JPS6260676B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for replacing control rod patterns in a boiling water nuclear reactor without shutting down the reactor.

Generally, during operation, every other control rod in each row and column of a boiling water reactor is inserted into the reactor core to control the core's output, and the control rods are controlled by a combination of the amount of control rods withdrawn and inserted. Several patterns are available. In order to equalize the combustion of each fuel assembly, a so-called on-power control rod pattern exchange is performed during operation of the reactor, changing the control rod pattern from the pattern shown in Figure 1 to the pattern shown in Figure 2. It can be done. In addition, the numbers at each position in Figures 1 and 2 indicate the number of notches on the control rod at that position when the period from full insertion to full withdrawal of the control rod is divided into 48 notches. The position indicates that the number of notches on the control rod at that position is 48, that is, the control rod is fully withdrawn. Conventionally, in order to perform such a control rod pattern exchange, for example, as shown in Fig. 1, the control rods B ₁ . Insert the control rods up to the number of notches corresponding to the pattern after replacement as shown in Fig. 2, then pull out the control rods B ₂ ... in other mirror-symmetrical positions, and then remove the control rods in the fully withdrawn state. The control rod pattern was replaced by inserting the number of notches corresponding to the pattern after replacement, and then repeating this operation. However, in this method, a plurality of control rods are simultaneously pulled out or inserted when changing the control rod pattern, resulting in large fluctuations in the output of the reactor. In particular, when replacing control rod patterns, the fluctuation of Xe, which is a neutron absorbing substance, is large, resulting in large output fluctuations, and in order to compensate for this, it is necessary to operate other control rods or adjust the core flow rate. arise. For this reason, it was troublesome to replace the control rod pattern, and it was difficult to maintain the output during the replacement work, causing problems such as interfering with the power supply plan. In addition, in such a method, for example, in section C of Fig. 1, a control rod located at the lower right of the four control rods B around the four fuel assemblies D as shown in Fig. 3 is used. When control rod _B2 with 28 notches is pulled out, only one control rod B is inserted around these four fuel assemblies D, as shown in Figure 4, and the output of this part is increases locally, resulting in uneven power distribution and impairing the integrity of the fuel. Additionally, there were problems such as sudden changes in the output signal of the in-core movable neutron detector E located at the center of these four fuel assemblies D, making the control of the reactor unstable.

The present invention has been made based on the above circumstances, and its purpose is to create a control rod pattern that is simple in process, has little fluctuation in reactor output when changing patterns, and does not impair the integrity of the fuel. It's about finding a way to exchange.

The present invention will be explained below with reference to an embodiment shown in FIGS. 5 to 21. First, FIG. 5 shows the control rod pattern before the core 1 is replaced, and FIG. 6 shows the control rod pattern after the replacement. Note that the numbers in the figure indicate the number of notches on the control rod 2, and the control rods with no numbers indicate that the number of notches is 48, that is, the control rod is in a fully withdrawn state. First, in this method, the pattern exchange of the control rods 2 is performed sequentially along the rows or columns of the core 1, starting from the end of each row or column.
The choice of whether to perform pattern exchange along a row or along a column is made when the control rod that was deeply inserted before the pattern exchange, that is, the control rod with a small number of notches, is pulled out in the row direction. If control rods (48 notches) are inserted deeply after pattern exchange, pattern exchange is performed along the row, and the control rods in the withdrawn state located in the column direction of the control rods that were deeply inserted before pattern exchange are When inserting deeply after pattern exchange, pattern exchange is performed along the row. The reason for this selection is to minimize the amount of adjustment when adjusting the insertion amount of the control rods in accordance with the control rod pattern after replacement, as will be described later, to suppress fluctuations in the output distribution. That is, in a nuclear reactor, the output is generally low at the end of the core because neutrons leak to the sides, and the output increases toward the center of the core. For this reason,
Even if the amount of control rod operation is the same, the change in power at the end of the core is smaller than the change in power at the center of the core. Therefore, when control rod operations are started from the end of the reactor core, by the time the control rods in the center of the core are operated, the control rods have already been inserted into the adjacent position, so This means that output fluctuations during pattern exchange can be suppressed to a lower level than when operating the central control rod. Therefore, in this embodiment, as can be seen from FIGS. 5 and 6, pattern exchange starts from the left end of the bottom row.

First, the movement state of the control rod is shown as 0 notches when fully inserted into the reactor core, and 48 notches when fully withdrawn, and control rods are generally configured to move in increments of 2 notches. In FIGS. 7 to 18, the numbers in the control rods 2a to 2g shown in blocks indicate how many notches have been withdrawn from the core, and blank blocks indicate the fully withdrawn state. 7 is the initial state, and the control rods to be pulled out are 2b, 2d, and 2f. And control rod 2
The control rods a and 2g must be inserted deeper than the control rods 2b and 2f next to them, and the control rods 2c and 2g must be inserted deeper than the control rods 2b and 2f next to them.
The rod e is inserted deeper than the control rods 2b, 2d, and 2f next to it. From this, first,
As shown in Fig. 8, the control rod 2c needs to be inserted deeper than the control rods 2b and 2d on both sides, so the control rod 2c moves less than 16 notches into the reactor core, and the control rod moves in increments of 2 notches. Less than 14 notches have been pulled out from inside the core. Since the control rods are not fully inserted during normal operation, taking these into consideration, the control rods 2c are placed within the core within a range of 2 to 14 notches. For example, a state of 12 notches is shown here as being within this range.

Thereafter, using the method described above, a value between 2 and 26 notches is selected in order to insert the control rod 2a to the left of the control rod 2b shown in FIG. 9 deeper than the insertion position 28 notches of the control rod 2b. , here, as an example, it is configured to insert up to a value of 24 notches. Next, as shown in FIG. 10, the control rod 2b having 28 notches is pulled out. Next, the control rod 2a inserted on the left side
The control rod 2a is adjusted in accordance with the pattern to be replaced, but since the control rod 2a should be in the withdrawn state in the pattern after replacement, the control rod 2a is adjusted in the fully withdrawn state as shown in FIG. Pull it out until
The next control rod to be pulled out is control rod 2d, which has 16 notches and is located in the center of this row.
Since the control rod 2c to the left of d has already been inserted, the control rod 2e to the right of the 16-notch control rod 2d is inserted up to 12 notches, as shown in FIG. Next, as shown in Figure 13, the number of notches is 16.
Pull out the control rod 2d. Next, the number of notches of the control rod 2c to the left of the control rod 2d that has been pulled out is adjusted according to the pattern after replacement, and in this embodiment, the number of notches on the control rod 2c should be 18. Therefore, as shown in FIG. 14, six more notches are pulled out to make the number of notches 18. The next control rod to be pulled out is the control rod 2f with 28 notches located on the right side of this row, but since the control rod 2e to the left of this control rod 2f has already been inserted, this control rod 2f is Connect the control rod 2g on the right as shown in Figure 15.
Insert up to 24 notches. Next, as shown in FIG. 16, the control rod 2f having 28 notches is pulled out. Next, as shown in FIG. 17, the control rod 2g on the right of the pulled out control rod 2f is pulled out. Next, the number of notches on the inserted control rod 2e on the left side is adjusted according to the pattern after replacement, but in the case of this embodiment, the number of notches is 18, so as shown in FIG. 18, the number of notches is adjusted. Pull out 6 notches to make 18 notches.
As described above, the control rod pattern of this bottom row is replaced, and the control rod patterns of the other rows are replaced in the same manner as above. When replacing the control rod pattern using the method of this embodiment, the control rods on both sides of the control rod are inserted before the control rod to be pulled out, so there is little fluctuation in the reactor's output, and local output There is nothing that will lead to an increase. That is, in the initial state of insertion of the control rods for section F in FIG. 5, as shown in FIG. 20, the four control rods 2 located around the four fuel assemblies 3...
The book is fully drawn out, and the other two have 42 and 28 notches, respectively. Since the control rods 2a and 2c on both sides are inserted before the control rod 2b with 28 notches is pulled out, the control rods 2b with 28 notches are inserted as shown in FIG.
Even if the control rod 2a is pulled out, the control rod 2a next to it on the left is inserted, and the four fuel assemblies 3 around the four fuel assemblies 3...
Two of the control rods 2 are in the inserted state.
Therefore, a local increase in the output of this part is prevented, and sudden changes in the output signal of the in-core movable neutron detector 4 placed in the center of these four fuel assemblies 3 are also prevented. This allows the furnace output to be maintained stably. Note that FIG. 19 shows the results of an experiment conducted to confirm the effect of such control rod pattern exchange. In the figure, the solid line shows the fluctuation in the reactor output when changing the control rod pattern according to an embodiment of the present invention, and the broken line shows the fluctuation in the reactor output in the case of the conventional method. As is clear from these results, according to one embodiment of the present invention, fluctuations in reactor output during control rod pattern exchange can be significantly reduced compared to the conventional method.

As described above, in the present invention, if a control rod in a withdrawn state located in the row direction of the control rods that was deeply inserted before the pattern exchange is inserted deeply after the pattern exchange, the pattern exchange is performed in each row along the row direction of the reactor core. If the control rods in the withdrawn state are inserted deeply after the pattern exchange, the pattern exchange is performed from the end of the reactor core in the column direction of the control rods that were deeply inserted before the pattern exchange. The control rods in the withdrawn state located on both sides of the control rods to be withdrawn from the core at the time of pattern exchange are inserted deeper than the control rods to be withdrawn, and the control rods to be withdrawn after insertion of the control rods are inserted deeper than the control rods to be withdrawn. is pulled out from the core, and the amount of control rods to be inserted after the control rods are pulled out is adjusted in accordance with the control rod pattern after replacement. Therefore, even if a control rod is pulled out, the increase in reactor output can be suppressed by the inserted control rods on both sides, and local output fluctuations will not occur. Furthermore, when adjusting the control rod insertion amount to correspond to the control rod pattern after replacement, the amount of adjustment can be minimized to suppress power distribution fluctuations, so the reactor output when the control rod pattern is replaced can be reduced. The effects are great, such as being able to maintain stability.

[Brief explanation of the drawing]

Figures 1 to 4 show conventional examples, Figure 1 is a control rod pattern diagram before replacement, Figure 2 is a control rod pattern diagram after replacement, and Figures 3 and 4 are parts at the time of pattern replacement. Fig. 3 is a typical control rod pattern diagram. 5 to 21 show an embodiment of the present invention, and FIG. 5 is a control rod pattern diagram before replacement;
FIG. 6 is a control rod pattern diagram after replacement, FIGS. 7 to 18 are control rod pattern diagrams showing the pattern replacement procedure in one row, and FIG. 19 is a comparison between an embodiment of the present invention and a conventional example. Figures 20 and 21 are partial control rod pattern diagrams at the time of pattern exchange. 1... Core, 2, 2a... 2g... Control rod, 3... Fuel assembly.

Claims (1)

[Claims] 1. When the control rod pattern of a boiling water reactor is replaced without stopping the operation of the reactor, the control rods in the withdrawn state located in the row direction of the control rods that were deeply inserted before the pattern replacement are removed after the pattern replacement. If the control rods are inserted deeply, pattern exchange is performed from the end of each row along the row direction of the reactor core, and the control rods in the withdrawn state located in the column direction of the control rods that were deeply inserted before the pattern exchange will be inserted deeply after the pattern exchange. When inserted, the pattern exchange is performed from the end of each row along the column direction of the reactor core, and the control rods in the withdrawn state located on both sides of the control rods to be pulled out from the core at the time of pattern exchange are replaced by the control rods to be pulled out. The control rods that should be inserted deeper than the control rods and pulled out after the control rods are inserted are pulled out from the reactor, and the insertion amount of the control rods that should be inserted after the control rods are pulled out is adjusted in accordance with the control rod pattern after replacement. A control rod pattern exchange method characterized by: