JPS61288196A - Fuel aggregate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a fuel assembly, and more particularly to an improvement in a fuel assembly for a pressure tube type nuclear reactor.

[Background of the invention]

Prior to explaining the present invention, the overall structure of a pressure tube type nuclear reactor fuel spacer will be explained based on FIG. It is shown.

A pressure tube type nuclear reactor fuel assembly is loaded inside a circular tube called a pressure tube (indicated by reference numeral 11 in FIG. 13), and as shown in FIG. 1...
The circular tube elements 5, 5, . . . are arranged concentrically. To make sure that the fuel rods 1, 1, . . . are cooled uniformly, a predetermined distance is maintained between each fuel rod 1, 1. In addition to this.

The fuel spacer 2 is required to have a certain level of structural strength for the purpose of reducing fluid vibrations during transportation of fuel assemblies, loading into the reactor core, and during reactor operation, that is, reducing fluid vibrations due to passage of coolant.

That is, the fuel spacer 2 is installed between a cylinder 3 located at the center of the spacer 2, rib members 4, 4, . A circular tube element 5.5 for holding the rods 1, 1...;
The fuel spacer 2 surrounds the small circular tubes 6, 6, etc., which are disposed within the circular tube elements 5, 5, and serves as a support member, and the above-mentioned fuel spacer inner structure, that is, the cylinders 3 to 6. and an outer ring 7 to maintain structural strength.

FIG. 12 is a partially cutaway plan view of 1/6 (60 degree range) of a fuel assembly 8 for a pressure tube type nuclear reactor that has been used in practical use. The same parts as in the figure are given the same reference numerals.

As is clear from FIG. 12, on the inside of the circular tube elements 5, 5... are two convex parts 9, 9 that support the fuel rods 1, 1..., respectively, and one leaf spring part. 10 are provided. Therefore, when the inner diameter of the pressure tube shown by reference numeral 11 in FIG. The gap is about 2W11. As will be described later, the pressure pipe indicated by the reference numeral 11 in FIG. ing. Nine, the inside of the pressure pipe 11 is filled with light water 1, which is a coolant.
2 flows. As mentioned above, the fuel rods 1, 1, . . . are arranged concentrically within the fuel spacer 2. However, the fuel rods 1, 1, . . . are forced into a narrow area. As a result, the cylindrical elements 5, 5, . . . supporting the fuel rods 1, 1, . . . are arranged in a very dense manner within the fuel space f'2. On the other hand, when focusing on one fuel rod 1.

The fuel rod 1? The plate spring portion 10 of the circular tube element 5 to be held is
It is formed integrally with the cylindrical element 5 by stamping the cylindrical element 5 itself.

For comparison, considering the internal structure of a fuel assembly for a light water reactor, the fuel rods for a light water reactor are arranged in a square lattice between the - fuel rod and other fuel rods adjacent to it. Since it is possible to secure a minimum spacing of about 4 mm, in the production of the actual aircraft, the leaf spring part of the circular tube element, which is a component of the fuel spacer, is made of Inconel material with high spring strength, and the other parts are made of Inconel material with high spring strength. Even if they were all made of a zirconium alloy with low neutron absorption and the two were mechanically connected, there would be no problem in terms of space to load them into the reactor core.

However, in the pressure tube reactor, as mentioned above, the cylindrical elements 5, 5, which are one component of the fuel spacer 2,
The leaf spring part 10 and the other circular tube element 5, which are arranged in a very dense manner in the fuel spacer 2, are molded from different materials, and the two are mechanically connected. Unfortunately, loading this into the reactor core is somewhat unreasonable in terms of space, and the reality is that it has not been put to practical use despite various research efforts.

From this table, in pressure tube nuclear reactors that have been put into practical use, the fuel spacer 2 was developed in 1982, for example, in order to ensure the spring strength of the spacer 2.
As stated in the "Power Reactor Technical Report" published in December 2015, page 44, page 41, all of the fuel rods are made of Inconel material, but since Inconel material has a high neutron absorption, one fuel rod
In the conventionally proposed pressure tube reactor using 12 fuel spacers 2, 2... in the axial direction, the fuel spacers 2
The burn-up loss due to ゜2... was approximately 1800 MWd/l, which was a very large value.

[Purpose of the invention]

The present invention was made as a result of various studies in order to solve the problems of the prior art described above, and its purpose is to significantly increase the burnup of a pressure tube reactor compared to the conventional technology. It is an object of the present invention to provide an improved fuel assembly that can be improved.

[Summary of the invention]

Ninth to achieve the above object, the present invention comprises a plurality of fuel rods and a fuel spacer that maintains the spacing between the fuel rods, and the fuel spacer includes a plurality of fuel rods into which each fuel rod is inserted. In the structure of a fuel assembly for a pressure tube nuclear reactor comprising a cylindrical body and a ring member surrounding the bundle outer periphery of the cylindrical body, the ring member constituting the fuel spacer may be replaced with another fuel spacer configuration. It is made up of a member that has a smaller neutron absorption cross section than the cylindrical member, and the cylindrical member is made of a member that has a larger spring force than the ring member, and both are fixed integrally. It is characterized by:

That is, the present invention was made as a result of careful review and study of the structure of the fuel assembly, especially the fuel spacer, incorporated in a pressure tube nuclear reactor. As shown, fuel rods 1, 1 inserted into circular tube elements (see numbers 5, 5, . . . in FIG. 12) of a fuel spacer (see numbers 2 in FIGS. 11 and 12).
... are loaded into the pressure pipe 11 through which the coolant (light water) 12 flows. In FIG. 13, reference numeral 13 indicates a calandria tank whose outside is filled with heavy water 14 as a moderator. The curve indicated by the symbol X in FIG. 13 shows the thermal neutron flux distribution in the area of heavy water 14 and in the pressure tube 11. Because it exists outside of
The outside of the calandria tank 13, that is, the pressure pipe 1
The thermal neutron flux outside of 1 is large. On the other hand, inside the pressure tube 11, the thermal neutron flux decreases toward the center. The reason for this is that neutrons are absorbed by the fuel rods 1, 1... and the light water 12 which is a coolant.

By the way, in a pressure tube reactor, the gap shown by the symbol g in FIG. 13, that is, the inner wall of the pressure tube 11 and the fuel rods 1/, 1/... ...
Among them, the gap g between the two fuel rods (fuel rods located on the outer periphery) is 3w or more, which is larger than the gap between the fuel rods 1 and 1 (2wm). Therefore, the thermal neutrons flowing into the pressure pipe 11 from the area of the heavy water 14 are divided into the light water 12 existing in the gap g and the fuel rods 1', 1', and ..., and as a result, the thermal neutron flux inside the pressure tube 11 rapidly decreases in the gap g,
Fuel rods 1.1 located toward the center of the pressure pipe 11...
・The number of thermal neutrons reaching .

Therefore, in order to increase the burnup of this type of reactor, taking into consideration the thermal neutron flux distribution within the pressure tube 11, it is necessary to prevent a sudden decrease in the thermal neutron flux in the vicinity of the pressure tube 11. However, as mentioned above, the rapid decrease in thermal neutron flux near the pressure pipe 11 is caused by the gap g between the inner wall of the pressure pipe 11 and the fuel rods 1/, 1/, etc. located inside the pressure pipe 11. Light water existing in fuel rods 1/, 1/・
...The number of thermal neutrons absorbed per unit volume is given by the following formula.

A=Σ, Xφ, k (1)
Here, A is the number of thermal neutrons absorbed per second in a unit volume Σ1 is the thermal neutron absorption cross section (cy-') φtb is the thermal neutron flux [numbers/an''·sec].

As shown in equation (1), the number of thermal neutrons absorbed is expressed as the product of the absorption cross section Σ of the substance and the thermal neutron flux φtk. That is, in a pressure tube reactor, even if all the fuel spacers are made of the same material, the thermal neutron flux is high in the portion near the inner wall of the pressure tube 11, so neutron absorption is large; In the direction toward the center of the pressure tube 11, contrary to the above, neutron absorption becomes smaller. In pressure tube reactors that have been put into practical use, Figures 11 and 1
It was previously mentioned that all of the fuel spacers indicated by the reference numeral 2 in Fig. 2 are made of Inconel material, but when the conventional Inconel fuel spacers 2i are placed in the core, the weight ratio of the outer ring 7 is The neutron absorption rate of the entire spacer 2 is about 25 cm, whereas the neutron absorption rate of the fuel spacer 2 is
As a result, in the previously proposed pressure tube reactor that uses 12 fuel spacers 2, 2, etc. in the axial direction of the fuel rod 1, as described above, As shown, the burnup loss due to fuel spacers 2, 2... is approximately 1800
It reached a very large value of MWd/l.

The present invention solves the above points, that is, nuclear calculations that in the pressure tube reactor proposed in the past, a large proportion of neutron absorption by the fuel spacer 2 was accounted for by the outer ring 7, which is a component of the spacer 2. This was done based on the results, and a material with low neutron absorption was selected for the outer ring member of the fuel spacer, and the cylindrical body to which the fuel rods were attached was located inside the outer ring member, and the supporting function of the fuel spacer was improved. By selecting a material with a large spring force that maintains It is designed to reduce the degree of absorption of the reactor, thereby significantly improving the burnup of the entire reactor core compared to conventional methods.

[Embodiments of the invention]

Hereinafter, the present invention will be explained based on an embodiment shown in FIGS. 1 to 5. FIG. 1 is a partially cut away plan view of 1/6 of a fuel assembly for a pressure tube type nuclear reactor. Figure 2 is a plan view showing the overall configuration of the fuel spacer inner structure;
The figure is a side view partially showing the outer ring of the fuel spacer.
Figure 4 is a sectional view taken along line A-A in Figure 3, and Figure 5 is a view taken along arrow B in Figure 1. In Figures 1 to 5, the same reference numerals as in Figures 11 to 13 are the same. part, i.e. 1°1・
... is a fuel rod, 1', 1' is the fuel rod located at the outermost periphery of the fuel rods 1, 1, 2 is a fuel spacer to which the fuel rods 1, 1... are attached, 3 is a fuel spacer of the fuel spacer 2. A cylinder located in the center, 4, 4... are lip materials extending radially around the cylinder 3, 5, 5... are lip materials 4, 4... installed between the fuel rods 1, A circular tube element holding 1..., 6, 6
. . . is a small circular tube as an auxiliary member disposed within the circular tube elements 5, 5 . Outer ring for holding, 8#-j, general term for fuel assembly 8 for pressure tube type nuclear reactor, 9゜9 is two convex parts provided inside the circular tube elements 5, 5..., 10 is There is one plate spring part installed inside the circular tube elements 5, 5..., and each fuel rod 1, 1... has the two convex parts 9, 9 and one plate spring. The parts 10 and 10 are supported at 120 degree intervals. Therefore, the fuel spacer inner structure shown as a single unit in FIG. Molded from Inconel material with large spring force. The outer ring 7 shown as a single unit in FIGS. 3 and 4 is made of a zirconium alloy having a smaller neutron absorption cross section than the above-mentioned fuel spacer inner structure, that is, the cylinder 3 or the small circular tube 6. , in FIG. 1, among the fuel spacer inner structure constituted by the cylinder 3 or the small circular pipe 6,
The circular tube elements S/, S/, etc. located at the outermost periphery thereof and the outer ring 7 are mechanically coupled at a point P by means described later.

In FIG. 3, which shows the outer ring 7 alone, the height H of the ring 7 is the same as that of the circular tube elements 5, 5... and the lip material 4,
The height is the same as that of 4. By the way, in FIG. 3, 7a and 7b are the windows 15 bored in the outer ring 7.
These are a pair of upper and lower protrusions located nearby and provided inside the ring 7, and their cross-sectional shape is shown in FIG. Note that the window portion 15 described above.

The outer ring of the fuel spacer in this conventional nuclear reactor is also perforated, and the outer ring 7 has a window 15.15.
By drilling ..., the neutron absorption cross-sectional area of the ring 7 is reduced, and conventionally, the circular tube element located at the outermost periphery of the fuel spacer inner structure and the outer ring are Welding was performed through the window portion 15.

In FIG. 5, which is a view taken in the direction of arrow B in FIG. The projections 7a and 7b of the outer ring 7 are fitted together, and by caulking or melting and deforming the tips of the projections 7a and 7b of the outer ring 7, the outer ring 7 made of zirconium alloy and the Inconel material can be separated. At this time, the amount of protrusion of the protrusions 7a and 7b of the outer ring 7 (the fourth
(indicated by the symbol t in the figure) is predetermined to have a size that does not interfere with the fuel rod 1 inserted into the single circular tube element 5. In addition, even in the circular tube element of the conventional fuel spacer, the above-mentioned vertical grooves 16 are provided in three pieces with a width of about 3.5 mm at 120 degree intervals for each circular tube element. There are longitudinal grooves 16, 16... on the peripheral wall of the circular tube elements 5, 5...
By providing the window portions 15, 15 on the outer ring 7,
. . , the neutron absorption cross section of the circular tube elements 5, 5 . . . can be made small.

The present invention is as described above. Next, the effects of the above embodiment will be explained. In the embodiment, the fuel spacer 2
Since the outer ring 7 located at the outermost periphery of the fuel spacer 2 is made of a zirconium alloy with a small neutron absorption cross section, the degree of neutron absorption in the periphery of the fuel spacer 2 is reduced, and the inner wall of the pressure pipe 11 shown in FIG. A sudden decrease in the thermal neutron flux distribution X in the vicinity can be alleviated.

Then, of the thermal neutrons coming from outside the pressure pipe 11, the proportion of thermal neutrons that reach deep inside the fuel assembly 8 increases, increasing the burnup of the fuel assembly 8 as a whole. According to calculations, it has been confirmed that the burnup when the fuel assembly according to the present invention is loaded into a pressure tube reactor increases by approximately 800 MWd/ compared to the conventional one. I'm sorry.

Further, as shown in the illustrated embodiment, a cylinder 3, rib members 4, 4, . . ., cylindrical elements 5, 5 located inside the outer ring 7,
... and the small circular tubes 6, 6... and using Incone e material with a large spring force, the thermal expansion in the radial direction of the fuel rods 1, 1... during reactor operation can be easily prevented. Can be absorbed.

FIG. 6 shows another embodiment of the present invention, which corresponds to FIG. 5 in the embodiment @1. In the embodiment shown in FIGS. 1 to 5, a pair of upper and lower protrusions 7a and 7b are provided inside the ring 7, located near the window 15 formed in the outer ring 7, and the protrusions 7a and 7b are provided inside the ring 7. and 7b are fitted and fixed in the vertical groove 16 of the circular tube element 5', but in the embodiment shown in FIG. Groove 16
located near the upper and lower pair of protrusions 5/a and 5
'b and correspondingly the window 15 of the outer ring 7
A pair of upper and lower grooves 17a and 17b are provided adjacent to the grooves 17a and 17b of the outer ring 7, and the projections 5/a and s/b of the circular tube element 5' are fitted and fixed into the grooves 17a and 17b, respectively. Ta.

Still other embodiments of the present invention are shown in FIGS. 7 to 9, in which FIG. 7 is a side view partially showing the outer ring 7 of the fuel spacer 2, and FIG. 8 is a cross section taken along line C-C in FIG. FIG. 9 is a partially cutaway plan view showing a state in which the fuel rod 1 is attached to the fuel spacer 2. In the embodiment of FIGS. 7 to 9, projections 7a and 7b are provided on the inner side of the outer ring 7, located near the upper and lower ends of the outer ring 7, and correspondingly protrusions 7a and 7b are provided on the inside of the outer ring 7. Near the upper and lower ends 1c, 5'a and 5'b are provided, and projections 7a and 7 of the outer ring 7 are inserted into the grooves 5/a and 5'b of the circular tube element 5', respectively.
The case where bi-fitting and fixing is shown. In FIG. 9, reference numeral 18 indicates a welded portion between the circular tube element 5' and the rib material 4. In the embodiments shown in FIGS. 7 to 9, protrusions 7a and 7b are provided inside the outer ring 7 near the upper and lower ends of the outer ring 7, and the circular tube element 5 is provided in correspondence with the cracks. ′
Although the case where the grooves 5/a and 5'b are provided near the upper and lower ends of the tube element 5' has been illustrated, contrary to the above,
Protrusions are provided on the outside of the circular tube element 5', located near the upper and lower ends of the outer ring 7, and corresponding grooves are provided near the upper and lower ends of the outer ring 7. The protrusion of the element 5' may be fitted and fixed.

Figure 10 shows the outer filtration structure shown in Figure 7.
FIG. 10 is a diagram equivalent to FIG. 8 showing a modified example of
A case is shown in which projections 7a and 7b are provided approximately midway between the upper and lower ends of the outer ring 7 and the window portion 15.

〔Effect of the invention〕

The present invention is as described above, and as is clear from the description of the illustrated embodiments, the present invention provides an improved and carefully designed system that can significantly improve the burnup of a pressure tube reactor compared to the conventional one. A fuel assembly can be obtained.

[Brief explanation of drawings]

1 to 5 show an embodiment of the pressure tube type nuclear reactor fuel assembly according to the present invention, and FIG. 1 shows 1/1/2 of the entire assembly.
FIG. 2 is a plan view showing the overall structure of the fuel spacer inner structure; FIG. 3 is a side view partially showing the outer ring of the fuel spacer; FIG. 3 is a sectional view taken along line A in FIG. 3, FIG. 5 is a view taken along arrow B in FIG. 1, and FIG.
7 to 9 show still other embodiments of the fuel assembly of the present invention, FIG. 7 is a side view partially showing the outer ring of the fuel spacer, and FIG. 8 is the side view shown in FIG. 7. 9 is a partially cutaway plan view showing a state in which fuel rods are attached to the fuel spacer, and FIG. 10 is a modified example of the outer ring shown in FIG. 7. FIG. 11 is a plan view schematically showing the overall configuration of a fuel spacer for a pressure tube type nuclear reactor, and FIG. 12 is a diagram corresponding to a fuel assembly for a pressure tube type nuclear reactor that has been put into practical use in the past. Figure 13 shows the thermal neutron flux distribution characteristics when the reactor is operated with the conventional fuel assembly shown in Figure 12 loaded into the reactor core. It is an explanatory diagram. 1 and 1'...Fuel rod, 2...Fuel spacer, 3
...Cylinder, 4...Lip material, 5 and 5'...Cylindrical element, 5/a and 5'b...Protrusion, 6...Small circular tube, 7...Outer ring, 7a and 7b...Protrusion, 8
... fuel assembly, 9 ... convex part, 10 ... plate spring part, 11 ... pressure pipe, 12 ... coolant (light water), 13
... Calandria tank, 14... Moderator (heavy water)
, 15... Window, 16... Vertical groove, (1 other person) 3rd item 4th item 6th item 9th item

Claims (1)

[Claims] 1. It is equipped with a plurality of fuel rods and a fuel spacer that maintains the spacing between the fuel rods, and the fuel spacer includes a plurality of cylindrical bodies into which each fuel rod is inserted, and a spacer between the cylindrical bodies. In the structure of a pressure tube type nuclear reactor fuel assembly comprising a ring member surrounding the outer periphery of the bundle, the ring member constituting the fuel spacer has a neutron absorption cross-sectional area larger than that of the cylindrical body constituting the other fuel spacer components. 1. A fuel assembly characterized in that the cylindrical body is made of a member having a larger spring force than the ring member, and the two are integrally fixed.