent to form a trap vane (14) instead of the dimple (13), which is also included in the present invention.

In the case of the vane (14), the back side of the cell adjacent to the vane (14) is generally formed with a hard stop (9) of the support lattice or the dimple (13).

In addition, the vane (14) is provided with a hole (15) as shown in FIG.

FIG. 4 further illustrates the trap vane. In this case, the vane (14) is provided together with the dimple (13).

Alternatively, as shown in FIG. 5, a foreign matter trapping blade (16) may be provided near the intersection of the straps. In this case, it is preferable that the blade (16) is provided below the strap because the trap position can be further lowered.

In each of the above examples, the distance relationship between the dimple, vane, or blade closest to the fuel rod and the upper surface of the lower nozzle (5), which is expected to have a foreign matter trapping effect, is at the lowermost position where l> L. Of course, a grid is attached.

Further, it goes without saying that the present invention is not limited to the above examples, and may be appropriately modified without departing from the object of the present invention.

(Effects of the Invention) As described above, the present invention lowers the position of the lowermost support lattice in the nuclear fuel assembly to the vicinity of the lower nozzle and provides dimples for trapping foreign matter or vanes for trapping foreign matter at the lower part of the strap, and The position where the relationship between the distance (L) between the portion closest to the dimple or the fuel rod having a similar structure and the upper surface of the lower nozzle and the length (l) of the fuel rod lower end plug becomes l> L. Since the lowermost support grid is attached, when the primary coolant flows from the lower part to the upper part, there is no need to add a separate support grid, so that a pressure loss does not occur. Even if foreign matter enters the coolant, the foreign matter is trapped not by the cladding tube but by the lower end plug of the fuel rod because of the above-described structure. This has a remarkable effect of stably preventing fuel damage without generating fuel, and is expected to be highly effective in terms of improving the safety of reactor operation.

[Brief description of the drawings]

FIG. 1 is a schematic structural view of the vicinity of a lowermost support lattice which forms a main part of the fuel assembly of the present invention, FIG. 2 is a schematic structural view according to a modified embodiment of FIG. 1, and FIG. 4 (b) and FIGS. 4 (a) and 4 (b) are further simplified schematic diagrams according to the modified embodiment of FIG. 1, and each figure (b) is AA, B of each figure (a). −
It is an arrow B top view. FIG. 5 is a top view showing still another modified embodiment of the dimple effect structure for foreign matter trap of the present invention, FIG. 6 is a schematic view showing a basic configuration of a known nuclear fuel assembly, and FIG. FIG. 4 is an explanatory view of a foreign matter trap state of the lower nozzle portion of the fuel assembly. (1) ... upper nozzle, (3) ... support grid, (4) ...
... fuel rod, (5) ... lower nozzle, (6) ... fuel assembly, (7) ... lower nozzle water hole, (8) ... support grid soft stop, (9) ... support grid hard stop ,
(10) ... foreign matter, (11) ... support lattice strap, (1
2)… fuel rod lower end plug, (13)… dimple for foreign matter trap, (14)… vane for foreign matter trap, (16)…
... A blade for foreign matter trapping.

Claims (4)

(57) [Claims] 1. A nuclear fuel assembly in which a number of parallel fuel rods supported by a support grid are held between an upper nozzle and a lower nozzle, the lowermost support grid is lowered to a position near the lower nozzle,
A dimple for trapping foreign matter or a structure having the same effect is provided on the strap of the lowermost support lattice toward the fuel rod side, and is provided so as to protrude with a slight gap between the fuel rod and the fuel rod.
The distance (L) between the position of the lowermost support lattice closest to the dimple or the fuel rod having the same structure and the upper surface of the lower nozzle is 1> L with respect to the length (l) of the fuel rod lower end plug.
The fuel assembly characterized in that the fuel assembly is specified and attached at a certain position. 2. The nuclear fuel assembly according to claim 1, wherein the structure having the same effect as the foreign matter trapping dimple is a vane obtained by bending a lower end of a lowermost support lattice strap. 3. The nuclear fuel assembly according to claim 2, wherein the vanes have holes for reducing pressure loss of the support grid. 4. The nuclear fuel assembly according to claim 1, wherein the structure having the same effect as the dimples for trapping foreign matter is a trap blade provided near the intersection of the straps of the lowermost support lattice.