JPH0376438B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a device for predicting changes in the power distribution and thermal margin of a reactor core in advance during startup and shutdown, load following, etc. The present invention relates to a nuclear reactor power distribution prediction device that determines the calculation time interval by taking into consideration the degree of deviation of xenon concentration and iodine concentration from the equilibrium concentration.

[Prior art] A reactor power distribution prediction device is known as a device that predicts changes in the power distribution and thermal margin of the reactor core in advance (Enomoto, Nishizawa, et al.: Nuclear Power Magazine, 22 (11),
821 (1980)), but in the case of conventional methods, the burden on the operator may increase in order to improve the prediction accuracy.

Here, the usage of a conventional reactor power distribution prediction device using a reoperator or the like and its device configuration will be explained using a specific example of operation in a boiling water reactor.

In other words, the cases where prediction of the reactor power distribution is required include during start-up and shutdown, during load following, and when changing the control rod insertion pattern, and in these cases, control rods and core flow rates are appropriately manipulated. It's becoming like that. FIG. 1 shows a specific example of operation in such a case. According to this, during the control rod operation period shown as 〓, the operator adjusts the thermal output by withdrawing and inserting the control rods, but before that operation, the reactor power distribution prediction device predicts the power distribution after the control rod operation. It is designed to predict and calculate such things. The control rods to be operated and the amount of operation are corrected if necessary according to the predicted calculation results. On the other hand, when adjusting the core flow rate, the operator adjusts the heat output by adjusting the core flow rate at times other than the times marked with a cross mark. Specifically, as shown by the cross mark, adjustments are made to keep the thermal output constant while the concentration of xenon, a fission product, changes transiently, and adjustments are made to keep the thermal output constant until the thermal output reaches the target value. Adjustments are made to increase or decrease.

Thermal output, core flow rate, and power distribution are calculated by a reactor power distribution prediction device, and FIG. 2 shows the functional configuration of an example of the device. According to this, the entire system consists of an input section 1, a processing section 2, and an output section 3. In the input section 1, settings for predictive calculation by an operator etc. are sent via a calculation condition receiving section 10, and heat balance data and control rod data are input to the input section 1. Current core state data regarding the position and the like is imported into the processing unit 2 via the data acquisition unit 11. In the processing section 2, the current thermal output, core flow rate, and power distribution are calculated by the current setting section 20 based on the current core state data from the input section 1, and the time mesh division section 21 calculates the calculation conditions and the current state. A time interval, ie, a time mesh, is set for comparing and using the nuclear thermal hydraulic property analysis model for predictive calculation. For each time mesh, the level prediction unit 22 predicts and calculates the thermal output or core flow rate, and the power distribution prediction unit 23 predicts and calculates the power distribution. The prediction calculation results obtained in this manner are appropriately edited by the output editing section 30 and then output to the outside.

In the reactor power distribution prediction apparatus constructed as described above, the time mesh dividing section is one of the elements that influences the accuracy of the prediction calculation results. The purpose of the time mesh dividing section is to reduce the burden on operators and the like, and to improve prediction accuracy without unnecessarily increasing the time required for prediction calculations. However, in the past, although the setting of the time mesh was based on a rough guideline, it largely depended on the experience of the operator, etc., which resulted in problems such as an inability to improve prediction accuracy.

The configuration of an example of the time mesh division section in FIG. 2 will now be described as shown in FIG. 3. That is, the thermal power/core flow rate change calculation part 21
0, the rough rate of change in thermal output or core flow rate with respect to time between the current time and the predicted time is determined, and in response to this, the time mesh setting section 211
The time mesh is now set. The larger the rate of change, the smaller the time mesh is set. Specifically, the time mesh τ is set as follows.

τ=ε/(|Δx/Δt|) ...(1) However, ε is the input constant (error value), and Δx
indicates the difference in thermal power or core flow between the current time and the predicted time. The input constant ε is almost determined at the reactor design stage, but the operator etc. can set it within a certain range around this value.

Therefore, assuming that the core flow rate at point C in Figure 1 is predicted and calculated from point B in Figure 1, thermal output P _B at point B, core flow rate f _B , time t _B are found to be 46%, 47%, and 14 hours (h), respectively, from the figure, and the target heat output P _C time t _C at point C is 75% and 65 hours, respectively, so the time mesh τ(h) is obtained as follows, with the input constant (%) ε being 5.

τ=5/{(75−46)/(65−14)≒8.8……(2) In other words, in this example, the level prediction unit uses the nuclear thermal hydraulic property analysis model to reach point C every 8.8 hours. The change in the core flow rate is discrete, and furthermore, the power distribution prediction unit predicts the change in the power distribution.

FIG. 4 is an enlarged view of a part of FIG. 1, and the reason for the existence of the time mesh division section can be explained as follows.

That is, the operator etc. calculates the time t _C and heat output at point C.
If P _C is input, the time mesh τ is determined to be approximately 8.8 hours, and from this, the predicted value of the core flow rate from point B to point C is determined as a black circle. On the other hand, the solid line between points B and C indicates the prediction result when the time mesh division part does not exist, but as is clear from the figure, the prediction result by the time mesh division part is the actual value (displayed by the broken line). It can be seen that they match. That is, the time mesh dividing section can improve prediction accuracy.

However, even if the time mesh division section attempts to perform predictive calculation between points A and B in FIG. 1, the predictive calculation cannot be improved. This means that the rate of change in heat output is approximately 0.11 from equation (1).
%/h, which becomes extremely small, resulting in a large value of time mesh τ. Xenon Xe, which is actually a fission product
Despite the fact that the core flow rate changes as shown by the broken line in Figure 4 due to transient changes in Similar results can be obtained.

In order to solve this problem, the value of the input constant ε should always be set to a small value, or the operator should understand that the xenon Xe concentration is changing transiently and set the time mesh in such a case. It is conceivable to set the value of the input constant ε to a small value in order to reduce the value. Prediction points are always taken finely, or prediction points are taken finely as necessary, but the former takes a lot of time to calculate even when there is no transient change in the xenon concentration, while the latter takes a lot of time. Not only does this require the operator to be aware of changes in the xenon concentration, which is a heavy burden, but the input constant ε cannot be set optimally, making it impossible to improve prediction accuracy.

[Purpose of the invention]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a nuclear reactor power distribution prediction device that can obtain prediction results with high accuracy without requiring much time for prediction calculations and without placing a large burden on the operator.

[Summary of the invention]

To this end, the present invention provides a means for determining the degree of deviation of the xenon and iodine concentrations from the equilibrium concentration, i.e.
The degree of non-equilibrium is also taken into consideration, and the time mesh is generally set smaller as the degree of non-equilibrium increases.

[Embodiments of the invention]

The present invention will be explained below with reference to FIGS. 5 to 7.

First, the theoretical background of the present invention will be explained with reference to FIG. Figure 5 shows the target heat output (50% in this example)
This figure shows how the xenon Xe concentration and iodine concentration change over time when the thermal output is changed in steps up to As shown in the figure, even when the thermal output changes stepwise from a certain value to the target value (equilibrium point) of 50%, it can be seen that the xenon concentration and iodine concentration do not immediately reach the concentrations corresponding to the equilibrium point. This is because the concentration of iodine, a fission product, is almost proportional to the thermal output, but it changes to xenon with a time delay while emitting beta rays, while xenon absorbs neutrons and converts it into other nuclides. This is due to the behavior that has changed. For example, if we assume that the thermal output is changed stepwise from 100% to 50%, the xenon concentration will rise due to the decrease in neutrons, then gradually decrease until it settles at the concentration corresponding to 50% thermal output. It is something. Therefore, by determining the degree of non-equilibrium between the xenon concentration and the iodine concentration, the current position on the map can be determined, and furthermore, the subsequent trajectory to the equilibrium point and the change in the xenon concentration over time can be determined. Since there is a one-to-one correspondence between xenon concentration and heat output via the degree of reactivity, it is possible to know changes in heat output over time rather than changes over time in xenon concentration. On the other hand, as can be seen from equation (1), the basic idea of time mesh division is to use a nuclear thermal-hydraulic characteristic analysis model every time the thermal output or core flow rate changes by a certain amount. Therefore, it is possible to optimally set the time mesh using the temporal change in xenon concentration known from the xenon-iodine map. Note that the xenon-iodine map shown in Figure 5 is not obtained from experimental data;
Although not explained in detail here, it can be obtained from differential equations through theoretical analysis (U _MEGAKI K.,
et al.: J.Nud.Sci & Tech., 19(7), 513
(1982)). Of course, equilibrium points other than 50% can also be obtained in the same way. Therefore, it is necessary to prepare and store as many such maps as data in advance in the reactor power distribution prediction device. Alternatively, by calculating the optimum time mesh, it is possible to automatically set the optimum time mesh with variable input constants.

Now, the nuclear reactor power distribution prediction device according to the present invention will be specifically explained. The configuration of the device is almost the same as that shown in FIG. 2, but the only difference is the configuration of the time mesh division section. As can be seen from the previous explanation, the time mesh should be set to be smaller as the degree of non-equilibrium between xenon and iodine increases, but as shown in FIG. For simplicity, time meshes are roughly divided into two types:
Usually, predictions are made using a large time mesh based on the rate of change of thermal output or core flow rate.
When the degree of non-equilibrium for xenon and iodine is large, predictive calculations are performed using a small time mesh. As shown in FIG. 6, in the non-equilibrium degree calculation part 212, the non-equilibrium degree with respect to the equilibrium point of xenon and iodine is calculated, and depending on whether the non-equilibrium degree is above a certain level, a time mesh that is small or a time mesh that is larger than a large time mesh is calculated. A time mesh (for example, an infinite time mesh) is set in the time mesh setting section 213. On the other hand, apart from this, in the time mesh section 211, a time mesh is set based on the rate of change of thermal output or core flow rate in the same manner as in the conventional case, but the smaller of these two set time meshes is selected as the time mesh selection section 214. Therefore, the time mesh is selected as the optimal time mesh. Here, if the time mesh to be selected is γ', the time mesh γ _oew will be selected by the following selection logic.

γ _oew = M _io (γ, γ′) ...(3) Now, if we explain specifically how the time mesh γ′ is set, it will be the simplest process and the one that is necessary and sufficient for effectiveness. It is based on the following formula.

_{γ = γ _ _} In the case of I _eq ) −1 _| δ _I ...(4) However, δ _X , δ _I , and τ Further, X _eq and I _eq represent the xenon concentration and iodine concentration, respectively, assuming an equilibrium state at the present time.

In this case, if the time mesh τ′ is ∞, the time mesh τ is selected according to equation (3), and if the time mesh τ′ is τ _XI (<τ), the time mesh τ _XI is selected as the time mesh τ _oew . Prediction calculations are performed with high accuracy using the optimally set time mesh τ _oew .

To explain specifically the effectiveness of the xenon-iodine map, point A in Figure 1 indicates that although the control rods have been operated before, approximately the thermal output has been reduced from about 100% to about 50%. It can be considered that 5 hours (h) have passed since the step-like change. That is, the xeson-iodine map shown in FIG. 5 can be used. FIG. 7 shows a trajectory corresponding to the heat output change extracted from the various trajectories shown in FIG. 5, and points A and B are located on the trajectory as shown. Therefore δ _X = δ _I
= 0.25, _τ It is. Outside the thick solid line frame in the figure, formula (4)
Therefore, τ' = 2(h), and therefore τ = 2(h), so the change in core flow rate between points A and B is the fourth
This makes it possible to accurately predict the actual values indicated by the broken lines in the figure.

Finally, other methods for setting the time mesh τ' will be explained.

First, it is conceivable to set it in stages according to the degree of non-equilibrium. This can be expressed as a formula as follows.

τ' ₌ τ _XI (k ₎ (δ _X (k)<|( _X /X _eq ) −1| _δ k+1) ...(5) It is also possible to set a time mesh every time a prediction calculation is performed.After one prediction calculation is performed using the currently set time mesh, a new non-equilibrium degree is set. After calculating, the time mesh is reset using equation (4) or equation (5).

Furthermore, although not directly related to the present invention, it is also conceivable to set based on simplified prediction results. A simplified nuclear thermal-hydraulic characteristic analysis model used for predictive calculations is used to perform predictive calculations with a fine time mesh of, for example, 15 to 30 mm, and to obtain a rough estimate of thermal output and core flow rate. The goal is to find a trajectory of change. After that, a time mesh is set based on the trajectory according to the rate of change in thermal output and core flow rate, and the power distribution is predicted and calculated using a nuclear thermal-hydraulic characteristics analysis model.

Using these methods, the calculation time will be further reduced.

〔Effect of the invention〕

As explained above, in the case of the present invention, the time mesh is set in consideration of the non-equilibrium degree of the canon concentration and the iodine concentration, so the prediction calculation does not require much time.
Furthermore, there is an effect that prediction results can be obtained with high accuracy without placing a large burden on the operator.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a specific example of operation in a boiling water reactor, FIG. 2 is a diagram showing the functional configuration of an example of a reactor power distribution prediction device according to the prior art, and FIG.
The figure shows the functional configuration of the time mesh dividing section in the configuration, FIG. 4 shows an enlarged view of a part of FIG. 1, and FIG. 5 shows an example of the xenon-iodine map according to the present invention. Figure shown, No. 6
This figure shows the functional configuration of an example of the time mesh dividing section according to the present invention, and FIG. 7 is a diagram showing a part of the trajectory extracted from FIG. 5. 1...Input section, 20...Current setting section, 21...
Time mesh division section, 22...Level prediction section,
23...Power distribution prediction section, 210...Thermal power/core flow rate change rate calculation section, 211, 213...Time mesh setting section, 212...Non-equilibrium degree calculation section, 214...Time mesh selection section.

Claims (1)

[Claims] 1 In response to external requests, changes in the future reactor power distribution and thermal margin are calculated using the current state of the reactor as an initial value, and are calculated based on the rate of change in thermal output and core flow rate. In a reactor power distribution prediction device that predicts using a force characteristic analysis model, the degree of non-equilibrium with respect to the equilibrium state of the concentration is calculated based on the xenon concentration and iodine concentration from the xenon/iodine concentration calculation means, and then the degree of non-equilibrium is calculated based on the It has a means for setting a new time mesh according to the above, and either a time mesh set as small as the degree of non-equilibrium from the means is large or a time mesh set based on changes in thermal output and core flow rate. A nuclear reactor power distribution prediction device characterized by a configuration in which a small time mesh is selected by a selection means and used for prediction.