Reactor CHF evaluation method and calculation device

Abstract

A reactor CHF assessment method comprises the steps that temperature curves of a plurality of test points of a CHF test device are provided, the heating rates of the test points are calculated according to the temperature curves, and the test points comprise all positions where CHF risks exist; drawing a heating rate curve and carrying out moving average treatment; screening CHF moments with the heating rate exceeding a given heating rate threshold in all the heating curves, and determining the earliest CHF moment as a characteristic moment; and determining fluid conditions at the characteristic moment, and calculating the heat flux density at the characteristic moment to obtain a CHF value. According to the method, the CHF occurrence time and the CHF value can be determined through quantitative calculation, and the efficiency and accuracy of reactor CHF evaluation are improved. The invention further provides a reactor CHF evaluation and calculation device.

Description

Technical Field

[0001] The present invention belongs to the field of nuclear power, and in particular relates to a reactor CHF evaluation method and a calculation device. Background Art

[0002] In nuclear power plants with a liquid core loop, the coolant exchanges heat across the fuel element surface, removing the heat generated by the fuel. When the heat flux (heat flux) on the fuel element surface reaches a certain level, the liquid coolant vaporizes locally on the fuel element surface, transforming into bubbles and causing nucleate boiling. As the heat flux increases further, the bubbles connect to form a gas film covering a portion of the surface, leading to film boiling. Due to the poor heat transfer capacity of the gas film, the presence of a gas film on the fuel element surface can cause a rapid temperature increase, leading to fuel element burnout. The heat flux at this point is called the critical heat flux (CHF). In pressurized water reactors, the CHF value is a key limiting thermal parameter. Currently, the transition to boiling on the fuel element surface relies primarily on CHF testing. However, the onset of the CHF is primarily determined manually by observing the temperature-time curve of the heating rod. This is inefficient, highly random, and prone to overestimation or underestimation of the CHF value. Therefore, developing an accurate CHF assessment method is of great significance for improving reactor safety management. Summary of the Invention

[0003] The present invention aims to provide a reactor CHF estimation method to improve the estimation accuracy of CHF occurrence time and CHF value. The present invention also provides a reactor CHF estimation calculation device.

[0004] According to an embodiment of one aspect of the present invention, a method for evaluating reactor CHF is provided, the method comprising the following steps:

[0005] Step a): providing temperature curves of multiple test points in a CHF test device, and calculating the heating rate of the test points according to the temperature curves of the test points, wherein the test points include all locations where there is a risk of CHF under the test conditions;

[0006] Step b): plotting the heating rate data of the test points into a heating rate curve, and performing a sliding average process on the heating rate curve;

[0007] Step c): determining a heating rate threshold, and searching for a moment exceeding the heating rate threshold in the heating rate curves of all the test points as a CHF moment;

[0008] Step d): selecting the earliest CHF moment as the characteristic moment, and taking the test point at which the heating rate at the characteristic moment exceeds the heating rate threshold as the characteristic point;

[0009] Step e): determining the fluid condition at the characteristic moment and calculating the heat flux density at the characteristic moment.

[0010] This method can quantitatively determine the onset time and specific CHF value of CHF, improve the accuracy of CHF assessment, eliminate the error of manual judgment, and facilitate automated processing.

[0011] Furthermore, in some embodiments, in step a), the heating rate is calculated as follows: dT = (T2 - T1) / Δt, where dT is the heating rate, and T1 and T2 are the temperatures at two moments separated by Δt on the temperature curve.

[0012] Furthermore, in some embodiments, in step b), the sliding average processing is calculated using 10-100 data points.

[0013] Furthermore, in some embodiments, in step c), the threshold of the heating rate is 0.1°C / s-10°C / s.

[0014] Furthermore, in some embodiments, in step e), the heat flux density at the characteristic moment is calculated as follows: CHF =f(z)Q / A, where P CHF is the heat flux density at the characteristic moment, f(z) is the axial power factor of the characteristic point, z is the axial position of the characteristic point, Q is the total power of the heating rod of the CHF test device where the characteristic point is located, and A is the heating section area of ​​the heating rod.

[0015] Furthermore, in some embodiments, the fluid conditions include inlet fluid temperature, mass flow rate, and outlet pressure.

[0016] Furthermore, in some embodiments, the test points include all positions where there is a risk of CHF under the test conditions, and at least four measuring points are axially arranged on each heating rod of the CHF test device.

[0017] Furthermore, in some embodiments, the reactor CHF assessment method is used for pressurized water reactor CHF assessment.

[0018] According to another embodiment of the present invention, a reactor CHF assessment calculation device is provided, which includes a memory and a processor, wherein the memory stores a calculation program, and when the calculation program is executed by the processor, the reactor CHF assessment method provided in any of the aforementioned embodiments can be implemented. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1Schematic diagram of the test point temperature and heating rate curve in one embodiment;

[0020] Figure 2 Schematic diagram of the test system state curve in one embodiment.

[0021] The purpose of the above drawings is to provide a detailed description of the present invention so that those skilled in the art can understand the technical concept of the present invention, and is not intended to limit the present invention. DETAILED DESCRIPTION

[0022] The present invention will be further described in detail below through specific embodiments with reference to the accompanying drawings.

[0023] Mention of "embodiment" herein means that the specific features, structures or characteristics described in conjunction with the embodiment may be included in at least one embodiment herein. The phrase appearing in various places in the specification does not necessarily refer to the same embodiment, nor is it limited to mutually exclusive independent or alternative embodiments. It should be understood by those skilled in the art that the embodiments herein can be combined with other embodiments under the premise that no structural conflict occurs. In the description herein, terms such as "first" and "second" are only used to distinguish different objects, and cannot be understood as indicating relative importance or limiting the number, specific order or primary and secondary relationship of the technical features described. In the description herein, the meaning of "multiple" is at least two.

[0024] Heat flux refers to the amount of heat that passes through a unit cross-sectional area on the surface of a heating element per unit time. For a solid-liquid heat exchange interface, the liquid medium primarily achieves relatively stable heat exchange through convection.

[0025] However, in actual engineering, the heat exchange conditions in a reactor core with a liquid circuit are more complex. For example, in a pressurized water reactor (PWR) reactor, in the reactor's primary circuit, the primary cooling water exchanges heat with the surface of the fuel element. Due to the high heat flux density on the fuel element surface, the cooling water, under high temperature and pressure, partially vaporizes and forms bubbles on the fuel element surface. This state is called nucleate boiling. Under certain operating conditions, as the heat flux density on the fuel element surface increases further, the bubble density increases until the bubbles connect to form a gas film covering part of the surface, entering a film boiling state. In this film boiling state, because the thermal conductivity of the gas film is significantly weaker than that of liquid cooling water, deteriorating heat transfer while the heat flux density on the fuel element surface remains unchanged can lead to a rapid increase in the fuel element temperature and subsequent burnout. Therefore, monitoring the heat exchange conditions on the fuel element surface to prevent deviation from nucleate boiling into film boiling is crucial for reactor operational safety. The critical heat flux density (CHF) is the heat flux density that causes the fuel element surface to deviate from nucleate boiling and enter film boiling. The core should not deviate from nucleate boiling under normal reactor operation or moderate frequency accident conditions. The CHF value is an important limiting thermal parameter for determining whether deviation from the nucleate boiling state occurs.

[0026] Due to the complex causes and specific processes that lead to deviations from nucleate boiling, current research on the critical phenomena of boiling state transitions on the fuel element surface relies primarily on CHF tests. CHF tests typically use a rod bundle test piece consisting of a small-scale grid structure that characterizes the fuel assembly grid design and a full-length electric heating rod that simulates the heat release process of the core fuel rods. These tests are conducted in a high-temperature, high-pressure test loop under varying pressures, inlet coolant flow rates, and inlet coolant temperatures to obtain a time-varying curve of the heating rod surface temperature. This allows for analysis and determination of the CHF value at which heat transfer deteriorates on the heating rod surface. However, this process primarily relies on manual selection of the CHF onset time by observing the trend of the heating rod temperature-time curve. This manual selection is inefficient, highly random, and poorly repeatable, making it prone to overestimation or underestimation of the CHF value, making further improvement in CHF assessment difficult.

[0027] To solve the above problems, an embodiment of the present invention provides a reactor CHF assessment method that can accurately assess the CHF occurrence moment and the CHF value at the moment of heat transfer deterioration, thereby improving the efficiency and accuracy of CHF assessment.

[0028] The method comprises the following steps:

[0029] Step a): providing temperature curves of multiple test points of a CHF test device under given working conditions, and calculating the heating rates of the test points according to the temperature curves.

[0030] Specifically, the temperature curve can be obtained through actual measurement of a CHF test device. The CHF test device can adopt a scaled-down test device that can reflect the characteristics of the reaction fuel assembly, which has multiple full-length electric heating rods for simulating the fuel assembly in the reactor. Multiple test points are selected on the electric heating rods, and each test point is provided with a thermocouple for measuring the temperature of the test point. The setting position of the thermocouple covers all risk positions where CHF may occur. The position of the test point can be determined by conducting multiple tests statistically, or it can be determined based on the results of simulation calculations. The temperature change of the test point over time under the test conditions is measured to obtain a temperature curve.

[0031] In some embodiments, the temperature curve of the test point may also be derived from temperature state data under different accident conditions generated according to historical tests or based on a probabilistic safety model.

[0032] In a preferred embodiment, the heating rate dT is calculated as follows: dT = (T2-T1) / Δt, where T1 and T2 are the temperatures at two moments separated by Δt on the temperature curve. The selection of Δt can be determined based on the frequency of thermocouple data acquisition.

[0033] Step b): plotting the heating rate data of the test points into a heating rate curve and performing a sliding average process.

[0034] Due to data noise or other interference factors, the smoothness of the directly calculated heating rate curve may be poor and needs to be smoothed first. The heating rate curve is sampled at multiple points and then averaged to remove burrs.

[0035] The number of data points used for sliding average processing is determined according to the sampling frequency, and is preferably 10-100 data points.

[0036] Step c): determining a heating rate threshold, and searching the heating rate curves of all test points for a moment exceeding the heating rate threshold as the CHF moment of the heating rate curve.

[0037] The heating rate threshold is determined based on the actual test power loading rate. In a preferred embodiment, the threshold range is 0.1°C / s to 10°C / s. A heating rate exceeding the threshold at a test point indicates that the heating rod wall temperature at that location has transitioned from a slow rise to a rapid rise, a typical sign of a CHF. The earliest data point in a heating rate curve where the heating rate exceeds the threshold is considered the CHF moment for that test point.

[0038] Step d): Select the earliest CHF moment as the characteristic moment, and take the test point where the heating rate at the characteristic moment exceeds the heating rate threshold as the characteristic point.

[0039] The characteristic point is the location where CHF first appears under the test condition, and the characteristic moment is the time when CHF occurs under the test condition.

[0040] Step e): Determine the fluid condition at the characteristic moment and calculate the heat flux density at the characteristic moment.

[0041] The CHF test device measures system parameters such as fluid inlet temperature, mass flow rate and outlet pressure at different times. After determining the characteristic time, the system parameters such as inlet temperature, mass flow rate and outlet pressure when CHF occurs can be further obtained.

[0042] In a preferred embodiment, the CHF value can be calculated as follows:

[0043] P CHF =f(z)Q / A.

[0044] Among them, P CHF is the critical heat flux density, f(z) is the axial power factor of the heating rod where the characteristic point is located, z is the axial position of the characteristic point on the heating rod, Q is the total power of the heating rod where the characteristic point is located, and A is the heating section area of ​​the heating rod.

[0045] Therefore, the CHF occurrence moment, i.e., the CHF value, of the test device under given operating conditions can be determined by quantitative calculation, effectively improving the efficiency and accuracy of reactor CHF assessment.

[0046] The above method can be implemented through manual calculation or automatically executed by a computing device. The computing device's memory stores a pre-written calculation program, which, when executed by the computing device's processor, can implement the CHF assessment method provided in the above embodiment. The computing device can be a general-purpose computer, a dedicated computing device, a cloud computing device, or a virtual machine; the calculation program can be written using general-purpose computing software such as MATLAB, or a specially constructed dedicated calculation program. During the analysis and calculation, the test data used in the above method can be manually input into the computing device, pre-stored in a database and automatically retrieved by the computing device, or connected to a CHF test device signal to allow the computing device to read the corresponding test data and perform calculations synchronously with the progress of the test.

[0047] In a preferred embodiment, the CHF test device uses a 5×5 typical grid cell non-uniform heating device with a total of 25 non-uniform electric heating rods. Each heating rod has seven test points set along the axial direction. The test device is placed in a high-temperature, high-pressure water environment to conduct a CHF simulation test, and a total of 175 temperature curves are collected. It is difficult to effectively and effectively identify the CHF moment from 175 temperature curves manually. CHF assessment is performed using the following methods:

[0048] The temperature curve measured by thermocouple at one of the test points is as follows: Figure 1 As shown, curve 1 is the measured temperature curve, the heating rate is calculated according to dT = (T2-T1) / Δt and the heating rate curve 2 is plotted, and the heating rate curve is subjected to a 10-point sliding average process to obtain curve 3.

[0049] According to the heating rod power set in the experiment, the heating rate threshold is set to 0.1℃ / s. Figure 1 Curve 3 in the figure is screened to determine that the moment when the heating rate reaches the heating rate threshold is point A, which is 149s. This is the moment when CHF occurs. Based on the temperature curve, it can be further determined that the temperature when CHF occurs is 380.2℃.

[0050] Repeat the above steps for the remaining 174 temperature curves, determine the earliest CHF occurrence time as the characteristic time, and use the corresponding test point as the characteristic point.

[0051] According to P CHF=f(z)Q / A to calculate the CHF value of the characteristic point, and combined with the measurement results of the CHF test device, we can get the following Figure 2 The system state curve is shown. Curve 4 is the power curve of the electric heating rod, curve 5 is the wall temperature curve of the electric heating rod, curve 6 is the pressure curve, curve 7 is the inlet temperature curve, and curve 8 is the flow curve. The time represented by the dotted line B is determined to be the time when CHF occurs under the final test conditions.

[0052] In this way, the above method can obtain the time and location of the CHF phenomenon in the CHF test device under the test conditions, as well as system information such as the inlet temperature, pressure, mass flow rate under the corresponding state, so as to provide more sufficient support and guidance for the reactor safety performance design and management.

[0053] The reactor CHF assessment method provided by the above embodiment can more accurately determine the occurrence time of CHF, avoid the problem of overestimation or underestimation of CHF value caused by manual judgment, and is conducive to improving the efficiency of CHF assessment work with the help of a computing device.

[0054] The purpose of the above embodiments is to provide a further detailed description of the present invention in conjunction with the accompanying drawings so that those skilled in the art can understand the technical concept of the present invention. Within the scope of the present invention, optimization or equivalent replacement of the method steps involved, as well as combination of implementation methods in different embodiments without conflict of structure and principle, all fall within the scope of protection of the present invention.

Claims

1. A reactor CHF assessment method, characterized in that: The following steps are involved: Step a): providing temperature curves of multiple test points in a CHF test device, and calculating the heating rate of the test points according to the temperature curves of the test points, wherein the test points include all positions where there is a risk of CHF under the test conditions; Step b): plotting the heating rate data of the test points into a heating rate curve, and performing sliding average processing on the heating rate curve; Step c): determining a heating rate threshold, and searching for a time exceeding the heating rate threshold in the heating rate curves of all the test points as a CHF time; Step d): Select the earliest CHF moment as the characteristic moment, and take the test point at which the heating rate at the characteristic moment exceeds the heating rate threshold as the characteristic point; Step e): determine the fluid condition at the characteristic moment, and calculate the heat flux density at the characteristic moment.

2. The reactor CHF evaluation method according to claim 1, characterized in that: In the step a), the heating rate is calculated as follows: dT=(T2-T1) / Δt, wherein dT is the heating rate, and T1 and T2 are the temperatures at two moments separated by Δt on the temperature curve.

3. The reactor CHF evaluation method according to claim 1 or 2, characterized in that: In the step b), the sliding average process is calculated using 10-100 data points.

4. The reactor CHF evaluation method according to claim 1 or 2, characterized in that: In the step c), the threshold of the heating rate is 0.1°C / s-10°C / s.

5. The reactor CHF evaluation method according to claim 1 or 2, characterized in that: In the step e), the heat flux density at the characteristic moment is calculated as follows: CHF =f(z)Q / A, where P CHF is the heat flux density at the characteristic moment, f(z) is the axial power factor of the characteristic point, z is the axial position of the characteristic point, Q is the total power of the heating rod of the CHF test device where the characteristic point is located, and A is the heating section area of ​​the heating rod.

6. The reactor CHF evaluation method according to claim 1 or 2, characterized in that: The fluid conditions include inlet fluid temperature, mass flow rate, and outlet pressure.

7. The reactor CHF evaluation method according to claim 1 or 2, characterized in that: The test points include all positions where there is a risk of CHF under the test conditions, and at least four measuring points are axially arranged on each heating rod of the CHF test device.

8. The reactor CHF evaluation method according to claim 1 or 2, characterized in that: The reactor CHF evaluation method is used for pressurized water reactor CHF evaluation.

9. A reactor CHF evaluation calculation device, comprising a memory and a processor, characterized in that: The memory stores a calculation program, and when the calculation program is executed by the processor, the reactor CHF estimation method according to any one of claims 1 to 8 can be implemented.

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