JPH028789A - Fuel assembly for nuclear reactor - Google Patents

Abstract

PURPOSE:To increase a value of a neutron multiplication factor and to improve a thermal performance by forming an outer circle of water rod with a member having a recessed part the interval between the water rod and a fuel rod adjacent to the water rod is kept wider than a specific distance. CONSTITUTION:A main part of a fuel assembly 11 is constituted with a channel box 12, small diameter fuel rods 13 arranged in the box 12 and larger diameter fuel rods 14 arranged at a central part of a horizontal cross section in the box 12. An outer shape 14a of the rod 14 has a shape which is connected arcs each of which is larger than the fuel rod 13a, and is shaped around centers of diameters of the fuel rods 13a being adjacent each other, as its center point. With shaping the rods 14 to be of such a cross sectional shape, an adjacent flow pass areas can be diminished at around 45% and a cross sectional area of the water rod can be increased at around 45%. In this way, the cross sectional area of the water rod can be increased and a coolant flow in a fuel element can be homogenized as well therefore a neutron multiplication factor can be increased and thermal performances can be much improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a nuclear reactor fuel assembly, and particularly to a fuel assembly for a water-moderated nuclear reactor.

(Prior art) In recent years, in water-moderated reactors, especially light water-moderated reactors, which are the mainstream, in order to reduce the operating costs of nuclear couplers, long-term cycle operation and economical combustion methods of fuel assemblies to achieve this have been developed. is being considered.

FIG. 10 is a diagram showing an example of the cross-sectional shape of a fuel assembly employed in a conventional boiling water light water reactor.
fuel rods 3 arranged in a square lattice shape of 8 rows and 8 columns;
2 placed in the center of the horizontal section inside the channel box
It consists of a main water rod 4.

This water rod 4 allows non-boiling water to flow through the internal oil,
The neutron moderating effect of this water plays a role in alleviating the decrease in thermal neutron flux at the center of the fuel assembly and improving the efficiency of thermal neutron utilization.

The structure of the fuel assembly in such a boiling water light water reactor must be constructed in order to achieve efficient combustion of fuel.

Conventionally considered means include increasing the cross-sectional area of the water rods by increasing the number of water rods or increasing the diameter of the rods.

That is, by increasing the cross-sectional area of the water rods, the H/U ratio (hydrogen atoms to fissile uranium atoms) in the core is increased, effectively slowing down (thermalizing) neutrons, and increasing the neutron multiplication rate. It aims to increase the...

For example, as shown in FIG.
In addition, as shown in FIG. Inside the channel box 2, small diameter fuel rods 6 are arranged in a square lattice shape of 9 rows and 9 columns, and instead of the 9 small diameter fuel rods placed at the center of the horizontal section inside this channel box 2, one large diameter water rod is arranged. A fuel assembly having a configuration in which rods 7 are arranged is being considered. In fuel assemblies that use water rods with a large cross-sectional area, compared to fuel assemblies without water rods, there is an increase in surface heat flux due to a decrease in the number of fuel rods in the fuel assembly, and a non-heat generating effect. Since the thermal limit output decreases due to non-uniformity of the coolant flow rate due to the introduction of parts, countermeasures such as flattening the radial output distribution are being taken.

In addition, when increasing the cross-sectional area of the water rod, the fuel rods adjacent to the water rod become thermally harsh due to higher burnup. It is necessary to maximize the cross-sectional area of the rod.

(Problems to be Solved by the Invention) By the way, the disadvantages of a fuel assembly having a structure in which large-diameter water rods are installed in fuel rods arranged in a square lattice as shown in FIGS. 11 and 12 are as follows. As the water rod diameter increases, the flow path area around the large diameter water port and rod increases compared to a fuel assembly without a large diameter water rod, and the thermal equivalent diameter around the large diameter water port and rod also decreases. , the proportion of the total coolant flow flowing around the large diameter water rod increases, and the coolant spent on cooling the fuel rods is relatively reduced.

This tendency can be explained by the fact that two fuel rods 6 as shown in FIG.
When large-diameter water rods 8 are arranged in fuel assemblies arranged with different fuel rod pitches PI and P2, it amplifies the decrease in the cooling l flow rate in the flow path with a narrow fuel rod pitch, and It is thought that the critical output power is lower than that of the uniform-Rakuko arrangement.

There is a problem in that such a reduction in the thermal limit power reduces the number of valleys in the core design, which may lead to deterioration in operability.

The present invention was made to solve the above-mentioned problems, and it is possible to increase the cross-sectional area of the water rod, equalize the flow rate of coolant in the fuel assembly, have a large neutron multiplication factor, and The objective is to provide a nuclear reactor fuel assembly with good physical performance.

[Structure of the Invention] (Means for Solving the Problems) The nuclear reactor fuel assembly of the present invention includes a large number of fuel rods housed in a channel box, and a water rod arranged inside the fuel rods. In the nuclear reactor fuel assembly, the side surface of the water rod is configured by a set of curved surfaces that maintain a substantially constant gap between the water rod and the fuel rod adjacent to the water rod.

(Function) By configuring the outer periphery of the water rod with a concave shape that maintains a gap between the water rod and the fuel rod adjacent to the water rod above a certain level, the cross-sectional area of the water rod is increased and Adjacent coolant flow paths can be prevented from becoming large. Considering that the fuel rod has a cylindrical shape, the concave shape that forms the outer periphery of the water rod is designed so that the narrow part of the gap between the water rod and the fuel rod adjacent to the water rod is kept above a certain level. The shape should be a smooth curve such as an arc or an ellipse.

The concave shape of the water rod that maximizes the surface area is based on a concentric circle whose radius is the radius of the fuel rod adjacent to the water rod plus the minimum gap between the water rod and the adjacent fuel rod, with the outer periphery of the adjacent fuel rod as the center. The outer periphery of the water wood is constructed by a curve connecting the arcs of the water rod.With this configuration, the length of the minimum gap along the circumference of the water rod becomes the longest, and the cross-sectional area of the water rod becomes the largest. become.

Further, the concave shape that maximizes the area of the water rod may be formed by a collection of convex curved surfaces circumscribing the concave curved surface.

In this case, the convex curve forming the outer circumference of the water rod is
A circular arc that circumscribes a curve that connects concentric circles whose radius is the radius of the fuel rod adjacent to the water rod plus the minimum gap between the water rod and the adjacent fuel rod, centered on the outer periphery of the adjacent fuel rod. Let be a convex curve connecting arcs of circles having the same radius as the concentric circle.

In this way, while increasing the cross-sectional area of the water rod, the increase in the cross-sectional area of the coolant flow path around the water wood is suppressed, and the coolant flowing inside the assembly is not unevenly distributed around the water rod, and the coolant flowing in the aggregate is prevented from being unevenly distributed around the water rod. Cooling of the fuel rods can also be ensured at the same time.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing the cross-sectional structure of the fuel assembly of the example.
The fuel assembly 11 includes a channel box 12, small-diameter fuel rods 13 arranged in a square lattice shape of 9 rows and 9 columns in the channel box by inserting uranium dioxide pellets into a zirconium alloy cladding tube, and these fuel rods 13. The main part consists of one large-diameter water rod 14 placed in the center of the horizontal cross section of the channel box 12 in place of nine small-diameter fuel rods.

The cross-sectional shape of the large-diameter water rod 14 has an outer peripheral shape 14a that maintains the minimum gap between the shape a of the conventional large-diameter water rod and the adjacent fuel rod 13a.

That is, the outer peripheral shape 14a of the large diameter water rod 14 is the same as the fuel rod 1 drawn with the center of the diameter of the adjacent fuel rod 13a as the center point.
It has a shape formed by connecting arcs larger than 3a.

By making the large-diameter water rod 14 into such a cross-sectional shape, the flow path area adjacent to the large-diameter water rod 14 can be reduced by about 45%, and the cross-sectional area of the power hydraulic rod can be increased by about 45%. Become.

Note that the outer peripheral shape of the large-diameter water rod 14 does not have to be a series of circular arcs, and may be, for example, a part of an ellipse or a gentle curve.

FIG. 2 is a diagram showing another embodiment of the present invention, in which a fuel assembly 1
1 has a structure in which 8 sets of fuel rods 13 arranged in 3 rows and 3 columns are housed in a channel box 12. A large diameter water rod 16 of corresponding size is arranged.

The cross-sectional shape of the large-diameter water rod has an outer circumferential shape 16a formed by connecting arcs so as to maintain a minimum gap with the adjacent fuel rod 13a, as in FIG.

In this fuel assembly 11 of 1-inch construction, the large diameter water rod 16
The flow area adjacent to the water rod can be reduced by about 37%, while the water rod cross-sectional area can be increased by about 54%.

Thus, in the embodiment described above, the flow rate ratio W /W used for cooling the internal fuel rods (where rod
Lot W: All channel flow rate, W: All channels tot
Since the total flow rate increases, the thermal limit output increases as shown by curve (b) in FIG.

In addition, according to this example, as shown by curve (B) in Figure 4, due to the increase in the cross-sectional area of the water rod, the infinite neutron multiplication factor becomes higher than that of the conventional device shown by curve (A). , can increase the efficient combustion of fuel.

By the way, a mechanism for making the flow rate distribution of the coolant uniform within the fuel assembly according to this example will be described below.

As shown in Figure 7 above, the cross section of the assembly is divided into subchannels A (subchannels) at the point where the gap between the fuel rods is the smallest, and for simplicity, the fuel flows through subchannels A (subchannels). It is assumed that the profile of the coolant flow rate G per unit area is determined such that ΔP in the single-phase part is equal. Then,
The coolant flow rate Gl per unit area of each subchannel is given by the following expression Ve (1) using the average coolant flow per unit area ff1G.

G1-Fg*G, vo ・・−・−・(
1) F − [A * (D,) 4] g tot Compared to the conventional device shown by curve (A), / [
nA (D) t+n, Ay (Dhv) ~ h +n A (D)1+nwlRAwlRehc +D] ・
・・・・・・・・・(2) W/R Where, A: Cross-sectional area of the aggregate flow path tot A: nA+n, A, +ncAcot +0wlRAwlR n: Number of internal subchannels “W/R” adjacent to the water rod Number of subchannels: Number of outer subchannels (excluding n): Number of corner subchannels: Internal subchannel hydraulic diameter: Subchannel hydraulic diameter adjacent to water rod: Outer subchannel hydraulic diameter (excluding n): Corner subchannel hydraulic diameter: Internal subchannel passage area A: Subchannel adjacent to water rod W/R passage area AI,: Outer subchannel passage area Ao: Corner subchannel passage surface However, D is inside Represents the hydraulic diameter of the subchannel.

Therefore, the flow distribution per unit area is proportional to the square root of the hydraulic diameter per subchannel.

In addition, the ratio W /W of the total channel flow rate W and the flow rate used for cooling the internal fuel rods excluding the flow rate used for cooling the fuel rods adjacent to the water rods from the total channel flow rate W is W /W The following rods
It is given as the rod tot expression.

W /W -1/ll+[A (
D) 1 rod tot
W/RV/R/[A (D)
~ ] ) ... (3) rod ro
d Here, the subscript indicates the quantity related to the flow path excluding the subchannel adjacent to the water rod.

Usually, (D)'/(D)' is greater than 1, W
/Rrod Since the channel area ratio A /A is about 0.1, the flow rate used for cooling the fuel rods inside W/Rrod is estimated to be 90% or less of the total channel flow rate.

The effect of this example is that (D '') ~/(D
) ~ V/Rrod The flow rate ratio W used for cooling the internal fuel rods by reducing both the flow path area ratio A /A V/Rrod
/W can be increased.

rod tot By the way, when considering irradiation growth, etc., it is necessary to consider the outer circumferential shape of the water rod so that the intersection points of each arc do not have acute angles. In such a case, as shown in Figure 5, Alternatively, the water rod 17 may have an outer peripheral shape 17a with the intersection points cut off.

Furthermore, as another embodiment of the present invention, as shown in FIG. ) The present invention can also be applied to. in general,
In fuel assemblies that employ conventional square water ports and throats, the flow rate ratio used for cooling internal fuel rods is not small compared to water ports and throats that have circular cross-sectional shapes. By adopting No. 18, it is expected that the cooling efficiency of the internal fuel rods will be improved.

Incidentally, in each of the above-mentioned embodiments, the side shape of the water rod is constituted by a set of concave curved surfaces that maintain a substantially constant gap with the fuel rod adjacent to the water rod, but the present invention is not limited to this. The side shape of the rod may be constituted by a set of convex curved surfaces circumscribing a concave curved surface that maintains a substantially constant gap from the fuel rod adjacent to the water rod.

Hereinafter, such an embodiment will be described.

Note that the same parts as in FIG. 1 are given the same reference numerals, and the explanation of the overlapping parts will be omitted.

FIG. 7 is a diagram showing the cross-sectional structure of the fuel assembly of this embodiment, and the cross-sectional shape of the large-diameter water rod 21 maintains the minimum gap between the shape a of the conventional large-diameter water rod and the adjacent fuel rod 13a. It has an outer circumferential shape 21a formed of a convex curve formed by connecting a concave curve and circumscribed circular arcs.

That is, the cross-sectional shape 21a of the large-diameter water rod 21 is first defined by the fuel rod 1 with the center point of the diameter of the adjacent fuel rod 13a as the center point.
It is formed so that a concave curved line is drawn by connecting arcs larger than 3a, and then a convex curved shape is drawn by connecting arcs that are circumscribed to this concave curved line.

By making the large-diameter water rod 21 into such a cross-sectional shape, the heavy viscosity of the flow path adjacent to the large-diameter water rod 21 can be reduced by about 15%, and the sheer viscosity of the power hydraulic rod can be increased by about 15%. It becomes possible.

Note that the shape of the curved line does not need to be a combination of circular arcs, and may be, for example, a part of an ellipse or a gentle curve.

In addition, by configuring the outer peripheral shape of the water rod cross section with a convex curve, the liquid film flow rate in the annular two-phase flow mode can be made smaller than in the case of a concave curve. This makes it possible to reduce the flow rate of coolant that does not contribute to cooling the fuel rods.

By the way, as another example of this example, as shown in FIG. ) can also be applied to this example. By employing the water rod 22 to which this example is applied in this manner, it is expected that the cooling efficiency of the internal fuel rods will be improved.

Further, as another embodiment of the present invention, as shown in FIG. 9, three concentric circular arcs are drawn, each centering on three diagonally located fuel rods 13b among the adjacent fuel rods 13a. , a concave curve connecting these circular arcs may be included in the outer peripheral shape 23a of the water rod 23. In this case, the flow path area of the four gap portions 23a at diagonal positions of the water rod 23 can be reduced, and the cooling of the fuel can be further reduced. The proportion of coolant flow rate contributing to this can be increased.

[Effects of the Invention] As explained above, according to the present invention, it is possible to provide a nuclear reactor fuel assembly with a large neutron multiplication factor and good thermal performance.

[Brief explanation of the drawing]

Fig. 1 is a sectional view showing an embodiment in which the present invention is applied to a large-diameter water rod of a fuel assembly in which fuel rods are arranged in a lattice shape with 9 rows and 9 columns, and Fig. 2 is a sectional view showing an embodiment in which fuel rods are arranged in a lattice shape with 9 rows and 9 columns. Fig. 3 is a cross-sectional view showing an embodiment in which the present invention is applied to a large-diameter water rod of a fuel assembly with a fuel rod pitch of 2FIi type. Figure 4 is a diagram showing the effect of the example in terms of the relationship between the neutron infinite multiplication factor and the atomic ratio H/U ratio of moderator and fuel, and Figure 5 is
6 is a sectional view showing another embodiment, FIG. 7 is a sectional view showing still another embodiment, and FIG. 8 is a sectional view showing another embodiment. 9 is a sectional view showing another embodiment, FIG. 10 is a sectional view of a conventional fuel assembly, and FIG. 11 is a fuel assembly arranged in a lattice shape of 8 rows and 8 columns. Figure 12 is a cross-sectional view showing a conventional fuel assembly in which large diameter water rods are installed in the fuel assembly. 13 is a cross-sectional view showing a conventional fuel assembly with two types of fuel rod pitches. 11...Fuel assembly 12...Channel box 13...
...Fuel rod 14.16.17.18.21.22.23...
...Water rod applicant: Japan Atomic Energy Corporation
Toshiba Corporation Representative Patent Attorney Satoshi Suyama - Flow rate figure 3 H/U ratio figure 4 n figure 7, 11

Claims (1)

[Scope of Claim] A nuclear reactor fuel assembly including a large number of fuel rods housed in a channel box and a water rod arranged in the group of fuel rods, wherein the side shape of the water rod is defined as described above. A nuclear reactor fuel assembly characterized by comprising a collection of curved surfaces that maintain a substantially constant gap between a water rod and a fuel rod adjacent to the fuel rod.