JPH0524476B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a nuclear reactor core, and more particularly to a method for arranging control rod clusters in a reactor core simultaneously loaded with two or more types of fuel having significantly different reaction effects on neutrons. [Prior art] In a reactor core loaded simultaneously with two or more types of fuel that have significantly different reaction effects against neutrons, such as uranium oxide fuel, gadolinia-containing uranium oxide fuel, or uranium-plutonium mixed oxide fuel (MOX: Mixed Oxide), In order to safely shut down the reactor using the control rods, and to ensure the safety of the reactor even if a control rod fails, various restrictions have been placed on the core fuel loading system, including the fuel type configuration. The problem was that the design lacked flexibility. As an example, the case where uranium oxide fuel and MOX fuel are simultaneously loaded into a pressurized water reactor (PWR) is as follows. In other words, when determining the fuel loading method for the reactor core, it is necessary to ensure that even if one of the control rod clusters cannot be inserted into the reactor core due to sticking or the like, the reactor can be shut down with sufficient time using the remaining control rods, and It is necessary to satisfy the condition that the reactor will not be damaged even if one of the control rod clusters flies out of the reactor when the control rods are inserted. To do this, the neutron absorption effect of each control rod cluster must be It is desirable that there be no difference in the neutron absorption effect compared to the neutron absorption effect of the fuel. Traditionally, it has been normal to use one type of control rod cluster placed in the reactor core, but when loading uranium oxide fuel and MOX fuel at the same time,
Since the neutron absorption effect of MOX fuel is greater than that of uranium oxide fuel, there is a difference in the neutron absorption effect between control rod clusters of the same type, and depending on the fuel loading method, the above-mentioned reactor shutdown margin may be reduced. There are also problems in that safety conditions may not be satisfied. [Problems to be Solved] The present invention solves the above-mentioned problems of the prior art and changes the type and arrangement of control rod clusters depending on the fuel loading method, thereby ensuring the safety of a nuclear reactor. Relaxing restrictions on fuel loading methods,
The aim is to provide a nuclear reactor core with flexibility in fuel loading methods. [Means for Solving Problems and Effects] According to the present invention, a reactor core with a neutron absorption effect is placed at a predetermined control rod cluster position in a nuclear reactor core which is simultaneously loaded with a plurality of types of fuel having greatly different reaction effects on neutrons. Two or more different types of control rod clusters are arranged to provide optimal reactor safety depending on the fuel loading scheme. in this case,
The arrangement pattern is selected so that the neutron absorption effect of each control rod cluster differs as little as possible from the neutron absorption effect of the fuel into which it is inserted, and according to one embodiment, a fuel region with a large neutron absorption effect is inserted. A control rod cluster with a large neutron absorption effect is placed at a position where a control rod cluster with a small neutron absorption effect is inserted, and a control rod cluster with a small neutron absorption effect is placed at an insertion position of a fuel region with a small neutron absorption effect. In the present invention, differences in neutron absorption effects within the reactor core due to simultaneous loading of different types of fuel are equalized by the complementary arrangement of different types of control rod clusters, thereby ensuring sufficient reactor shutdown margin and flattening power peaking. It is possible to provide flexibility in the fuel loading method while maintaining a high level of safety. Examples of the present invention will be shown below for a better understanding of the present invention. [Example] Figures 1 to 3 show an example of the present invention when 60 units of MOX fuel and 61 units of uranium oxide fuel are loaded in the core of a pressurized water reactor (PWR).
The figure is a schematic cross-sectional view showing the control rod cluster insertion position in the reactor core with ○ marks, and Figure 2 shows MOX fuel (x marks).
Fig. 3 is a schematic cross-sectional view showing the loading method of uranium oxide fuel (unmarked), and Fig. 3 is a schematic cross-sectional view showing the insertion positions of different control rod clusters (marked with ○ and ◎) arranged according to the present invention. be. As shown in FIG. 1, fuel loading is performed in the pattern shown in FIG. 2 with respect to the control rod cluster insertion position determined in advance. Here, even if the same type of control rod crust is inserted at each insertion position, the neutron absorption effect of the control rod cluster inserted into the uranium oxide fuel region at the periphery of the core will be absorbed by the MOX fuel region at the center of the core. Because the control rod cluster is relatively large compared to that of the control rod cluster, in the event of an ejection accident of the control rod cluster in the surrounding area, the degree of loss of its neutron absorption effect is large, leading to a rapid increase in reactivity and severe output. The safety of the reactor cannot be ensured due to peaking, etc. In contrast, in the present invention, as shown in FIG. 3, a plurality of types of control rod clusters having different neutron absorption effects are arranged and inserted in accordance with the loading method of different types of fuel. That is, in the embodiment shown in FIG. 3, five control rod clusters 21 having a large neutron absorption effect are arranged in the area of the MOX fuel 11 having a large neutron absorption effect in the center, and four control rod clusters 21 near the periphery of the center are arranged. MOX fuel 11a
Similar control rod clusters 21 are arranged for each of the control rod clusters 21 to 11d, and the control rod clusters 21 are arranged in the peripheral region of the reactor core formed by the uranium oxide fuel 12, which has a relatively small neutron absorption effect. A total of 20 control rod clusters 22 of a different type having a smaller neutron absorption effect than the control rod clusters 21 have a substantially rectangular shape. In a PWR loaded with uranium oxide fuel and MOX fuel distributed approximately half and half, as shown in Figure 2, there are two types of control rod clusters, one in which all the control rod clusters are of the same kind, and the other in accordance with the present invention, which have different neutron absorption effects. An example of a comparison of the reactor shutdown margin and output peaking in the event of a cluster ejection accident when using a control cluster is shown in the table below. It can be seen that according to the present invention, the safety of the reactor can be ensured in a dissimilar fuel loading system, which cannot be achieved with the use of homogeneous control rod clusters.

【table】

〔Effect of the invention〕

As described above, according to the present invention, in a reactor core where multiple types of fuel with different reaction effects to neutrons are simultaneously loaded, multiple types of control rod clusters with different neutron absorption effects are arranged according to the fuel loading method. Therefore, while ensuring the safety of the reactor, the restrictions on different fuel loading methods can be relaxed, and flexibility can be achieved.

[Brief explanation of drawings]

The drawings relate to one embodiment of the present invention, and Fig. 1 is a schematic cross-sectional view of the reactor core with O marks indicating the predetermined insertion positions of control rod clusters in the reactor core, and Fig. 2 shows MOX fuel (x marks) and FIG. 3 is a schematic cross-sectional view of the core showing the loading method of uranium oxide fuel (unmarked), and FIG. 3 is a schematic cross-sectional view of the core showing the insertion positions of heterogeneous control rod clusters arranged according to the present invention. 11, 11a-d: MOX fuel, 12: Uranium oxide fuel, 21: Control rod cluster with relatively large neutron absorption effect, 22: Control rod cluster with relatively small neutron absorption effect.

Claims (1)

[Scope of Claims] 1. In a nuclear reactor core in which uranium oxide and MOX fuel are simultaneously loaded, which have greatly different reaction effects against neutrons, the neutron absorption effects differ depending on the loading method of the uranium oxide and MOX fuel. A control rod cluster with a large neutron absorption effect is arranged at the insertion position of the MOX fuel region with a large neutron absorption effect so that the difference in neutron absorption effect is equalized by the complementary arrangement of the control rod clusters, A nuclear reactor core characterized in that a control rod cluster having a small neutron absorption effect is arranged at an insertion position of a uranium oxide fuel region having a small effect.