JPS6133393B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to boiling water nuclear reactors, and more particularly to improvements in core configuration.

In boiling water reactors, control rods are used to adjust core reactivity. Considering the time (referred to as one cycle) from the initial stage of reactor operation until the reactor is shut down for fuel exchange, control rods are inserted to suppress excess reactivity in the reactor core. The control rods in a boiling water reactor are cross-shaped, and the progress of combustion of fuel in the fuel assembly (hereinafter referred to as control rod cell) at the position where the control rod is inserted is determined by the direction in which the control rod is inserted. The rate of fission of uranium-235 is therefore lower than that of a bundle without control rods. The density of control rods inserted into the reactor core decreases as the core surplus reactivity decreases, but the longer the period in which control rods are inserted throughout the cycle, the more
The progress of fuel combustion slows down. Therefore, the longer a control rod is inserted into a fuel assembly, the smaller the reactivity loss due to combustion of that fuel assembly is compared to that of a fuel assembly in which no control rod is inserted. The output of the time rod cells increases rapidly, and sometimes the fuel assembly may produce the maximum output in the entire core, which is unfavorable for the health of the fuel.

Therefore, in order to avoid a state in which a specific control rod is inserted for a long period of time during one cycle, it is necessary to drastically change the arrangement of control rods (hereinafter referred to as pattern). There is an operation to pull out a control rod, but a sudden increase in the output of the cell of the control rod that is being pulled out cannot be avoided.
For this reason, measures have been taken to reduce the core power when operating the control rods, but this is a factor that lowers the capacity utilization rate. On the other hand, the power distribution of fuel assemblies in the core is changed from the center of the core to the outermost periphery of the core.
The power distribution when viewed in the radial direction of the core (hereinafter referred to as radial power distribution) is large at the center of the core and lowest at the outermost periphery of the core. This is because the leakage of neutrons from the fuel assemblies at the outermost periphery of the core to the outside of the core is maximum within the core. Therefore, the greater the gradient of the radial power distribution between the center of the core and the outermost periphery, the more
A large amount of power will be concentrated in the fuel assembly at the center of the core, which is also unfavorable in terms of fuel integrity.

The present invention has been made based on the above circumstances, and includes:
During the cycle, a specific control rod can be inserted for a long period of time, reducing the number of pattern changes, thereby improving capacity utilization.At the same time, the output of the control rod cell can be kept low even when the control rod is withdrawn.
The purpose is to improve the health of the fuel by making the slope of the radial power distribution gentler.

In the present invention, the control rods that are inserted and withdrawn during one cycle are fixed, and the fuel enrichment of these control rod cells is lower than the fuel enrichment of the bundle in which no control rods are inserted. , it is possible to keep the output low when the control rod is withdrawn. In addition, the control rod cells have two or more types of fuel enrichment, and the control rod cells with the lowest enrichment are used near the center of the core when viewed in the radial direction of the core, and the control rod cells with the highest enrichment are used in concentric circles toward the outer periphery of the core. By arranging the control rod cells, the radial power distribution can be made even. However, as described above, the enrichment of two or more types of control rod cells is lower than that of a fuel assembly in which no control rods are inserted.

Figure 1 schematically shows the cross section of the reactor core. The cross-shaped symbol is the control rod 10, and the numbers 1, 2, and 3 written in each quadrant of the cross-shaped symbol are different types. Fuel assemblies 1, 2, and 3 are shown.

Of the three types of fuel assemblies, fuel assembly 1 has the highest enrichment degree, and the control rods in contact with fuel assembly 1 remain unoperated and withdrawn during reactor operation. In Figure 1, fuel assemblies 2 and 3 are control rod cells, fuel assembly 3 near the core center has the lowest enrichment fuel, and fuel assembly 2 near the core is more enriched than fuel assembly 3. Although the concentration is high, the fuel enrichment is lower than that of fuel assembly 1.

In FIG. 2, the horizontal axis represents the core radial direction connecting the core center and the outermost periphery of the core, and the vertical axis represents the fuel assembly output (relative value) when no control rods are inserted. In FIG. 2, curve A is the power distribution when the core is constructed using only one type of fuel assembly without using low enrichment control rod cells in the core. Curve B is the fuel assembly output distribution when one type of control rod cell (when fuel assembly 2 and fuel assembly 3 have the same enrichment). The concave portion of curve B corresponds to the control rod cell. Comparing the convex portion of curve B and the concave portion of curve A, curve B has a gentler radial output distribution.

This is because the bundle output of the control rod cells in the center of the reactor core decreases due to the presence of the control rod cells, which also lowers the bundle output around the control rod cells. However, when one type of control rod cell is used, low-density fuel is put into the control rod cells in the vicinity of the core, which originally share a small amount of power, and which causes a large reduction in output. Therefore, during actual operation, the surrounding area of the furnace (Fig. 2 γ
or δ), the output of the control rod cell (γ or δ) decreases too much, making it impossible to obtain a significant effect in flattening the radial output distribution.

Curve C in FIG. 2 shows the case when the control rod cell according to the present invention is used, and two types of control rod cells are used. As can be seen by comparing curves C and B, curve C has a larger output around the core than curve B, and the effect of flattening the radial power distribution is more pronounced than in curve B. .

This is because in the case of the present invention, the control rod cells (γ or δ) in the periphery of the reactor are provided with fuel assemblies with higher enrichment than the control rod cells (α or β) in the center of the reactor. Compared to the above-mentioned uniform fuel core and a core using one type of control rod cell, in the case of the present invention, the fuel assembly output of the control rod cell and the fuel assembly around the control rod cell are both In addition, the fuel assembly output at the center of the core is relatively lower in the present invention than in the former two cores, and the effect of flattening the radial power distribution is remarkable.

Furthermore, even when operating with control rods inserted during actual operation, the output of the control rod cells (γ or δ) in the peripheral area of the reactor does not decrease as much as in a core using one type of control rod cell.

Therefore, the effect of flattening the power distribution in the radial direction is most significant for the former two cores.

Figure 3 shows the enrichment of fuel assembly 1 shown in Figure 1 at 2.2w/o, and the enrichment of fuel assembly 2 at 1.8w/o.
o, the enrichment of fuel assembly 3 is 1.2w/o, 110
Operation plan (control rod arrangement (pattern)) for a 10,000 KWe class boiling water reactor for the period (called a cycle) from the initial stage of combustion until the reactor shuts down for fuel replacement.
) is shown for 1/4 core,
Figure 3 a, b, c, d, e, f, g, h, and i are burnup levels of 0 GWd/t and 0.2 GWd/t, respectively.
1.5GWd/t, 3.0GWd/t, 4.5GWd/t,
6.0GWd/t, 7.5GWd/t, 9.0GWd/t,
This is the control rod pattern at 10GWd/t.

Note that Figure 3 shows 1/4 of the reactor core, and the remaining 3/4 is omitted. Also, one □ indicates a unit cell that accommodates 4 fuel assemblies around 101 control rods, and the number shown in this unit cell indicates the rate at which the control rods are withdrawn. Full withdrawal, 0
indicates full insertion. Furthermore, unit cells without numbers indicate complete extraction.

Figure 4 shows the change in control rod density under the above operation plan. M is the control rod cell in the center, N
represents control rod cells at the periphery. As shown in Figures 3 and 4, control rods from two types of control rod cells are used at the beginning and middle of the cycle, but near the end of the cycle, only the control rods from the central control rod cell are inserted. All control rods have been pulled out.

Figure 5 focuses on the maximum linear power density (the allowable maximum linear power density in this example is 13.4KW/ft) in the thermal limit values when the operation plan shown in Figure 3 is adopted. It is something.

Here, in Fig. 5, the horizontal axis shows the average burnup of the core in one cycle (GWD/t), and the vertical axis shows the maximum power density of the fuel assembly (KW/ft). This shows an example of the effect when two types of cells are used. P, Q, and R in FIG. 5 correspond to fuel assemblies 1, 2, and 3 in FIG. 1, respectively. As can be seen from Figure 5, the power densities Q and R of the control rod cells are lower than P, and even if only a specific control rod is operated for a long period of time, sufficient thermal power can be maintained without impairing the fuel integrity. This shows that you can drive comfortably.

As described above, according to the present invention, the power distribution in the radial direction of the reactor core (fuel assembly power distribution) is kept flat throughout the cycle, and the control rods to be used during operation are identified. By gradually withdrawing the control rods to adjust the reactivity of the core toward the end of the cycle, it is possible to adjust the reactivity of the reactor core without having to change the pattern of the control rods, which was previously necessary. This significantly reduces the output drop when changing the hot control rod pattern, making a large contribution to improving the capacity utilization rate.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of a reactor core schematically showing an embodiment of the present invention, Fig. 2 is an explanatory diagram of the effects of the present invention, and Fig. 3 is a control rod pattern in an operation plan to which the present invention is applied. FIG. 4 is a diagram showing the change in control rod density with respect to burnup, and FIG. 5 is a diagram illustrating the effect of the present invention. 1... Fuel assembly, 2... Fuel assembly, 3...
Fuel assembly, 10...control rod.

Claims (1)

[Claims] 1. A control rod, and 4.
In a nuclear reactor in which the control rods are arranged in approximately one fuel assembly for each of the four fuel assemblies, and the core is arranged in the main part, the fuel assembly There are three or more types of enrichment of uranium-235 stored in the reactor, and the area around the control rods that are not operated to change the pattern throughout the reactor operation period is composed of the first fuel assembly with the highest enrichment, and the reactor The area around the control rods, which are operated to change the pattern throughout the operating period and are close to the central axis of the reactor core, is composed of a third fuel assembly with the lowest enrichment. A nuclear reactor characterized in that the area around the control rods near the periphery is constituted by a second fuel assembly having a higher enrichment than the third fuel assembly. 2 The enrichment level A of the first fuel assembly, the enrichment level B of the second fuel assembly, and the enrichment level C of the third fuel assembly are characterized by a relationship of A>B>C. A nuclear reactor according to claim 1.