JP4008131B2 - Reactor core performance calculator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a reactor core performance calculation apparatus for monitoring and predicting critical eigenvalues, power distribution and the like in a reactor core by calculation.
[0002]
[Prior art]
For monitoring and prediction of the reactor core, in order to obtain the nuclear thermohydraulic characteristics such as eigenvalue, three-dimensional power distribution, neutron flux distribution, etc. A 3D core simulator based on a physical model is provided. As a physical model, neutrons are classified into high-speed groups, thermal outer groups, thermal groups, etc. based on energy, and after deriving diffusion equations that follow the neutron flux of each group, a nuclear calculation model that solves the grouped modified one-group diffusion equation Is often used.
[0003]
This diffusion equation is generally solved by dividing one fuel assembly into one grid and dividing it into 10 to 20 nodes in the axial direction and discretizing it. The nuclear calculation model uses nuclear constants arranged according to the moderator void fraction, burnup, control rods, etc. In general, this nuclear constant is calculated by an infinite system that is specularly reflected at the boundary of each fuel assembly.
[0004]
In order to operate a nuclear reactor safely and efficiently, it is important to accurately monitor and grasp the power distribution in the reactor and to predict changes in the power distribution and the like accompanying the operation in advance. Core performance prediction calculation predicts core characteristics such as power distribution and fuel assembly thermal margin for future operation such as control rod operation and core flow rate adjustment using 3D core simulator Is.
[0005]
The critical eigenvalue should be 1.0 at first, but it is a three-dimensional core simulator due to the use of a modified one-group diffusion equation for the neutron diffusion equation and the calculation of nuclear constants using an infinite system. Due to this error, the critical eigenvalue does not become 1.0, but changes around 1.0 throughout the operation.
[0006]
The core performance prediction calculation has a function for obtaining the core characteristics after a certain operation. In general, this prediction calculation is performed using parameters such as core thermal power, core flow rate, control rod pattern, prediction date and time. At this time, the user sets and inputs the target critical eigenvalue based on the past operation performance tracking analysis of the reactor.
[0007]
As a conventional technique related to the core performance prediction calculation, Japanese Patent Laid-Open No. 7-209473 discloses the eigenvalues obtained in the repeated calculation process of the nuclear characteristics and thermal hydraulic characteristics of the initial reactor in the trial calculation in a critical state. Store and calculate the final convergence value of the eigenvalue based on the obtained eigenvalue during the iterative calculation, reduce the number of iterations required to derive the final convergence value of the eigenvalue, and calculate the core performance prediction A technique for reducing the calculation time required is described.
[0008]
[Problems to be solved by the invention]
In the prediction calculation, the core performance is predicted by repeatedly performing the calculation until the eigenvalue of the core converges to a critical eigenvalue indicating that the core is critical. Conventionally, the critical eigenvalue used at this time is an input value by the user. The target critical eigenvalue set by the user is determined by the user from the past operation result data, and is automatically calculated within the prediction calculation and is not set.
[0009]
An object of the present invention is to provide a core performance calculation apparatus that automatically calculates and sets a target critical eigenvalue instead of a user input value in a core performance prediction calculation.
[0010]
[Means for Solving the Problems]
In order to achieve the above objective, in the core performance prediction calculation that predicts the core performance of the reactor at the prediction target point after the prediction start point by the three-dimensional core calculation , the four bodies surrounding the control rods to be inserted are Using the value based on the change from the prediction start point of the relative heat output in the core at the prediction target point calculated by the three-dimensional core calculation for the axial position of the fuel assembly control rod adjacent to the prediction The target critical eigenvalue at the target point is calculated internally.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The index used for setting the target critical eigenvalue needs to be a good representation of changes in the core state due to operation (core flow rate operation, core heat output operation, control rod operation, etc.). The relationship between the index used in the present invention and the core state will be described below.
[0012]
First, the core average burnup will be described. In the prediction calculation, there is always a time lapse between the prediction start point and the prediction target point, and therefore a difference occurs in the core average burnup. This change in burnup affects the material composition of the fuel assembly. This material composition affects the type of fission reaction occurring in the core, the neutron flux distribution, etc., and the eigenvalue.
[0013]
Next, the core average moderator void ratio will be described. The moderator void ratio changes mainly after the core flow rate operation or when the core thermal output changes due to control rod operation. As the moderator void fraction changes, the number of neutrons with adequate energy to cause a fission reaction changes, and the core eigenvalue is affected.
[0014]
Next, the relative core thermal output will be described. The relative core thermal power is a value indicating the thermal power of the nuclear reactor as a value relative to the rated thermal power. This change in relative core thermal output is accompanied by a change in the temperature of the fuel assembly. The temperature change of the fuel assembly affects the number of neutrons absorbed by resonance absorption occurring in the fuel assembly, and affects the eigenvalue of the core.
Next, the value determined by the relative heat output in the core at the axial node position adjacent to the control rods of the four fuel assemblies around the inserted control rod will be described. The operation for adjusting only the depth of the inserted control rod is generally performed in a short time without a change in the core flow rate without changing the position where the control rod is inserted. For this reason, in the above-mentioned indexes (core average burnup, core average moderator void ratio, core heat output), such control rod depth adjustment has little change.
[0015]
The change accompanying the control rod operation appears in the core relative heat output at the axial node position adjacent to the control rods of the four fuel assemblies 11 surrounding the inserted control rod 10 shown in FIG. Therefore, if the value based on the change in core relative heat output at the axial node position adjacent to the control rods of the four fuel assemblies 11 surrounding the inserted control rods 10 is used as an index, the core flow rate can be reduced in a short time. Even in the control rod depth adjustment in which there is no change and there is no change in the position where the control rod is inserted, a change occurs before and after the control rod depth adjustment. The insertion of control rods affects the number of thermal neutrons present in the core and affects the eigenvalues of the core.
[0016]
Next, the core average xenon concentration will be described. When the power of the reactor changes, the neutron flux changes, so that the amount of xenon produced and extinguished in the core changes. Since xenon has a large thermal neutron absorption cross section, changes in the core average xenon concentration affect the eigenvalue of the core.
[0017]
In the reactor core performance calculation apparatus, coefficients, formulas, and the like for evaluating changes in eigenvalues accompanying changes in the above-described index are obtained from the results of the operation performance tracking analysis of the reactor and stored in advance. Then, in the prediction calculation, the amount of eigenvalue that changes in accordance with the change in the above-mentioned index is obtained, and the target critical eigenvalue is internally calculated.
[0018]
Next, the reactor core performance calculation apparatus of the present invention will be described with reference to FIG. In FIG. 1, 1 is a reactor core performance calculation device, 2 is an input processing unit, 3 is a three-dimensional core calculation unit, 4 is a core performance record data storage unit, 5 is a target critical eigenvalue calculation unit, and 6 is a target critical eigenvalue calculation. Database, 7 is a nuclear reactor, 8 is a core, and 9 is an in-core neutron measuring instrument.
In the core performance monitoring calculation, information on the distribution of the neutron flux in the reactor such as the control rod insertion pattern and the core flow rate from the reactor 7 is taken into the input processing unit 2 from the in-core neutron measuring instrument 9, and the three-dimensional core calculation unit 3 The core performance is calculated, and the core performance record data is stored in the core performance record data storage unit 4.
[0019]
On the other hand, in the core performance prediction calculation, the user inputs parameters necessary for the prediction calculation (core thermal output, core flow rate, control rod pattern, prediction date, etc.) to the input processing unit 2, and the three-dimensional core calculation unit 3 Based on the result and the core performance record data of the prediction start point stored in the core performance record data storage unit 4 using the data from the target critical eigenvalue calculation database 6, A target critical eigenvalue is calculated by the eigenvalue calculation unit 5.
[0020]
FIG. 4 shows a flowchart of the prediction calculation of the present invention. In the core performance prediction calculation, first, parameters necessary for the prediction calculation (core thermal output, core flow rate, control rod pattern, prediction date, etc.) are set. At this time, an estimated value is set for a parameter to be predicted. After that, 3D core calculation is performed by 3D core simulator to calculate power distribution and eigenvalue. This calculation is repeated until the output distribution becomes a distribution consistent with the neutron flux distribution, and the convergence calculation is performed. Then, based on the converged core state, the target critical eigenvalue is internally calculated and set by the aforementioned index.
[0021]
Then, it is determined whether the eigenvalue by the 3D simulator when the power distribution has converged and the target critical eigenvalue calculated internally based on the core state where the power distribution has converged. If it does not match within the predetermined range, the parameter to be obtained is reset based on the difference between the target critical eigenvalue and the eigenvalue, and a series of calculations is repeated. In the process of this iterative calculation, the value of the target critical eigenvalue calculated internally is updated.
[0022]
The eigenvalue change amount A corresponding to the core average burnup change amount is defined by the following equation 1.
[0023]
[Expression 1]
A = FA (E2) -FA (E1)
Here, E2 represents the core average burnup at the prediction target point, E1 represents the core average burnup at the prediction start point, and FA (E) represents the assumed value of the eigenvalue in the core average burnup E. The FA values shown here are stored in the target critical eigenvalue calculation database 6 in a format arranged for each combustion point.
[0024]
An eigenvalue variation B corresponding to a change in the relative core thermal output and the core average moderator void ratio is defined by Equation 2.
[0025]
[Expression 2]
B = FB (P2, V2) -FB (P1, V1)
Here, P1 is the relative core heat output at the prediction start point, P2 is the relative core heat output at the prediction target point, V1 is the core average moderator void rate at the prediction start point, and V2 is the core average moderator void rate at the prediction target point. FB (P, V) represents the relative value of the eigenvalues in the relative heat output P and the core average moderator void ratio V, and is defined by the following formula 3 in this embodiment.
[0026]
[Equation 3]
FB (P, V) = P * (FV (1) * V + FV (2) * V * V)
FV (1) and FV (2) represent the correlation coefficient between the relative core thermal output, the core average moderator void ratio and the critical eigenvalue, and are stored in the target critical eigenvalue calculation database 6.
[0027]
The eigenvalue change amount C with respect to the change in the relative thermal power in the core at the axial node position adjacent to the control rods of the four fuel assemblies surrounding the inserted control rod is defined by the following equation 4.
[0028]
[Expression 4]
C = FC (R2) -FC (R1)
Here, R1 is a value calculated based on the relative heat output in the core at the axial node position adjacent to the control rods of the four fuel assemblies surrounding the control rod to be inserted at the prediction start point, and R2 is the prediction target point The value calculated on the basis of the relative thermal power in the core at the axial node position adjacent to and at FC (R) is the value of the axial node position adjacent to the control rod of the four fuel assemblies surrounding the inserted control rod. This represents an eigenvalue in the value R calculated based on the relative heat output in the core. The value of FC (R) is stored in the target critical eigenvalue calculation database 6.
[0029]
The eigenvalue change amount D with respect to the change in the core average xenon concentration is defined by the following equation 5.
[0030]
[Equation 5]
D = FX (X2) −FX (X1)
Here, X2 represents the core average xenon concentration at the prediction target point, X1 represents the core average xenon concentration at the prediction start point, and FX (X) represents an assumed value of the eigenvalue in the core average xenon concentration X. The FX value shown here is stored in the target critical eigenvalue calculation database 6.
[0031]
The target critical eigenvalue of the prediction target point is internally calculated by the following equation 6 by the change of each index described above.
[0032]
[Formula 6]
K2 = K1 + A + B + C + D
Here, K2 is a target critical eigenvalue calculated internally at the prediction target point, K1 is a critical eigenvalue at the prediction start point, A is an eigenvalue change amount with respect to core average burnup change, and B is a change in core thermal output and core average void ratio. , C is the eigenvalue change amount with respect to the change calculated based on the relative heat output in the core at the axial node position adjacent to the control rods of the four fuel assemblies surrounding the inserted control rod, and D is the core It represents the eigenvalue change amount with respect to the change of the average xenon concentration.
[0033]
FIG. 3 shows the effect of the present invention. In FIG. 3, the horizontal axis represents time, the vertical axis represents the critical eigenvalue, the solid line represents the critical eigenvalue based on the operation performance tracking analysis, and the alternate long and short dash line represents the target critical eigenvalue calculated internally by the present invention. Both agree well, and the prediction calculation can be performed with high accuracy without the user inputting the target critical eigenvalue.
[0034]
【The invention's effect】
According to the present invention, an internal calculation of a target critical eigenvalue can be performed using an index representing a core state, and a core performance prediction calculation can be performed with high accuracy.
[Brief description of the drawings]
FIG. 1 is a block diagram of a reactor core performance calculation apparatus according to the present invention.
FIG. 2 is a view showing a control rod to be inserted and four fuel assemblies surrounding the control rod.
FIG. 3 is a characteristic diagram of an internally calculated target critical eigenvalue showing the effect of the present invention.
FIG. 4 is a flowchart in the prediction calculation of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Reactor core performance calculation apparatus, 2 ... Input processing part, 3 ... Three-dimensional core calculation part, 4 ... Core performance result data storage part, 5 ... Target critical eigenvalue calculation part, 6 ... Target critical eigenvalue calculation database, 7 ... reactor, 8 ... core, 9 ... in-core neutron measuring instrument, 10 ... control rod, 11 ... four fuel assemblies surrounding the control rod 10.

Claims (3)

In the core performance prediction calculation that predicts the core performance of the reactor at the prediction target point that has elapsed from the prediction start point by the three-dimensional core calculation , adjacent to the control rods of the four fuel assemblies surrounding the inserted control rods using a value based on a change from the predicted starting point incore relative thermal output of the target axial node position in the prediction target point calculated by the three-dimensional core calculation of the target in the prediction target point Reactor core performance calculation device characterized in that critical eigenvalues are internally calculated and set.   The reactor core performance calculation apparatus according to claim 1, wherein a plurality of indexes representing a core state used when the target critical eigenvalue is internally calculated and set are used.   The reactor core performance calculation apparatus according to claim 1 or 2, wherein the core average burnup, relative core thermal output, core average deceleration are used as indices for expressing the core state used when the target critical eigenvalue is internally calculated and set. Reactor core performance calculation system using material void ratio and core average xenon concentration.

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