JPH0414317B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a core for a fast reactor in which the arrangement of fuel assemblies is improved so as to equalize the power distribution of the core within a reactor vessel.

[Technical Background of the Invention] Conventionally, a fast reactor as shown in FIG. 1 has been used. In the figure, reference numeral 1 denotes a reactor vessel, and this reactor vessel 1 is filled with liquid sodium 2 as a coolant. This reactor vessel 1
A reactor core 3 is disposed at the center of the reactor, a core upper mechanism 4 is disposed on the reactor core 3, and a primary coolant pump 5, an intermediate heat exchanger 6, etc. are disposed around the reactor core 3.

In addition, a roof slab 7 as an upper shield is provided at the upper opening of the reactor vessel 1 and hermetically sealed, and an inert gas is provided in the plenum space between the roof slab 7 and the liquid level of the liquid sodium 2. Cover gas is included. A control rod drive mechanism 8 is supported in the center of the roof slab 7.

As shown in FIG. 2, this control rod drive mechanism 8 is composed of a drive section disposed on the roof slab 7, and an upper guide tube 12 and an extension tube 13 disposed on the lower surface side of the roof slab 7. There is. In addition, the second
The figure schematically shows the drive mechanism 8 for inserting the control rods into the reactor core 3 in the reactor vessel 1 in FIG. 1 and the reactor core 3 in an enlarged manner.

The control rod drive mechanism 8 controls the reactivity of the reactor core 3 by inserting and withdrawing the control rods 10 into the core fuel 9 of the reactor core 3.

This core 3 includes a large number of outer core fuel assemblies each containing a large amount of fissile material, an inner core fuel assembly containing less fissile material than the outer core fuel assemblies, and a neutron absorbing material to control the fission reaction. It is formed by the control rod.

FIG. 3 is a schematic plan view showing an example of the arrangement of the core 3 of the fast reactor shown in FIG. 1 from above.

That is, the core 3 is composed of an outer core region 14 (outside the thick line in the figure) made up of outer core fuel assemblies, and an inner core region 15 made up of inner core fuel assemblies. , a large number of control rods 10A, 10B, 10C, and 10X are inserted and arranged in a dispersed manner in these regions 14 and 15 within the core 3.

Among the many control rods, the control rods 10A, 10B, and 10C are main reactor shutdown control rods used for the purpose of starting, stopping, and controlling the power of the reactor core 3. Among the many control rods, the control rod 10X is used only for the purpose of emergency shutdown of the reactor, and is a backup reactor shutdown control rod that is always fully withdrawn from the reactor core 3 during normal operation.

Among these main reactor shutdown control rods, 10A is located at the center, 10B is located at the second distance from the center, and 10C is located at the boundary between the inner core region 15 and the outer core region 14. The main reactor shutdown control rod 10 is shown.

Further, as shown in FIG. 3, the control rod 10C disposed at the farthest distance from the reactor center is normally located at the boundary between the inner core region 15 and the outer core region 14.

In the operation of a fast reactor, in order to continue operation over a long period of time, it is necessary to control the excess reaction rate that the reactor has at the beginning of the operation cycle. is partially inserted into the reactor core 3 for operation.

[Problems with the background art] By the way, two control rods 10 separated by a certain distance
When comparing the case where the control rods 10 are inserted into the core fuel 9 to produce a certain output, and the case where the control rods 10 are pulled out and the same output operation is performed, the output of the core fuel 9 near the control rods 10 is lower than that of the control rods 10. It is lower in the inserted state than in the withdrawn state. On the contrary, the output of the core fuel 9 at the midpoint between the two control rods 10 is higher when the control rod 10 is in the inserted state than when it is in the withdrawn state.

This state is expressed in the power distribution curve shown in Figure 4, where the horizontal axis represents the position and the vertical axis represents the core fuel 9.
The output of is shown. In addition, in the figure, the solid line A indicates the control rod 10 is in the inserted state, and the dotted line B indicates the control rod 10 is in the inserted state.
The cases in which it is in the pulled out state are shown in each case. Note that FIG. 4 corresponds to the output distribution along A-A' in FIG. 3.

As is clear from FIG. 4, when operating with all of the main reactor shutdown control rods 10C in FIG. 3 of the conventional example partially inserted into the reactor core at the beginning of the operation cycle, The output of the outer core fuel assembly 16A or 16B located at the midpoint between the two is very high.

This usually occurs at the beginning of the operation cycle when the local maximum power control rods are partially inserted into the core fuel, and at that time the most thermally severe condition occurs. In this case, if the thermal limit conditions are exceeded, it is necessary to reduce the total power of the core or increase the number of core fuel assemblies.

However, doing so is undesirable not only in terms of fuel economy but also in terms of plant economy.

Additionally, in general, when designing the core of a fast reactor, it is necessary to equalize the power distribution within the core as much as possible and reduce the required use of fissile material in order to improve the economic efficiency of the core. The placement of the reactor core is still not sufficient.

[Objective of the Invention] The present invention has been made in view of these points, and an object thereof is to provide a core for a fast reactor that can be operated in a uniform state without disturbing the power distribution of the core as much as possible. do.

[Summary of the Invention] That is, the present invention provides an outer core region consisting of an outer core fuel assembly containing a large amount of fissile material, and an inner core region consisting of an inner core fuel assembly containing less fissile material than the outer core fuel assembly. area and
In a fast reactor core composed of control rods that contain neutron absorbing materials and are inserted into and withdrawn from the inner core region to control nuclear fission reactions, one of the plurality of control rods located farthest from the center of the core. All inner core fuel assemblies are arranged at the midpoints of the straight line connecting two control rods located at the shortest distance from each other, and among the fuel assemblies in contact with the control rod located farthest from the core center, The feature is that outer core fuel assemblies are used for all the fuel assemblies located on the side of the control rods opposite to the core center, which makes it possible to equalize the power distribution of the core.

[Embodiments of the Invention] Hereinafter, an embodiment of a fast reactor core according to the present invention will be described based on FIG. 5, in which the same parts as in FIG. 4 are denoted by the same reference numerals.

FIG. 5 is a plan view showing the arrangement of a fast reactor core when the present invention is used.

As shown in FIG. 5, the fuel assembly 1 is sandwiched between the main reactor shutdown control rods 10C, which are located farthest from the core center, and is located at the midpoint between the control rods 10C.
7A, or 17B, all belong to the inner core region 15 by using inner core fuel assemblies containing less fissile material than the outer core fuel assemblies.

In general, if the environmental conditions surrounding the fuel, such as neutron flux, are the same, the smaller the amount of fissile material contained in the fuel assembly, the lower the output of the fuel assembly. When using the reactor core arrangement, even when operating the fast reactor with the control rods 10C partially inserted into the reactor core, the local maximum power that has conventionally been generated will no longer be generated in the fuel assembly 17A or 17B.

FIG. 6 shows the power distribution in the core fuel sandwiched between the partially inserted control rods 10C when the present invention is used. This figure corresponds to the output distribution along A-A' in FIG.

In the conventional example, a large power is locally generated in the core fuel section at the midpoint between the partially inserted control rods 10 (Fig. 4), but in the embodiment of the present invention, the local power is suppressed. It is clear that this is true (Figure 6). This is because the inner core fuel assembly containing a small amount of fissile material is placed at the midpoint between the partially inserted control rods 10C.

To explain further, if the maximum output per 1 cm of axial length of the core fuel pin is called the maximum linear output, the maximum linear output in the conventional example is 445 W/cm, but in this example it is 425 W/cm. The maximum output value decreases by approximately 5%.

FIG. 7 shows another embodiment of the invention.

In Fig. 7, fuel assemblies 18A, 18B, 18
C, 18D uses an inner core fuel assembly,
Compared to the embodiment shown in FIG. 5, the inner core region 15 is made wider. The depth of partial insertion of the control rod 10C at the beginning of the operation cycle varies depending on individual designs such as the excess reactivity the reactor has at the beginning of the operation cycle and the amount of neutron absorbing material contained in the control rod 10C. The shape of the area power distribution sandwiched by 10C varies depending on the individual design.

Therefore, if the output of the core fuel at the midpoint between the control rods 10C becomes significantly large, inner core fuel assemblies may also be arranged in the fuel assemblies 18A and 18B as in the embodiment of the present invention. The local maximum output can be suppressed by

FIG. 8 shows yet another embodiment of the invention.

Code 19A, 19B, 19C, 19D, 19E
is an inner core fuel assembly, which suppresses local maximum power generation when operating with the control rod 10C partially inserted at the beginning of the operation cycle.

Furthermore, as described above, the depth of partial insertion of the control rod 10C at the beginning of the operation cycle varies depending on the individual design, so the output of the fuel assembly in contact with the control rod 10C is significantly reduced due to the insertion of the control rod 10C. In this case, as in the embodiment of the present invention, the fuel assemblies 20A, 20B, 20C, 20D,
For 20E and 20F, it is more effective to use outer core fuel assemblies containing more fissile material than the inner core fuel assemblies in terms of uniformity of power distribution.

In general, in fuel assemblies containing the same amount of fissile material per unit volume, the output is higher when placed near the center of the reactor core than when placed further away. For the fuel assemblies 20B and 20F arranged on the center side, inner core fuel assemblies with a low content of fissile material are used, but in this embodiment, the power decreases significantly due to the partial insertion of the control rod 10C. It uses an outer core fuel assembly that contains a large amount of fissile material. This prevents the output from decreasing in the fuel assemblies 20B and 20F and makes the output distribution uniform.

Similarly to the embodiment shown in FIG. 7, inner core fuel assemblies are used in the fuel assemblies 19A, 19B, 19C, 19D, and 19E, so that localized large-scale damage caused by partial insertion of the control rod 10C is possible. The generation of output is suppressed.

[Effects of the Invention] As explained above, the present invention partially inserts the control rods located farthest from the core center into the reactor core at the early stage of the operation cycle, thereby reducing localized damage occurring between the control rods. Since the large power is suppressed by arranging the inner core fuel assemblies, a uniform power distribution is achieved and the economic efficiency of the core is improved.

[Brief explanation of drawings]

Figure 1 is a longitudinal side view showing the fast reactor, Figure 2 is a configuration diagram showing the control rod drive mechanism in Figure 1, and Figure 3 is a diagram showing the control rod drive mechanism in Figure 1.
The figure is a plan view showing an example of the core arrangement of a conventional fast reactor, and Figure 4 shows a control rod located farthest from the core center that is partially inserted into the core at the beginning of the operation cycle in the case of a conventional fast reactor core. Figure 5 is a plan view showing an embodiment of the core arrangement according to the present invention, and Figure 6 is the core arrangement according to the present invention. Figures 7 and 8 are characteristic diagrams showing the power distribution shape of the core fuel around the control rod when the control rod located farthest from the core center is partially inserted into the core at the beginning of the operation cycle. FIG. 7 is a plan view showing still other embodiments of the present invention. 1...Reactor vessel, 3...Reactor core, 10A, 10
B, 10C, 10X...control rod.

Claims (1)

[Scope of Claims] 1. An outer core fuel assembly containing a large amount of fissile material, an inner core fuel assembly containing less fissile material than the outer core fuel assembly, and a control rod that includes a neutron absorbing material and controls a nuclear fission reaction. In a fast reactor core configured in this manner, the inner core fuel is located at the midpoint of a straight line connecting two control rods located at the shortest distance from each other among the plurality of control rods located farthest from the center of the core. Outer core fuel assemblies are arranged for all fuel assemblies located on the opposite side of the core from the control rod among the fuel assemblies in contact with the control rod located farthest from the core center. A fast reactor core characterized by the use of a human body. 2. The core for a fast reactor according to claim 1, wherein the inner core fuel assembly is arranged so as to be in contact with the core center side on the opposite side.