JPH0439639B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to the startup of nuclear reactors, particularly light water boiling nuclear reactors.

[Background of the invention]

When operating a nuclear reactor, so-called PCIOMR operation is used, which is a method of running-in to avoid sudden increases in control rod output in order to maintain fuel integrity. In PCIOMR operation, in order to avoid a sudden increase in output, the threshold value (P _CR ) of the linear power density of fuel due to control rod operation and the rate of change per unit time (P) of the subsequent increase in output are specified. Although this is an effective operating method for maintaining the integrity of the reactor, the load factor of the reactor will decrease slightly.

In other words, in light water boiling reactors, void distribution occurs in the axial direction, and this effect tends to distort the power distribution downwards. Conventionally, the fuel linear power density threshold (P _CR ), resulting in the reactor output being suppressed to a low value, and since the subsequent output increase rate P is also specified, it takes time for the output to increase.

On the other hand, for example, Japanese Patent Application Laid-open No. 53-40188 proposes a fuel that flattens the power distribution by adding high and low reactivity levels to the top and bottom of the fuel assembly in consideration of void distribution characteristics. This eliminates the need to adjust the power distribution 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 power increase at startup, the accumulation of xenon (Xe) is not sufficient and the power distribution expands downward due to the effect of void distribution, so a large number of shallowly inserted control rods are used. Or, the control rod operation is stopped at low output, and the output is increased by adjusting the flow rate.
In order to promote Xe accumulation, a method was used in which the operation was repeated several times to reach a predetermined output.

For this reason, control rod operations were complicated and it took a long time to start up the system.

[Purpose of the invention]

The present invention flattens the output distribution during output increase,
The purpose of this study is to provide a method for starting a light water boiling nuclear reactor that can shorten the output rise time.

[Summary of the invention]

The present invention provides a method for starting a light water boiling nuclear reactor in which fuel assemblies loaded in the reactor are started by operating control rods. Four of the control rods are fully inserted or close to it, and the neutron absorbing parts of more than half of the control rods inserted into the reactor core have a control rod pattern in which more than 2/3 of the total length is inserted into the reactor core. It is characterized by an intermediate control rod pattern.

When a nuclear reactor starts up, there is generally no fission product Xe, so it is necessary to accumulate this Xe in an appropriate location within the reactor as quickly as possible. Generally, control rods are withdrawn from the upper part of the reactor to the lower part, so the peak of the power distribution also shifts to the lower part of the reactor core. In addition, due to the void distribution and unaccumulated Xe, the power distribution peaks downward, especially when the control rods are withdrawn from the lower part of the core.

For this reason, the axial power distribution is flattened.
It is necessary to increase the output corresponding to the threshold output R _CR of PCIOMR operation as high as possible to accumulate Xe in the entire core.

Figure 1 shows the relationship between time and Xe concentration using the achieved output as a parameter, with time (hr) and Xe concentration plotted on the horizontal and vertical axes, respectively.
The achieved output increases in the order of A ₁ , A ₂ , and A ₃ ,
Xe has the characteristic that the higher the output, the faster it accumulates.

FIGS. 2 and 3 schematically show a method for flattening the axial power distribution, and show the relationship between the control rod position and the relative power distribution. FIG. 2 shows the axial distribution when the shallowly inserted control rods are withdrawn with priority, and the axial power distribution has a downward bulge. On the other hand, Figure 3 shows the use of shallowly inserted control rods and the appropriate amount of withdrawal of deeply inserted control rods, i.e., at low power when only 0 to 1/4 of the total length of the neutron absorption part of the control rod is pulled out of the reactor. The axial power distribution is flat, indicating that the downward bulge in the axial power distribution at low power can be flattened by control rod operation. Therefore, by withdrawing the control rod by an appropriate amount, the output level reaching _PCR can be increased. As shown in FIG. 3, since the deeply inserted control rods are also withdrawn, the output level is adjusted by the insertion amount of the outermost control rods.

Additionally, in addition to flattening the power distribution, the reduction in power due to the insertion of the outermost control rods is used to reduce the number of control rods inserted inside the reactor core, reducing the accumulation of Xe in areas where no control rods are inserted. This will reduce Xe deviation in the axial direction of the core.

In this way, the present invention makes it possible to start up a nuclear reactor by increasing output in a short period of time while maintaining fuel integrity.

[Embodiments of the invention]

Examples will be described below.

In the reactor of this example, the fuel assemblies loaded in the reactor are 1/3 and 7/12 of the total length from the lower end in the axial direction.
It is a light water boiling reactor that is divided into two regions, upper and lower, between the two regions, and the infinite effective multiplication factor in the upper region is higher than the infinite effective multiplication factor in the lower region.

Fig. 4 shows an example of the output increase at startup of one embodiment, with time (hr) and output (%) plotted on the horizontal and vertical axes, respectively, with 1 (solid line) indicating the embodiment and 2 (Dotted line) is the case of the conventional example, parts 1a, 2a, 1d, and 2d are output increase by control rod operation, 1b, 2b, 1e, and 2e are output maintenance by control rod operation or flow rate adjustment, 1c, 2c, and 1
f and 2f indicate an increase in output due to flow rate adjustment. 1a, 2a and 1d, 2d or 1b, 2b
Control rod operations at 1e and 2e are performed below _PCR .

FIG. 5 shows the accumulation of Xe when the output is increased along the output increase line in FIG. 4.
Time (hr) and Xe amount (relative value) are plotted on the horizontal and vertical axes, respectively.
shows the case of the conventional example. In this example, the output is increased while flattening the axial output distribution.
As the output that reaches R _CR , that is, the value corresponding to 1b in Figure 4, increases, the amount of accumulated Xe also increases,
Many control rods can be withdrawn before the output increases, that is, by the end point 1b in FIG. 4, and the control rods can be easily operated when forming the final target control rod pattern, that is, at the stage 1d in FIG. 4.

Figure 6 shows an example of the configuration of a control rod pattern used to carry out this reactor startup method, where a is the target pattern, b is the intermediate pattern 1, and c is the intermediate pattern 2. shows a conventional control rod pattern. In these figures, the numbers written in the squares representing each control rod represent the amount of withdrawal. For example, 0 indicates full insertion of the control rod, 10 indicates 10/48 withdrawal, and no entry indicates full withdrawal. . In FIG. 6b showing intermediate pattern 1, 9 excluding the outermost periphery
When the shallowly inserted control rods reach a state where 5/6 of their total length is withdrawn, or before that, the 21 deeply inserted control rods are pulled out of the reactor by 0 to 1/3 of the total length of their neutron absorption sections. FIG. 6c, which shows the control rod pattern and shows intermediate pattern 2, shows the control rod pattern with the deeply inserted control rod withdrawn to 8/48 or 10/48. This control rod pattern corresponds to the control rod pattern from the end point 1b in FIG. 4 to the intermediate control rod pattern 1d in FIG. 4. FIG. 7 shows a control rod pattern in a conventional nuclear reactor startup method for comparison.

Figure 8 shows the axial output distribution when the output is increased by such an intermediate pattern, with the horizontal and vertical axes representing the axial average relative output and axial height, respectively. C and D correspond to FIG. 6 a, b and c, respectively, and E corresponds to the conventional example of FIG. 7.

In the case of the embodiment in which the output is increased using the intermediate pattern, the axial output distribution is flattened as shown in Figure 8, and as a result, the output level that becomes _PCR becomes high, and the load at startup is rate loss can be reduced. On the other hand, in the conventional example, since the intermediate pattern is not used, the axial output distribution swells downward, so the output level that becomes R _CR becomes low and it takes time to start up.

The control rod inserted at the outermost periphery in intermediate pattern 2 in FIG. 6c is pulled out in response to the increase in output shown in FIG. 4 1d. Since the fuel output at the outermost circumference is low, it is difficult to withdraw control rods at the outermost circumference.
The _PCR threshold output is sufficient for extraction.
The target pattern shown in FIG. 6a is formed by withdrawing the outermost control rod and withdrawing the deeply inserted control rods to the target pattern. After forming the target pattern, the output is increased by adjusting the flow rate to reach a predetermined output as shown in 1f of FIG. 4.

Next, the control rod operations to reach the target pattern configuration will be explained based on the control rod withdrawal sequence.
FIG. 9a is an explanatory diagram of the control rod withdrawal sequence, showing the number of control rods withdrawn in each group at each step, with 48 indicating total withdrawal. Figure 9b shows the position of the control rods corresponding to the group numbers in Figure 9a.
~5). The control rod inserted at the outermost circumference is set as 3, and the other control rods are set as 1,
They are numbered 2, 4, and 5. FIG. 10 shows the state in which groups 1 and 2 of step 1 of the control rod withdrawal sequence of FIG. 9a have been withdrawn.

In the control rod withdrawal sequence shown in Figure 9a, groups 1 and 2 belonging to step 1 are pulled out first, then groups 1, 2 and 4 belonging to step 2
Control rod operations are performed by sequentially pulling out the deep-inserted control rods (groups 4 and 5) and shallowly-inserted control rods (groups 1 and 2). Control rods are operated by pulling out. FIG. 6b corresponds to the control rod withdrawal up to step 3 and group 5, FIG. 6c corresponds to step 6 and control rod withdrawal up to group 2, and step 10 and group 4
The control rod pattern shown in FIG. 6a corresponds to the control rod withdrawal up to.

In this example, the fuel assembly is divided into two regions, upper and lower, between 1/3 and 7/12 of the total length from the lower end in the axial direction, and the infinite effective multiplication factor in the upper region is the same as the infinite multiplication factor in the lower region. Although the results are shown for the case where the ratio is higher than the multiplication factor, it is also applicable to cases where other fuel assemblies are used.

As is clear from this example, when the power of the reactor increases, at least four control rods at or adjacent to the outermost periphery are fully inserted (note that the same effect occurs when the control rods are inserted to a state close to that). ,
In addition, the desired purpose can be achieved by setting a control rod pattern in which the neutron absorbing parts of most of the control rods inserted into the reactor core are inserted into the reactor core at least 2/3 of the total length as an intermediate control rod pattern. I know it will happen.
In addition, when control rods are further withdrawn beyond the state where 3/4 of the control rods in the core have been fully withdrawn, excluding the outermost control rods or the adjacent control rods, the outermost control rods or the adjacent control rods are excluded. 2/5 to 3/5 (approximately half) of the control rods inserted into the reactor core are located at or near the outermost periphery before being pulled out of the reactor by more than 5/6 of the total length of the neutron absorption section. When using an intermediate control rod pattern in which 2/5 to 3/5 (approximately half) of the control rods other than the control rods are pulled out of the reactor by 0 to 1/3 of the total length of the neutron absorption part, Particularly effective.

By operating the control rods in the manner described above, the output distribution in the axial direction is always kept flat, and the output can be increased by operating the control rods at a relatively high output, thereby reducing the number of days required to increase the output at startup.

〔Effect of the invention〕

The present invention flattens the output distribution during output increase,
This provides a method for starting a light water boiling nuclear reactor that can shorten the output rise time, and has great industrial effects.

[Brief explanation of drawings]

Figure 1 is a characteristic diagram showing the accumulation characteristics of Xe, Figure 2 is a characteristic diagram showing the accumulation characteristics of Xe.
3 and 3 are explanatory diagrams schematically showing a method for flattening the axial power distribution, and FIG. 4 shows a comparison of the power increase state of an embodiment of the nuclear reactor startup method of the present invention with the conventional method. 5 is a characteristic diagram showing the state of change in Xe amount corresponding to the power increase state shown in FIG. 4, and FIG. 6 is a control rod pattern of an embodiment of the reactor startup method of the present invention. Figure 7 is an explanatory diagram of the control rod pattern in the conventional nuclear reactor startup method, and Figure 8 is a characteristic diagram showing the power distribution when the control rod patterns of Figures 6 and 7 are used. , FIG. 9 is an explanatory diagram of a control rod withdrawal sequence in one embodiment of the nuclear reactor startup method of the present invention, and FIG. 10 is an explanatory diagram of a control rod pattern in another embodiment.

Claims (1)

[Claims] 1. In a light water boiling nuclear reactor startup method in which fuel assemblies loaded in the reactor are started by operating control rods, when the output of the reactor increases, the outermost control rod or the adjacent control rod However, at least four of the control rods are fully inserted or close to it, and the neutron absorbing parts of more than half of the control rods inserted into the reactor core are inserted into the reactor core at least 2/3 of their total length. A nuclear reactor startup method characterized by using a control rod pattern. 2. When the intermediate control rod pattern is further withdrawn beyond the state in which 3/4 of the control rods in the reactor core, excluding the outermost control rods or the control rods adjacent thereto, are fully withdrawn, the outermost control rod pattern or before 2/5 to 3/5 of the control rods inserted into the reactor core, excluding the adjacent control rods, are pulled out of the reactor at least 5/6 of the total length of the neutron absorption part. Other 2/5 excluding control rods on or adjacent to the periphery
2. The method for starting up a nuclear reactor according to claim 1, wherein the control rods have a pattern in which 0 to 3/5 of the control rods are pulled out of the reactor by 0 to 1/3 of the total length of the neutron absorption part. 3. The fuel assembly is 1/1/1 of the total length from the lower end in the axial direction.
3 and 7/12 into two upper and lower regions, and the infinite effective multiplication factor in the upper region is higher than the infinite effective multiplication factor in the lower region. How to start up a nuclear reactor as described in section.