Method for determining low-flow CHF test working condition of fuel assembly

Abstract

The invention mainly relates to the technical field of nuclear power reactors, in particular to a method for determining a low-flow CHF test working condition of a fuel assembly. The method comprises the steps that aiming at the working condition that the flow range is 20-300 kg / (m < 2 >. S), a plurality of first CHF values are calculated through a plurality of first CHF relational expressions; the first CHF relational expression with the minimum deviation between the first CHF value and the calculation result of the WLOP relational expression based on the sub-channel program is selected as the CHF relational expression of the low-flow working condition; a plurality of low-flow CHF test working conditions are constructed according to the CHF relational expression of the low-flow working conditions and the flow range of 20-300 kg / (m < 2 >. S); and screening the plurality of low-flow CHF test working conditions according to a working condition effectiveness screening criterion to obtain a final CHF test working condition.

Description

Technical Field

[0001] The invention mainly relates to the technical field of nuclear power reactors, and in particular to a method for determining a low-flow CHF test condition of a fuel assembly. Background Art

[0002] Critical heat flux (CHF) refers to the amount of heat transferred per unit surface area per unit time when the boiling heat transfer mechanism changes, resulting in deterioration of heat transfer on the surface of the heating element. It is a key thermal performance parameter of the fuel assembly and an important limiting thermal parameter related to reactor safety. Its size directly affects the safety and economy of the reactor.

[0003] The CHF test of fuel assemblies usually uses a heating rod to simulate the nuclear heat release process, and monitors the temperature change at a specific position on the surface of the heating rod. The CHF test of fuel assemblies usually uses a direct heating electric heating rod. Under low pressure (usually considered to be below 10MPa) and low flow (usually considered to be below 1000kg / (m2·s)) conditions, the CHF of fuel assemblies usually occurs in the high gas content area, which is prone to flow instability. Under low pressure, the vaporization of the coolant causes its volume change range to increase, and the vaporization area blocks the flow channel, which may cause the coolant to further vaporize, forming a positive feedback effect. In the above process, if the test personnel are inexperienced and fail to take timely actions, the heating rod of the fuel assembly test piece may burn out due to the soaring temperature.

[0004] Therefore, the CHF test of fuel assemblies under low pressure and low flow conditions has a high risk. In order to reduce the test risk, ensure the reliability of the CHF test conditions of fuel assemblies under low flow test conditions, and provide technical support for the smooth implementation of the low flow CHF test of fuel assemblies, it is necessary to establish a method for determining the low flow CHF test conditions of fuel assemblies. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a method for determining a low-flow CHF test condition of a fuel assembly, thereby providing a technical guarantee for the smooth implementation of a low-flow CHF test of a fuel assembly.

[0006] To solve the above technical problems, the present invention provides a method for determining a low-flow CHF test condition of a fuel assembly, comprising: for a condition with a flow range of 20 to 300 kg / (m2·s), using multiple first CHF relationship equations to calculate multiple first CHF values; selecting the first CHF relationship equation with the smallest deviation between the first CHF value and the calculation result of the WLOP relationship equation based on the sub-channel program as the CHF relationship equation for the low-flow condition; constructing multiple low-flow CHF test conditions based on the flow range of 20 to 300 kg / (m2·s) and the CHF relationship equation for the low-flow condition; screening the multiple low-flow CHF test conditions according to the condition validity screening criteria to obtain the final CHF test condition.

[0007] Optionally, the plurality of first CHF relationship equations include: a look-up table method of rod bundle correction, a Griffith-Zuber relationship equation and a MacBeth-CE relationship equation.

[0008] Optionally, the Griffith-Zuber relationship is:

[0009]

[0010] In the formula,

[0011] α: vacuolar fraction;

[0012] h fg : Latent heat of vaporization, J / kg;

[0013] ρ g : saturated vapor density, kg / m3;

[0014] ρ f : saturated liquid density, kg / m3;

[0015] σ: surface tension, N / m;

[0016] g: acceleration due to gravity, 9.8 m / s2;

[0017] q CHF : Critical heat flux, W / m2.

[0018] Optionally, the MacBeth-CE relationship is:

[0019]

[0020] In the formula,

[0021] CHF: critical heat flux,

[0022] G L : average mass flow rate,

[0023] hfg : Latent heat of vaporization,

[0024] d e : Thermal equivalent diameter,

[0025] P h : Heating circumference,

[0026] P w : Wetted perimeter,

[0027] x L : Steam content,

[0028] f axial : Axial non-uniformity factor.

[0029] Optionally, the operating condition effectiveness screening criteria include:

[0030] (1) The working condition converges normally;

[0031] (2) The location of CHF is reasonable;

[0032] (3) The absolute value of the deviation of MDNBR from 1 is less than 5%;

[0033] (4) The upper limit of the local steam content is 0.9, and the lower limit of the local steam content is -0.5;

[0034] (5) The local steam content is evenly distributed.

[0035] Optionally, it also includes: for zero flow conditions, using multiple second CHF relationship formulas to calculate multiple second CHF values; comparing the multiple second CHF values ​​with TRIGA test data, and selecting one of the second CHF relationship formulas as the CHF relationship formula for the zero flow condition based on conservatism and condition connectivity; constructing multiple zero flow CHF test conditions based on the zero flow condition and the CHF relationship formula for the zero flow condition; screening the multiple zero flow CHF test conditions according to the condition validity screening criteria to obtain the final CHF test condition.

[0036] Optionally, the plurality of second CHF relationship equations include: an El-Genk relationship equation, a Mishima and Ishii relationship equation, and a Block and Wallis relationship equation.

[0037] Optionally, the El-Genk relationship is:

[0038]

[0039] In the formula,

[0040] Δh i : Inlet subcooling,

[0041] L: Heating length,

[0042] h fg : Latent heat of vaporization,

[0043] d e : Hydraulic diameter.

[0044] Optionally, the Mishima and Ishii relationship is:

[0045]

[0046] In the formula,

[0047] D: hydraulic diameter,

[0048] C0: constant,

[0049] h fg : Latent heat of vaporization,

[0050] Δh i : Inlet subcooling,

[0051] ρ g : saturated vapor density;

[0052] g: acceleration due to gravity;

[0053] G: mass flow rate;

[0054] Δρ: density difference between saturated vapor and liquid.

[0055] Optionally, the Block and Wallis relationship is:

[0056]

[0057] In the formula,

[0058] D: hydraulic diameter;

[0059] Δρ: saturated vapor-liquid density difference;

[0060] g: acceleration due to gravity;

[0061] ρ g : saturated vapor density;

[0062] ρ f : saturated liquid density;

[0063] h fg : Latent heat of vaporization;

[0064] C: Import coefficient, take 1.

[0065] Compared with the prior art, the present invention has the following advantages:

[0066] The present invention provides a method for determining a low-flow CHF test operating condition for a fuel assembly. Based on the above method, the reliability of the CHF test operating condition of the fuel assembly under low-flow test conditions can be ensured, providing technical guarantee for the smooth implementation of the low-flow CHF test of the fuel assembly. The present invention proposes a method for determining an operating condition for a low-flow CHF test of a fuel assembly, which can more reasonably formulate an operating condition matrix for the low-flow CHF test of the fuel assembly, and has very important practical significance for ensuring the feasibility and safety of the test. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The accompanying drawings are included to provide a further understanding of the present application. They are included and constitute a part of the present application. The accompanying drawings illustrate embodiments of the present application and together with the present specification serve to explain the principles of the present application. In the accompanying drawings:

[0068] Figure 1 4 is a flow chart of a method for determining a low flow CHF test condition of a fuel assembly according to an embodiment of the present invention.

[0069] Figure 2 yes Figure 1 Flow chart of the method for determining low flow CHF test conditions for fuel assemblies of an optimized embodiment.

[0070] Figure 3 Schematic diagram of a method for determining a low flow CHF test condition of a fuel assembly according to an embodiment of the present invention. DETAILED DESCRIPTION

[0071] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for the description of the embodiments. Obviously, the drawings described below are only some examples or embodiments of the present application. For ordinary technicians in this field, the present application can also be applied to other similar scenarios based on these drawings without creative work. Unless it is obvious from the language environment or otherwise explained, the same reference numerals in the figures represent the same structure or operation.

[0072] The method for determining the low flow CHF test working condition of a fuel assembly of the present invention is mainly based on a single channel analysis model, considers using relationship expressions corresponding to different flow rates, and determines the low flow CHF test working condition of the fuel assembly according to working condition effectiveness screening criteria.

[0073] Figure 1 1 is a flow chart of a method for determining a low flow CHF test condition of a fuel assembly according to an embodiment of the present invention. Figure 1 As shown, the method 100 for determining the low flow CHF test condition of a fuel assembly includes:

[0074] Step S11: For a flow rate range of 20 to 300 kg / (m 2 ·s), multiple first CHF values ​​are calculated using multiple first CHF relationship expressions.

[0075] Optionally, the plurality of first CHF relationship equations include: a look-up table method of rod bundle correction, a Griffith-Zuber relationship equation and a MacBeth-CE relationship equation.

[0076] The table lookup method for rod bundle correction refers to looking up the corresponding CHF value from a preset table based on the flow value.

[0077] Alternatively, the Griffith-Zuber relation is:

[0078]

[0079] In the formula,

[0080] α: vacuolar fraction;

[0081] h fg : Latent heat of vaporization, J / kg;

[0082] ρ g : saturated vapor density, kg / m3;

[0083] ρ f : saturated liquid density, kg / m3;

[0084] σ: surface tension, N / m;

[0085] g: acceleration due to gravity, 9.8 m / s2;

[0086] q CHF : Critical heat flux, W / m2.

[0087] Alternatively, the MacBeth-CE relation is:

[0088]

[0089] In the formula,

[0090] CHF: critical heat flux,

[0091] G L : average mass flow rate,

[0092] h fg : Latent heat of vaporization,

[0093] d e : Thermal equivalent diameter,

[0094] P h : Heating circumference,

[0095] P w : Wetted perimeter,

[0096] x L : Steam content,

[0097] f axial : Axial non-uniformity factor.

[0098] Step S12: Selecting the first CHF relationship equation with the smallest deviation between the first CHF value and the calculation result of the WLOP relationship equation based on the sub-channel program as the CHF relationship equation for the low flow condition.

[0099] The WLOP relationship is a relationship fitted by Westinghouse Electric Company of the United States through the CHF data of the rod bundle. The coefficients of the WLOP relationship are not disclosed. The WLOP relationship is configured in the subchannel program.

[0100] Deviations between the calculation results of the plurality of first CHF values ​​and the WLOP relationship based on the subchannel program are calculated, and the first CHF relationship with the smallest deviation between the calculation results of the first CHF value and the WLOP relationship based on the subchannel program is selected as the CHF relationship for the low flow condition.

[0101] Through experiments, it is found that the first CHF value calculated by the lookup table method with rod bundle correction has the smallest deviation from the calculation result of the WLOP relation based on the subchannel program. Therefore, the lookup table method with rod bundle correction is selected as the CHF relation for low flow conditions.

[0102] Optionally, the method further includes comparing the multiple first CHF values ​​with the TRIGA test data, and selecting one of the first CHF relationship equations as the CHF relationship equation for the low flow condition based on conservatism and working condition connectivity.

[0103] The TRIGA reactor is a research reactor with unique performance and wide application, which was researched and developed by General Atomics of the United States and is monopolized by it. The TRIGA reactor, also known as the uranium-zirconium hydrogen reactor, is a research reactor with high inherent safety that uses a uniform mixture of zirconium hydride and enriched uranium as fuel. The TRIGA test data is the CHF data obtained from the TRIGA reactor test.

[0104] Step S13: According to the flow rate range of 20 to 300 kg / (m 2 ·s) and the CHF relationship of low flow conditions to construct multiple low flow CHF test conditions.

[0105] Taking the lookup table method of rod bundle correction as an example, multiple low-flow CHF test conditions can be constructed according to the flow range and the lookup table method of rod bundle correction.

[0106] Step S14: screening multiple low-flow CHF test conditions according to the condition validity screening criteria to obtain the final CHF test condition.

[0107] Optionally, the operating condition effectiveness screening criteria include:

[0108] (1) The working condition converges normally;

[0109] (2) The location of CHF is reasonable;

[0110] (3) MDNBR is around 1 (the absolute value of the deviation is less than 5%);

[0111] DNBR is the abbreviation of Departure from Nucleate Boiling Ratio. The DNBR value varies with the axial position z of the coolant channel, and its minimum value is called MDNBR. If there is no error in the calculation formula of the critical heat flow, when MDNBR=1, it means that the fuel element has burned. Therefore, MDNBR is usually a design criterion for water reactors. For steady-state conditions and expected accident conditions, the value of MDNBR must be determined separately, and its specific value is related to the selected calculation formula.

[0112] (4) The upper limit of the local steam content is 0.9, and the lower limit of the local steam content is -0.5;

[0113] (5) The local steam content is evenly distributed.

[0114] The purpose of the present invention is to propose a method for determining a low-flow CHF test condition of a fuel assembly. Based on the above method, the reliability of the CHF test condition of the fuel assembly under low-flow test conditions can be ensured, providing technical guarantee for the smooth implementation of the low-flow CHF test of the fuel assembly.

[0115] Figure 2 yes Figure 1 Flow chart of the method for determining the low flow CHF test conditions of the fuel assembly of the optimized embodiment. Figure 2 As shown, the fuel assembly low flow CHF test condition determination method 200 also includes:

[0116] Step S15: for the zero flow condition, multiple second CHF values ​​are calculated using multiple second CHF relationship equations.

[0117] Optionally, the plurality of second CHF relational expressions include: an El-Genk relational expression, a Mishima and Ishii relational expression, and a Block and Wallis relational expression.

[0118] Alternatively, the El-Genk relationship is:

[0119]

[0120] In the formula,

[0121] Δh i : Inlet subcooling,

[0122] L: Heating length,

[0123] h fg : Latent heat of vaporization,

[0124] d e : Hydraulic diameter.

[0125] Alternatively, the Mishima and Ishii relationship is:

[0126]

[0127] In the formula,

[0128] D: hydraulic diameter,

[0129] C0: constant,

[0130] h fg : Latent heat of vaporization,

[0131] Δh i : Inlet subcooling,

[0132] ρ g : saturated vapor density;

[0133] g: acceleration due to gravity;

[0134] G: mass flow rate.

[0135] Δρ: Saturated vapor-liquid density difference, kg / m 3 .

[0136] Alternatively, the Block and Wallis relation is:

[0137]

[0138] In the formula,

[0139] D: hydraulic diameter;

[0140] Δρ: saturated vapor-liquid density difference;

[0141] g: acceleration due to gravity;

[0142] ρ g : saturated vapor density;

[0143] ρ f : saturated liquid density;

[0144] h fg : Latent heat of vaporization

[0145] C: Import coefficient, take 1.

[0146] Step S16: compare the multiple second CHF values ​​with the TRIGA test data, and select one of the second CHF relationship equations as the CHF relationship equation for the zero flow condition based on conservatism and working condition connectivity.

[0147] Preferably, the Block and Wallis relationship is selected as the CHF relationship for zero flow conditions.

[0148] Step S17: construct multiple zero flow CHF test conditions according to the zero flow condition and the CHF relationship of the zero flow condition.

[0149] Step S18: Screen multiple zero-flow CHF test conditions according to the working condition validity screening criteria to obtain the final CHF test condition.

[0150] Similarly, the working condition effectiveness screening criteria include:

[0151] (1) The working condition converges normally;

[0152] (2) The location of CHF is reasonable;

[0153] (3) MDNBR is around 1 (the absolute value of the deviation is less than 5%);

[0154] (4) The upper limit of the local steam content is 0.9, and the lower limit of the local steam content is -0.5;

[0155] (5) The local steam content is evenly distributed.

[0156] Figure 3 FIG. 1 is a schematic diagram of a method for determining a low flow CHF test condition of a fuel assembly according to an embodiment of the present invention. Figure 3 As shown, the method for determining the low flow CHF test conditions of the fuel assembly is mainly aimed at establishing test conditions at low flow (20-300 kg / (m2·s)) and zero flow.

[0157] For the flow range of 20 to 300 kg / (m2·s), equations 1 to 3 are used and the calculated results are compared with the WLOP equation calculation results based on the subchannel program and the TRIGA test data.

[0158] For zero flow conditions, equations 4 to 6 are used for calculations, and the calculation results are compared with the TRIGA test data.

[0159] Preferably, by comparing the rationality of the deviation and the feasibility of covering the applicable operating conditions through the calculation results, the lookup table method of the bundle correction and the Block and Wallis relationship are suitable for the CHF prediction of the fuel assembly under low flow rate (20-300kg / (m2·s)) and zero flow rate, respectively.

[0160] Preferably, the following working condition effectiveness screening criteria are used:

[0161] (1) Normal convergence of working conditions

[0162] (2) Reasonable location of CHF

[0163] (3) MDNBR is around 1 (the absolute value of the deviation is less than 5%)

[0164] (4) The upper limit of the local steam content is 0.9, and the lower limit of the local steam content is -0.5

[0165] (5) The local steam content should be distributed as evenly as possible

[0166] The low flow CHF test conditions of the fuel assembly can be finally determined based on the lookup table method of the rod bundle correction and the Block and Wallis relationship.

[0167] Flowcharts are used in the present application to illustrate the operations performed by the system according to the embodiments of the present application. It should be understood that the preceding or following operations are not necessarily performed accurately in order. On the contrary, various steps may be processed in reverse order or simultaneously. At the same time, other operations may be added to these processes, or one or more operations may be removed from these processes.

[0168] The basic concepts have been described above. Obviously, for those skilled in the art, the above invention disclosure is only used as an example and does not constitute a limitation of the present application. Although not explicitly stated herein, those skilled in the art may make various modifications, improvements and amendments to the present application. Such modifications, improvements and amendments are suggested in the present application, so such modifications, improvements and amendments still belong to the spirit and scope of the exemplary embodiments of the present application.

[0169] At the same time, the present application uses specific words to describe the embodiments of the present application. For example, "one embodiment", "an embodiment", and / or "some embodiments" refer to a certain feature, structure or characteristic related to at least one embodiment of the present application. Therefore, it should be emphasized and noted that "one embodiment" or "an embodiment" or "an alternative embodiment" mentioned twice or more in different positions in this specification does not necessarily refer to the same embodiment. In addition, some features, structures or characteristics in one or more embodiments of the present application can be appropriately combined.

[0170] As shown in this application, unless the context clearly indicates an exception, the words "a", "an", "a kind" and / or "the" do not refer to the singular, but also include the plural. Generally speaking, the terms "include" and "comprise" only indicate the inclusion of the steps and elements that have been clearly identified, and these steps and elements do not constitute an exclusive list. The method or device may also include other steps or elements.

[0171] Unless otherwise specifically stated, the relative arrangement, numerical expressions and numerical values ​​of the parts and steps set forth in these embodiments do not limit the scope of the application. Meanwhile, it should be understood that, for ease of description, the sizes of the various parts shown in the accompanying drawings are not drawn according to actual proportional relationships. The technology, methods and equipment known to those of ordinary skill in the relevant art may not be discussed in detail, but in appropriate cases, the technology, methods and equipment should be considered as a part of the specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters represent similar items in the following drawings, and therefore, once a certain item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0172] In the description of the present application, it should be understood that the directions or positional relationships indicated by directional words such as "front, back, up, down, left, right", "lateral, vertical, perpendicular, horizontal" and "top, bottom" are usually based on the directions or positional relationships shown in the drawings, and are only for the convenience of describing the present application and simplifying the description. Unless otherwise specified, these directional words do not indicate or imply that the device or element referred to must have a specific direction or be constructed and operated in a specific direction, and therefore cannot be understood as limiting the scope of protection of the present application; the directional words "inside and outside" refer to the inside and outside relative to the contours of each component itself.

[0173] For ease of description, spatially relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below other devices or structures". Thus, the exemplary term "above" can include both "above" and "below". The device can also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used here are interpreted accordingly.

[0174] In addition, it should be noted that the use of words such as "first" and "second" to define components is only for the convenience of distinguishing the corresponding components. If not otherwise stated, the above words have no special meaning and cannot be understood as limiting the scope of protection of this application. In addition, although the terms used in this application are selected from well-known and commonly used terms, some terms mentioned in the specification of this application may be selected by the applicant at his or her discretion, and their detailed meanings are explained in the relevant parts of the description of this article. In addition, it is required to understand this application not only by the actual terms used, but also by the meaning implied by each term.

[0175] It should be understood that when a component is referred to as being "on another component," "connected to another component," "coupled to another component," or "contacting another component," it may be directly on, connected to, coupled to, or contacting the other component, or there may be intervening components. In contrast, when a component is referred to as being "directly on another component," "directly connected to," "directly coupled to," or "directly contacting" another component, there are no intervening components. Similarly, when a first component is referred to as being "electrically in contact with" or "electrically coupled to" a second component, there is an electrical path between the first component and the second component that allows current to flow. The electrical path may include capacitors, coupled inductors, and / or other components that allow current to flow, even without direct contact between conductive components.

[0176] Similarly, it should be noted that in order to simplify the description of the disclosure of this application and thus help understand one or more embodiments of the invention, in the above description of the embodiments of the present application, multiple features are sometimes combined into one embodiment, figure or description thereof. However, this disclosure method does not mean that the subject of the present application requires more features than the features mentioned. In fact, the features of the embodiment are less than all the features of the single embodiment disclosed above.

[0177] In some embodiments, numbers describing the number of components and attributes are used. It should be understood that such numbers used in the description of the embodiments are modified by the modifiers "about", "approximately" or "substantially" in some examples. Unless otherwise specified, "about", "approximately" or "substantially" indicate that the numbers are allowed to vary by ±20%. Accordingly, in some embodiments, the numerical parameters used in the specification are approximate values, which may change according to the required features of individual embodiments. In some embodiments, the numerical parameters should take into account the specified significant digits and adopt the general method of retaining the digits. Although the numerical domains and parameters used to confirm the breadth of the range in some embodiments of the present application are approximate values, in specific embodiments, the setting of such numerical values ​​is as accurate as possible within the feasible range.

[0178] Although the present application has been described with reference to the current specific embodiments, ordinary technicians in this technical field should recognize that the above embodiments are only used to illustrate the present application, and various equivalent changes or substitutions may be made without departing from the spirit of the present application. Therefore, as long as the changes and modifications to the above embodiments are within the essential spirit of the present application, they will fall within the scope of the present application.

Claims

1. A method for determining a fuel assembly low flow CHF test condition, characterized in that: include: For flow range of 20~300kg / (m 2 ·s), using multiple first CHF relationship formulas to calculate multiple first CHF values; Selecting the first CHF relational expression corresponding to the calculation result of the first CHF value and the WLOP relational expression based on the sub-channel program with the smallest deviation as the CHF relational expression for the low flow condition; According to the flow range of 20 ~ 300kg / (m 2 s) and the CHF relationship of the low flow condition to construct a plurality of low flow CHF test conditions; The multiple low-flow CHF test conditions are screened according to the working condition validity screening criteria to obtain the final CHF test condition.

2. The method for determining the low flow CHF test condition of a fuel assembly according to claim 1, characterized in that: The plurality of first CHF relations include: a look-up table method of rod bundle correction, a Griffith-Zuber relation and a MacBeth-CE relation.

3. The method for determining the fuel assembly low flow CHF test operating condition as claimed in claim 2, characterized in that: The Griffith-Zuber relationship is: In the formula, α: vacuolar fraction; h fg : Latent heat of vaporization, J / kg; ρ g : saturated vapor density, kg / m3; ρ f : saturated liquid density, kg / m3; σ: surface tension, N / m; g: acceleration due to gravity, 9.8 m / s2; q CHF : Critical heat flux, W / m2.

4. The method for determining the low flow CHF test condition of a fuel assembly as claimed in claim 2, characterized in that: The MacBeth-CE relationship is: In the formula, CHF: critical heat flux, G L : average mass flow rate, h fg : Latent heat of vaporization, d e : Thermal equivalent diameter, P h : Heating circumference, P w : Wetted perimeter, x L : Steam content, f axial : Axial non-uniformity factor.

5. The method for determining the low flow CHF test condition of a fuel assembly according to claim 1, characterized in that: The working condition effectiveness screening criteria include: (1) The working condition converges normally; (2) The location of CHF is reasonable; (3) The absolute value of the deviation of MDNBR from 1 is less than 5%; (4) The upper limit of the local steam content is 0.9, and the lower limit of the local steam content is -0.5; (5) The local steam content is evenly distributed.

6. The method for determining the low flow CHF test condition of a fuel assembly according to claim 1, characterized in that: Also includes: For the zero flow condition, multiple second CHF values ​​are calculated using multiple second CHF relationship equations; Compare multiple second CHF values ​​with TRIGA test data, and select one of the second CHF relations as the CHF relation for zero flow condition based on conservatism and working condition connectivity; Constructing a plurality of zero flow CHF test conditions according to the zero flow condition and the CHF relationship of the zero flow condition; The multiple zero-flow CHF test conditions are screened according to the working condition validity screening criteria to obtain a final CHF test condition.

7. The method for determining the low flow CHF test condition of a fuel assembly as claimed in claim 6, characterized in that: The plurality of second CHF relational expressions include: an El-Genk relational expression, a Mishima and Ishii relational expression, and a Block and Wallis relational expression.

8. The method for determining the low flow CHF test condition of a fuel assembly as claimed in claim 7, characterized in that: The El-Genk relationship is: In the formula, Δh i : Inlet subcooling, L: Heating length, h fg : Latent heat of vaporization, d e : Hydraulic diameter.

9. The method for determining the low flow CHF test condition of a fuel assembly according to claim 7, characterized in that: The Mishima and Ishii relationship is: In the formula, D: hydraulic diameter, C0: constant, h fg : Latent heat of vaporization, Δh i : Inlet subcooling, ρ g : saturated vapor density; g: acceleration due to gravity; G: mass flow rate; Δρ: density difference between saturated vapor and liquid.

10. The method for determining the low flow CHF test condition of a fuel assembly according to claim 7, characterized in that: The Block and Wallis relationship is: In the formula, D: hydraulic diameter; Δρ: saturated vapor-liquid density difference; g: acceleration due to gravity; ρ g : saturated vapor density; ρ f : saturated liquid density; h fg : Latent heat of vaporization; C: Import coefficient, take 1.

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