JPH01250791A - Fuel assembly - Google Patents

Abstract

PURPOSE:To plan the reduction of pressure loss of the upper part of a fuel assembly by widening the interval of a spacer above the point, at which two- phase coolant makes the transition to ring flow, more than the spacer interval therebelow. CONSTITUTION:For example, one of spacers 6 heretofore is lowered at a position lower than the upper end of an effective fuel range (heat generating range) 2a of a fuel rod 2 while the intervals between an upper tie plate 4 and the spacer 6 and between the spacers 6 are arranged 1.2-1.3 times wider than the conventional spacers. At the lower position where support force of the fuel rod 2 is required, the support force is held by the spacer intervals as while the pressure loss by the spacers 6 is reduced. Therefore, the efficiency of a reactor can be improved.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to a fuel assembly installed in a nuclear reactor.

(Conventional technology) FIG. 3 shows a conventional fuel assembly.

This fuel assembly has a cylindrical fuel channel 1 in which a plurality of fuel rods 2 and at least one water rod 3 are accommodated. Further, an upper tie plate 4 and a lower tie plate 5 are fitted to the upper and lower parts of the fuel assembly 1, respectively, and both ends of the fuel rods 2 and water rods 3 are fixed to the upper tie plate 4 and the lower tie plate 5, respectively. has been done. Among these, the water rod 3 has orifices 3a and 3b on its upper and lower side walls. This water rod 3 forms a coolant passage within the fuel assembly, and has the function of flattening the power distribution within the fuel assembly and suppressing the generation of voids in the upper part of the fuel assembly. A plurality of (usually 7) spacers 6 are placed at predetermined intervals in the direction.
are attached, and the fuel rods 2 are aligned and supported by the spacers 6, and the curvature of the fuel rods 2 is suppressed.

Next, the function of this fuel assembly will be explained.

The coolant flows in from the holes 5a provided in the lower tie plate 5, flows from the bottom to the top in the fM fuel channel 1, receives heat from the fuel rods 2 on the way, and flows to the top with voids mixed in. It flows out from the hole 4a provided in the tie plate 4. On the other hand, a part of the coolant flows into the water rod 3 from the orifice 3b provided on the lower side wall of the water rod 3, and flows from the bottom to the upper side of the water rod 3. It flows out through the orifice 3a.

However, in this fuel assembly, the spacing between the fuel rods is narrow, and the cross-sectional area of the coolant flow path is narrowed by the spacer 6, so a pressure loss occurs, and this pressure loss accumulates as it moves toward the upper part of the fuel assembly. and becomes excessive. This pressure loss, combined with the large number of voids present in the upper part of the fuel assembly, reduces the efficiency of the reactor.

Therefore, attempts were made to reduce the pressure loss by reducing the number of spacers, but implementation was put off due to concerns that reducing the number of spacers would weaken the support for the fuel rods.

(Problems to be Solved by the Invention) The present invention has been made in view of the above circumstances, and it is possible to reduce the pressure loss in the upper part of the fuel assembly without causing any problem in supporting the fuel rods. The purpose is to provide fuel assemblies.

[Structure of the invention]

(Means for Solving the Problems) The present invention provides that the positions in the fuel assembly that require more fuel rod support are the lower liquid single-phase region and the vicinity of the boiling start position, and the upper liquid single-phase region and the vicinity of the boiling start position. There is no problem in widening the spacer spacing to about 1.25 times the conventional spacer spacing, and since there is a non-heat generating zone between the topmost spacer and the upper tie plate, it can be made wider. This was done based on the knowledge of

That is, the present invention includes a plurality of fuel rods having both ends fixed to an upper tie plate and a lower tie plate that are fitted into the upper and lower parts of a cylindrical fuel channel, and which are spaced apart along the axial direction inside the fuel channel. In the fuel assembly, the fuel assembly includes a spacer that is provided to align and support the fuel rods, and at least one water rod that forms a coolant passage in the fuel assembly and supports the spacer. It is supported at a total of eight locations along the direction by an upper tie plate, a lower tie plate, and a spacer, and the spacing between the spacers is such that the spacing between the spacers above the point where the coolant in the two-phase flow transitions to the annular flow is equal to the spacing between the spacers. The present invention relates to a fuel assembly characterized in that the spacing is wider than the spacing between the spacers located below.

(Function) In the fuel assembly of the present invention, the number of spacers is 6, which is one less than the conventional example, which normally has 7, so the pressure loss is first reduced by that amount. Furthermore, at the same time, due to the above-described arrangement of the spacer, the spacer position above the effective fuel area is lower than before, so that the pressure loss is further reduced.

In addition, the intervals between the six spacers are not all equal as in the past, and the intervals between the spacers above the point where the two-phase coolant transitions to the annular flow are the same as the intervals between the spacers below. Since it is wider, the supporting force is not weakened lower than before, where more support is required for the fuel rods.

Therefore, even if the number of spacers is reduced, the supporting force of the fuel rods does not weaken.

Therefore, according to the present invention, it is possible to reduce the pressure loss in the upper part of the fuel assembly without weakening the fuel rod supporting force.

(Example of the invention) Hereinafter, the present invention will be explained based on embodiments shown in the drawings.

Incidentally, the same parts as those of the conventional one (FIG. 3) are given the same reference numerals, and the explanation thereof will be omitted.

FIG. 1 is a diagram showing the spacer position in comparison of the present invention with a conventional example. FIG. 1(A) shows the embodiment of the present invention, and FIG. 1 CB) shows the conventional example.

A plurality of fuel rods 2 and at least one water rod (not shown) are housed in a cylindrical fuel channel 1 in a state where they are arranged in 8 rows and 8 columns as in the conventional case. Support is provided by at least one water rod.

As shown in the figure, in this embodiment, one spacer 6 is provided below the upper end of the effective fuel area (heat generating area) 2a of the fuel rod 2, one less than the conventional spacer 6 (in the figure, 2b is a non-heat generating area). ), the spacing between the lower tie plate 5 and the spacer 6 or the lower spacer 6 and the spacer 6 is the same as before, and the spacing between the upper spacer 6 and the spacer 6 or the spacer 6 and the upper tie plate 1 to 4 is 1.2 to 1. It is arranged so that it is about 1.3 times wider.

In the fuel assembly of this embodiment, the spacers are arranged at the intervals as described above, so that the spacer spacing is the same as that of a conventional fuel assembly in the lower part where supporting force of the fuel rods 2 is required, and sufficient support is provided. It has power. On the other hand, in the upper part, which does not require as much supporting force, the spacer spacing is wider than that in the lower part, but the supporting force required by design is naturally maintained.

By the way, the total pressure loss ΔPT of the fuel assembly is equal to the water head ΔP
eV+ Friction ΔPf + Local pressure loss ΔPL due to the upper and lower tie plates and spacers is expressed as ΔpT = ΔPoV + ΔP, +ΔPl-, but the above local loss ΔTp is ΔpL = ΔptTp + ΣΔPgPi + ΔpuTp ■ Where ΔPLTP: Local pressure loss due to the lower tie plate ΔPspi: Upper Local pressure loss due to the i-th spacer 8Putp: This is expressed as the local pressure loss due to the upper tie plate, and it is known that these increase as the steam quality (steam weight ratio) at the point where they are located increases. . Steam quality is higher in the upper boiling range, so
The upper spacer has a large pressure loss.

ΔPaps > ΔPspa > ΔP11P3 >...
Therefore, according to the embodiment of the present invention, the number of spacers is one less than that of the conventional fuel assembly, and at the same time, the position of the spacer above the effective fuel area (heat generation area) 2a is lower than that of the conventional fuel assembly. This results in a lower pressure loss than that experienced at conventional spacer locations.

That is, conventional ΔpL=Δp+−Tp+ゑΔPSPi+ΔPUT
PThis example ΔPL=ΔPLTP+ΣΔP8Pl+ΔPu
Tp where Δpt, Tp = Δpt, tpΔPs
px <ΔPQP□ △ps□kuΔPliP□ Δ'sp3<ΔPI! .

ΔPsp4<ΔP8P4 ΔPsps <ΔPsps (=ΔP8P4)ΔPsp
It is clear that s<ΔPsp-("ΔPsps)~0<ΔPsp7(=ΔPsp-)Δl'LTP<ΔPLTP, and ΔPL<ΔPL.

Figure 2 shows the results of calculating the above relationship using a detailed analysis code. In the figure, the broken line represents this embodiment, and the solid line represents the conventional example.

On the other hand, there was a concern that widening the distance between the spacers would degrade the critical output characteristics, but the results of experiments and calculations using an analysis code confirmed that there was almost no degradation.

〔Effect of the invention〕

As explained above, one aspect of the present invention is to reduce the number of spacers compared to the conventional one, and at the same time, make the supporting interval of the spacers wider above the point where the two-phase coolant transitions to an annular flow than below. This has the effect that the local pressure drop due to the spacer can be reduced compared to the conventional spacer, and the fuel rod support function that the conventional spacer had can be maintained as it is, and the limit output characteristics are not deteriorated.

[Brief explanation of the drawing]

Fig. 1 is for comparing and explaining the spacer position I between the embodiment of the present invention and the conventional example. 2 is a diagram showing the relationship between total pressure loss and flow rate for an embodiment of the present invention and a conventional example, and FIG. 3 is a schematic vertical sectional view of a conventional fuel assembly. 1...Fuel channel 2...Fuel rod 3...
・Water rod 4... Upper tie plate 5
...Lower tie plate 6...Spacer agent Patent attorney Nori Ken Chika Ken Shu Daikomaru (B) (A) Fig. 1 Fig. 2) a Fig. 3

Claims (1)

[Claims] a plurality of fuel rods having both ends fixed to an upper tie plate and a lower tie plate fitted in the upper and lower parts of the cylindrical fuel channel; and a plurality of fuel rods provided at intervals along the axial direction inside the fuel channel. In a fuel assembly having a spacer for aligning and supporting the fuel rods, and at least one water rod for forming a coolant passage in the fuel assembly and supporting the spacer, the fuel rods are arranged along the axial direction. Supported at a total of eight locations by an upper tie plate, a lower tie plate, and a spacer, the spacing between the spacers is such that the spacing between the spacers is above the point where the two-phase coolant transitions to an annular flow, and the spacing between the spacers is below the point where the coolant in the two-phase flow transitions to an annular flow. A fuel assembly characterized in that the interval is wider than the interval between spacers.