JPS581758B2 - core support plate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a core support plate for a nuclear reactor, and particularly to a core support plate for a fast reactor that determines the configuration of a core portion of a fast breeder reactor.

Generally, the core of a fast breeder reactor is constructed as shown in FIGS.

That is, nuclear fuel material, absorbent material, etc. are sintered and sealed in a thin-walled cylindrical cladding tube to form a fuel element or absorbent material element 10, and a plurality of these elements are bundled and placed in a wrapper tube 6, which is usually a hexagonal thin-walled tube. The fuel assembly, control rod assembly, neutron shielding body with a shielding block provided in the trumpet tube 6, etc. are loaded into the reactor core.

In this case, a cylindrical tubular entrance nozzle 7 for coolant intake (orifice 5) is provided at the lower end of each assembly, while a core support plate 4a consisting of two disks at the bottom of the core,
4b, and the entrance nozzle 7 is fitted into the connecting pipe 5 which connects the nozzle 4b to make it independent.

However, when the power of the reactor is increased and the reactor is put into operation, each assembly receives a large thermal load and bends, causing the core as a whole to deform into a drum shape.

This deformation is due to the following.

That is, in the above configuration, in the reactor core 1, except for the control rods, core fuel assemblies loaded with fissile material are arranged,
In the radial blanket part 2 surrounding this, radial blanket fuel assemblies loaded with a parent substance that hardly causes nuclear fission are arranged, and in the neutron shielding part 3 surrounding the outside of the radial blanket fuel assembly. Arrange the above neutron shields.

In this arrangement, the power distribution inside the reactor is such that the power is greater toward the center of the core, so even when considering a single aggregate layer, there is a temperature difference between the core side wall and the outer wall of the trumpet tube 6, resulting in thermal expansion and swelling expansion. The difference tends to cause the upper end to curve outward.

At the same time, if we compare the reactor core part 1 with the diameter blanket part 2 and the neutron shield part 3, which generates almost no heat, we find that the temperature distribution decreases with a steep slope outward between each part. The temperature difference between the assembly wall surfaces becomes extremely large, and is particularly noticeable in the innermost radial blanket fuel assembly of the radial blanket portion 2 near the reactor core 1.

Therefore, in the absence of any restraint, an extremely large curvature will occur in the neutron shield, especially in the diameter blanket fuel assembly, compared to the core fuel assembly.

In this way, the core fuel assembly is curved and displaced toward the outside of the reactor where the neutron flux distribution is low, which may jeopardize the criticality of the reactor core.

In order to prevent this, a core restraint mechanism is usually provided on the outer periphery of the neutron shield 3 to limit the amount of curved displacement.

However, in this case, as a result of restricting the displacement of the top of the neutron shield, its large free bending force adversely affects the inner core fuel assembly through the pads 8a and 8b,
Since the core fuel assembly moves in the direction of high neutron flux distribution, positive reactivity is injected, causing problems such as melting of the fuel as the reactor output increases.

Furthermore, in order to forcibly limit this extremely large free bending force, it is necessary to consider that a large load will be applied to the pad portion of each component.

As described above, in the design of conventional reactor cores, it is desirable to reduce the free curve load as much as possible in terms of power control and ensuring the integrity of each part.

As one solution, the wrapper tube 6 of the diameter blanket section 2
It is possible to make the wall thickness thinner than that of the reactor core, but
It becomes necessary to manufacture two types of wrapper tubes 6 and to separate the core fuel assembly and the diameter blanket fuel assembly, which poses problems in production automation and management.

It is also possible to uniformly increase the arrangement pitch of the fuel assemblies throughout the reactor in order to absorb the curvature of each fuel assembly, but if the arrangement pitch of the core fuel assemblies in the core section 1 is increased, nuclear fission The generated fast neutrons are slowed down or reduced by the coolant.

As a result, there is a problem in that the breeding ratio, that is, the ratio of uranium-238 converted to plutonium-239 and used as fuel again, decreases, resulting in a decrease in the reactor's capacity.

The present invention was made in view of the above circumstances, and in order to obtain a core for a fast breeder reactor that has a small amount of curvature, a sufficiently small bending load, and does not complicate manufacturing automation, the core support is the basis of its performance. This is an improved version of the board.

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

FIG. 3 is a plan view of a conventional core support plate 4a, and FIG. 4 is a plan view of the core support plate 4 of the present invention.

21, 22. Reference numeral 23 indicates each hole into which the entrance nozzle 7 of the fuel assembly is inserted.

In the conventional core support plate, the arrangement pitch of the core fuel assembly holes 21 in the core part 1, the diameter blanket fuel assembly holes 22 in the diameter blanket 2, and the neutron shield holes 23 in the neutron shield part 3 is Everything was the same.

In the support plate 4 of the present invention, the arrangement pitch of the diameter blanket fuel assembly holes 22 and the neutron shielding holes 23 is set larger than the arrangement pitch of the core fuel assembly holes 21.

Further, the boundary pitch in the radial direction between the core fuel assembly holes 21 and the diameter blanket fuel assembly holes 22 is set to be larger than that of each core fuel assembly hole 21.

The effects of the above configuration are as follows.

In the conventional core support plate (Figure 3), the pitches between all the fuel assemblies are naturally the same, and the core deforms as shown in Figure 5, with the maximum curvature being the innermost assembly of the diameter blanket section 2. The mechanism by which the large curvature of the diameter blanket part 2 and the neutron shield part 3 propagates to the core part through the butt parts 8a and 8b is as described above.

If the pitch arrangement is as in the core support plate 4 of the present invention, the pitch between the aggregates of the radial plunket part 2 and the neutron shield part 3 will be coarser than in the core part 1, so Interaction becomes smaller.

Further, this effect will be explained with reference to FIG.

In the figure, the solid line shows the arrangement of the fuel assembly of the present invention, and the broken line shows the arrangement of the conventional fuel assembly.

As shown by the broken line in the same figure, in the conventional fuel assemblies, the fuel assemblies are provided at equal pitch l1 from the reactor center radially outward, so that the deformation of the core fuel assemblies is sequentially caused by the It is transmitted to the fuel assembly and curves to the outermost blanket fuel assembly.

In contrast, in the present invention, the pitch l2 of the outer fuel assembly is formed larger than the pitch l1 of the core fuel assembly, so even if the fuel assembly in the core part curves, the outer fuel assembly becomes difficult to convey.

Looking at this with respect to the curvature restraining force of the upper pad 8a that restrains the curvature of the fuel assembly, we can see that the fuel assembly
The height L above a is 3 m, and the second moment of area ■ is 1.
8 x 106 mm4, Young's modulus E is 1. 6 X
The bending restraining force P in a typical case of 104 kg/mm2 is as follows.

Here l is the pinch of adjacent fuel assemblies.

Therefore, in Fig. 6, the difference P1 in the bending restraining force acting on the fifth and sixth fuel assemblies from the reactor center is P1 = 16 (l2 - l1) ...... (2) from equation (1). .

Here, if the difference in pitch between l2 and l1 is, for example, 4, then according to equation (2), if the pitch is increased by 4 mm, the bending restraint force will be reduced by about 64 kg.

This reduction in the curvature restraint force reduces the shearing force acting on the pad 8a, which is highly effective in ensuring the safety of the core.

From this, it can be seen that by increasing the pitch between the core part 1 and the diameter blanket part 2 in particular, the effect of reducing the curvature restraining force and preventing the curvature from being transmitted to the outside becomes greater.

Even if the pitch is increased, it is only a few millimeters, so it does not affect the insertion of the aggregate.

In addition, the load on the pad is sufficiently small, and in terms of manufacturing,
By varying the pitch of the holes in the core support plate, each fuel assembly can be manufactured without changing existing manufacturing automation processes.

As described above, by using the core support plate of the present invention,
It is possible to design a safe core with a small pad load and easy output control without changing the automated fuel assembly manufacturing process, which is a great feature in that it contributes to improving the functionality of fast breeder reactors.

[Brief explanation of the drawing]

Fig. 1 is a plan view showing the configuration of the fast breeder reactor core, Fig. 2 is an enlarged partial longitudinal cross-sectional view of the same part, Fig. 3 is a plan view of a conventional core support plate, and Fig. 4 is a plan view of the present invention. FIG. 5 is a plan view showing an embodiment of the core support plate, FIG. 5 is a side view showing the curved state of the fuel assembly in the conventional core part, and FIG. 6 is a skeleton diagram showing the curved state of the fuel assemblies of the present invention and the conventional example. It is a skeleton diagram shown in FIG. 1...Reactor core part, 2...Diameter blanket part, 3...Neutron shield part, 4.4a, 4b...Core support plate, 5...Connecting pipe, 6...Trumpet tube, 7...Entrance nozzle , 8
a, 8b...pad, 9...orifice, 21, 22.
23...hole.

Claims (1)

[Claims] 1. Consists of two upper and lower discs, with a core fuel assembly in the center, a diameter blanket fuel assembly on the outside, and a large number of inserting and supporting entrance nozzles of a neutron shield on the outside. 1. A core support plate having holes, the arrangement pitch of the diameter blanket fuel assembly holes and the neutron shielding holes being set to be larger than the arrangement pitch of the core fuel assembly holes. 2. A patent claim characterized in that the radial boundary pitch between the core fuel assembly holes and the diameter blanket fuel assembly holes is set larger than the radial pitch of the core fuel assembly hole arrangement. The core support plate according to scope 1.