JPH0517514B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a method and apparatus for monitoring thermal-hydraulic oscillations (channel stability) of fuel bundles in a boiling water nuclear reactor, and particularly relates to a method and apparatus for monitoring thermo-hydraulic oscillations (channel stability) of fuel bundles in a boiling water nuclear reactor. After limiting the number of fuel bundles that are expected to have poor stability based on the flow rate, axial power distribution, etc., the fuel bundle with the least stability is determined from among those fuel bundles by stability analysis. The present invention relates to a method and apparatus for monitoring channel stability, which is a local phenomenon, by displaying the signal of a local power detector assembly near the fuel bundle on a control panel together with the signal of the core average power detector.

[Technical background of the invention and its problems] In general, the coolant that passes through the fuel bundles arranged in the core of a boiling water nuclear reactor forms a two-phase flow of water and steam to dissipate the heat generated in the core. However, this two-phase flow includes the amount of vapor bubbles (void amount) in the coolant,
It is known that there is a possibility of thermo-hydraulic oscillations based on feedback between pressure drop and flow rate.
There are many fuel channels in the reactor core, and they form parallel flow paths. Therefore, for each fuel channel, its thermo-hydraulic oscillation is caused by other fuel channels under a constant differential pressure between the inlet and the outlet. It can be considered that this occurs independently of the fuel bundle. Furthermore, since feedback via nuclear properties is considered to be secondary due to the locality of vibration, it can be ignored as a first approximation. Such a vibration mode is called "channel stability." On the other hand, the mode of vibration in which the feedback phenomenon of the two-phase flow in the fuel channel is related to the nuclear characteristics of the reactor core and the dynamic characteristics of the recirculation flow path is called "core stability." Normally, oscillations in core stability appear as oscillations in neutron flux (or power) throughout the core.

Core stability during actual nuclear power plant operation can be monitored by global signals such as core average power detector signals and recirculation flow rates, but channel stability can be monitored locally. Because it is a phenomenon, although many local power detector assemblies are installed in the reactor core, they are not used for constant monitoring like the core average power detector signal, so vibrations must be constantly monitored. I can't. Therefore, channel stability is sufficiently analyzed at the design stage to determine the range in which stable operation can be achieved.

The operating range at the design stage is determined based on the most severe conditions, so in normal cases there is considered to be a considerable margin of stability. During this period, operating conditions may change due to changes in fuel type, operating constraints, etc. In such cases, stability analysis is naturally performed to redetermine the operating range. It is no exaggeration to say that the stability margin for channel stability in an actual nuclear reactor changes from moment to moment, including small changes that do not lead to this, such as minute deformation of the fuel channel.

Therefore, the stability margin of channel stability, that is, the stability, can be evaluated at regular intervals for all fuel bundles under actual operating conditions, and the fuel bundle with the least stability can be selected. If possible, channel stability can be constantly monitored by monitoring the signal of a local power detector assembly near the fuel bundle. As a result, if it is determined that vibration has occurred beyond the stability limit, it is possible to immediately take measures such as inserting a nearby control rod to reduce the output.

However, there is a problem in that it takes time and is inefficient to analyze channel stability for all fuel bundles and determine the fuel bundle with the least stability.

[Objective of the Invention] The present invention has been made in response to the above situation, and it is possible to evaluate the stability of channel stability without performing stability analysis on all fuel bundles,
By efficiently detecting the fuel bundle with the lowest channel stability and displaying the signal from the local power detector assembly placed near that fuel bundle on the control panel, all local power in the core can be detected. It is an object of the present invention to provide a method and apparatus for monitoring channel stability of a nuclear reactor, which allows channel stability to be monitored more easily than by displaying and monitoring signals from a detector assembly.

[Summary of the Invention] That is, the present invention selects a plurality of fuel bundles with poor channel stability based on the power ratio of the fuel bundles arranged in the reactor core to the core average and the axial power distribution shape, From the stability analysis performed on these fuel bundles, the fuel bundle with the worst channel stability is determined, and a local output detector set near this fuel bundle is selected. A method for monitoring channel stability in a nuclear reactor, characterized in that the channel stability is monitored by displaying a signal from a nuclear reactor and a signal from a core average power detector on a control panel, and A core performance calculation device that performs nuclear thermal-hydraulic calculations, a stability evaluation parameter calculation device that inputs the calculation results of this core performance calculation device and calculates parameters for evaluating channel stability of each fuel bundle, and a stability evaluation parameter calculation device that calculates parameters for evaluating channel stability of each fuel bundle. a fuel bundle selection device that selects a plurality of fuel bundles with poor channel stability based on the calculation results of the evaluation parameter calculation device; and output data of the core performance calculation device regarding the fuel bundles selected by the fuel bundle selection device. A stability calculation device that calculates the stability of channel stability using A local output detector aggregate selection device that selects a plurality of local output detector aggregates arranged near the bundle, and a signal of the local output detector aggregate selected by the local output detector aggregate selection device. This is a channel stability monitoring device for a nuclear reactor, characterized by comprising a display device that displays a signal from a core average power detector.

[Example of the Invention] The present invention will be described in detail below using an example.

FIG. 1 shows the configuration of an embodiment of the channel stability monitoring device of the present invention. In the figure, reference numeral 1 indicates a nuclear reactor. For example, each detector installed in the core 2 of the reactor 1
inputs process variables such as core flow rate, core pressure, core power, control rod pattern, etc., and calculates the output, flow rate, and axial power distribution of each fuel bundle.

The stability evaluation parameter calculation device 4 uses the calculation results of the core performance calculation device 3 to calculate stability evaluation parameters.

The fuel bundle selection device 5 takes into consideration the stability evaluation parameter calculated by the stability evaluation parameter calculation device 4 and other factors that affect channel stability, and selects a fuel bundle that is considered to have low channel stability. Select several fuel bundles in order from those listed.

The stability calculation device 6 performs stability analysis on several fuel bundles selected by the fuel bundle selection device 5 using data from the core performance calculation device 3, and determines the fuel bundle with the lowest degree of channel stability. Moreover, its stability is calculated.

The local output detector assembly selection device 7 automatically selects four local output detector assemblies near the position of the fuel bundle determined as a result of the stability analysis by the stability calculation device 6. The display device 8 on the control panel displays the signals of the four local power detector aggregates selected in this way and the core average power detector 9.
and the signal are displayed together.

Here, the stability evaluation parameters calculated by the stability evaluation parameter calculation device 4 will be described in detail. Although there are many parameters that can generally affect channel stability, the one that is of concern within the normal operating range of a nuclear power plant is the hole where the coolant flows into the fuel bundle (see below). (called an orifice), the output and flow rate of each fuel bundle, and the axial output distribution shape.

Normally, the coolant flowing into the core is unsaturated and has a single phase flow. Therefore, the size of the orifice area affects the pressure loss in the single phase section. That is, if the area is small, the resistance to flow (hereinafter referred to as orifice coefficient) is large and the pressure loss becomes large. The channel stability becomes more stable as the ratio of pressure loss in the single phase portion to the total pressure loss increases, so it can be said that the greater the orifice coefficient, the more stable the channel stability becomes. In a normal core, the outermost fuel bundles have a larger orifice coefficient than the inner fuel bundles, and the output of the outermost fuel bundles is also small, so there is almost no problem in terms of channel stability.

Next, regarding the output and flow rate, the higher the output and the lower the flow rate, the lower the channel stability becomes. Therefore, in general, the higher the output and the lower the flow rate of operation, the lower the stability. In addition to these, there are other things that have a big impact on channel stability.
An example is the output distribution type in the axial direction.

In other words, this axial power distribution shape not only has a great influence on channel stability, but also changes considerably in actual operating conditions due to the effects of the nuclide Zenon associated with control rod insertion and removal and combustion. and,
Since each fuel channel has a different shape, it is necessary to take into account the effect of the axial power distribution shape in order to evaluate stability in accordance with actual operating conditions.

By the way, in general, the relationship between the axial power distribution shape and the stability of the channel stability is that the higher the peak height of the axial power distribution shape, and the closer the peak position is to the core (fuel It can be said that the lower the bundle (bundle), the lower the stability.

In other words, if the peak height is high and the peak position is at the bottom, it means that the amount of heat generated is large at the bottom, and if the amount of heat generated at the bottom is large, the boiling point of the coolant will be at the bottom. , it can be considered that stability decreases because the region of two-phase flow, which is a destabilizing factor, becomes longer.

In this embodiment, taking the above points into consideration, channel stability is monitored by focusing on the output, flow rate, and axial output distribution of the fuel bundle.

That is, the output coefficient P ^r _j of the bundle is used as a parameter representing the magnitude of the output of the j-th fuel bundle among the large number of fuel bundles accommodated in the reactor core.
Use.

This P ^r _j can be expressed by the following formula.

P ^r _j = Q _j 1 (Q/N) where Q _j is the output of the j-th fuel bundle, Q
is the power of the entire reactor core, and N is the number of fuel bundles in the reactor core.

Furthermore, the peaking coefficient and peak position of the axial output are used to characterize the axial output distribution.

In general, the axial power distribution shape is determined by dividing the fuel bundle into a large number of nodes, for example 1 to 24, along the axial direction, as shown in FIG. 2, and this will be followed here as well.

In Fig. 2, the horizontal axis shows the output, and the vertical axis shows the axial nodes, and each curve (A),
(B) and (C) each show the output distribution of the fuel bundle.

Now, if the axial output coefficient at the i-th node in the axial direction of the j-th fuel bundle is P ⁱ _j , P ⁱ _j can be expressed by the following equation.

P ⁱ _j =Q ⁱ _j /(Q _j /Nax) where Q ⁱ _j is the output at the i-th node of the j-th fuel bundle, and Nax is the number of nodes in the axial direction.

The largest one among these P ⁱ _j is defined as the axial peaking coefficient P ^ax _j .

Therefore, it can be expressed as P ^ax _j = ^max _j (P ⁱ _j ).

Using this P ^ax _j , the height of the peak of the axial output distribution type can be evaluated.

That is, the larger P ^ax _j becomes, the higher the peak height becomes.

Next, the following quantities are defined as parameters for expressing the position of the peak of the axial output distribution.

P ^l _j = (Nax − N ^p _j )/Nax Here, N ^p _j is the node number where the peak of the j-th fuel bundle exists, and the node numbers are assigned upward from the bottom of the fuel bundle. do.

According to this P ^l _j , it can be seen that the larger the value of P ^l _j, the lower the peak position is in the fuel bundle.

The axial power distribution shape of the j-th fuel bundle is expressed by P p j defined as P ^p _j = P ^ax _j・P ^l _j as the product of the two parameters P ^ax _j and ^{P l} _j defined above _. can be evaluated based on

In addition, the following parameter Pj including the output size
Define.

Pj = P ^r _j · P ^p _j = P ^r _j · P ^ax _j · P ^l _j The degree of channel stability of each fuel bundle can be roughly predicted by the size of the parameter Pj. That is, as a general tendency, it is considered that the larger the stability evaluation parameter Pj is, the worse the stability is.

Next, the operation of the channel stability monitoring device configured as described above and its channel stability monitoring method will be explained.

The above-mentioned stability evaluation parameter Pj can be calculated very easily and in a short time by using the data of the core performance calculation device 3 that calculates nuclear thermal and hydraulic power, so the stability evaluation parameter calculation device 4 calculates the The stability evaluation parameters Pj are calculated for all fuel bundles by inputting the calculation results of the core performance calculation device 3, which periodically performs nuclear thermal and hydraulic calculations. Then, in the first step, the fuel bundle selection device 5 uses the calculation results to select a number of fuel bundles with poor channel stability among all the reactor cores in descending order of the value of the stability evaluation parameter Pj. . In this case, we will exclude the outermost fuel bundle with a fairly large orifice coefficient, and if different types of fuel bundles coexist in the core, we will select several fuel bundles with large Pj for each type. be done. Stability analysis is performed by the stability calculation device 6 on the several fuel bundles selected in this way, and the fuel bundle with the worst channel stability and the stability at that time are determined. Next, the local output detector assembly selection device 7 selects four local output detector assemblies near the fuel bundle with the worst channel stability, and the signals of these local output detector assemblies are displayed on the display device 8. Displayed and monitored along with the core average power detector signal.

Here, FIG. 3, FIG. 4, and FIG. 5 respectively show the installation position of the local power detector assembly in the horizontal direction in the core, the vertical arrangement of the local power detector in the local power detector assembly, and the local The horizontal arrangement of the power detector assembly throughout the core is shown. In these figures, numerals 10, 11, and 12 are a fuel bundle, a control rod, and a local power detector assembly, respectively;
A, b, c, and d are local output detectors arranged at equal intervals in the vertical direction in the local output detector assembly 12.

In the local output detector assembly selection device 7, four local output detector assemblies are selected within the square formed by connecting these four local output detector assemblies, and the one with the worst channel stability determined by the stability calculation device 6 is selected. The fuel bundles shall be automatically selected for entry so that the degree of spread within the core of any power fluctuations can also be monitored. However, when displaying, the signal of the local output detector assembly closest to the fuel bundle with the worst channel stability and the other three
The signal of the local power detector assembly closest to the fuel bundle with the worst channel stability and the signal of the core average power detector are displayed on the display device 8 so as to be distinguishable from the signals of the two local power detector aggregates. It should be supervised at least. The other three signals are then used for reference.

Note that calculations are performed periodically by the core performance calculation device 3 at fixed time intervals, but may also be performed at the request of an operator when changing the core state, such as when operating a control rod. The fuel bundle with the lowest channel stability and the local power detector assembly in its vicinity shall be determined each time, and as a result, the position of the local power detector assembly to be monitored may be moved. In this case, the display signal will be automatically switched.

By the method described above, the core performance calculation device 3
At the time the calculation is performed, the stability analysis determines the location of the fuel bundle with the worst channel stability and the stability at that time, and until the next calculation, the output detector signal from the instrument on the control panel is determined. The display allows channel stability monitoring.

In addition, if the stability analysis results show that the channel stability is very low and close to the stability limit, refrain from operations that will worsen the stability, such as increasing the output by withdrawing the control rod. do. In addition, in the event that vibrations occur beyond the stability limit, it goes without saying that we will immediately take measures to reduce the output, such as by inserting nearby control rods, and this will be done automatically. You can also.

[Effects of the Invention] As is clear from the above explanation, according to the present invention, by selecting several fuel bundles in advance depending on the magnitude of the stability evaluation parameter Pj, Rather than analyzing the channel stability of all fuel bundles to determine the least stable fuel bundle, it is much more efficient and quick to find the fuel bundle with poor stability. The local output detector clusters near the fuel bundles with the worst channel stability are selected and their signals are displayed.
Compared to displaying and monitoring the signals of all local power detector assemblies in the core, there is less chance of oversight, and it is the same as monitoring core stability using core average power detector signals. This has the advantage that local output fluctuations can be monitored fairly easily.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing one embodiment of the channel stability monitoring device of the present invention, Fig. 2 is a graph showing the axial power distribution of the fuel bundle, and Fig. 3 is the fuel bundle, control rod, and local power detector. 4 is a cross-sectional view showing the arrangement of the assembly in the horizontal direction within the core; FIG. 4 is a perspective view showing the arrangement of the local power detectors in the vertical direction within the local power detector assembly; and FIG. FIG. 3 is a plan view of the core showing the horizontal arrangement of the output detector assembly. 1... Nuclear reactor, 10... Fuel bundle, 11...
...Control rod, 12...Local output detector assembly.

Claims (1)

[Claims] 1. A plurality of fuel bundles with poor channel stability are selected based on the power ratio of the fuel bundles arranged in the reactor core to the core average and the power distribution shape in the axial direction, and these fuel bundles are The fuel bundle with the worst channel stability is determined from the stability analysis performed on the bundle, a local output detector assembly near this fuel bundle is selected, and the signal of the local output detector assembly selected in this way is determined. A method for monitoring channel stability of a nuclear reactor, characterized in that channel stability is monitored by displaying on a control panel a signal from a core average power detector and a signal from a core average power detector. 2 The axial power distribution shape is the ratio of the maximum power to the average power in the axial direction of the fuel bundle, and the ratio of the power peak to the total number of nodes when the fuel bundle is uniformly divided into many nodes along the axial direction. The method for monitoring channel stability of a nuclear reactor according to claim 1, wherein the channel stability monitoring method for a nuclear reactor is evaluated by the product of the position and the ratio of the number of nodes above the position. 3 A core performance calculation device that performs nuclear thermal-hydraulic calculations in the reactor core, and a stability evaluation parameter calculation that inputs the calculation results of this core performance calculation device and calculates parameters for evaluating the channel stability of each fuel bundle. a fuel bundle selection device that selects a plurality of fuel bundles with poor channel stability based on calculation results of the stability evaluation parameter calculation device; and a fuel bundle selection device that selects a plurality of fuel bundles with poor channel stability based on the calculation results of the stability evaluation parameter calculation device; A stability calculation device that calculates the stability of the channel stability using the output data of the performance calculation device and determines the fuel bundle with the least stability and the stability at that time, and A local output detector assembly selection device that selects a plurality of local output detector aggregates arranged near the fuel bundle with the lowest stability; and a local output detector selected by the local output detector assembly selection device. A display device for displaying a signal from a reactor assembly together with a signal from a core average power detector. 4 The parameters for evaluating the channel stability of a fuel bundle are the power ratio of the fuel bundle to the core average, the ratio of the maximum power to the average power in the axial direction, and the uniform division of the fuel bundle into a number of nodes along the axial direction. 4. The channel stability monitoring device for a nuclear reactor according to claim 3, which is expressed as a product of the ratio of the number of nodes above the position of maximum output to the total number of nodes when