JPH024875B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of operating a boiling water reactor (BWR), and in particular, a method of operating a boiling water reactor suitable for reactor startup and output change operation control such as pattern exchange. It is related to.

In BWR, from the viewpoint of preventing fuel rod damage due to interaction between fuel pellets and cladding, control rods should be withdrawn only when the linear power density of the fuel rods is below a certain threshold (hereinafter referred to as the threshold) or below. It is limited to operating below the linear power density (hereinafter referred to as envelope) that the fuel rods have already experienced. Furthermore, power increases above a threshold or envelope are achieved by increasing the core flow rate, and the rate of increase is also limited.

In order to comply with the above operating conditions,
In the conventional example disclosed in Japanese Patent No. 129290, an operating method is adopted in which the envelope is expanded during operation to increase the reactor output to the set output, which is the final target.
This envelope expansion requires all fuels to experience a high power level at least once, but since the rate of power increase through core flow control to achieve this high power level is limited as mentioned above, it takes a long time. I need it. For example, electrical output 784MW,
Power density 51KW/l, control rod withdrawal threshold
8KW/ft, output increase rate limit 0.06KW/ft/h
For BWR, the startup time is approximately 21 days.
In other words, the time required for startup and pattern exchange will be longer.
This is a cause of deterioration in reactor operation rate.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for operating a boiling water reactor that eliminates the drawbacks of the prior art described above, prevents fuel damage, and improves reactor operating efficiency.

The feature of the present invention is that the target control rod pattern further includes one
Operation with a control rod pattern in which one or more control rods are inserted into the periphery of the core is performed only in a core flow rate range of 50% or less of the core flow rate rated value.

The present invention was made in consideration of the following two facts. First, fuel failure is limited to fuel assemblies adjacent to operational control rods that experience linear power densities above a threshold or envelope. This is JP-A-51-
This is clear from what is shown in FIG. 6 of Publication No. 141990. In other words, the control rod 17C that is pulled out
The fuel rod at position a in the fuel assembly 29C adjacent to the control rod changes greatly from curve 33 to curve 34 near the tip of the control rod 17C, and the risk of damage to that fuel rod is extremely large. Therefore, the axial power distribution of the fuel rod at position b of the fuel assembly 29B, which is adjacent to the fuel assembly 29C and is slightly farther away than the control rod 17C, remains in the state of curve 32 even when the control rod 17C is withdrawn. The risk of breakage of the fuel rod is significantly smaller than that of the fuel rod in position a described above. Based on these facts, in regions with large power peaking, control rod operations should be performed at a time when the reactor power is low and the linear power density is low (the time when the linear power density remains below the threshold even after control rod withdrawal). After that, the reactor can be operated to increase the reactor output by withdrawing the control rods in a region where the output peaking is small. In other words, in this operating method, when the control rods are withdrawn in a region with small power peaking, there are fuel assemblies whose linear power density exceeds the threshold value from the perspective of the entire reactor core, as will be described later. The linear power density in the fuel assemblies adjacent to the rods can be below a threshold value, thereby eliminating fuel failure. The second problem is that neutron leakage is large around the core. In general, the linear power density is lower around the core, and if this is used, it is easy to pull out the control rods below the threshold (although the power peak when operating the control rods is generally larger).

Based on the above two considerations, in the boiling water reactor operating method according to the present invention, the control rods are first inserted into the periphery of the reactor core and the reactor output is reduced, and then the output peaking is reduced. Control rods are operated in the center of the large reactor core. This means that during the first start-up in the operating cycle after a new fuel assembly is loaded into the core, to prevent fuel failure, and after the reactor scram in the middle of the operating cycle when the envelope has formed. When restarting, the control rods inserted in the center of the reactor core are pulled out below the envelope, and the target control rod pattern is achieved in advance in the center of the reactor core. After that, all the control rods inserted around the core where the linear power density is low are pulled out to realize a target control rod pattern that can increase the reactor power to the final target set level. During this series of control rod operations, the fuel assembly adjacent to the operated control rod does not exceed the threshold value. Therefore, maintenance of fuel integrity can be expected.

In summary, the present invention provides an additional step to the target control rod pattern before pulling out the control rods to the target control rod pattern (the control rod pattern that is finally being achieved in the output increase operation of interest). Operation with a control rod pattern in which one or more control rods are inserted into the periphery of the core is performed only in a core flow rate range of 50% or less of the core flow rate rated value.

The present invention has an operation period with a control rod pattern in which control rods are inserted around the core, and is similar to the above-mentioned Japanese Patent Application Laid-Open No. 54-129290 (Japanese Patent Application No. 53-35492).
has the same characteristics. However, JP-A-54-
The invention disclosed in Publication No. 129290 involves increasing the reactor core flow rate to 60% or more of the rated value with control rods inserted around the core in order to expand the envelope in the process of increasing the reactor power to the final target set level. It is essential to drive safely. In contrast, the present invention does not aim at expanding the envelope as in the conventional example, and therefore operates with a control rod pattern in which control rods are inserted into the core periphery added to the target control rod pattern. It is unique in that it is carried out only in the core flow rate range below 50% of the rated value.
In this invention, the control rod withdrawal operation until the target control rod pattern is formed is performed at a rate of 50% of the core flow rate rated value.
% or less, and control rod withdrawal is not performed in the core region where the core flow rate exceeds 50% of the rated value, so it is possible to prevent fuel damage due to control rod withdrawal. . If a control rod around the core is pulled out when the core flow rate exceeds 50% of the rated value, there is a risk that the linear power density of the fuel assembly adjacent to the pulled control rod will exceed the threshold and cause damage. There is. In addition, the present invention provides a method for determining the rated value of the core flow rate when a control rod pattern in which control rods are inserted into the core periphery is added to the target control rod pattern.
There is no need for envelope expansion operations that increase core flow rate by more than 50%, significantly shortening the time required to increase reactor power to the set level, which is the ultimate goal.

The operating method of the BWR, which is a preferred embodiment of the present invention, has the same electric output of 784 MWe and power density as the conventional example.
51KW/l, control rod withdrawal threshold 8KW/ft,
An example of application to a BWR with an output increase rate limit of 0.06KW/ft/h will be explained.

The coordinate axes are the thermal output of the reactor and the core flow rate.
Figure 1 shows the trajectory during startup operation of the BWR. 1st
In the figure, the solid line shows the process of increasing thermal output during control rod operation, and the broken line shows the process of increasing thermal output during core flow rate control. Figure 2 shows the control rod pattern (1/4 core) at a typical point in time.
In FIG. 2, the numbers shown at the control rod positions indicate the rate of control rod insertion; 24 indicates full insertion, and 0 or blank space indicates full withdrawal. At startup of this embodiment, control rod withdrawal is performed at the minimum core flow rate (approximately 40% of the core flow rate rated value). That is, with the core flow maintained at the minimum core flow rate and the control rods inserted in the core periphery, the control rods in the center of the core, where the power peaking is large, are pulled out to achieve the control rod pattern shown in Figure 2a (Figure 1). Operating point 1). In other words, in this example, the control rods in the center of the core are withdrawn by a predetermined amount with the core flow rate at 40% and all the control rods inserted in the core periphery, so the linear power density is below the threshold of 8KW/ft. It is possible to realize a rated output control rod pattern (a control rod pattern for producing rated output) in the center of the core without causing fuel damage, and a control rod pattern with fully inserted control rods in the periphery of the core. The control rod pattern shown in FIG. 2a is a pattern in which the control rods in the center of the core are pulled out more than the control rods in the periphery. As the control rods are withdrawn in the process of forming the control rod pattern shown in Figure 2a, the thermal output of the reactor increases along the solid line in Figure 1;
The thermal output when the control rod pattern is formed is the thermal output at operating point 1 in FIG. Thereafter, the control rods inserted into the periphery of the core are sequentially pulled out to form the rated output control rod pattern shown in FIG. 2b (operation 2 in FIG. 1). Figure 2b after pulling out the control rods around the core.
By forming the control rod pattern, the thermal output of the reactor increases from operating point 1 in FIG. 1 to operating point 2 in FIG. 1 along the solid line. FIG. 3 shows the change in linear power density when the control rods around the core are pulled out from the control rod pattern shown in FIG. 2a. curve 5
shows the change in linear power density in the fuel assembly adjacent to a control rod that is fully inserted in the core periphery when the control rod is moved from fully inserted to fully withdrawn. . Curve 4 shows the linear power density in the fuel assembly that has the maximum linear power density in the center of the core when all the control rods (24 control rods in this example) inserted in the periphery of the core are pulled out. It shows change. Control rods around the core are withdrawn in sequence in 1/24 increments so that they are symmetrical with respect to the center of the core. The black circles in Curve 4 indicate the linear power density of the fuel assembly in the center of the core when all the control rods inserted in the periphery of the core are pulled out to the control rod insertion amount corresponding to the black circles. For example, the second black circle from the left in Figure 3 shows the linear output of the aforementioned fuel assembly when all 24 control rods inserted around the core are pulled out to the 20/24 position. It is density. The linear power density of the fuel assembly adjacent to the control rods that are pulled out from the core periphery is small, even when all the control rods are pulled out, because the leakage of neutrons in the core periphery is large as described above.
It is less than 8KW/ft (Curve 5 in Figure 3). Therefore, even in fuel assemblies located in the periphery of the reactor core and adjacent to the withdrawn control rods, where the change in linear power density is greatest when the control rods are withdrawn, no damage to the fuel rods occurs. However, by pulling out the control rods in the core periphery from the control rod pattern in Figure 2a, the fuel assembly existing in the center of the reactor core has a linear power density of 8KW/8KW as shown in curve 4 in Figure 3. Something that exceeds ft occurs. This is the total amount of change in linear power density until all control rods inserted around the core are fully withdrawn. As mentioned above, the control rods inserted in the periphery of the reactor core are pulled out one by one in order of 1/24 so that they are point symmetrical with respect to the center of the reactor core.
As shown in Figure 6 of the publication, the influence of control rod withdrawal does not affect the fuel assemblies located far from the control rods, so the linear power density of the fuel assemblies located in the center of the reactor core is It does not increase significantly due to withdrawal operations of control rods around the core. In other words, the linear power density of the fuel assembly placed in the center of the core remains the same even if the control rods around the core are pulled out.
The increase rate is about 0.06KW/ft/h. Therefore, even if the control rods around the core are pulled out, the fuel assembly at the center of the core will not be damaged. In addition,
In the vicinity of the reactor core, after a predetermined amount of one control rod has been withdrawn, the next control rod is withdrawn after a predetermined period of time has elapsed in order to accumulate xenon.

In this way, in this example, the rated output control rod pattern was achieved while observing the operational restrictions that took into account the first fact described above to prevent fuel rod damage. After that, from operating point 2 onwards, if the control rod pattern shown in Figure 2b is maintained and the core flow rate is increased while maintaining the rate of change of 0.06 KW/ft/h, 100% (rated) output can be achieved (1st The total time required for the example of operation point 3) in the figure is about 7 days, which is reduced to one third of the example shown in the prior art.

Note that if the control rods around the core are pulled out from the control rod pattern shown in Figure 2a with the core flow rate exceeding 50% of its rated value, the linear output of the fuel assembly adjacent to the pulled control rods will be reduced. The density exceeds the threshold. Therefore, when withdrawing the control rods around the core using the control rod pattern shown in Figure 2a, it is necessary to reduce the core flow rate to 50% or less of its rated value.

FIG. 4 shows a case where the rated control rod pattern is different from the control rod pattern shown in FIG. 2. The control rod pattern shown in Figure 4b is the target control rod pattern that realizes the rated output, and the control rod pattern shown in Figure 4a is the target control rod pattern with multiple control rods around the core. This is a pattern with . The operating method at this time is the same as that shown in the first embodiment, and by this method, 100% (rated) output can be achieved while observing the operating restrictions that take Fact 1 above into consideration.

The number of control rods to be inserted into the periphery of the reactor core and the depth of their insertion may be determined to be sufficient to reduce the power level so that the threshold value is 8 KW/ft or less. The standard is power density 51KW/l, electrical output
It is a 784MW BWR with a total of 20 inserted control rods. The number of inserted control rods can be considered to be proportional to the maximum linear power density of the control rod pattern that produces the rated output.

Furthermore, each of the embodiments described above can be applied to pattern exchange of control rods, etc. In other words, as shown in Example 1, a state in which the peripheral control rods are inserted into the target pattern for control rod pattern exchange is achieved after reducing the core flow rate (~40% of the rated core flow rate), and then the peripheral control rods are pulled out. All you have to do is use the following driving techniques.

In this specification, the core peripheral control rods refer to the outermost control rods of the core and the control rods adjacent thereto.

As explained above, if the operating method of the present invention is used, the target output operating state can be achieved in a short time without impairing the integrity of the fuel, and the effect of improving the operating rate is large and the industrial effect is great. It is.

[Brief explanation of drawings]

1 and 2 are explanatory diagrams of an embodiment of the present invention, and FIG. 3 is a control rod pattern a of FIG.
A diagram showing the change in the maximum linear power density of the fuel adjacent to the operating control rods and the maximum linear power density at the center of the core when the peripheral control rods are pulled out to control rod pattern b (target control rod pattern) in Fig. 2. FIG. 4 is a diagram showing another embodiment of the present invention. 1... Operating point when realizing a control rod pattern in which peripheral control rods are further inserted into the target control rod pattern,
2... Operating point when the target control rod pattern is achieved, 3... Rated power operating point, 4... Maximum linear power density at the center of the core, 5... At the fuel assembly adjacent to the operating control rod. Maximum linear power density.

Claims (1)

[Claims] 1 A target control rod pattern in which a predetermined amount of control rods are inserted in the central part of the reactor core, and all control rods are withdrawn in the peripheral part of the core surrounding the central part of the reactor, one or more control rods are inserted in the peripheral part of the reactor core. Operation with a control rod pattern with additional rods inserted is performed only in a core flow rate range of 50% or less of the core flow rate rated value, and then in a core flow rate range of 50% or less of the core flow rate rated value, The control rods inserted into the peripheral area of the reactor core are pulled out to form the target control rod pattern, and in this state, the core flow rate is increased so as to maintain a predetermined limit value for the rate of increase in power, thereby achieving the final target set level. A method of operating a boiling water reactor that increases reactor output to .