JPS61219893A - Support lattice for reactor fuel aggregate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to improvements in support grids for nuclear reactor fuel assemblies.

(Prior art) As shown in Fig. 9, in a nuclear reactor fuel assembly, fuel rods (1) are securely held between upper and lower nozzles (3) (41) at a predetermined interval, and during reactor operation, A plurality of support grids (2) are provided in the longitudinal direction of the fuel rods so that the fuel rods can exhibit sufficient performance without any problems.

This support grid typically serves to align a large number of fuel rods axially in parallel, maintaining the necessary gaps between the fuel rods to remove heat from the heat-generating surfaces of the fuel rods, and to ensure coolant flow. Conventionally, iron-chromium-nickel superalloys (trade name Inconel) and zirconium alloys (
Zircaloy-2 or Zircaloy-4), usually 0
．． A thin plate of approximately 0.25 mm to 0.6 mm is formed into a predetermined shape by press processing, and as shown in Fig. 10, it is equipped with elastic protrusions (7) and fixed protrusions (7). After assembling the two groups of strip plates marked with the middle plate (51) into a lattice shape by fitting the assembly slits (6) and (6') together, the upper and lower parts where the middle plates intersect are They are manufactured by bonding and integrating them using metallurgical joining methods such as TIG welding, laser welding, and electron beam welding, as well as welding and brazing.

By the way, such support grids are required to have the strength necessary for their performance due to the above-mentioned function, and also to have as little resistance as possible to the flow of coolant, but these two characteristics are usually contradictory. becomes.

That is, if the thickness of the material is increased in order to increase the strength, the cross-sectional area of the coolant flow path decreases, and the flow resistance, that is, the value of the coolant pressure loss increases.

Therefore, support grids usually have to be designed with a balance between the two, but the optimal combination of these two contradictory characteristics requires consideration of the plate thickness and the shape of the protrusions that support the fuel rods. Therefore, we were forced to search within an extremely narrow range of design conditions.

In this case, for the middle plate, the intersecting parts of the metal thin plates are melted to form a welding nugget (9) as shown in Fig. 3, and the orthogonal temporary members are joined and fixed. However, although the dimensions of this weld nugget have traditionally been assumed to be a factor that affects the support grid properties, efforts have been made to actively control this, even if it is simply to obtain a good weld without defects. The idea of doing so was never expressed.

(Problems to be Solved by the Invention) The present invention addresses the above-mentioned current situation, focuses on the dimensional relationship of the weld nugget, which has not been considered in the past, and controls the dimensions of the weld nugget. The purpose of this invention is to obtain a support grid with low coolant flow resistance and high strength by balancing two contradictory characteristics.

(Means for Solving the Problems) That is, the feature of the present invention that satisfies the above-mentioned object is that two groups of mutually orthogonal strips made of thin metal plates shown in FIG. <6, which are formed by combining them in a lattice shape through the respective assembly slits (6) and (6'), and each square lattice cell has a projection ( 7) (In the support grid for a reactor fuel assembly equipped with 71, the assembly slits shown in FIGS. 1 to 3) (6) (6') are fitted together at the intersection portions (8) of the middle plates. ) is fixed by welding, and the dimensions of the weld nugget (9) of the welded part are made larger at the welded part at the outer edge of the support grid and at the welded part at the center part. be.

Here, the welding location on the outer edge of the support grid is usually one or two rows on the outer periphery of the intersection of the middle plates to be fitted, but the welding location is not limited to this. . As for the weld nugget dimensions of these welding points at the outer edges, the weld nugget dimensions of the welding points at the center are 3 times the thickness of the middle plate.
The preferable range is approximately 5 to 5 times, while 6 to 8 times is preferred.

Note that the outer edge and the center are not limited to two stages in a strict sense;
Considering the strength, if the design allows, divide it into three or more stages,
It is also possible to make the difference stepwise, which is included in the present invention.

The thin metal plate material constituting the support grid of the present invention includes:
Examples include iron-chromium-nickel superalloys and zirconium-based alloys, which are appropriately selected and used.

(Function) Next, as for the effect of the above-mentioned configuration, a plurality of the support grids are used in the fuel assembly in the longitudinal direction, like conventional support grids.

At this time, considering the characteristics of the support grid as described above, it is preferable that the flow passage cross-sectional area be large in terms of coolant pressure loss.

However, as is clear from FIG. 3, the weld nuggets at the intermediate plate intersection welds narrow the coolant flow path, so the smaller the weld nuggets, the better.

On the other hand, from the viewpoint of support grid strength, the larger the weld nugget, the stronger the joint strength, which in turn increases the strength of the support grid as a whole.

Therefore, we first considered the strength of the support grid, and found that
The same can be said about the strength against static loads,
It has been found that the strength against shock loads that act on the support grid, especially in the event of an earthquake, is largely dependent on the strength of the joints on the outer periphery of the support grid.

Figure 11 shows that as a result of such an impact test, the support grid was damaged;
It shows a deformed state, and from this it can be understood that the strength of the outer peripheral portion is particularly important.

Furthermore, in the case of a pressurized water reactor fuel assembly, as shown in Figure 1, it is normal to provide a control rod guide tube insertion cell, indicated by a single circle, inside the third row. It is necessary to secure a passage for the control rod guide tube so that the control rods can be freely inserted in the event that the reactor has to be stopped urgently due to a loss of coolant accident, etc. It is important to have a strong structure and protect it.

On the other hand, considering the coolant pressure loss, the coolant pressure loss is approximately determined by the overall cross-sectional area of the flow path, so it goes without saying that it is preferable that the cross-sectional area of the flow path is large, that is, the weld nugget size is small. is very little affected by its placement.

Thus, the configuration of the support grid of the present invention is such that the weld nugget size of one row, preferably two rows of intermediate plate intersection welding points on the outer periphery is increased to the extent permitted by the design, while At the welding location, by making the weld nugget size as small as possible, the mechanical strength is greater than that of a support grid in which the weld nugget size is uniform throughout, and the coolant pressure loss is reduced.

(Example) Hereinafter, specific examples of the support grid of the present invention will be further described with reference to the accompanying drawings.

Figure 1 shows the positions of welding points in the support grid of the present invention, and (2) shows the support grid with two groups of mutually orthogonal strips made of thin metal plates as the middle plate (5) (5). The control rods are assembled in a grid pattern through the respective assembly slits 61 (<6) as shown in FIG. The guide tube also has an instrumentation guide tube insertion cell (1
2) are each inserted with an instrumentation guide tube shown in double light.

The intersecting points (8) of the intermediate plates (8) that are fitted to each other through the assembly slits from the intermediate plates (5) of the two groups are TI.
It is welded and fixed by known welding means such as G welding, laser welding, and electron beam welding, and a weld nugget (9) as shown in FIG. 3 is formed at the welding location.

The present invention has an important feature in this welding nugget, in which the dimensions of the welding nugget at the outer edge of the support grid indicated by small circles in FIG. made larger. For example, the weld nugget size of the welding location on the outer edge, that is, the third
(a) in the figure is 6 to 8 times the thickness of the middle plate (1), and on the other hand, the weld nugget size at the welding point in the center is 3 to 5 times the thickness of the middle plate (1). .

Note that this weld nugget size is not fixed but can be controlled as appropriate, and is achieved by appropriately selecting the magnitude of welding heat input using normal welding conditions, that is, welding current and welding time as parameters.

In addition, as shown in FIGS. 4 and 5, a welding tab aωQOI for supplying molten metal material for forming a welding nageft is provided.
If the intermediate plate (5) is prepared in advance by TGT press working, it will be easier to control the welding tab αω06 by adjusting its dimensions and shape according to the size of the welding nugget to be obtained. be able to.

Next, a support grid for a pressurized water reactor fuel assembly was made using Inconel-718 with a thickness of 0.4 tm, and its strength and coolant pressure loss values were tested. Figures 6 and 7 show the results. Shown below.

Note that TIG welding was used as the welding method. Further, in the figure, the broken line and the two-dot chain line represent welding points for one row and two rows, respectively, of the intersection of the outer edge middle plate according to the present invention.

The behavior is shown when the weld nugget size is fixed at 8t (t is the thickness of the middle plate) and the weld nugget at the central welding point on the inside is changed to the size shown on the horizontal axis.On the other hand, the solid line is A comparative example is shown in which the dimensions of the weld nugget remain uniform inside and outside, but the dimensions change.

First, from Figure 6, which shows the behavior of the support grid strength, in the comparative example, when the thickness of the middle plate is t, the strength increases rapidly until the nugget size reaches around 3t, and is relatively low between 3t and 8t. The strength increases gradually, and the degree of strength increase increases from around 10t, but in the case of the present invention, the strength increases at the outer edge 2.
If the welding nugget for the row is 8t, the center part will be 2t.
It is understood that almost no decrease in strength is observed up to t, and that there is virtually no decrease in strength beyond 3 even in the case of one row.

On the other hand, regarding the coolant pressure loss value, as shown in Fig. 7, in the case of the comparative example, the curve shape is almost parabolic, and the nugget size is 8t.
However, in the case of the present invention, compared to the case where the dimensions of the weld nuggets are uniform and small on both the inner and outer circumferences, there is a sharp rise from around the outer circumference of the outer edge.

It can be seen that even when the nugget size in both rows is 8t, it does not increase so much.

Therefore, it can be seen that the support grid according to the present invention is superior in the balance of properties of the support grid when the conventional nugget size is uniform.

FIG. 8 is a map of the strength and pressure loss values when the weld nugget dimensions are changed between the outer edge and the center, with two rows of the outer periphery as the outer edge and the inner side as the center.

From this figure, the optimal combination is that the outer edge weld nugget size is 6t~8t, and the center weld nugget size is 3t~
It can be said that it is 5t. This range of preferable conditions is shown by crossed diagonal lines in FIG.

Here, the lower limit of the center weld nugget size was set at 3t because when welding strips of thin metal plates, such as middle plates for support grids, welds that are too small tend to have wide variations.
This is because it is difficult to carry out large quantities of welding of uniform quality for product production.

The above explanation is for the case where Inconel-718 is used, but a separate test using a support grid made by joining Zirl Caloy-4 medium plates with a thickness of 0 and 45 m using a laser welding method yielded exactly the same results. Obtained.

Thus, it can be seen from the above that the support grid according to the present invention has excellent properties.

(Effects of the Invention) As described above, the present invention provides a support grid for a fuel assembly.
Focusing on the weld nugget size of the welded part, the weld nugget size at the outer edge and center of the support grid is made different, and the former is made larger than the latter. By controlling and making it larger at the outer edge and smaller at the center, we are able to meet the two contradictory properties required for support grids: high mechanical strength (high mechanical strength, low coolant pressure drop). As for the characteristics, the strength is guaranteed at the outer edge and the pressure loss is guaranteed at the center, respectively.Although the two characteristics are both guaranteed, the result is a remarkable effect that satisfies both the safety and function of support grids for fuel assemblies. It is highly anticipated that this will be effective.

[Brief explanation of the drawing]

Fig. 1 is a schematic plan view of a support grid to which the present invention is applied, Fig. 2 is a partial plan view of a state in which slits for assembling the support grid middle plate are fitted, and Fig. 3 is a section after welding the intersection points of the support grid middle plate. A plan view, FIGS. 4 and 5 are partial views showing the welding tab shape of each strip forming the support grid intermediate plate, FIG. 6 is a chart showing the relationship between weld nugget dimensions and support grid strength, and FIG. Figure 8 is a chart showing the relationship between weld nugget dimensions and coolant pressure loss values due to support grids; Figure 8 is a chart showing the influence of outer edge and center weld nugget dimensions on support grid strength and coolant pressure loss values; FIG. 9 is an overall schematic diagram of the fuel assembly, FIG. 10 is an explanatory diagram of a state in which slits for assembling the support grid intermediate plate are fitted, and FIG. 11 is an explanatory diagram of the state of deformation after the support grid strength test. (1)...Fuel rod. (2)...Support grid. (3)...Top nozzle. (4)...Lower part, (51(5'l...Middle plate (band plate). (61tj+...Slit for assembly. (7)...Elastic protrusion. (7)...Fixing protrusion. (8)...Intersection part. (9)...Weld nugget, ・α0)06...Weld tab. ゝζ-L7 Figure 1 Figure 6 Atsushi 7I21 Figure 8 Grass II Figure

Claims (1)

[Scope of Claims] 1. It is formed by combining two orthogonal groups of strips made of metal thin plates into a lattice shape through assembly slits as middle plates, and each square lattice cell has a fuel cell. In a support grid for a nuclear reactor fuel assembly that is equipped with projections that laterally hold rods, etc., the intersecting points of the middle plates where the assembly slits are fitted together are fixed by welding, and a weld nugget is formed at the welded portion. 1. A support grid for a nuclear reactor fuel assembly, characterized in that the dimensions of a welded location on the outer edge of the support grid and a welded location in the center of the support grid are larger than those of the latter. 2. The welding point of the outer edge of the support grid is at the intersection of the middle plate,
The support grid for a nuclear reactor fuel assembly according to claim 1, which is one to two rows on the outer periphery. 3. The size of the weld nugget at the welded location on the outer edge is 6 to 8 times the thickness of the middle plate, and the size of the welded nugget at the welded location in the center is 3 to 5 times the thickness of the middle plate. A support grid for a nuclear reactor fuel assembly according to item 1 or 2. 4. A support grid for a nuclear fuel assembly according to claim 1, 2 or 3, wherein the metal thin plate is an iron-chromium-nickel superalloy thin plate. 5. The support grid for a nuclear fuel assembly according to claim 1, 2, or 3, wherein the thin metal plate is a zirconium-based alloy thin plate.