JPH0328794A - Fuel assembly for boiling water reactor and its water pipe for cooling water flow - Google Patents

Abstract

PURPOSE:To make a pressure drop small by nearly equalizing overall axial length to the fuel effective length of a fuel rod and making the tube plate thickness in a lower area having its border at a height position which is about 1/3 - 2/3 time the overall axial length larger than the tube plate thickness in an upper area. CONSTITUTION:A short-sized W channel has maximum deformation due to creeping at the position P of about 1/3 height from the short-sized channel. This results from that the acceleration factor of fast neutrons affects the creep deformation fo the short-sized channel in addition to the inside/outside pressure difference. For the purpose, the plate thickness (d) in the upper area U of the short-sized W channel is so determined that the creep deformation quantity at the part is nearly equal to or less than the maximum deformation quantity. For example, when the plate thickness (d) in the lower area U is 0.7mm, the plate thickness (d) in the lower area D is 1.5mm. Consequently, the pressure drop in an in-channel area to a two-phase flow is reduced and channel stability is improved.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention generally relates to a fuel assembly for a boiling water reactor (BWR), and particularly relates to a cooling water distribution water pipe (round type , regarding the improvement of square-shaped U-Saiichita channel).

Further, the difference in details relates to the structure of the fuel assembly for increasing the flow passage cross-sectional area of the two-phase flow section in the fuel assembly and improving the collective rest pressure loss characteristics, and the structure of the cooling water flow pipe used therein.

That is, the technical field of the present invention includes improving the channel stability and core stability of a BWR by reducing the pressure drop in a two-phase flow section where the pressure drop is large due to the occurrence of voids in the sludge aggregate.

[Prior art] In this section, we will first discuss an 8×8 fuel assembly as a conventional BWR fuel assembly.
The configurations of the fuel rods, cooling water pipes, and the entire assembly of the fuel assembly will be explained, and then the 9x9 fuel assembly will be explained.

<Fuel rod> FIG. 6(A) shows the configuration of the fuel rod.

The fuel rod 8l is made by filling a Zircaloy cladding tube 1b with sintered uranium dioxide pellets 1a, and arranging a Blenheim spring 1c in the Blenheim area above the cladding tube 1b, and using an inert gas with good thermal conductivity ( Helium) is sealed, and the lower end of the cladding tube 1b is filled with an upper end plug 1d and a lower end plug 1, respectively.
It is constructed by sealing welding with e. Here, the loaded fuel effective length (uranium portion) of the uranium pellet 1a is usually about 370 cm, and the blennium length (axial length of the blennium region) is usually about 30 cm.

<Water pipe for cooling water distribution> FIG. 6(B) shows the configuration of the water pipe for cooling water distribution (in the illustrated example, a Usaichi tarod).

The quarter rod wr (hereinafter abbreviated as "W rod") is hollow and does not contain any fuel material inside, and upper and lower end plugs 1d1e are welded to the upper and lower parts of the main body, similar to the fuel rod 81.
An appropriately controlled amount of unboiled water flows from the lower part to the upper part through an upper orifice hole h1 and a lower orifice h2 provided near these end plugs 1d and Ie, respectively.

<8x8 type fuel assembly> Figure 7 shows the cross-sectional structure of the 8x8 type fuel assembly in the core loaded state.

In the figure, the 8×8 type fuel assembly, indicated by the symbol 8o as a whole (hereinafter referred to as “8×8 fuel゜゜)” is NO.1.
A fuel bundle is constructed by arranging the above-mentioned fuel rods 81 of 62 wood of ~N0.62 and the above-mentioned W lot of 2 wood in a square grid of 8 rows and 8 columns, and these are placed in a channel box 85 made of Zircaloy. As shown in the figure, the cross-shaped control rods Crk: are loaded into the reactor adjacent to each other. During power operation of the nuclear reactor, cooling water flows from the bottom of the fuel toward the top, removing heat generated by the fuel rods 81. Note that the symbol "i" in the figure indicates an instrumentation tube.

The overall structure of this 8×8 fuel 80 is as shown in FIG.

In FIG. 8, a fuel rod 81 and a W lot are fixed by an upper tie plate 82 and a lower tie plate 83. Furthermore, spacers 84 are arranged at regular intervals along the assembly, so that the spacing between the fuel rods is kept constant.

<9x9 type fuel assembly> By the way, in recent years, 9x9 type fuel assemblies have been adopted in place of the above-mentioned 8x8 fuel 80 for the purpose of increasing the burnup of fuel (for example, Please refer to Publication No. 1988-62118297>
.

FIG. 9 shows an example of a configuration of a 9×9 fuel assembly (hereinafter referred to as "conventional 9×9 fuel"). Components 91, 9 of a conventional 9x9 fuel, indicated generally at 90 in the figure.
2. . . , 95 correspond to the constituent elements 81, 82, . . . , 85 of the 8×8 fuel, respectively.

In the illustrated configuration example, a single wooden 3×3 type U-shaped channel W (hereinafter abbreviated as "w channel'゜) is provided as a water pipe, and 72 fuel rods 91 are arranged. The structure of the rod 91 is similar to the 8x8 fuel rod 81 except for the fuel rod diameter and fuel enrichment.On the other hand, the W channel promotes the moderation of neutrons to increase the reactivity of the fuel assembly. , which is an important component for controlling the water uranium ratio and optimizing the void coefficient, is detailed as follows regarding the W channel cooling water flow rate.

If the flow rate of cooling water flowing through the W channel in the reactor power operating state is too large, the flow rate of the in-channel used for cooling the fuel rods 91 will decrease, which is dangerous because the cooling capacity of the fuel rods 91 will decrease. On the other hand, if the cooling water flow rate is low, the cooling water and gamma rays in the W channel. Vapor bubbles (voits) are generated within the W channel due to heat generation due to reaction with neutrons and heat transfer through the W channel side wall, which hinders neutron moderation.

Therefore, it is necessary to avoid the occurrence of voids in the W channel during reactor power operation. For this reason, conventionally the 10th
As shown in the figure, upper and lower orifice holes h3 and h4 are provided in the upper and lower end plugs W1 and W2 of the W channel, so that the flow of cooling water is controlled in an appropriate amount.

In addition, conventional water pipes (W channel, W lot) have a total axial length that is almost the same as the total axial length of the fuel rod, and the plate thickness is constant in the axial direction (for example, 0.7 mm for the W channel).
It becomes.

[Problems to be Solved by the Invention] Compared to the 8×8 fuel 80, the 9×9 fuel 90 as described above achieves higher fA compression and at the same time increases the number of fuel rods. It has the advantage of reducing the output per fuel rod and reducing the temperature under the same output conditions.

However, as the number of fuel rods increases, pressure loss also increases. Furthermore, in the 9x9 fuel 90, in order to increase the area of the non-boiling water region in the center of the assembly in response to the high enrichment of the fuel, a large diameter water pipe (for example, a large diameter W lot or The W channel shown in the example of FIG. 10) is adopted. By increasing the diameter of the water pipe, the flow area of the in-channel region becomes smaller and the pressure drop becomes higher. Therefore, with the 9×9 fuel 90, the unstable phenomenon of channel stability, which is a problem in BWR, becomes more prominent.

Here, channel stability refers to the degree of effectiveness in suppressing flow rate fluctuations.

On the other hand, core stability refers to the degree to which the entire core region produces sustained local power oscillations in coordination with either flow rate oscillations or power oscillations. However, since this core stability a is subject to change as the channel stability is changed, the following explanation will focus exclusively on the channel stability.

This unstable phenomenon of channel stability is unique to BWR, and it has been pointed out that the greater the pressure drop in the two-phase flow section due to the generation of voids, the more unstable it becomes (for example, in the article by Enoki et al. ``Recent findings and future prospects on water-type light water reactor stability'', Journal of Japan Atomic Energy Society, Vol. 27, No. 10. No. 89
0, 1985). In addition, especially when the reactor power is about 50% or more of the rated power and the recirculation flow rate is relatively low,
It is also known that it becomes unstable at 1.

*1 For example, natural circulation flow rate (approximately 30% of rated flow rate) or recirculation pump speed during reactor startup, low power adjustment during load following operation, or core flow rate reduction due to recirculation bomb failure. is operating at approximately 20% of the rated value.

When this unstable phenomenon occurs, the flow rate oscillates greatly,
In the worst case, the fuel may be damaged due to insufficient heat removal. Furthermore,
The vibration of the flow rate is caused by the vibration of the coolant vapor volume ratio (Voight rate),
The resulting oscillations in the nuclear reaction rate cause oscillations in the entire reactor core or local neutron flux, eventually leading to a reactor scram.

Therefore, ensuring channel stability in BWR is an important issue.

Furthermore, conventional water pipes such as the above-mentioned W lot and W channel have the problem that they deform as they are used. This problem will be explained using the W channel of 9×9 fuel 90 as a representative.

The driving pressure of the cooling water flowing through the W channel is the pressure difference between the W channel and the in-channel.
The distribution of the W channel 'l'llll is not expressed in white, but is as shown by the dashed dotted line in FIG. That is, at the W channel entrance (lower orifice hole h4), the inside of the W channel is under negative pressure based on the pressure of the in-channel, and the cooling water is forced into the W channel side. On the other hand, there is a positive pressure inside the W channel at the W channel outlet (upper orifice hole h3), and the cooling water is pushed out by the pressure loss resistance pile of the two-part orifice hole h3. This pressure difference between the inside and outside of the W channel causes deformation of the W channel, and the details are as follows.

The cross-sectional shape of the W channel in the manufactured state is shown in Figure 12 (A
) as shown in the square shape. However, during use (during reactor operation), the pressure of the cooling water acts on the side of the W channel, and at the axial lower part of the W channel, the pressure increases to W.
Because it is higher in the in-channel region than inside the channel,
The cross-sectional shape is deformed inwardly as shown in FIG. 12(B). This amount of deformation increases with time due to creep deformation that is accelerated by irradiation. - If the amount of deformation becomes excessive, the engagement with the smoother 94 (spring contact) will be disengaged, causing fretting due to fluid vibration, and in the worst case, there is a risk that cracks will occur in the W channel.

The present invention has been made in view of the above-mentioned problems of the conventional technology, and its purpose is to reduce the pressure loss for the two-phase flow in the in-channel region, and to improve the channel stability and core stability of the BWR. B is suitable for operating conditions with particularly high pressure drop, such as low flow rate operation.
We also provide fuel assemblies for W'R and their cooling water distribution pipes. Furthermore, in addition to this purpose, it is also part of the object of the wooden invention to reduce the creep deformation of the water pipe for cooling water distribution and improve safety.

[A Thousand Steps to Solve the Problems] In order to achieve the above object, the present invention provides a buntle configuration consisting of a plurality of fuel rods in a square lattice arrangement, in which a part of the fuel rods is replaced. In the fuel assembly equipped with a cooling water canal water pipe, the total axial length of the water pipe is approximately equal to the effective fuel length of the fuel rod, and the upper part of the total axial length 11 12 is about 1/3 to 2 Of the two upper and lower regions bounded by a height position of /3 or more, the tube plate thickness in the lower region is thicker than the tube plate thickness in the upper region, and the upper end in the axial direction almost coincides with the upper end of the effective fuel length. It is arranged like this.

The present invention also provides a cooling water flow in which one or more fuel rods are arranged in place of some of the fuel rods in a buntle configuration consisting of a plurality of fuel rods in a square lattice arrangement in a fuel assembly for a boiling water reactor. In the service water pipe, the fuel rod has a relatively short overall length dimension that is approximately equal to the effective fuel length of the fuel rod that constitutes the bundle, and has a height position of about 1/3 to 2/3 or more of the upper part of the overall axial length. Of the two upper and lower bordering areas, the thickness of the bamboo plate in the lower area is thicker than the thickness of the tube plate in the upper area.

In either case, the water tube may include a region formed in a tapered cross-sectional shape so that the tube plate thickness becomes gradually thinner at the boundary portion from the lower region to the upper region.

[Operation] In the present invention, the total axial length of the water tube of the fuel assembly is made approximately equal to the effective length of the fuel. In other words, the axial length of the water tube in the present invention is shorter than that of the conventional water tube by the length corresponding to the blemish region of the fuel rod. In other words, no water tube exists in the axial region of the aggregate corresponding to the Blenheim region. Therefore, the flow path area (
Hereinafter, the ``Blenheim channel flow path area'') is expanded by the cross-sectional area of the water pipe.

A two-phase flow with a high void ratio occurs in the in-channel region corresponding to the Blenheim region, but a low pressure drop can be achieved by expanding the flow path area. Therefore, in a BWR loaded with the fuel assembly of the present invention, the channel stability is significantly improved.

In addition, the cooling water distribution water pipe in the fuel assembly of the present invention has a constant plate thickness in the axial direction, whereas the conventional water pipe has a constant thickness in the axial direction.
The thickness of the lower part in the axial direction is thicker than that of the second part. Here, the lower part of the water pipe is made thicker to increase the resistance to creep deformation due to the pressure difference of the cooling water.On the other hand, in the water pipe section, the "C" is fixed on the outside or with a smoother. Because of this, there is no problem of nodding like in some water pipes.

Therefore, there is no need to thicken the water pipe section.

The boundary point for increasing the thickness of the water pipe is approximately the upper part of the water pipe in the axial direction, which is the position where -c/lT- is zero in the distribution of the pressure difference between the inside and outside as shown in Fig. 11. A height of 1/3 to 2/3 or more is appropriate. In this case, depending on the thickness of the water pipe, it may be formed so as to smoothly change into a flat, flat or flat shape.

In order to clarify the features and advantages of the wooden invention, some preferred embodiments will be described with reference to the accompanying drawings.

[Example] Since the combustion angle assembly of the wood invention can achieve a low IEE trt, it is currently effective for achieving a 9x9 fuel efficiency with a high E[tjl. An example of this is the case where the invention is applied to 9x9 fuel, but of course the application of the present invention is not limited to 9x9 fuel.

An example of the configuration of a 9×9 fuel according to the present invention is shown in FIGS. 1 and 2.

The 9×9 fuel 90A (Example A) shown in FIG.
O. It is a type equipped with 473 wooden fuel rods 91 and two large-diameter water rods WR.

On the other hand, the 9×9 fuel 90B (Example B) shown in FIG.
) HaNO. I ~N0.7 2 (7), il72
Wooden fuel rod 91 and one 3x3 type Usaichi channel W
It is a format with Configuration i of this 9×9 fuel 90B
It is the same as the conventional 9x9 fuel 90, except for the J, U and 4-T Channel W.

For each 9x9 fuel 90A, 90B: l3 +'J drainage pipe (
Dog caliber water rod WR, water channel W
), does it have any characteristics in the overall axial length and thickness as explained in the section of the function? - As an example, for 9x9 fuel 90B. An example of the configuration of the JJ Rir Uta Channel W (hereinafter not referred to as ``short W channel'') is shown in FIGS. 3 and 4.

In the figure, the total length of the short W channel in the axial direction is equal to the fuel efficiency (approximately 370 cm in this embodiment). From this square-shaped W channel θ)lj,:,4-scaled V, the Brenna l\-indian channel flow area of 9×9 paddy 90B is expanded as described above. Specifically, the 9×9 fuel 90B of the present invention and the conventional 9×9 fuel 90
The inch tunnel area is approximately 95cm' according to the present invention.
Since the cross-sectional area of the short W channel and the conventional W channel in J5 is about 15 cm"C, the plenum channel flow area of the 9x9 fuel 90B of the present invention is smaller than that of the conventional 9x9 fuel 90. This results in an expansion of approximately 16%.This corresponds to an increase in equivalent hydraulic diameter (4 x flow path area/wetted edge length) of approximately 25% as t.f ages. Due to the shortening of the W Buchennel, the conventional W
The shank that was set in the second part of "Channel" (Part 2)
Part 18 (H, 1rh, the engaging protrusion on the upper part of the shuttle W1) shown in Figure O has been deleted. Conventionally, this shank has only been fitted to the upper type L No. 1/16 with some play, so there is no particular disadvantage in a configuration in which the shank is removed. The maximum deformation occurs at a height position P of approximately 1/3 from the lower end of the length-shaped W channel. This is short i{! The creep deformation of the W channel is affected not only by the pressure difference between the inside and outside but also by the accelerated concentration of fast neutrons. Therefore, the plate thickness dU of the -1 partial region [J of the 11-inch type W channel is set to be equal to or less than the amount of creep deformation in that portion or the maximum amount of deformation described above. To give an example, when the plate thickness du of the L region U shown in FIG. 1 5... Here-
The cross-sectional shape from the bottom region D to the bottom region U may be formed in the shape of a retainer as described above.

A graph showing the effect of the tree embodiment in terms of the limit output of channel stability is shown in FIG. Here, the analysis conditions were a natural circulation state where the core flow rate was approximately 30% of the rated value. Further, the pressure loss ratio on the horizontal axis of the graph is the ratio of the two-phase flow section pressure loss to the single-phase section pressure loss of the fuel assembly. In the figure, mark ●. ◆marks are 9×9 fuel 90 respectively
The characteristics of A and 90B are shown, and the O and ◇ marks represent the characteristics of a conventional configuration example (one in which the water pipe is not shortened and the wall thickness is not changed) corresponding to the 9×9 fuels 90A and 90B, respectively.

As is clear from this figure, the 9X9 fuel 90 of this example
A, 90B channel stability limit output is conventional 9×
This is about 7% higher than 9 fuel.

[Effects of the Invention] As explained above, according to the present invention, the pressure loss in the two-phase flow section is reduced by shortening the water tube, and channel stability and core stability can be significantly improved. Furthermore, since the lower part of the water pipe is made thicker, creep deformation of the water pipe is reduced and safety is improved.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional view showing a 9x9 fuel plate according to one practical example of the present invention, Fig. 'fJ2 is a cross-sectional view showing a 9x9 fuel plate according to another embodiment of the wood invention, and Fig. 3 Figure 4 (A) is a perspective view showing the short water channel used for the 9x9 fuel shown in the previous figure.
is a sectional view taken along line A-A in the previous figure, FIG. 4(B) is a cross-sectional view taken along line B-B, and FIG. Diagram showing the relationship with the sex furnace output in comparison with the conventional example, Figure 6 (A). (B) are each 8×
8 Fuel rods for fuel. An internal transparent vertical cross-sectional view showing the configuration of an 8x8 fuel water lot, Figure 7 is a cross-sectional view of the 8x8 fuel loaded in the reactor core, and Figure 8 is the entirety of the 8x8 fuel shown in the previous figure. An internal transparent perspective view showing the configuration, Figure 9 is a conventional 9x9
FIG. 10 is an internal perspective view showing the overall configuration of a fuel assembly. FIG. 10 is an internal perspective view showing the configuration of a conventional water channel. FIG. 11 is a diagram showing the inside of a conventional water channel during reactor power operation. Diagram showing calculation results of external coolant pressure distribution, Figure 12 (A). (B) is a cross-sectional view showing the shape of the water channel during manufacture and use, respectively. [Explanation of symbols of main parts] 9OA 90B...9x9 fuel W...Short water channel WR...Short large diameter water rod U...Upper region p...Lower region P... The boundary points are the same in each figure. Codes indicate the same or equivalent parts.

Claims (4)

[Claims] (1) In a fuel assembly comprising one or more water pipes for cooling water distribution replacing a part of the fuel rods in a bundle configuration consisting of a plurality of fuel rods in a square lattice arrangement, the water pipes are The total length in the axial direction is approximately equal to the effective fuel length of the fuel rod, and the upper third of the total length in the axial direction is approximately equal to the effective fuel length of the fuel rod.
Of the two upper and lower regions bordered by a height position of ~2/3 or more, the tube plate thickness of the lower region is thicker than the tube plate thickness of the upper region, and the upper end in the axial direction is approximately the same as the upper end of the effective fuel length. A fuel assembly for a boiling water reactor, characterized in that the fuel assembly is arranged so as to match. (2) The water tube includes a region formed in a tapered cross-sectional shape so that the tube plate thickness gradually becomes thinner at the boundary portion from the lower region to the upper region.
A fuel assembly for a boiling water reactor as described in . (3) In a fuel assembly for a boiling water reactor, one or more water pipes for cooling water distribution are arranged in place of some of the fuel rods in a bundle configuration consisting of a plurality of fuel rods arranged in a square lattice arrangement. , having a relatively short overall length approximately equal to the effective fuel length of the fuel rods constituting the bundle, and bordering on a height position of about 1/3 to 2/3 or more of the upper part of the entire axial length. A cooling water distribution water pipe characterized in that, of the two upper and lower regions, the tube plate thickness in the lower region is thicker than the tube plate thickness in the upper region. (4) The water pipe for cooling water distribution according to claim 3, characterized in that it includes a region formed in a tapered cross-sectional shape so that the tube plate thickness becomes gradually thinner at the boundary portion from the lower region to the upper region.