JP2751078B2 - Spacer and fuel assembly for reactor fuel assembly - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spacer for a reactor fuel assembly for holding rod-shaped elements such as fuel rods in a state of being bundled in a square lattice arrangement with a space between each other. In particular, the present invention relates to improvement of burnout resistance of fuel rods in a boiling water reactor.

[Prior Art] In a fuel assembly loaded in a boiling water reactor (BWR), for example, as shown in FIG. 5, a square array of rods including a required number of fuel rods A and water pipes W is arranged. The upper and lower ends thereof 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 a plurality of positions at intermediate height positions. Generally, the interval between the rod-shaped elements is maintained at a predetermined interval. 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 the fuel rods A in the core R, and a seat for the core structure is provided. During the operation of the reactor, cooling water is fed into the channel box 4 from the lower orifice (not shown) of the seat to move upward between the fuel rods in each fuel assembly 1. The cooling water is boiled by the reaction heat of the fuel rods A to take out heat to the outside and to remove the heat of the fuel assembly 1. In FIG. 5, C indicates a control rod between the fuel assemblies in the core R.

Conventionally, in such a fuel assembly, as a spacer 5 for maintaining the arrangement interval of rod-shaped elements such as fuel rods at a predetermined value, for example, a grid-like grid set of strips 51 as shown in FIG. A plurality of rod-shaped element insertion cells 52 are formed, and elastic / rigid support protrusions 53 projecting toward the center of the cell to support the rod-shaped element substantially in the radial direction are provided on the four side walls of each cell. Opposed arrangements are known, and such spacers 5 are arranged at, for example, seven locations at an interval in the axial direction of the fuel assembly 1.

Since BWR is a direct cycle system that generates steam in the furnace, boiling is allowed in the core. Therefore, cooling of the fuel is performed under the condition of two-phase flow of liquid phase / steam (void), and the cooling water flow pattern above the fuel assembly having a high void ratio is a film-like liquid as shown in FIG. phase (liquid film) a ₁ forms a continuous annular flow at the outer peripheral surface of the inner wall surface and the fuel rods a of the channel box 4 and between and between the fuel rods between the channel box and fuel rods droplet a ₂ This is a so-called annular flow in which the vapor phase b accompanied by the liquid film a _{1 on} the outer peripheral surface of the fuel rod
Plays an important role in fuel rod cooling.

[Problems to be Solved by the Invention] However, if the fuel assembly is placed in a thermally intense state due to, for example, an over-power state or the like, and a transition from a so-called nucleate boiling state to a film boiling state occurs, the eighth problem occurs. since the fuel rods a liquid film a ₁ is erased by heat removal effect of the outer peripheral surface of the as shown in FIG rapidly deteriorates, the temperature in the liquid film disappears region of the fuel rods a rises rapidly, eventually parts thereof As a result, combustion loss (burnout) occurs. Burnout caused by the loss of the liquid film characteristic of BWR fuel is called dryout, and the load on the entire fuel assembly where burnout occurs is called marginal output.

Generally, both the 8 × 8 type and 9 × 9 type fuels have seven spacers in one fuel assembly, but the fuel rods in the fuel assembly are thermally most severe above the assembly. It has already been confirmed by experiments that the portion is located above the portion immediately below the sixth spacer (downstream of the cooling water flow). When attention is paid to the flow path portion (sub-channel) f divided by the fuel rod A or the like in the cross section of the fuel assembly as shown in FIG. 10, generally, as shown in FIG. since the outer subchannels f ₁ and the side rod a outside the sub-channel f ₅ the friction pressure loss of the cooling water flow _s corner rod a _c adjacent to the channel box 4 is larger mass flow rate is less, than the contrary In the inner sub-channels f _{2 to} f ₄ and f _{6 to} f ₈ , the mass flow rate tends to be relatively large. And by actually at the corner rods A _c in contact with the outer subchannels f ₁ in the cooling water flow rate is smallest corners, then withdrawal of the cooling water liquid film in the side rods A _s in contact with the outer subchannels f ₅ of the side portion It has been confirmed that dryout is likely to occur.

Thus, increasing the dry-out output of the corner rod A _c and the side rod A _s is, 8 × 8 type fuel and 9 ×
Regardless of the type 9 fuel, it has the significance of raising the critical output and improving the burnout resistance of these fuel assemblies.

By the way, the outer size of the fuel assembly is determined by the outer size of the spacer projecting outward in the channel box.Conventionally, in general, the outer size of the spacer is generally determined by the channel box in order to fit the fuel assembly smoothly in the channel box. The lower limit value (the minimum value of the inner dimension tolerance dimension) in consideration of the manufacturing tolerance of the inner dimension is determined to be smaller with further allowance. Therefore, when the fuel assembly is housed in the channel box, the fuel assembly 1 is eccentric in a certain direction from the center of the channel box 4 as shown in FIG. 6a, and is not necessarily as shown in FIG. 6b. The water gaps on the four sides located at the center of the channel box 4 were not always symmetric. The deviation of the cross-sectional position of the fuel assembly in the channel box results in that the water gap between the inner wall of the channel box and the fuel assembly is large on a certain side and small on a certain side. The unevenness particularly reduces the burnout resistance of a certain corner rod or side rod, and in turn, lowers the burnout resistance of the entire fuel assembly.

Conventionally, to improve the burnout resistance of fuel rods, a plurality of fixed blades for stirring the coolant flow are provided on the side plate around the spacer, and the liquid in the flow is mixed downstream of the spacer by mixing of the coolant flow. Drops are replenished to the liquid film on the fuel rod surface, and this replenishment effect is used to improve the fuel rod dryout output.
As shown in Fig.6a, the fuel assemblies are eccentrically arranged in the channel box, so that the burnout resistance characteristics of particular corner rods or side rods according to the direction of eccentricity are particularly deteriorated compared to others. However, since the direction of the eccentricity has not been determined in the reactor core, it has been difficult to take countermeasures.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and a technique is devised for the spacer so that the fuel assembly is housed in the center of the channel box as much as possible without substantially increasing the flow path resistance to the coolant flow. Therefore, the deviation of the water gaps on the four sides is minimized, and as a result, the burnout resistance of the corner rod or the side rod at a certain position is not particularly reduced. It is an object of the present invention to provide a reactor fuel assembly spacer capable of preventing the occurrence of a fuel cell.

[Means for Solving the Problems] In the spacer for a nuclear reactor fuel assembly according to claim 1, rod-like elements such as fuel rods are bundled in a square lattice array at intervals from each other in a rectangular tubular channel box. In the spacer for a nuclear reactor fuel assembly that is held in the state, when placed in the channel box on each of the four sides of the outer peripheral side plate of the grid set of the strips forming the plurality of rod-shaped element insertion cells of the mesh shape, Protrusions are provided to abut against the inner wall surface of the channel box so that the distance between the inner wall surface of the channel box and the outer surface of the outer peripheral side plate is substantially symmetrical between four opposite sides, and each of the protrusions is substantially A rigid projection, and an outer dimension between the opposed rigid projections is substantially equal to a minimum value of a corresponding inner dimension tolerance dimension of the channel box. This has solved the above-mentioned problem.

According to the second aspect of the invention, in addition to the configuration of the first aspect, at least two adjacent sides of the protrusion include the rigid protrusion and the substantially elastic protrusion. The present invention provides a reactor fuel assembly spacer in which the elastic projection has a projection dimension larger than the rigid projection.

According to the third aspect of the present invention, there is provided a spacer for a nuclear reactor fuel assembly which holds rod-shaped elements such as fuel rods in a square lattice array in a rectangular cylindrical channel box at intervals. On each of four sides of the outer peripheral side plate of the grid set of strips constituting a plurality of mesh-shaped rod-shaped element insertion cells, the inner wall surface of the channel box and the outer surface of the outer peripheral side plate when disposed in the channel box. Are provided so as to abut each other on the inner wall surface of the channel box so that the distance between the opposite sides is substantially symmetrical between the opposing sides of the four sides. Each of the adjacent two sides of the channel box comprises a substantially elastic projection, and an outer dimension between the opposing rigid projection and the elastic projection is a corresponding inner dimension of the channel box. It is intended to provide a nuclear reactor fuel assembly-body spacer, characterized in that is greater than or equal to the minimum value of the difference sizes.

According to a fourth aspect of the present invention, there is provided a fuel assembly for a boiling water reactor, comprising at least one spacer according to any one of the first to third aspects. is there.

[Operation] In the spacer for a nuclear reactor fuel assembly according to the first aspect of the present invention, rod-like elements such as fuel rods are bundled in a square lattice array at intervals from each other in a rectangular tubular channel box. In the spacer for a reactor fuel assembly to be held in a state, on each of four sides of an outer peripheral side plate of a grid set of strips constituting a plurality of mesh-shaped rod-like element insertion cells, when placed in the channel box, Since the protrusion between the inner surface of the channel box and the outer surface of the outer peripheral side plate is provided so as to be in contact with the inner wall surface of the channel box so that the opposing sides of the four circumferences are substantially symmetrical, the fuel assembly in the channel box is The inner wall surface of the channel box and all four sides are abutted and supported by the protrusions of the spacers. The size of the water gap around the fuel assembly in the fuel cell is determined by the projecting dimension of the abutting portion of the projecting portion on each of the four sides, thereby providing the fuel assembly with symmetry of the sectional position in the channel box. Is guaranteed.

As described above, the outer dimensions of the conventional spacer were determined to be smaller than the lower limit in consideration of the manufacturing tolerance of the inner dimensions of the channel box, so that the fuel assembly was located inside the channel box as shown in FIG. 6a. Eccentrically, and the resulting dimples D on two adjacent sides of the spacer side plate abut on the inner wall surface of the channel box 4. The water gap with the channel box is smaller than when the fuel assembly is located in the center of the channel box as shown in FIG. 6b.

In this way, when the fuel assembly equipped with the conventional spacer is eccentrically housed in the channel box, the dry output of the corner rods and side rods relatively far from the inner wall of the channel box increases. However, on the contrary, the dry output of the corner rods and side rods on the side close to the inner wall of the channel box is considerably reduced, and the burnout resistance of the fuel assembly as a whole is such that the fuel assembly is located in the center of the channel box. Lower than if you were.

That is, since the voids generated by the heat removal of the fuel rods generally tend to move (drift) to the flow path having a large mass flow rate, the sub-channel (FIG. 9: f ₁ , At f ₂ ), as shown in FIG. 7, part of the generated void moves to other internal subchannels (f ₃ and f _6, etc.), and most of the liquid phase has negligible heat generation Due to the cold wall effect of the channel box as small as possible, it flows as a thick liquid film covering the inner wall of the channel box, and a part of the liquid phase flows as a liquid film thinned by evaporation on the surface of the corner rods and side rods. Become.

The flow rate of the liquid film flowing over the inner wall of the channel box increases almost as the gap between the corner rods or side rods and the inner wall of the channel box increases, and this substantially increases the sub-channel at the periphery of the fuel assembly. The cross-sectional area of the flow channel reduces the frictional pressure loss, increases the coolant flow rate in these peripheral sub-channels, and tends to cause most of the liquid phase to flow along the inner wall of the channel box due to the cold wall effect Because.

The liquid film flowing over the inner surface of the channel box is a supply source of droplets to the surfaces of the corner rods and side rods. The liquid film on each surface of the rod also changes.

In the fuel assembly equipped with the conventional spacer, the eccentricity in the channel box causes the sub-channel on the side where the fuel assembly approaches the inner wall of the channel box to be so small that it cannot be ignored. , The dry-out output of the corner rods and side rods facing it decreases considerably due to the decrease in the liquid film flow rate on the inner wall surface of the chamber.

In the spacer of the present invention, when the fuel assembly is viewed as a whole, the minimum value of the gap between the corner rod or the side rod in the channel box and the inner wall surface of the channel box is increased to almost the average value. The minimum value of the liquid film flow rate on the inner wall of the channel box also increases to almost the average value, and the liquid film on the fuel rod surface that contributes the most to the heat removal of the fuel rod is extremely reduced at the corner rod and side rod at any position. Without decreasing
As a result, it is possible to improve the dry-out resistance of the corner rod and the side rod.

As described above, since the fuel assembly equipped with the spacer of the present invention prevents eccentricity in the channel box,
The burnout resistance of the fuel assembly as a whole is reduced because the corner rods, side rods, and the inner wall of the channel box do not get too close to each other, and the dry output of the corner rods and side rods is not unbalancedly reduced. This is significantly better than if the fuel assemblies were skewed in the channel box.

In the present invention, the protruding portions provided on each of four sides of the outer peripheral side plate of the spacer are arranged on the inner wall surface of the channel box and the outer peripheral side plate when the fuel assembly equipped with the spacer is disposed in the channel box. It protrudes with a dimension such that the distance from the outer surface is substantially symmetrical between the four opposite sides.

The protrusions have a uniform protrusion dimension on each side that provides a spacer outer dimension that is basically greater than or equal to the nominal inner dimension value of the channel box, thereby ensuring symmetry of the cross-sectional position of the fuel assembly within the channel box. .

For this reason, in the present invention, each of the protrusions comprises a substantially rigid protrusion, and the outer dimension between each of the opposing rigid protrusions is substantially equal to the minimum value of the corresponding inner tolerance of the channel box. I have. Specifically, in order to guide and prevent vibration at the time of insertion into the channel box on the spacer side plate, usually, one or two or more projection dimples are formed by punching. The dimensions should be the same as the channel box dimensions. Thereby, it is guaranteed that the distance, that is, the size of the water gap around the fuel assembly in the channel box is equal to or larger than the protrusion size of the rigid protrusion on four sides.

In the spacer of the present invention, when the tolerance of the inner size of the channel box is negative, the dimple is less likely to be deformed. Therefore, when the outer dimension between the rigid protrusions is set to be equal to or more than the minimum value of the corresponding inner dimension tolerance dimension of the channel box. When the fuel assembly is inserted into the channel box, the inner wall surface of the channel box may be damaged, or an excessive force may be applied to the spacer to cause displacement or deformation thereof. For this reason, it is necessary to strictly control the dimensional tolerance with respect to the inner size tolerance of the channel box, and when the inner size tolerance is positive, the fuel assembly may vibrate in the channel box due to the coolant flow. In the spacer according to claim 2,
This problem can be solved by providing at least two adjacent protrusions with the rigid protrusions and the substantially elastic protrusions, and making the elastic protrusions larger in size than the rigid protrusions. ing. In this case, the rigid projection and the elastic projection may be combined at the same position, or may be individually provided at different positions, and the rigid projection and the elastic projection may be provided on all four sides of the spacer. Both may be provided. In the spacer according to claim 2,
It suffices that the outer dimension between the rigid projections is not more than the minimum value of the corresponding inner dimension tolerance dimension 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 projection section. Since the extra length of the spacer is absorbed or absorbed by the four sides of the spacer to support the inner wall surface of the channel box by each projection, and at least two sides are elastically performed,
Vibration of the fuel assembly in the channel box due to coolant flow is also prevented.

In another aspect of the present invention, each of the protrusions is a substantially elastic protrusion, and an outer dimension between the opposing elastic protrusions is set to be equal to or more than a minimum value of a corresponding inner dimension tolerance of the channel box. Even in this case, even if the inner dimension tolerance of the channel box is positive, the extra length of the outer dimension of the spacer is absorbed by the elastic deformation of the elastic projections preferably evenly on the four sides of the spacer, and the channel by each elastic projection is formed. The abutting support on the inner wall surface of the box ensures the symmetry of the cross-sectional arrangement position of the fuel assembly, and also damages the inner wall surface of the channel box and causes the spacer to shift when the fuel assembly is inserted into the channel box. There is no fear of doing it.

In the spacer according to the present invention, the positioning of the fuel assembly in the channel box is somewhat uncertain because the protrusions on all four sides of the spacer are elastic projections, and the fuel assembly in the channel box depends on the elasticity of the elastic projection. However, in the invention according to claim 3, the adjacent two sides are substantially rigid projections, and the other adjacent two sides are formed with the respective projections. This is solved by providing substantially elastic projections and making the outer dimension between the opposed rigid projections and elastic projections equal to or more than the minimum value of the corresponding inner dimension tolerance of the channel box. This is because the spacer and the projection according to the first aspect complement each other's functions of the spacer consisting of a substantially elastic projection, thereby obtaining a synergistic effect of the advantages of both.

Further, the invention according to claim 4 is a fuel assembly provided with the spacer according to the present invention. In this case, the fuel assembly includes at least one spacer according to any one of claims 1 to 3 as a fuel assembly. An object of the present invention is to provide a corner rod and a side rod having improved burnout resistance as described above. Of course, a single fuel assembly provided with a plurality of spacers in combination with the spacers according to claims 1 to 5 is also included in the scope of the present invention.

As a preferred embodiment of the present invention, a spacer for a fuel assembly in which fuel rods are arranged in a square array of 9 rows and 9 columns and one square large-diameter water channel having a cross-sectional area of 9 cells is arranged in the center thereof. Will be described below with reference to the drawings.

Embodiment FIG. 1 shows a 9 × 9 type fuel assembly spacer according to an embodiment of the present invention. The channel box 4, fuel rods A and water channels W are the same as those in FIG.

As shown in FIG. 1, a spacer 10 is formed by assembling a square grid by welding around a grid plate 11 formed by assembling a plurality of strips, surrounded by side plates 12 each formed of a similar slightly thicker strip on four sides. A plurality of cells 13 are formed in the inside by the lattice assembly, and fuel rods A are inserted into each of the cells 13, and a large space for nine cells is provided in the center. The bore water channel W can be inserted. A clip-type spring member 14 is attached to four sides of each cell 13 so as to engage with a through hole (not shown) provided in the lattice plate 11 on the side wall of the cell. Is a well-known configuration made of an Inconel spring band plate material formed into a clip shape as an elastic support projection portion and the other side as a rigid support projection portion. It is engaged in the hole. A similar clip-type spring member 15 is also mounted on the spacer side plate 12, but most of the clip-type spring member 15 has only the elastic support protrusion on the cell side.

In the vicinity of each corner of the four sides X ₁ , X ₂ , Y ₁ , Y ₂ of the spacer side plate 12, dimples 16 with vertically split slits are provided by protruding outward and stamped out. ₁ , a dimple 16 of Y ₁ has another clip-type spring member 17 attached to its slit, and this clip-type spring member 17 has elastic support protrusions 18 and 19 on both the cell side and the outside of the spacer. ing. That is, the dimple 16 with the vertically split slit is a rigid dimple D which constitutes a main part of the present invention.
The elastic support protrusion 19 outside the clip type spring member 17 attached to the slit has a larger protrusion size than the dimple 16 as an elastic protrusion which also constitutes a main part of the present invention. Have the following protruding dimensions. In this case, the outer dimension between the dimples 16 on the opposite side is equal to or less than the minimum value of the inner dimension in consideration of the manufacturing tolerance of the channel box 4 and includes the elastic support protrusion 19 outside the spring member 17. The outer dimension of the spacer is larger than the maximum value of the inner dimension in consideration of manufacturing tolerance of the channel box 4 when the spacer is not inserted into the channel box 4.

FIG. 2a is an enlarged plan view of a portion of the dimple 16 with a slit and the clip type spring member 17 mounted on the slit, and FIG. 2b is a longitudinal sectional view of the same.

2a and 2b, the clip-type spring member 17 has a width substantially equal to the width of the slit 16a of the dimple 16, and the elastic supporting protrusion is correlated with its material and plate thickness.
18, 19 are molded to have the required elastic modulus.
The elasticity of the outer elastic support projection 19 can be set to the required elasticity by a projecting shape such as a mountain shape as shown in FIG.
In FIG. 2B, both skirts of the mountain shape are accommodated in the slit 16a of the dimple 16, but when the dimple 16 does not have the slit 16a, the elastic support protrusions on both sides are provided as shown in FIG. 3A.
The channel box 4 may be formed by making the protrusions 18 and 19 substantially symmetrical and changing only the amount of protrusion, or by forming the outer elastic support protrusion 19 into a mountain shape that contacts the dimple 16 at the skirt as shown in FIG. 3b. The elasticity may be adjusted so that the pressing force against the inner wall surface becomes appropriate.

The elastic support projection 18 on the cell side of the clip type spring member 17 is connected to the fuel rod A together with the clip type spring member 14 of the lattice plate 11 and the other clip type spring members 15 of the side plate 12.
In this case, the mounting direction of each clip-type spring member is determined such that the elastic support protrusions and the rigid support protrusions face each other on the four sides of the cell side wall of one cell 2 in one cell 2. The fuel rod A is sandwiched between the opposed protrusions at the center of the cell with an appropriate pressing force.

The clip-type spring member 17 has an elastic support protrusion.
Since the projecting dimension of 19 is relatively large, it is conceivable that the posture stability becomes unstable due to mechanical interference with the channel box when the fuel assembly is inserted into the channel box 4. In order to prevent the clip type spring member 17 from inclining with respect to the spacer side plate 12, a wide support piece or the like for stabilizing the posture is integrally formed with the spring member 17 at a contact portion with the side plate 12, or the clip type spring member 17 is engaged with the side plate 12. It is preferable to adopt a matching structure so as not to cause displacement.

For the 9 × 9 fuel assembly equipped with the conventional spacer shown in FIG. 6a or the spacer according to the embodiment of the present invention shown in FIG. 1, the cap between the channel box 4 and the fuel assembly, more precisely, 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 the side rod. This fuel assembly includes No. 1 to No. 72 fuel rods, and the limit output was determined by the No. 1 corner rod, N
o.2 side rod, No. 11 second row inner rod, N
There are four center rods adjacent to the water channel W of o.22.

In FIG. 4, the horizontal axis represents the interval δ, and the vertical axis represents
As shown in FIG. 6a, when the fuel assembly 1 equipped with the conventional spacer 5 is eccentrically arranged with the dimple D of the spacer side plate in contact with the inner wall surface of the channel box 4 (interval δ = δ _CON ). 5 shows the dryout output ratio at an arbitrary interval δ based on the dryout output of the fuel rod. _ΔSYM on the horizontal axis is the nominal gap when the fuel assembly is symmetrically arranged in the channel box 4 without eccentricity as shown in FIG. 6b by the spacer according to the present invention, and is expressed by the following equation.

δ _SYM = (L _CH −L _SP ) / 2 where L _CH is the nominal value of the inner dimension of the channel box, L _SP is the nominal value of the outer dimension of the current spacer, and in the spacer of the present invention, _ΔSYM is ensured on four _sides by the amount of protrusion of the above-mentioned protruding portion when it is inserted into the.

_δSYM is generally a small value of 1 mm or less.
As can be seen from the figure, the effect on dryout resistance is significant. Corner rod No. 1 is most affected by the flow rate of the liquid film on the inner wall surface of the channel box, and its dry-out output increases about 6% in the case of δ _{SYN in} a symmetrical arrangement compared to the case of δ _CON. Approx. 3 for side rod No.2
% Increase in dryout output. Since the inner rod is far from the inner wall of the channel box, the symmetrical arrangement has little effect on improvement.
In the case of 1, the dryout output is improved by about 1%.

In this case, even without performing a complete symmetrical arrangement, by reducing the degree of eccentricity in the channel box of the fuel assembly so that δ of each side approaches δ _SYM from δ _CON ,
As shown in FIG. 4, it is possible to increase the dryout output of the peripheral rods, thereby improving the burnout resistance performance of the entire fuel assembly.

The above-described embodiment is merely an example of the present invention, and the same effect can be obtained for an 8 × 8 fuel assembly or a fuel assembly including a plurality of water channels. It goes without saying that various other modifications are possible.

[Effects of the Invention] As described above, in the fuel assembly equipped with the spacer of the present invention, eccentricity in the channel box is prevented, so that the corner rods and side rods at any positions approach the inner wall surface of the channel box. Without passing
Since the uneven decrease in the dryout output of the corner rods and side rods is suppressed, the burnout resistance characteristics of the fuel assembly as a whole are improved as compared with the case where the fuel assembly is housed unevenly in the channel box. Things.

[Brief description of the drawings]

FIG. 1 is a schematic plan view showing a spacer according to a first embodiment of the present invention, FIG. 2a is an enlarged plan view of a main part, FIG. 2b is an enlarged vertical sectional view of the same main part, and FIG. 3b is an enlarged sectional view showing a main part of another modification, and FIG. 4 is a 9 × 9 type fuel equipped with a conventional spacer or the spacer according to the embodiment of the present invention. A diagram showing the relationship between the distance δ between the channel box and the fuel assembly and the limit power of the corner rod or side rod for the assembly, and FIG. 5 shows a general BWR core plane layout and a partially cutaway structure of the fuel assembly. FIG. 6a is a schematic plan view showing a state of eccentric arrangement in a channel box of a fuel assembly equipped with a conventional spacer, and FIG. 6b shows a state in which the fuel assembly is symmetrically arranged without eccentricity. Schematic plan view, Fig. 7 shows cooling near corners FIG. 8 is an enlarged sectional view showing a state of a two-phase flow of materials, FIG. 8 is an explanatory view showing a state of a coolant flow on a fuel rod surface and a temperature distribution, and FIG. 9 is a coolant flowing in a fuel assembly in a channel box. FIG. 10 is a diagram showing the difference in mass flow rate depending on the position of the coolant flowing in the fuel assembly. (Explanation of reference numerals of main parts) 10 ... spacer, 11 ... lattice plate, 12 ... spacer side plate, 13 ... cell, 16 ... dimple with rigid slit (rigid support protrusion), 17 ... clip type Spring member, 19: elastic support protrusion, W: water channel, 4: channel box, A: fuel rod.

Claims (4)

(57) [Claims] Claims: 1. A spacer for a nuclear reactor fuel assembly for holding rod-shaped elements such as fuel rods in a square cylindrical channel box at a distance from each other and bundled in a square lattice arrangement, comprising: On each of four sides of the outer peripheral side plate of the lattice assembly of the strips constituting the rod-shaped element insertion cells, when placed in the channel box, the interval between the inner wall surface of the channel box and the outer surface of the outer peripheral side plate is four oppositions. Projections are provided so as to be in contact with the inner wall surface of the channel box so as to be substantially symmetrical between sides, and each of the projections comprises a substantially rigid projection, and an outer dimension between the opposed rigid projections Is substantially equal to the minimum value of the corresponding internal tolerance of the channel box. 2. At least two adjacent sides of the protrusion are
2. The reactor fuel assembly according to claim 1, comprising the rigid protrusion and a substantially elastic protrusion, wherein the elastic protrusion has a protrusion dimension larger than the rigid protrusion. 3. spacer. 3. A spacer for a nuclear reactor fuel assembly for holding rod-shaped elements such as fuel rods in a rectangular cylindrical channel box at a distance from each other and bundled in a square lattice arrangement, comprising: On each of four sides of the outer peripheral side plate of the lattice assembly of the strips constituting the rod-shaped element insertion cells, when placed in the channel box, the interval between the inner wall surface of the channel box and the outer surface of the outer peripheral side plate is four oppositions. Protrusions are provided so as to be in contact with the inner wall surface of the channel box so as to be substantially symmetrical between the sides, and the protrusions on two adjacent sides are formed of substantially rigid protrusions and are formed on the other two adjacent sides. Each of the protrusions comprises a substantially elastic protrusion, and an outer dimension between the opposing rigid protrusion and the elastic protrusion is equal to or less than a minimum value of a corresponding inner tolerance of the channel box. Reactor fuel assembly-body spacer, characterized in that it is in. 4. A fuel assembly for a boiling water reactor, comprising at least one spacer according to claim 1.