JPH0445796B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for starting up a boiling water reactor (BWR), and in particular to a method for increasing power in a power increasing method that combines control rod manipulation and core flow rate adjustment. Concerning a BWR startup method suitable for shortening time.

[Background of the invention]

In BWR operation, from the perspective of maintaining fuel integrity, PCIOMR operation is carried out in which a so-called "breaking-in operation" is performed to avoid sudden increases in fuel rod output when output increases. PCIOMR is designed to avoid sudden spikes in output.
It stipulates the threshold value (P _CR ) of increase in fuel line power density due to control rod operation and the rate of change in fuel line power density per unit time (P〓) after reaching this threshold.

PCIOMR operation, which performs a break-in operation, is an effective operation method for maintaining fuel integrity, but it takes a considerable amount of time to increase output and has the disadvantage of slightly lowering the reactor load factor. In other words, in BWR, the void distribution occurs in the axial direction, so the power distribution tends to be biased downward, and as a result, the core power that reaches the above threshold P _CR of the fuel line power density at reactor startup becomes a low value. The output increase rate P〓
Because of this, it took a considerable amount of time to increase the output.

In recent years, in consideration of such void distribution characteristics, improved fuels have been proposed that flatten the power distribution by creating high and low reactivity levels in the upper and lower regions of the fuel assembly (Japanese Patent Laid-Open Publication No.
53-40188 "Fuel assembly"). Adoption of this improved fuel eliminates the need for power distribution adjustment using shallowly inserted control rods during normal output operation, and allows operation with thermal margins using only deeply inserted control rods.

However, in the low power state during the power increase at startup, the amount of xenon (Xe) accumulated is not sufficient, and the core power is relatively high compared to the core flow rate, so the power distribution expands downward due to the effect of the void distribution. Conventionally, the accumulation of Zenon is promoted by using many shallowly inserted control rods, or by stopping control rod operation at low power and increasing the power only by adjusting the core flow rate. A procedure is adopted in which the process of operating the control rods is repeated several times (so-called break-in operation) to reach a predetermined output.

In other words, in order to keep the fuel line power density due to control rod operation below the threshold P _CR , control rod operation when power increases is usually performed at the lowest core flow rate due to forced circulation. Since the core power is relatively high compared to the core flow rate, resulting in a downward bulge in the power distribution, conventionally, in order to compensate for this, prior to configuring the target control rod pattern to achieve the rating, so-called It was necessary to perform a sufficient break-in operation.

This made the operation complicated and required a considerable number of days to increase the output at startup.

[Object of the Invention] In view of the above drawbacks of the conventional startup method, the object of the present invention is to avoid the downward expansion of the core power distribution in the region where the core power is relatively high relative to the core flow rate, and shorten the power rise time. The objective is to provide a method for starting BWR that can be used.

[Summary of the invention]

Fission products are commonly used in BWR startups. Since there is no xeno (Xe), it is necessary to accumulate it in the appropriate place in the core as quickly as possible. need to be increased as soon as possible.

However, in a BWR, the control rods are generally withdrawn from above the core to below, so the peak of the power distribution also tends to move downwards as the control rods are withdrawn.
Moreover, at startup, the control rod withdrawal operation is generally performed in the lowest core flow rate state due to forced circulation as described above, so the core power becomes relatively high relative to the core flow rate, and the core power distribution becomes more downward due to the effect of the void distribution. It will expand to.

In view of the above points, the BWR startup method according to the present invention is to fully insert the outermost control rod or adjacent control rods that are fully withdrawn in the rated target control rod pattern, and to fully insert the outermost and adjacent control rods. Among the control rods other than the adjacent control rods, control rods that are not fully withdrawn in the rated target control rod pattern are set to intermediate control rod patterns with the same or nearby withdrawal position as in the rated target control rod pattern. When a pattern is adopted and the output is increased, first, by operating the control rods that form the intermediate control pattern, the core output is brought to the threshold of the specified fuel line power density at the minimum core flow rate in forced circulation or the core flow rate in the vicinity. The intermediate control rod pattern is completed at the point when the core output reaches the specified value, and after the completion of the intermediate control rod pattern, the core flow rate is increased while the control rod pattern remains fixed. Then, while keeping the core flow rate fixed, the control rod pattern is moved towards the target control rod pattern to reach the rating, while observing the specified fuel line power density change rate limit value only by withdrawing the control rods. This method is characterized in that the target control pattern for reaching the rated rating is completed through the process of further increasing the core power.

[Embodiment of the Invention] An embodiment of the BWR starting method of the present invention will be described below.

FIGS. 2a and 2b show the rated target control rod pattern and intermediate control rod pattern, respectively, in this embodiment. The numbers written in the diagram indicate that when the total stroke of the control rod is 48, the control rod has been pulled out from the fully inserted position by the stroke corresponding to that number. For example, the number 10 indicates that the control rod has been pulled out from the fully inserted position by The number 0 represents full insertion and the blank represents full removal. Total withdrawal is represented by the number 48,
It is shown as a blank to simplify the illustration.

As shown in Figure 2a, in the rated target control rod pattern, the outermost control rods and adjacent control rods are fully withdrawn, and several other control rods (in a symmetrical arrangement) are fully withdrawn. However, the others are completely removed. In addition, as shown in Figure b, in the intermediate control rod pattern, multiple control rods (in a symmetrical arrangement) at or adjacent to the outermost periphery are fully inserted; Among the other control rods, the plurality of control rods that are not fully withdrawn in the rated target control pattern are at the same extraction position as in the rated target control pattern,
The other control rods are fully withdrawn, similar to the rated target control rod pattern.

For comparison, FIG. 3 shows an intermediate control rod pattern employed in a conventional example in which the rated target control pattern is the same as that of this embodiment. One of the essential differences between the intermediate control rod pattern of the embodiment of the present invention (Fig. 2b) and that of the conventional example (Fig. 3) is that in the former, control rods other than the outermost control rod and adjacent control rods are The difference is that several of these control rods are in the same extraction position as the rated target control rod pattern, whereas in the latter they are fully inserted.

FIG. 4 is a graph showing the time course of the power increase at startup, and FIG. 5 is a graph showing this in terms of the relationship between core flow rate and core output (core flow rate - core output map). In Figure 5, 7 is the minimum core flow rate (approximately 20% of the rated value) line due to forced circulation, 8 is the line that provides 45% core flow rate due to forced circulation, and 9 is the line that provides the core flow rate of 45% due to forced circulation.
The APRM rod block line (control rod withdrawal prohibition limit line), 10 represents the 100% core flow control line, and 11 represents the minimum output line due to forced circulation characteristics.

In FIGS. 4 and 5, the solid line graph shows the case of this embodiment, and the broken line graph shows the case of the conventional example. In addition, the areas with many small horizontal lines on the graph represent the areas where control rod operations are performed, and the areas without lines represent areas where core flow rate adjustment is performed.

Horizontal sections 2' and 5' in FIG. 4 indicate power maintenance by control rod operation or core flow rate adjustment.

In this example, first, from point a, the control rods are operated to form the intermediate control rod pattern under the minimum core flow rate due to forced circulation, thereby achieving the prescribed fuel line power density as described above. threshold
The core power is increased to point b, where the core power reaches P _CR , and the intermediate control rod pattern is completed at point b. Next, from point b, by increasing the core flow rate while maintaining the intermediate control rod pattern, point c is reached, and from point c, the fuel line power density is fully inserted into the intermediate control rod pattern while observing the specified fuel line power density change rate P〓. By pulling out a certain outermost control rod or the adjacent control rod and operating the control rods again to form the target control rod pattern to reach the rating, point d on the 100% core flow control line 10 is reached, where the rated The target control rod pattern is completed. Thereafter, the rated output point e is reached by adjusting only the core flow rate while maintaining the rated target control rod pattern.

On the other hand, in the conventional example, by operating the control rods that form the intermediate control rod pattern (Fig. 3), at the lowest core flow rate of forced circulation, from point a to point b' where the fuel line power density threshold value P _CR is reached. The core power is increased to point C', and then the power is increased to point c' by adjusting the core flow rate while maintaining the intermediate control rod pattern, and then the power is decreased to point d' to promote Zenon accumulation.
Thereafter, the core power is increased from point d' to point b'' (the core power level must still be below the fuel line power density threshold P _CR mentioned above) by operating the control rods again, and then the rated power is reached. The formation of the target control rod pattern is completed, and the rated point e' is then reached only by adjusting the core flow rate.

According to the embodiment of the present invention, by adopting the above-mentioned intermediate control pattern, the power above the core is increased and the power distribution is flattened in a low power state in the middle of power increase, compared to the conventional example. As a result, the core power level at point b can be made relatively high, reaching the core power that becomes the fuel line power density threshold P _CR at the minimum core flow rate line 7 of forced circulation, and then the core power level at point b can be made relatively high. The process of completing the target pattern can be shortened and simplified. On the other hand, in the conventional example, the core power level at point b' where the threshold value P _CR is reached in the forced circulation minimum core flow line 7 is kept relatively low, and then the target control rod finally reaches the rating. Before reaching point b'', where the pattern is completed, it is necessary to perform a so-called break-in operation in which the core power is increased to point c' and decreased to point d', which is complicated and takes a long time.

FIG. 6 shows the end point b of the control rod operation section 1, which is the completion of the intermediate control rod pattern of this embodiment (FIG. 2 b), and the intermediate control rod pattern of the conventional example (the third
Fig. 12 shows the average relative power distribution in the core axis direction at the end point b' of the control rod operation section 1' when the control rod operation section 1' is completed, and also shows the power distribution when the target control rod pattern to reach the rating is completed. From this figure, the effect of flattening the power distribution due to the intermediate control rod pattern of the embodiment of the present invention can be clearly seen.

In the embodiment of the present invention, in order to avoid the conventional point b'' where the power distribution tends to swell downward, as described above, the control rods are first operated to reach point b, then the core flow rate is controlled from point b to point c, and then the control rods are operated to reach point b. The rated output is reached by increasing the output from point c to point d through rod operation and then from point d to finally point e through flow control.In this case, the control rod operation from c to d is performed by controlling the intermediate control rod. It is possible to increase the output by pulling out the outermost control rod from the pattern or the control rod adjacent to it, and it is possible to increase the output while complying with the specified output increase rate P〓 _l.In other words, by pulling out the outermost control rod or the control rod adjacent to it Since the increase in the linear power density of the fuel adjacent to the control rods can be suppressed to below the threshold power P _CR , it is sufficient to increase the power by suppressing the increase in the overall power of the core to below _P〓l .

The number of control rods to be fully inserted among the outermost control rods in the intermediate control rod pattern or adjacent to it is:
It is determined by the core characteristics of the target reactor, but generally speaking, inserting 20 tubes will give about 20% of the rated output, so the number of tubes to be inserted can be determined using this as a guide. It is generally desirable that there be four or more, and that they are arranged symmetrically with respect to the reactor core.

Pull out all the inserted control rods at or adjacent to the outermost circumference of the intermediate control rod pattern toward the target control rod pattern to achieve the rating (from point c to d in Figures 4 and 5).
(transition to a point) preferably using notch withdrawal. In the notch extraction, they may be pulled out in sets of about four, from full insertion to full withdrawal, or all of them may be pulled out so that the extraction positions are aligned.

In addition, it is preferable that the range of increase in core flow rate for the transition from point b to point c be approximately 10% or more of the rated core flow rate, and this 10% increase in core flow rate will improve the axial power peaking by approximately 3%. Ru.

FIG. 7 shows the power increase according to another embodiment of the present invention as a core flow rate-core power map.
In this embodiment, the power increase from the power increase at the lowest core flow rate line 7 due to forced circulation to the completion of the rated target control rod pattern on the 100% core flow rate control line 10 is determined from the outermost or This is performed in multiple stages by alternately performing the withdrawal operation of the adjacent fully inserted control rods and increasing the core flow rate. In the figure, 12 indicates the 30% core flow line. The amount of increase in core flow rate at each stage is preferably about 10% or more of the rated flow rate. In this example,
A power increase method that avoids as much as possible the area near the intersection of the minimum core flow rate line 7 due to forced circulation and the 100% core flow rate control line 10, which may become unstable, and also alleviates the stepwise increase in power due to control rod operation. It is.

Figure 8a shows that in the intermediate control pattern of the above embodiment, among the control rods other than the outermost and adjacent control rods, the control rods that are not fully drawn out in the rated target control pattern are in the rated target control rod pattern. Intermediate control is a control rod pattern in which the withdrawal position is the same as or near the withdrawal position in Regarding another embodiment of the present invention adopted as a bar pattern, the power distribution in the core axial direction at low flow rate and low power is shown. The left half of the figure schematically represents the positions of these control rods. According to this embodiment, the output distribution can be made even more flat.

For comparison, Figure 8b shows an intermediate control rod pattern in which some of the controls other than the outermost and adjacent ones are fully inserted and the remaining ones are fully withdrawn (as shown in Figure 3). The figure shows the power distribution in the axial direction of the reactor core in the case of a conventional example that adopts a similar pattern. The left half of the figure schematically shows these control rods.

Comparing FIG. 8a with FIG. 8b, it is clear that in this example, the core power distribution at low core flow rate and low core power during power increase is significantly flattened.

〔Effect of the invention〕

According to the BWR startup method of the present invention, compared to conventional startup methods, it is possible to avoid the region where the core power swells downward during the power increase, and to maintain the core flow rate close to the minimum core flow rate in forced circulation. It is possible to make the core power level relatively high at the point when the fuel line melt density threshold is reached, and also to avoid the minimum core flow rate line due to forced circulation, which may lead to instability.
While avoiding the area near the intersection with the 100% core flow control line, it is possible to shorten the time required to increase the output, and it is possible to shorten the start-up time by approximately 3 days from the time the generator is installed to reaching the rated output. be. Moreover, the BWR startup method of the present invention uses the same fuel line power density threshold P _CR and subsequent fuel line power density change rate as in the past.
It is possible to achieve the above effects while complying with the provisions of P〓.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the accumulation characteristics of Zenon as the ultimate output parameter, FIGS. 2 a and b are diagrams showing the rated target control rod pattern and intermediate control rod pattern, respectively, in an embodiment of the present invention. FIG. 3 is a diagram showing the intermediate control rod pattern of the conventional example, FIG. 4 is a graph showing the time course of the core power according to an embodiment of the present invention and the conventional example, and FIG. FIG. 6 is a graph showing the flattening effect of the core power distribution of the embodiment of the present invention in comparison with the conventional example, and FIG. 7 is a graph showing the power increase due to another embodiment of the present invention. The core flow rate-core power map, FIGS. 8a and 8b, is a diagram showing the core power distribution in another embodiment of the present invention and a conventional example, respectively.

Claims (1)

[Scope of Claims] 1. In the rated target control rod pattern, the outermost control rod or the adjacent control rods that are fully withdrawn are fully inserted, and the control rods other than the outermost control rod and the adjacent control rods are fully inserted. For the control rods that are not fully withdrawn in the rated target control rod pattern, the intermediate control rod pattern is set to the same or nearby withdrawal position as in the rated target control rod pattern, and when the output increases, First, by operating the control rods that form the intermediate control pattern, the core power is increased to the core power that reaches the specified fuel line power density threshold at the minimum core flow rate in forced circulation or a core flow rate close to it. The intermediate control rod pattern is completed when the core output reaches the core output, and after the intermediate control rod pattern is completed, the core output is further increased only by increasing the core flow rate while keeping the control rod pattern fixed. process, and then a process in which the core power is further increased while observing the specified fuel line power density change rate limit value by simply withdrawing the control rods toward the target control rod pattern to achieve the rating while keeping the core flow rate fixed. A method for starting a boiling water reactor characterized by completing a rated target control pattern through the following steps. 2. In the intermediate control rod pattern, among the control rods other than the outermost control rods and adjacent control rods, the control rods that are not in the specified extraction position are extracted by more than 75% of the control rod stroke. A method for starting a boiling water nuclear reactor as claimed in claim 1 in a position. 3. Claims 1 or 2, wherein the withdrawal operation of the control rods from the completion of the intermediate control rod pattern to the completion of the rated target control rod pattern is performed in multiple stages alternately with the increase operation of the core flow rate. The described method for starting a boiling water reactor. 4. The method for starting a boiling water reactor according to claim 1, 2, or 3, wherein the range of increase in core flow rate in the core flow rate increase operation is 10% or more of the rated core flow rate.