JPH03181887A - Spacer for reactor fuel assembly, and fuel assembly - Google Patents

Abstract

PURPOSE:To prevent the deterioration of a burnout-resistant characteristic of a corner rod and a side rod by reducing deviation of a water gap by providing a spacer with a projection. CONSTITUTION:A spacer 10 of a channel box 4 has a construction wherein a plurality of cells 13 are formed by combining lattice plates 11 in the shape of a lattice and surrounded by a side plate 12, and fuel rods A are held in an arrangement of a square lattice therein. In this construction, a clip-type spring member 14 is fitted on the four sides of the cell 13, and one side of a clip is formed to be an elastic supporting projection, while the other side is formed to be a rigid supporting projection. Besides, clip-type spring members 15 having elastic supporting projections respectively are fitted to the side plate 12, while dimples 16 with longitudinal slits are provided in corner parts respectively. A clip-type spring member 17 is fitted to each dimple 16 and elastic supporting projections 18 and 19 are provided on the member 17. According to this constitution, a corner rod and a side rod at any positions are prevented from being located too near to the inner wall surface of the channel box 4 and a burnout-resistant characteristic can be improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a spacer for a nuclear reactor fuel assembly that holds rod-like elements such as fuel rods bundled in a square lattice arrangement with mutual spacing. In particular, this invention relates to improving the burnout resistance of fuel rods in boiling water reactors.

[Prior Art] In a fuel assembly loaded into a boiling water reactor (BWR), rod-shaped elements including a required number of fuel rods A and water tubes W are arranged in a square arrangement, as shown in FIG. 5, for example. , their upper and lower ends are attached to the upper and lower tie plates 2 and 3 via end plugs, and spacers 5 made of a lattice of thin strips are arranged at predetermined intervals at multiple locations at intermediate height positions to connect each rod-shaped Generally, the intervals between the elements are maintained at predetermined intervals. In the reactor core R, such a fuel assembly 1 is housed in a rectangular cylindrical channel box 4 made of Zircaloy to secure cooling water for removing heat from each fuel rod A. The required number of fuel rods are loaded vertically in the seat, and during reactor operation, cooling water is fed into the channel box 4 from the lower orifice (not shown) of the catch to flow between the fuel rods in each fuel assembly 1. The cooling water is made to flow upward, and the reaction heat of the fuel rods A boils the cooling water, extracting the heat to the outside and removing heat from the fuel assembly 1. In addition, in Fig. 5, C is the reactor core R.
The l1lJa rods are shown between each fuel assembly in the figure.

Conventionally, spacers 5 for maintaining the arrangement interval of rod-like elements such as fuel rods at a predetermined value in such fuel assemblies have been used, for example, as shown in FIG. A plurality of rod-shaped element insertion cells 52 are constructed, and elastic/rigid support protrusions 53 are provided on four side walls of each cell, each protruding toward the center of the cell in order to support the rod-shaped element substantially in its radial direction. It is known that spacers 5 are arranged facing each other, and such spacers 5 are arranged at, for example, seven locations at intervals in the axial direction of the fuel assembly 1.

Since BWR is a direct cycle system that generates steam within the reactor, boiling is allowed within the reactor core.

Therefore, fuel cooling is performed under two-phase flow conditions of liquid phase and vapor (voids), and the flow pattern of cooling water in the upper part of the fuel assembly, which has a high void ratio, is a film-like liquid flow as shown in Figure 7. The phase (liquid film) a+ forms a continuous annular flow on the inner wall surface of the channel box 4 and the outer peripheral surface of the fuel rod A, and droplets a2 are present between the channel box and the fuel rods and between the fuel rods. This is a so-called annular flow in which a vapor phase flows, and the liquid film a1 on the outer peripheral surface of the fuel rod plays an important role in cooling the fuel rod.

[Problem to be Solved by the Invention 1] However, if the fuel assembly is placed in a thermally severe state for some reason, such as an overpower state, and a transition occurs from the so-called nucleate boiling state to the film boiling state, As shown in the figure, the liquid film on the outer circumferential surface of fuel rod A is n, h < f Wi 廖l, and the temperature in the liquid film disappearing region of fuel and fuel rod A is high. C suddenly rises, and eventually thermal burnout occurs in that part. Burnout due to loss of liquid film, which is characteristic of BWR fuel, is particularly called dryout, and the load on the entire fuel assembly at which burnout occurs is called critical output.

Generally, both 8x8 type and 9x9 type fuels have seven spacers in one fuel assembly, but the fuel rods in the fuel assembly are most thermally severe in the upper two spacers of the assembly. It has already been confirmed through experiments that this is the portion above (downstream side of the flow of cooling water) from the portion immediately below the sixth spacer. Also, as shown in Fig. 10, if we pay attention to the flow path portion (subchannel) f divided by fuel pJA etc. in the cross section of the fuel assembly, as shown in Fig. 10,
- In the outer subchannel f1 of the corner rod AC adjacent to the channel box 4 in the shell and the outer subchannel f of the side rod A, the friction pressure loss of the cooling water flow is large, so the mass flow rate is small; In the inner subchannels f2 to f4 and f6 to f, the mass flow rate tends to be relatively large. Then, in the corner rod Ae that contacts the outer subchannel f1 at a part of the corner where the cooling water flow rate is actually the smallest, then the outer subchannel f in the side part.

It has been confirmed that dryout due to separation of the cooling water film is likely to occur at the side rod A in contact with the cooling water.

In this way, increasing the dryout output of corner rod Ac and side rod A is possible with 8x8 type fuel and 9
Regardless of the x9 type fuel, it has the significance of increasing the limit output and improving the burnout resistance performance of these fuel assemblies.

By the way, the outer dimensions of the fuel assembly are determined by the outer dimensions of the spacer that protrudes outward most inside the channel box. Conventionally, the outer dimensions of the spacer were generally set based on the outer dimensions of the channel box in order to fit the fuel assembly smoothly inside the channel box. The lower limit value (minimum value of the inner dimension tolerance) that takes into account the manufacturing tolerance of the inner dimension (C is determined to be smaller considering the margin)
Ta. Therefore, when the fuel assembly is housed in the channel box, the fuel assembly 1 is eccentric in the direction of the center of the channel box 4 as shown in FIG. Located at the center of box 4, the water gaps on the four sides were not always symmetrical. The bias in the cross-sectional position of this fuel assembly within the channel box is large at the side where there is a water gap between the channel box inner wall surface and the fuel assembly.
This results in a decrease in the burnout resistance of the corner rod or side rod where the water gap is formed, which in turn causes a reduction in the burnout resistance of the entire fuel assembly. It was.

Conventionally, improvements in the burnout resistance of fuel rods have been made.
A plurality of fixed vanes are installed on the side plate around the outer circumference of the spacer to stir the coolant flow, and on the downstream side of the spacer, the main kissing of the coolant flow replenishes droplets in the flow to the liquid film on the surface of the fuel rod. Efforts are being made to improve the dryout output of fuel rods through the replenishment effect, but the
As shown in Figure a, the fuel assembly was arranged eccentrically in the channel box, so the burnout resistance characteristics of certain corner rods or side rods depending on the direction of eccentricity were particularly degraded compared to others. The problem was unresolved, and the direction of the eccentricity was not determined within the reactor core, so it was difficult to find a countermeasure.

This invention was made in view of the above-mentioned problems, and it is an object of the present invention to devise a spacer so that the fuel assembly is housed in the center of the channel box as much as possible without increasing the resistance to the coolant flow. In order to reduce the unevenness of the water gaps on the four sides as much as possible so that the burnout resistance of corner rods or side rods in uneven positions does not particularly deteriorate as a result, the burnout resistance of the entire fuel assembly is reduced. It is an object of the present invention to provide a spacer for a nuclear reactor fuel assembly that can prevent the above.

[Means for Solving 1 Problem] In the spacer for a nuclear reactor fuel assembly according to the invention described in claim 1, rod-like elements such as fuel rods are spaced apart from each other in a rectangular cylindrical channel box and arranged in a square shape. In a spacer for a nuclear reactor fuel assembly that is held in a bundled state in a lattice arrangement, the spacer is placed in the channel box on each of the four sides of the outer circumferential side plate of the lattice arrangement of band plates constituting a plurality of square-shaped rod-shaped element insertion cells. (The above-mentioned problem can be solved by providing protrusions that come into contact with the inner wall surfaces of the channel box so that the distance between the inner wall surface of the channel box and the outer surface of the outer circumferential side plate is approximately symmetrical between the four opposing sides.) This is the solution.

Further, in the invention according to claim 2, in addition to the configuration described in claim 1, each of the protrusions is a substantially rigid protrusion, and the outer dimension between the opposing rigid protrusions is the channel of the channel. The present invention provides a spacer for a nuclear reactor fuel assembly that is less than or equal to the minimum value of the corresponding internal tolerance of the box.

Further, in the invention according to claim 3, in addition to the configuration according to claim 2, the protrusion portions on at least two adjacent sides are provided with the rigid protrusion portion and a substantially elastic protrusion portion, The present invention provides a spacer for a nuclear reactor fuel assembly in which the elastic protrusion has a larger protrusion dimension than the rigid protrusion.

Further, in the invention according to claim 4, in addition to the configuration according to claim 1, each of the protrusions is a substantially elastic protrusion,
The present invention provides a spacer for a nuclear reactor fuel assembly in which the outer dimension between the opposing elastic protrusions is greater than or equal to the minimum value of the corresponding inner dimension tolerance of the channel box.

Further, in the invention set forth in claim 5, in addition to the configuration set forth in claim 1, the protrusions on two adjacent sides are substantially rigid protrusions, and each of the protrusions on the other two adjacent sides is For a nuclear reactor fuel assembly, the protruding portion is substantially an elastic protruding portion, and the outer dimension between the opposing rigid protruding portion and the elastic protruding portion is greater than or equal to the minimum value of the corresponding internal tolerance dimension of the channel box. It provides a spacer.

Furthermore, the invention according to claim 6 provides a fuel assembly for a boiling water reactor comprising at least one spacer according to any one of claims 1 to 5.

[Function] In the spacer for a nuclear reactor fuel assembly according to the invention as claimed in claim 1, rod-shaped elements such as fuel rods are bundled in a square lattice arrangement with mutual spacing in a rectangular cylindrical channel box. In a spacer for a reactor fuel assembly to be maintained in a state, when placed in the channel box, on each side of the outer peripheral side plate of a lattice of strips constituting a plurality of square-shaped rod-shaped element insertion cells, the above-mentioned Since the protrusions that come into contact with the inner wall surfaces of the channel box are provided so that the distance between the inner wall surface of the channel box and the outer surface of the outer peripheral side plate is almost symmetrical between the four opposing sides, the fuel assembly can be assembled in the channel box. is supported by the protrusion of the spacer on all four sides of the inner wall of the channel box, and the distance, that is, the size of the water gap around the fuel assembly in the channel box, is It is also determined by the protrusion size of the protrusion when they abut, thereby ensuring the symmetry of the cross-sectional position of the fuel assembly within the channel box.

As mentioned above, the conventional spacer outer dimensions were determined to be even smaller than the lower limit value considering the manufacturing tolerance of the channel box inner dimension, so the fuel assembly was set inside the channel box as shown in Figure 6a. This resulted in the projecting dimples D on two adjacent sides of the spacer side plate coming into contact with the inner wall surface of the channel box 4, but in this case, the corner rod or side rod on the dimple contact side The water gap between the fuel assembly and the channel box is narrower than when the fuel assembly is located in the center of the channel box as shown in Figure 6b.

In this way, the fuel assembly equipped with a conventional spacer is housed eccentrically in the channel box? ), the dryout output of the corner rods and side rods on the side relatively far from the inner wall of the channel box will increase, but conversely, the dryout output of the corner rods and side rods on the side closer to the inner wall of the channel box will increase. Since the output power is considerably reduced, the burnout resistance of the fuel assembly as a whole is lower than when the fuel assembly is located in the center of the channel box.

That is, in general, voids generated by heat removal from fuel rods tend to move (drift) to a flow path with a large mass flow velocity, so the subchannel sandwiched between the channel box and the fuel rod (Fig. 9: f).

f2), as shown in Figure 7, some of the generated voids move to other internal subchannels (f3, f6, etc.), and most of the liquid phase generates so little heat that it can be ignored. Due to the cold wall effect of the channel box, the liquid flows as a thick film covering the inner wall surface of the channel box, and part of the liquid phase flows on the surfaces of the corner rods and side rods as a thin liquid film due to evaporation.

The flow rate of the liquid film that flows over the inner wall of the channel box increases as the gap between the corner rod or side rod and the inner wall of the channel box increases, and this is due to the fact that the flow rate of the liquid film at the periphery of the fuel assembly is substantial. This is because the reduction in the cross-sectional area of the channel reduces the friction pressure drop and increases the coolant flow rate in these peripheral subchannels, and most of the liquid phase tends to flow along the inner wall surface of the channel box due to the cold wall effect. be.

tl l flowing over the inner wall of this channel box
li is the source of droplets to each surface of the corner rod and side rod, and therefore the liquid film on each surface of the corner rod and side rod changes depending on the flow rate of the liquid film flowing over the inner wall surface of the channel box. .

In a fuel assembly equipped with a conventional spacer, the subchannel on the side where the fuel assembly approaches the inner wall of the channel box is so narrow that it cannot be ignored due to eccentricity within the channel box, and this narrow subchannel Due to the decrease in the flow rate of the liquid film on the inner wall surface of the channel box, the dryout output of the corner rods and side rods facing the channel box decreases considerably.

The spacer of the present invention increases the minimum value of the gap between the corner rod or side rod in the channel box and the inner wall surface of the channel box to almost the average value when the fuel assembly is viewed as a whole. The minimum value of the flow rate of the liquid film on the inner wall of the channel box has also increased to almost its average value, and the liquid film on the surface of the fuel rod, which contributes most to the heat removal of the fuel rod, is extremely large at any position on the corner rod or side rod. As a result, it is possible to improve the dryout resistance performance of corner rods and side rods.

In this way, the fuel assembly equipped with the spacer of the present invention prevents eccentricity within the channel box.
Corner rods and side rods at any position do not come too close to the inner wall surface of the channel box, suppressing a biased drop in the dryout output of corner rods and side rods, which improves the burnout resistance of the fuel assembly as a whole. This is clearly an improvement over the case where the fuel assembly is housed unevenly within the channel box.

In the present invention, the protrusions provided on each of the four circumferential sides of the outer peripheral side plate of the spacer are arranged between the inner wall surface of the channel box and the outer peripheral side plate when the fuel assembly equipped with the spacer is placed in the channel box. The protrusion is dimensioned so that the distance from the outer surface is approximately symmetrical between the opposing sides of the four circumferences.

This protrusion basically guarantees the symmetry of the cross-sectional position of the fuel assembly within the channel box by having equal protrusion dimensions on each side that give the spacer outer dimensions greater than the nominal inner dimensions of the channel box. .

As one specific method for this purpose, in the spacer according to claim 2, each of the protrusions is made of a substantially rigid protrusion, and the outer dimension between the opposing rigid protrusions is It is less than or approximately equal to the minimum value of the corresponding internal tolerance dimension of the channel box. Specifically, by increasing the protrusion dimension of one or two protruding dimples, which are normally provided on the spacer side plate for guidance and vibration prevention when inserting into the channel box, compared to conventional ones, the spacer can be easily removed from the spacer. It is best to make the dimensions the same as the inside dimensions of the channel box. This ensures that the spacing, ie the size of the water gap at the periphery of the fuel assembly in the channel box, is equal to or greater than the protrusion dimension of the rigid protrusion on all sides.

In the spacer according to claim 2, since the dimples are difficult to deform when the tolerance of the inner dimensions of the channel box is negative, the outer dimension between the rigid protrusions is set to be greater than or equal to the minimum value of the corresponding inner dimension tolerance of the channel box. Then, when inserting the fuel assembly into the channel box, the inner wall surface of the channel box may be damaged, or excessive force may be applied to the spacer, causing its position to shift or deform. For this reason, it is necessary to strictly control the dimensions of the channel box with respect to its internal dimension tolerance, and if the internal dimension tolerance is positive, there is a possibility that the fuel assembly may vibrate within the channel box due to the flow of coolant. In the spacer according to claim 3, the protrusions on at least two adjacent sides include the rigid protrusion and the substantially elastic protrusion,
This problem is solved by making the elastic protrusion have a larger protrusion dimension than the rigid protrusion. In this case, the rigid protrusion and the elastic protrusion are combined at the same position,
Alternatively, they may be provided independently at different positions, or both rigid protrusions and elastic protrusions may be provided on all four sides of the spacer. In the spacer according to claim 3, it is only necessary that the outer dimension between the rigid protrusions be less than or equal to the minimum value of the corresponding inner dimension tolerance of the channel box, and even if the inner dimension tolerance of the channel box is positive, Due to the elastic deformation of the elastic protrusions, the excess length of the outer dimensions of the spacer is absorbed, and each protrusion on the four sides of the spacer achieves abutting support against the inner wall surface of the channel box, and at least two sides are elastically supported. This also prevents vibration of the fuel assembly within the channel box due to the coolant flow.

Further, in the invention according to claim 4, in addition to the configuration according to claim 1, each of the protrusions is a substantially elastic protrusion,
The outer dimensions between the opposing elastic protrusions are greater than or equal to the minimum value of the corresponding inner dimension tolerance of the channel box,
In this case as well, even if the inner dimensional tolerance of the channel box is positive, the excess length of the outer dimension of the spacer is preferably evenly absorbed by the elastic deformation of each elastic protrusion on the four sides of the spacer, and the excess length of the outer dimension of the spacer is absorbed by each elastic protrusion. The symmetry of the cross-sectional arrangement position of the fuel assembly is guaranteed by supporting the fuel assembly in contact with the wall surface, and there is no risk of damaging the inner wall surface of the channel box or misaligning the spacer when inserting the fuel assembly into the channel box. It also disappears.

In the spacer according to claim 4, since all of the protrusions on the four sides of the spacer are elastic protrusions, the positioning of the fuel assembly within the channel box may be somewhat inadequate. However, in the spacer according to claim 5, in addition to the configuration according to claim 1, the protrusions on two adjacent sides are substantially rigid protrusions, and , the protrusions on the other two adjacent sides are substantially elastic protrusions, and the outer dimension between the opposing rigid protrusions and elastic protrusions is the minimum value of the corresponding inner dimensional tolerance of the channel box. This is resolved by doing the above. This is intended to complement each of the functions of the spacer according to claim 2 and the spacer according to claim 4 to obtain a synergistic effect of the advantages of both.

Furthermore, the invention according to claim 6 is a fuel assembly provided with the spacer according to the present invention, and in this case, the fuel assembly is provided with at least one spacer according to any one of claims 1 to 5. As mentioned above, the present invention provides corner rods and side rods with improved knock-out resistance. Of course, a single fuel assembly equipped with a plurality of spacers according to claims 1 to 5 in combination is also included within the scope of the invention.

As a preferred embodiment of the present invention, a spacer for a fuel assembly has fuel rods arranged in a square array of 9 rows and 9 columns, and a square large-diameter water channel having a cross-sectional area of 9 cells is arranged in the center. This will be explained below along with the drawings.

[Example] Figure 1 shows a spacer for a 9x9 type fuel assembly according to an example of the present invention, and the channel box 4, fuel rods A, and water channel W are the same as those in Figure 5. .

As shown in FIG. 1, the spacer 10 is constructed by welding a lattice plate 11 made of a plurality of strips, surrounding the four circumferences of the lattice plate 11 with side plates 12 made of similar slightly thicker strips, into a square lattice shape. Inside, a plurality of cells 13 are formed by the latticework, and a fuel rod A is inserted into each cell 13, and a large space for nine cells is provided in the center. A large-diameter water channel W can be inserted therethrough. A clip-type spring member 14 is attached to the four sides of each cell 13 so as to engage with a through hole (not shown) provided in the grid plate 11 of the cell side wall, and this clip-type spring member 14 is attached to one side of the clip. It is of a known structure consisting of an Inconel spring strip material formed into a clip shape, with one side serving as an elastic support protrusion and the other side serving as a rigid support protrusion. engaged with the hole. The same applies to the spacer side plate 12 as well.Lip type spring member 1
5 is attached, but this clip-type spring member 15 mostly has only an elastic support protrusion on the cell side.

Four sides of spacer side plate 12 X I+X 2+Y 1. Y
t (7) A dimple 16 with vertical slits is provided near a portion of each corner by protruding outward and being punched out, one of the two adjacent sides X,,Y.

Another clip-type spring member 17 is attached to the dimple 16 in its slit, and this clip-type spring member 17 has elastic support protrusions 18, 19 both on the cell side and on the outer side of the base. That is, this dimple 16 with vertical slits serves as a rigid protrusion constituting the main part of the present invention, and is similar to the conventional dimple D (Nos. 6a and 6b).
Figure) The elastic support protrusion 19 on the outside of the clip-type spring member 17, which has a larger protruding dimension and is attached to the slit, is also larger than the dimple 16 as an elastic protrusion that constitutes the essential part of the present invention. It has a protruding dimension. In this case, the outer dimension between the dimples 16 on opposite sides is set to be equal to or less than the minimum value of the inner dimension considering manufacturing tolerances of the channel box 4, and includes the outer elastic support protrusion 19 of the spring member 17. The outer dimensions of the spacer are larger than the maximum inner dimensions of the channel box 4 in consideration of manufacturing tolerances when the spacer is not inserted into the channel box 4.

FIG. 2a is an enlarged plan view of the slit dimple 16 and the clip-type spring member 17 attached to the slit, and FIG. 2b is a longitudinal cross-sectional view.

2a and 2b, the clip-type spring member 17 has a width approximately equal to the slit 16a of the dimple 16, and the elastic support projections 18 and 19 have a required elastic modulus due to the correlation between the material and the plate thickness. It is molded to be. The elasticity of the outer elastic support protrusion 19 can be made to the required elasticity by using a protruding shape such as a chevron shape as shown in the figure, and in FIG.
Although it fits within the slit 16a of dimple 1
If the slit 16a does not exist in the slit 16a, as shown in Fig. 3a, the elastic support protrusions 18 and 19 on both sides are made almost symmetrical so that only the protrusion amount is changed, or as shown in Fig. 3b, the elastic support protrusions 18 and 19 on the outside are The support protrusion 19 may be formed into a mountain shape that contacts the dimple 16 at its hem, and the elasticity may be adjusted so that the pressing force against the inner wall surface of the channel box 4 is appropriate.

The cell-side elastic support protrusion 18 of the clip-type spring member 17 is connected to the clip-type spring member 14 of the lattice plate 11.
and other clip types of the side plate 12 and the bling member 15.
In this case, for each cell 2, each clip-type spring member is arranged such that the elastic support protrusion and the rigid support protrusion face each other on two opposite sides of the four sides of the cell side wall. The installation direction of the fuel rod A is determined, and the fuel rod A is held between these opposing protrusions at the center of the cell with an appropriate pressing force.

Note that in the clip-type spring member 17, since the protruding dimension of the elastic support protrusion 19 is relatively large, when the fuel assembly is inserted into the ζ channel box 4, mechanical interference with the channel box causes instability in its posture. However, as a countermeasure against this, a wide support piece or the like is installed at the contact portion of the clip-type spring member 17 with the side plate 12 to stabilize its posture so that the clip-type spring member 17 does not tilt relative to the spacer side plate 12. It is preferable to form it integrally with the side plate 17 or adopt a structure that engages with the side plate 12 to prevent positional displacement.

Regarding a 9×9 type fuel assembly equipped with the conventional spacer shown in FIG. 6a or the spacer according to the embodiment of the invention shown in FIG. FIG. 4 shows the relationship between the distance δ between the inner wall surface of the channel box 4 and the outer surface of the side plate 12 of the spacer 10 and the limit output of the corner rod or side rod. This fuel assembly is No. 1 to No. 7.
2 fuel rods are included, and the limit output was determined for corner rod No. 11, side rod No. 2, and No. 11.
There are four inner rondos in the second row of , the center rondo adjacent to No. 22 water channel W.

In FIG. 4, the horizontal axis indicates the spacing 6, and the vertical axis indicates that the fuel assembly 1 equipped with the conventional spacer 5 has the dimple D of the spacer side plate inside the channel box 4, as shown in FIG. 6a. The graph shows the dryout output ratio at an arbitrary interval δ based on the dryout output of the fuel rods when they are eccentrically arranged in contact with a wall surface (interval δ=δcoN). δ on the horizontal axis! TYM is the nominal gap when the fuel assembly is arranged symmetrically without eccentricity in the channel box 4 as shown in FIG. 6b using the spacer according to the present invention, and is expressed by the following equation.

δsym = (LCHLIIP)/2 However, in the above formula, LCH is the nominal value of the channel box inner dimension, Lsp
is the nominal value of the outer dimension of the currently used spacer, and in the spacer of the present invention, when inserted into the channel box, δS□ is ensured on all four sides by the amount of protrusion of the above-mentioned protrusion.

Although δ and □ are generally small values of 1 mm or less, as can be seen from FIG. 4, they have a large effect on the dryout resistance performance. Corner rod No. 1 is most affected by the flow rate of the liquid film on the inner wall of the channel box, and δ
. . In the case of the symmetrically arranged δsYM, the dryout output increases by about 6% compared to the case of No. 8, and an increase in the dryout output of about 3% is also observed for side rod No. 2. Since the internal rods are separated from the inner wall surface of the channel box, the improvement effect due to the symmetrical arrangement is small, but an improvement of approximately 1% in dryout output is still observed for internal rod No. 11.

In this case, even if the arrangement is not completely symmetrical, δ of each side
6°. By reducing the degree of eccentricity within the channel box of the fuel assembly so that it approaches δIIYM from 8, it is possible to increase the dryout output of the peripheral rods as shown in Figure 4, and by extension, the overall fuel assembly It is possible to improve the burnout resistance of

The embodiment described above is only one illustration of the present invention,
The same effect can be obtained with an 8×8 type fuel assembly or one including a plurality of water channels, and it goes without saying that the present invention can be modified in various other ways.

[Effects of the Invention] As described above, in the fuel assembly equipped with the spacer of the present invention, eccentricity within the channel box is prevented, so corner rods and side rods at any position are not close to the inner wall surface of the channel box. The f dimension is not too large,
Because the uneven drop in dryout output of corner rods and side rods is suppressed, the burnout resistance of the fuel assembly as a whole is improved compared to when the fuel assembly is housed unevenly in the channel box. It is something that

[Brief explanation of drawings]

Fig. 1 is a schematic plan view showing a spacer according to the first embodiment of the present invention, Fig. 2a is an enlarged plan view of the main part, Fig. 2b is an enlarged longitudinal cross-sectional view of the main part, and Fig. 3a is a modified example. Figure i3b is an enlarged sectional view showing the main parts of another modified example, and Figure 4 is a 9x9 fuel assembly equipped with a conventional spacer or a spacer according to an embodiment of the present invention. Figure 5 is a diagram showing the relationship between the interval δ between the channel box and the fuel assembly and the limit output of the corner rod or side rod. Fig. 6a is a schematic plan view showing the eccentric arrangement of fuel assemblies equipped with conventional spacers in a channel box, and Fig. 6b is a schematic diagram showing the state in which the fuel assemblies are arranged symmetrically without eccentricity. A plan view, Fig. 7 is an enlarged sectional view showing the state of two-phase flow of coolant near a part of the corner, Fig. 8 is an explanatory diagram showing the state of coolant flow and temperature distribution on the surface of the fuel rod, and Fig. 9 10 is an explanatory diagram showing the flow path of the coolant flowing inside the fuel assembly in the channel box, and FIG. 10 is a diagram showing the difference in mass flow rate depending on the position of the coolant flowing inside the fuel assembly. (Explanation of symbols of main parts) 10... Spacer, 11... Grid plate, 12...
・Spacer side plate, 13... Cell, I6... Dimple with vertical slit (rigid support protrusion), 17...
- Clip type spring member, 19... Elastic support protrusion, W... Water channel, 4... Channel box, A... Fuel rod.

Claims (1)

[Claims] 1. In a spacer for a nuclear reactor fuel assembly that holds rod-like elements such as fuel rods bundled in a square lattice arrangement with mutual spacing in a rectangular cylindrical channel box, On each of the four circumferential sides of the outer circumferential side plate of the latticework of strips constituting the plurality of rod-shaped element insertion cells, there is a space between the inner wall surface of the channel box and the outer surface of the outer circumferential side plate when placed in the channel box. A spacer for a nuclear reactor fuel assembly, characterized in that a spacer for a nuclear fuel assembly is provided with protrusions that abut each of the inner wall surfaces of the channel box so as to be substantially symmetrical between the four opposing sides. 2. Each of the protrusions is substantially composed of a rigid protrusion, and the outer dimension between the opposing rigid protrusions is less than or equal to the minimum value of the corresponding inner tolerance of the channel box. The spacer for a nuclear reactor fuel assembly according to claim 1, characterized in that: 3. The protrusion on at least two adjacent sides includes the rigid protrusion and a substantially elastic protrusion, and the elastic protrusion has a larger protrusion dimension than the rigid protrusion. Item 2. Spacer for a nuclear reactor fuel assembly according to item 2. 4. Each of the protrusions is substantially made of an elastic protrusion, and the outer dimension between the opposing elastic protrusions is greater than or equal to the minimum value of the corresponding inner tolerance of the channel box. The spacer for a nuclear reactor fuel assembly according to claim 1. 5. The protrusions on two adjacent sides are substantially made of rigid protrusions, and each of the protrusions on the other two adjacent sides is substantially made of elastic protrusions, and the rigid protrusions are opposite to each other. 2. The spacer for a nuclear reactor fuel assembly according to claim 1, wherein an outer dimension between the channel box and the elastic protrusion is greater than or equal to a minimum value of a corresponding inner dimension tolerance of the channel box. 6. A fuel assembly for a boiling water reactor, comprising at least one spacer according to any one of claims 1 to 5.