JPH01101495A - Fuel aggregate - Google Patents

Abstract

PURPOSE:To increase a thermal limit output at the time of normal operation and abnormal transient variation by providing two spacers which divide a fuel rod gap in section into an inside and an outside part and hold them at a constant interval, and overlapping at least four fuel rod intervals. CONSTITUTION:The fuel aggregate has one large-diameter water rod 2 in the center and many fuel rods 1 arranged regularly outside it and is held by the spacers 3 so that the fuel rod gap interval is constant; and its upper and lower ends are fixed by an upper tie plate 4 and a lower tie plate 5, and a channel box 6 is provided outside it. The spacers 30 each consist of an outside spacer 31 and an inside spacer 32; and the outside spacer 31 holds two outside arrays of fuel rods and the fuel rod at the most inside corner part and the inside spacer 32 holds one most inside array of fuel rods.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Field of Application) The present invention relates to a fuel assembly loaded into a nuclear reactor, and particularly to a fuel assembly equipped with a spacer that maintains a constant spacing between fuel rods.

(Prior Art) Spacers are provided to maintain constant spacing between fuel rods in a fuel assembly loaded into a nuclear reactor.

This spacer has two important factors in thermal design: pressure loss characteristics and thermal limit output characteristics.

Therefore, first, the pressure loss characteristics will be explained.

Spacers act as flow path obstructions and disrupt the flow.

It introduces an additional local pressure drop to the aggregate pressure drop profile and increases the aggregate inlet/outlet pressure drop. The proportion of fuel assembly pressure loss is as high as 30% under two-phase flow conditions. In consideration of such additional pressure loss, it is necessary to design the driving force in both the forced circulation method and the natural circulation method so that a constant coolant flow rate can be ensured.

Next, thermal limit output characteristics will be explained.

Regarding heat transfer properties, there is an effect on the condition where heat transfer on the fuel rod surface deteriorates, that is, on the boiling transition point. In the configuration without a spacer, a boiling transition occurs at the exit end of the heat generating section, but
It is known that when a spacer is installed, the position where boiling transition occurs tends to be limited to the immediate upstream side of the spacer. Therefore, by changing the spacer structure and installation interval,
The limit output characteristics also change.

The effects of the spacer configuration and installation interval will be described below regarding the above-mentioned pressure loss characteristics and thermal limit output characteristics.

Spacer pressure loss ΔP8 is evaluated by local loss element Δ
It can be considered separately into the Pie friction loss element ΔP. That is, Δpg = ΔP1+ΔP.

The local loss element ΔPA is given by a function of the spacer projected area ratio σ=Ap/Ax−s and the spacer shape factor Cg. Now, assuming that Cg is constant, the relationship between ΔP and σ is given as shown in FIG. Therefore, the smaller the spacer projected area ratio σ, the lower the local loss element.

Therefore, the influence of the spacer on the occurrence of boiling transition can be roughly classified into the following two points.

■ As an effect on the downstream side, mixing (mLx
ing) to homogenize the two-phase distribution within the flow path.

■ The effect on the upstream side is that turbulence on the upstream side increases as an obstacle to the flow.

By the way, the above item (2) is one of the positive effects on the heat transfer state, and has the effect of making the heat flow distribution in the flow path uniform and promoting heat removal efficiency.

The relationship between the axial spacer interval Ls and the boiling transition generation condition (thermal limit output) Qcp is given as shown in FIG.

That is, when the spacer interval is made smaller and the number of spacers installed in the axial direction is increased, the thermal limit output Qcp increases.

Current spacers have been improved and developed to have optimal characteristics, taking into consideration both pressure loss characteristics and thermal limit output characteristics. However, these two characteristics tend to be contradictory to each other.1 For example, as shown in Figure 7, if the spacer spacing is reduced, an aggregate with high critical output characteristics can be obtained, but on the other hand, in the axial direction If the number of spacers is increased, the pressure loss will increase.

(Problems to be solved by the invention) The present invention has been made in consideration of these circumstances, and
The object is to provide a fuel assembly with a fuel assembly spacer that is improved both in terms of pressure drop characteristics and critical power characteristics.

[Structure of the invention]

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a fuel assembly equipped with a spacer that holds a large number of fuel rods housed in a channel box in parallel with the longitudinal direction and at regular intervals. In the body, the fuel rod gap in the cross section is divided into two parts, an outer part and an inner part, and two spacers are provided to maintain each fuel rod gap part at a constant interval, and at least four fuel rod intervals overlap. It is characterized by being configured as follows.

Therefore, according to the present invention, pressure loss per spacer can be reduced and high limit output characteristics can be realized.

Next, the basic idea of the present invention will be explained.

Currently, the number of fuel assembly spacers loaded in nuclear reactors is 1.
For example, as shown in Fig. 8, the fuel rod 1 has a structure surrounded by cells 3, and the strain amplitude in the parallel and diagonal directions as loads acting within the cross section is relatively small, and the structure has a large strength. It becomes. Note that 2 is a large diameter water rod.

Therefore, it is possible to design within a range that can ensure sufficient strength as an aggregate even if it is divided into two parts within the cross section.
For example, a two-part spacer according to the present invention has a structure in which the strength is increased by partially overlapping fuel rod cells.

As the number of spacer installation locations in the axial direction increases, the aggregate limit output increases as shown in FIG. 7, but the pressure loss can be reduced by designing as described below.

That is, for simplicity, the conventional spacer 3 shown in FIG. 5(a) is divided within its cross section, and spacers 3a and 3b are each installed at two locations in the axial direction, as shown in FIG. 5(b). Then, the pressure loss changes as follows.

Now, if the spacer pressure loss is ΔPsP, then this ΔP
sP can be expressed by the following equation (D) using the coolant mass velocity GAP of the spacer portion.

Here, AsP = Aυ - AP Flow path area of the spacer section 8 To Ni spacer projected area i Flow path area of the tube group without spacer In the spacer section, the flow path is pinched and the mass velocity Gsp increases, so the pressure loss is larger than in the tube group section. .

When the projected area of the spacer is equally divided in the cross section and placed at two locations in the axial direction (see FIG. 5(b)), the pressure loss changes as follows.

ΔPb = ΔP1 + ΔP2 ...)
1 2...■ ΔP2=1(,,gaGb Here, the spacer divided into two has a projected area ratio σ of K
Since it is reduced by half compared to a, the local loss component becomes 1/2 or less from the relationship shown in FIG.

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

FIG. 1 is a longitudinal cross-sectional view of one embodiment of the present invention. As shown in the figure, the fuel assembly has one large-diameter water rod 2 in the center and a large number of fuel rods 1 on the outside. Spacers 3 are arranged regularly so that the fuel rod gap spacing is constant.
0, and its upper and lower ends are fixed by an upper tie plate 4 and a lower tie plate 5, respectively, and a channel box 6 is provided on the outside thereof. The spacer 30 is composed of an outer spacer 31 and an inner spacer 32, and the outer spacer 31 holds the two outer rows of fuel rods and the fuel rods at the innermost corner, as shown in the cross-sectional view of FIG. Also, the inner spacer 32
holds the innermost row of fuel rods. Therefore, the four fuel rods at the innermost corners are held by both spacers 31, 32, thereby increasing the mechanical strength. 1 In the above embodiment, the fuel rods held by both the inner and outer spacers are the fuel rods at the innermost corners, but the fuel rods are not limited to these four, and a larger number of fuel rods can be held by both the inner and outer spacers. It may be configured to hold the information.

Next, FIG. 4(a) compares the relationship between the limit output Qcp and the coolant flow rate W between the fuel assembly of the present invention and the conventional fuel assembly, and similarly, the assembly pressure loss Δ
Figure 4(b) shows a comparison of the relationship between P and coolant flow W.
As can be seen from these figures, the fuel assembly of the present invention (dotted line) has a higher assembly limit output and a lower pressure loss than the conventional fuel assembly (solid line).

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to increase the thermal limit output during normal operation and during abnormal transient changes, and to provide a fuel assembly with a large thermal margin. This has the excellent effect of expanding the driving range.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of one embodiment of the present invention, FIG. 2 and FIG.
The figures are cross-sectional views taken along lines 1-1 and -■ in Fig. 1, respectively, and Figs. 4 (a) and (b) show the limit output and pressure of the fuel assembly of the present invention and the conventional fuel assembly. Figures 5(a) and 5(b) are explanatory diagrams for comparing the pressure losses of the conventional spacer and the spacer of the present invention. Figure 6 is a diagram comparing the spacer local loss element and the projected area ratio. A diagram showing the relationship,
FIG. 7 is a diagram showing the relationship between the assembly limit output and the spacer axial spacing, and FIG. 8 is a cross-sectional view of a conventional fuel assembly. 1... Fuel rod 2... Large diameter water rod 3... Spacer 4... Upper tie plate 5... Lower tie plate 6... Channel box 30... Spacer 31... Outer spacer 32...Inner spacer (8733) Representative patent attorney Yoshiaki Inomata (and one other person) Figure 1 Figure 2 Figure 3 (a) (b) Figure 4 (a) Figure 5 O-6 Figure 7 Figure 8

Claims (1)

[Claims] (1) In a fuel assembly equipped with spacers that hold a large number of fuel rods housed in a channel box parallel to the longitudinal direction and at regular intervals, the fuel rod gap in the cross section is set to 2.
A fuel assembly characterized in that the fuel assembly is divided into an outer part and an inner part, provided with two spacers to maintain the respective fuel rod gap parts at a constant interval, and configured so that the intervals of at least four fuel rods overlap. body.