JPH0564755B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a core structure of a fast breeder reactor, and particularly to a core structure of a fast breeder reactor suitable for improving seismic performance of a fuel assembly.

[Background of the invention]

Improving the core seismic performance of fast breeder reactors (hereinafter referred to as FBR) will lead to the relaxation of the permissible vibration conditions of the core support section, rationalize the seismic structure of the reactor main vessel that supports the reactor core, and ultimately
Contributes to lower construction costs for the entire FBR plant. In order to improve the seismic performance of the core, it is effective to reduce the collision force generated in the spacer pads of the core components.

One of these methods is to avoid the dominant frequency component of the excitation acceleration at the core support and prevent core resonance (Japanese Patent Laid-Open No. 59-83082, 58-
No. 147683, No. 57-189095). As another method,
A method has been proposed in which a buffer member is provided in the spacer pad portion of the core component or in the portion that contacts the spacer pad within the core restraint frame installed on the outermost periphery of the core component (Japanese Unexamined Patent Application Publication No. 1988-1999).
No. 208691, No. 55-132988).

Even if a portion of the neutron shield among the core components inserted into the reactor core is damaged, the structural integrity will not be significantly affected. On the other hand, if the fuel assembly is damaged, the fuel element wrapped inside will be exposed or fall off.
Therefore, it is essential to improve the seismic resistance of the fuel assemblies, especially among the core components, in order to ensure structural integrity.

However, the above conventional methods are related to methods for reducing the overall vibration response of the core and methods for reducing the collision force between the core restraint frame and the outermost core component (usually a neutron shield). It does not indicate countermeasures for the core components, that is, the fuel assemblies themselves, where ensuring the integrity of the reactor is most strongly required.

[Purpose of the invention]

An object of the present invention is to provide an FBR core structure that effectively and rationally improves the structural integrity of the fuel assembly during an earthquake, for which ensuring the integrity of the fuel assembly is most strongly required among various core components. It is.

[Summary of the invention]

The present invention improves the structural integrity of fuel assemblies during earthquakes,
This is achieved by reducing the collision force of the lower spacer pad.

The top spacer pad portion of the fuel assembly has a thick wall structure on the inside to ensure strength against collision force. However, since the fuel element is located inside the lower spacer pad part, a thick structure cannot be adopted, and furthermore, damage to this pad part may directly lead to damage or falling off of the fuel element. Therefore, it is considered more effective to reduce the collision force at the lower spacer pad than at the top of the fuel assembly.

In addition, since the distribution of spacer pad collision force within the core during an earthquake tends to gradually increase from the core side to the outside, it is important to note that the spacer pad of the outer fuel assembly receives the greatest collision force among the fuel assemblies. It is better to reduce the collision force of the parts.

As a means of realizing these two principles, the present invention provides a spacer pad portion with compressive rigidity provided on the axially lower side of the neutron shield adjacent to the outer fuel assembly, so that the spacer pad portion is provided at the same position in the axial direction of the adjacent fuel assembly. The compression stiffness of the spacer pad is smaller than that of the spacer pad.
It provides the core structure of FBR.

The above-mentioned trend in the distribution of spacer pad impact force within the core during an earthquake is supported by the following experimental data. Figure 2A simulates the characteristics of an FBR core structure in which a large number of rods with hexagonal cross-sections are arranged in a triangular lattice arrangement through gaps in the liquid, supported at their lower ends and released at their upper ends. This is a plan view of the test specimen (composed of 127 approximately 1/2.5 scale models of core components with an electrical output of 1 million KW). As a result of performing an underwater vibration experiment on this specimen, the data shown in FIG. 2B was obtained as the top displacement behavior. In this experiment, excessive displacement in the outward direction is restrained at the top of the outermost peripheral element, so for example,
In model number 124, less displacement was observed.

From the results shown in Figure 2B, it is clear that in the nonlinear vibrations of the FBR core components, including liquid interaction and collision phenomena, a mode in which each component group vibrates as one in the same phase is excited. I understand. Therefore, the collision force generated in each component strongly depends on the amount of movement (displacement) of the component, and becomes larger for components outside the core whose displacement is smaller.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described with reference to FIG. Figure 1 shows an electrical output of 1 million KW class.
The outline of the FBR core structure is shown, A is a plan view, B
is a vertical sectional view. The reactor core includes a core fuel assembly 1 with a hexagonal cross section that mainly contains nuclear fuel material;
It mainly consists of a blanket fuel assembly 5 containing a nuclear fuel parent material therein, a control rod assembly 4 containing a neutron absorbing material therein, a neutron shielding body 6 containing a neutron shielding body therein, and the like. These assemblies and the like are usually called core components, and the core fuel assembly 1 and the blanket fuel assembly 5 are collectively called a fuel assembly.

Now, as shown in FIG. 3, the core fuel assembly 1 includes a plurality of fuel elements 18, a mantle 19 surrounding them, a lower nozzle 15 with a coolant inlet, and a head 20 with a coolant outlet. A top spacer pad 2 is provided on the top of the outer surface of the mantle 19, and a lower spacer pad 3 is provided on the axially lower side thereof.

The external structure of the blanket fuel assembly 5 and the control rod assembly 4 is basically the same as that of the core fuel assembly 1.

Neutron shield 6- adjacent to the outermost fuel assembly
As shown in FIG. 4, a includes a neutron shield 21, a mantle 19 surrounding it, a lower nozzle 15 with a coolant inlet, a head 20 with a coolant outlet, and the like. A top spacer pad is provided at the top of the outer surface of the spacer, and a lower spacer pad 3 is provided axially below the top spacer pad.

A feature of the present invention is that the compression rigidity of the lower spacer pad 3 portion of the neutron shield 6-a is made smaller than the compression rigidity of the lower spacer pad portion of the adjacent fuel assembly.

Other structures of the neutron shield 6-b, except for the shape and structure of the lower spacer pad 3, are as follows:
This is the same as that of the neutron shield 6-a.

These core components have their lower nozzles 15
is inserted and supported by the core support plate 17. The core curvature of each core component during normal operation and the radial displacement due to vibration during an earthquake are the top spacer pad 2
It is restrained by contact between core components at the lower spacer pad 3 part or by contact with core restraint frames 7 and 9 installed outside the outermost core component.

The purpose of restraining the displacement in this way is to suppress excessive displacement of the top part of the control rod assembly 4 in particular and ensure its insertability. This is to prevent excessive bending stress from occurring at the base of the lower nozzle 15. As this displacement restraint means, means other than the above-mentioned core restraint frames 7 and 9 may be used.

An example of the results of analyzing the vibration behavior when the core support plate 17 is vibrated with a sinusoidal acceleration of a frequency of 5 Hz and a maximum amplitude of 1.0 G in the core structure of this embodiment described above (partial collision force distribution of core component lower spacer pad) is shown in Figure 5. In this analysis, the compression rigidity of the lower spacer pad portion of the neutron shield 6-a was set to be approximately 1/3 that of other core components. The analysis is
As shown in the lower part of FIG. 5, a representative example of the inside of the reactor core was taken out and carried out. As mentioned above with reference to Figure 2, the FBR core components are excited to vibrate as one in the same phase, so even if the analysis ignores the interference effect between the columns, the essence of the result is not clear. This trend is considered to be reasonable.

FIG. 5 also shows the results obtained when the compressive stiffness of the conventional core structure, that is, the lower spacer pad 3 portion of the neutron shield 6-a, is made the same as that of other core components. As is clear from the comparison of the two characteristic curves, according to the present invention, the maximum value of the collision force generated in particular in the lower spacer pad 3 portion of the blanket fuel assembly can be reduced to approximately 30
% can be reduced.

What should be noted here is that in order to reduce this collision force, the structure on the fuel assembly side has not been changed from the conventional example. That is, the strength of the fuel assembly against collision force remains unchanged from the conventional example, and therefore, the reduction in collision force directly leads to an improvement in the structural integrity of the fuel assembly.

6 and 7 show other examples of shapes of the lower spacer pad 3 for reducing the compression rigidity of the lower spacer pad portion of the neutron shield 6-a. In FIG. 6, an axial slit 23 is provided in the lower spacer pad 3 to reduce its compression rigidity. The right half of Fig. 7 is the lower spacer pad 3.
The axial length of the reactor is made shorter than that of the other core components shown on the left, thereby reducing its compressive stiffness. Various other shapes and structures of the lower spacer pad 3 can be considered to reduce the compression rigidity of this portion, and are not limited to those shown in the drawings.

Another method for reducing the compression rigidity of the lower spacer pad 3 portion of the neutron shield 6-a is to change the shape or structure of the mantle 19 from that of the adjacent fuel assembly. For example, methods such as reducing the wall thickness of the mantle 19 of the neutron shield 6-a or providing an axial slit in the lower spacer pad 3 installation part of the mantle 19 can be considered.
Also in this regard, various shapes or structures can be adopted.

〔Effect of the invention〕

According to the present invention, in a fuel assembly whose integrity is most required among various core components,
The maximum impact force during an earthquake on the lower spacer pad is approximately 30
%, improving its structural integrity.
This improvement in vibration resistance will lead to a relaxation of the permissible vibration conditions for the core support section, resulting in rationalization of the seismic structure of the reactor main vessel that supports the core, which in turn will contribute to reducing the construction cost of the entire FBR plant.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing an example of the FBR core structure according to the present invention, Figure 2 is a vibration characteristic diagram of displacement obtained by a vibration experiment using a specimen simulating the FBR core structure, and Figure 3 is a diagram of the core structure. Fig. 4 is a structural diagram of a fuel assembly, and Fig. 5 is a structural diagram of a neutron shield according to the present invention.
The figure is a characteristic diagram of the analysis results of the earthquake force distribution between the lower spacer pads of the core components, and FIGS. 6 and 7 are structural diagrams showing other embodiments of the lower spacer pad portion of the neutron shield, respectively. DESCRIPTION OF SYMBOLS 1... Core fuel assembly, 2... Upper spacer pad, 3... Lower spacer pad, 4... Control rod assembly, 5... Blanket fuel assembly, 6...
Neutron shield, 7, 9... Core restraint frame, 15...
Lower nozzle, 18... Fuel element, 19... Mantle, 20... Head, 21... Neutron shield, 2
3...Axial slit.

Claims (1)

[Scope of Claims] 1. In a core structure of a fast breeder reactor having a plurality of fuel assemblies containing nuclear fuel material and nuclear fuel parent material therein, and a neutron shielding body surrounding the fuel assemblies in a layer, A spacer pad is provided at the same height as the axial top of the outer surface of the assembly and the neutron shield and below it, and compression of the spacer pad portion provided at the axial bottom of the neutron shield in the layer adjacent to the fuel assembly. A core structure for a fast breeder reactor, characterized in that its rigidity is smaller than the compressive rigidity of a spacer pad provided on the axially lower side of a fuel assembly. 2. Claim 1 provides that the compressive stiffness of the lower spacer pad portions is made different by changing the shape or structure of the spacer pads provided on the axially lower sides of the adjacent fuel assemblies and the neutron shield. Features of the core structure of a fast breeder reactor. 3. In claim 1, the compressive rigidity of the lower spacer pad portion is improved by changing the shape or structure of the outer mantle near the portion where the spacer pad is to be provided on the axially lower side of the adjacent fuel assembly and the neutron shield. The core structure of a fast breeder reactor is characterized by different characteristics.