A quantitative evaluation method for the independence of defense-in-depth layers in nuclear power plants

Abstract

The present invention relates to a method for quantitatively evaluating the independence of defense-in-depth levels of a nuclear power plant, comprising the following steps: (1) determining basic information for quantitatively evaluating the independence of defense-in-depth levels; (2) quantifying the basic information according to a probabilistic safety analysis model; (3) calculating the correlation of defense-in-depth levels according to qualitative and quantitative information of defense-in-depth levels, and obtaining a quantitative evaluation result of the independence of defense-in-depth levels. The method provided by the present invention can analyze and understand the degree of independence between defense-in-depth levels in a more specific and objective manner, make up for the deficiency of only qualitative understanding, and support the optimization of the design of defense-in-depth levels of nuclear power plants.

Description

Technical Field

[0001] The present invention belongs to nuclear power plant design technology, and in particular relates to a method for quantitatively evaluating the independence of a nuclear power plant's defense-in-depth hierarchy. Background Art

[0002] The main means to prevent accidents in nuclear power plants and mitigate their consequences is to apply the concept of defense in depth, which prevents accidents from causing harm to personnel and the environment through the combination of a series of continuous and independent defense levels. Specifically, it includes: the first level prevents deviations from normal operation and prevents failures of important safety items; the second level detects and controls deviations from normal operating conditions to prevent expected operating events from escalating into accident conditions; the third level assumes that an accident will occur and returns the nuclear power plant to a safe state through safety systems and procedures; the fourth level mitigates the consequences of accidents caused by failure of the third level of defense in depth; the fifth level, which is the final level of defense, aims to mitigate the radioactive consequences caused by potential radioactive releases that may be caused by accident conditions.

[0003] Among them, the third and fourth levels are achieved by setting up nuclear power plant safety systems (facilities). For pressurized water reactor nuclear power plants, the design quality of these two levels largely determines the safety level of the nuclear power plant.

[0004] The various levels of defense in depth must be as independent as possible to avoid the failure of one level of defense reducing the effectiveness of other levels. The defense in depth design of traditional nuclear power plants is mainly carried out by means of determinism and engineering judgment. The understanding of the independence of the defense in depth levels is qualitative, and there is a lack of quantitative evaluation methods and understanding. Summary of the invention

[0005] The purpose of the present invention is to provide a quantitative evaluation method for the independence of the defense-in-depth levels of nuclear power plants in view of the deficiency of only qualitative understanding of the defense-in-depth design of traditional nuclear power plants, so as to describe the independence degree of the defense-in-depth levels more specifically and objectively and support the defense-in-depth design and optimization of nuclear power plants.

[0006] The technical solution of the present invention is as follows: A method for quantitatively evaluating the independence of the defense-in-depth layer of a nuclear power plant comprises the following steps:

[0007] (1) Determine the basic information for the quantitative evaluation of the independence of the defense-in-depth layers;

[0008] (2) quantifying the basic information according to a probabilistic safety analysis model;

[0009] (3) Based on the qualitative and quantitative information of the defense-in-depth levels, the correlation of the defense-in-depth levels is calculated to obtain the quantitative evaluation results of the independence of the defense-in-depth levels.

[0010] Furthermore, as described above, the quantitative evaluation method for the independence of the defense-in-depth levels of a nuclear power plant comprises the following: the third level assumes that an accident will occur and returns the nuclear power plant to a safe state through safety systems and procedures; and the fourth level mitigates the consequences of the accident caused by the failure of the defense-in-depth level of the third level.

[0011] Furthermore, in the above-mentioned quantitative evaluation method for the independence of the defense-in-depth level of a nuclear power plant, the basic information in step (1) includes: each initiating event, the frequency of occurrence of the initiating event, the system that needs to be put into operation after the occurrence of each initiating event, the defense-in-depth level to which the mitigation system belongs, etc.

[0012] Furthermore, in the above-mentioned quantitative evaluation method for independence of the defense-in-depth layers of a nuclear power plant, the quantification of the basic information in step (2) includes: calculating the probability of failure of each system and the frequency of occurrence of accident sequences.

[0013] Furthermore, in the above-mentioned quantitative evaluation method for the independence of the defense-in-depth levels of a nuclear power plant, the correlation calculation described in step (3) can be performed at different levels, including the defense-in-depth level correlation of an accident sequence, the defense-in-depth level correlation of an initiating event, and the defense-in-depth level correlation of the entire plant.

[0014] Further, in the above-mentioned quantitative evaluation method for independence of defense-in-depth levels of nuclear power plants, the method for calculating the correlation of defense-in-depth levels in step (3) is as follows:

[0015] For a single accident sequence, the interval [0,1] is used to represent the correlation between levels, and it is calculated by the following formula:

[0016]

[0017] Where D is the calculated correlation, S is the frequency of the accident sequence calculated by the probabilistic safety analysis (PSA) model, P(A|IE) is the probability of failure of system A, P(B|IE) is the probability of failure of system B, and F IE is the frequency of the initiating event; System A and System B are systems belonging to two levels of defense in depth. In a single accident sequence, System A is first deployed to mitigate the accident, and then System B is deployed to mitigate the accident;

[0018] Based on the correlation of a single accident sequence, the correlation between the defense-in-depth levels for a certain initiating event or state (such as DEC-A) can be obtained by weighting according to the probability of system failure. The calculation formula is as follows:

[0019]

[0020] Among them, i represents the accident sequence under a certain initiating event or state, from 1 to n.

[0021] Considering multiple initiating events or states, weighting them according to the probability of occurrence of the initiating events or states can obtain the correlation between the in-depth defense levels of the entire plant for all initiating events or states. The calculation formula is as follows:

[0022]

[0023] in, k represents the possible initiating events or states of a nuclear power plant, ranging from 1 to n.

[0024] Furthermore, in the above-mentioned quantitative evaluation method for the independence of the defense-in-depth level of a nuclear power plant, in step (3), in the case where no countermeasures are set for the defense-in-depth level, it is considered that the correlation between this defense-in-depth level and the previous level is 1.

[0025] Furthermore, the above-mentioned method for quantitatively evaluating the independence of the defense-in-depth levels of a nuclear power plant further includes step (4), which is to provide optimization feedback to the safety system based on the quantitative evaluation results of the independence of the defense-in-depth levels of a nuclear power plant.

[0026] The beneficial effects of the present invention are as follows: Based on the deterministic theory and engineering judgment of the in-depth defense of traditional nuclear power plants, the present invention combines the insights of probabilistic safety analysis (PSA) to provide a quantitative evaluation method for the independence of the in-depth defense levels of nuclear power plants (mainly the third and fourth levels), and more specifically and objectively analyzes and understands the degree of independence between the in-depth defense levels, making up for the shortcomings of the only qualitative understanding, and supporting the optimization of the in-depth defense level design of nuclear power plants. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a flow chart of a quantitative evaluation method for independence of defense-in-depth layers of a nuclear power plant in a specific embodiment of the present invention;

[0028] Figure 2 This is an example diagram of nuclear power plant accident mitigation in a specific embodiment of the present invention;

[0029] Figure 3 It is a basic information diagram of a quantitative evaluation table of independence of defense-in-depth levels of a nuclear power plant in a specific embodiment of the present invention;

[0030] Figure 4 It is a schematic diagram of correlation calculation of the quantitative evaluation table of independence of defense-in-depth levels of nuclear power plants in a specific embodiment of the present invention;

[0031] Figure 5 This is a basic information diagram of the quantitative evaluation table of independence of the defense-in-depth level for a large LOCA accident in a nuclear power plant;

[0032] Figure 6This is a schematic diagram of the correlation calculation of the quantitative evaluation table of independence of the defense-in-depth level for a large LOCA accident in a nuclear power plant (3a and 3b levels);

[0033] Figure 7 Schematic diagram of the correlation calculation for the quantitative evaluation table of independence of the defense-in-depth levels for a large LOCA accident in a nuclear power plant (levels 3b and 4). DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0035] After an accident occurs in a nuclear power plant, safety-related systems are put into use to mitigate the accident. Using the event tree (ET) in probabilistic safety analysis (PSA), we can Figure 2 As shown. After the initiating event (IE) occurs, system A is put into use to mitigate the accident. If system A operates successfully, the accident is mitigated; if system A fails, system B is put into use to mitigate the accident; if both system A and system B fail, the accident cannot be mitigated. System A and system B are systems belonging to two levels of defense in depth.

[0036] In the above example, the initiating event (IE) has a corresponding probability of occurrence, that is, the frequency of occurrence of the initiating event F IE ; System A and System B have the possibility of failure (when the initiating event occurs), that is, the system failure probability P(A|IE) and P(B|IE); The possibility that the accident cannot be mitigated (sequence 3) is calculated according to the following formula:

[0037] F=F IE *P(A∩B|IE)

[0038] If systems A and B are completely independent, the probability that the accident cannot be mitigated is: F = F IE *P(A|IE)*P(B|IE). In this case, the accident mitigation levels formed by systems A and B are completely independent, and the degree of correlation (correlation) is recorded as 0. If system B fails after system A fails, the accident mitigation levels formed by systems A and B are completely correlated, and the correlation is recorded as 1.

[0039] In other cases, the correlation is between 0 and 1, so the interval [0,1] can be used to characterize the degree of correlation (correlation) between the defense-in-depth levels, and it can be calculated by the following formula:

[0040]

[0041] Where D is the calculated correlation, and S is the probability of sequence occurrence calculated by the PSA model.

[0042] Since there are many accident sequences in nuclear power plants, based on the correlation of a single accident sequence, the correlation between the defense-in-depth levels for a certain initiating event or state (such as DEC-A) can be obtained by weighting according to the system failure probability. The calculation formula is as follows:

[0043]

[0044] Because multiple systems may be put into use to mitigate accidents under a certain initiating event or state, such as A, B, C, D, etc., and the additional systems required after the failure of different systems may be different, there will be many accident sequences compared to the example of a single sequence. Here, i represents the accident sequence under a certain initiating event or state, from 1 to n.

[0045] Similarly, considering multiple initiating events or states, weighting them according to the probability of occurrence of the initiating events or states can obtain the correlation between the in-depth defense levels of the entire plant for all initiating events or states. The calculation formula is as follows:

[0046]

[0047] in, k represents the possible initiating events or states of a nuclear power plant, ranging from 1 to n.

[0048] In addition, for situations where a certain defense-in-depth level has no response measures, the above formula is still used in the relevant calculations, and it is considered that the correlation between this defense-in-depth level and the previous level is 1, but it is necessary to note that the high correlation is due to the lack of measures.

[0049] Based on the above calculation method, the evaluation steps of a quantitative evaluation method for independence of defense-in-depth layers of a nuclear power plant provided by the present invention are as follows: Figure 1 The specific implementation steps include:

[0050] (1) Fill in the basic information of the quantitative evaluation form of independence of defense in depth: fill in the qualitative information in the form according to the specific design characteristics of the nuclear power plant, including each initiating event, the frequency of occurrence of the initiating event, the system that needs to be put into use after the occurrence of each initiating event, and the defense in depth level to which the mitigation system belongs;

[0051] (2) Quantitative evaluation table for the independence of the defense-in-depth hierarchy: Based on the probabilistic safety analysis model, the probability of failure of each system and the probability of occurrence of each accident sequence are calculated;

[0052] (3) Calculate the correlation of the defense in depth levels: Based on the qualitative and quantitative information of the defense in depth levels, the correlation of the defense in depth levels is calculated based on the above formulas 1 to 3, that is, the quantitative evaluation results of the independence of the defense in depth levels are obtained. The correlation calculation can be performed at different levels, such as a certain accident sequence, a certain initiating event, and the correlation of the defense in depth levels of the entire plant.

[0053] The quantitative evaluation table of independence of the defense-in-depth layers involved in the implementation steps is as follows: Figure 3 and Figure 4 shown.

[0054] exist Figure 4 Note 1 is to calculate the correlation according to Formula 1, Note 2 is to calculate the correlation according to Formula 2 based on the calculation results at the accident sequence level, and Note 3 is to calculate the correlation according to Formula 3 based on the calculation results at the IE / state level.

[0055] Example

[0056] Taking a large LOCA accident in a pressurized water reactor nuclear power plant as an example, the implementation steps of the present invention are described.

[0057] 1) Fill in the basic information of the quantitative evaluation form of the independence of the defense-in-depth level

[0058] According to the design information of nuclear power plants, the relevant hierarchical mitigation systems after a large LOCA accident are as follows: Figure 5 The defense-in-depth levels include level 3a (DBA), level 3b (DEC-A), and level 4 (DEC-B). The accident mitigation system of level 3a (DBA) includes injection tanks, low-pressure injection, and containment spraying. The accident mitigation system of level 3b (DEC-A) includes passive containment heat removal (corresponding to the containment spraying accident sequence of level 3a). The accident mitigation system of level 4 includes severe accident mitigation measures composed of a series of systems.

[0059] 2) Quantitative evaluation table of the independence of the quantified defense-in-depth level

[0060] According to the probabilistic safety analysis model of a nuclear power plant, the probability of failure of each mitigation system after a large LOCA accident and the probability of occurrence of each accident sequence are as follows: Figure 5 shown.

[0061] 3) Calculate the correlation of defense-in-depth levels

[0062] The correlation between the depth defense levels of a nuclear power plant to cope with a large LOCA accident is calculated using formulas 1 to 3. The correlation between level 3a (DBA) and level 3b (DEC-A) is as follows: Figure 6 As shown in Figure 2, the correlation between level 3b (DEC-A) and level 4 (DEC-B) is as follows: Figure 7 shown.

[0063] In the design of nuclear power plants to cope with large LOCA accidents, the correlation between depth defense level 3a and level 3b is 0.82 (for most accident sequences, level 3b does not have response measures), and the correlation between depth defense level 3b and level 4 is 0.01. This shows that in the design of measures for nuclear power plants to cope with large LOCA accidents, due to the lack of response measures at level 3b, the independence of level 3a and level 3b is relatively low. However, due to the presence of a dedicated severe accident mitigation system, the independence of level 3b and level 4 is extremely high, and they are basically completely independent.

[0064] 4) Optimization feedback on safety systems (facilities)

[0065] Based on the above analysis results, the following optimization feedback is provided:

[0066] There is a high correlation between the defense-in-depth levels 3a and 3b. When the low-pressure safety injection fails, it is possible to consider making certain adjustments to the design parameters of the high-pressure safety injection system and investing in the high-pressure safety injection system to mitigate accidents and improve accident mitigation capabilities.

[0067] The correlation between the defense-in-depth levels 3b and 4 is extremely low. From the perspective of the configuration of the 4th level measures, some optimization can be considered, such as canceling the containment filtration and exhaust system, which has little impact on independence but can save project costs and improve economy.

[0068] It will be apparent to those skilled in the art that the structure of the invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-restrictive in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0069] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A quantitative evaluation method for independence of defense-in-depth layers of a nuclear power plant, characterized in that: The steps include: (1) Determine the basic information for the quantitative evaluation of the independence of the defense-in-depth level, including: each initiating event, the frequency of occurrence of the initiating event, the system that needs to be put into use after the occurrence of each initiating event, and the defense-in-depth level to which the mitigation system belongs; (2) quantifying the basic information according to the probabilistic safety analysis model, including: calculating the probability of failure of each system and the frequency of occurrence of accident sequences; (3) Based on the qualitative and quantitative information of the defense-in-depth level, the defense-in-depth level correlation is calculated to obtain the quantitative evaluation results of the independence of the defense-in-depth level. The method for calculating the defense-in-depth level correlation is as follows: For a single accident sequence, the interval [0,1] is used to represent the correlation between levels, and it is calculated by the following formula: Where D is the calculated correlation, S is the frequency of the accident sequence calculated by the probabilistic safety analysis model, P(A|IE) is the probability of failure of system A, P(B|IE) is the probability of failure of system B, and F IE is the frequency of the initiating event; System A and System B are systems belonging to two levels of defense in depth. In a single accident sequence, System A is first deployed to mitigate the accident, and then System B is deployed to mitigate the accident; On the basis of the correlation of a single accident sequence, the correlation between the defense-in-depth levels for a certain initiating event or state is obtained by weighting according to the probability of system failure. The calculation formula is as follows: Considering multiple initiating events or states, weighting is performed according to the probability of occurrence of the initiating events or states to obtain the correlation between the in-depth defense levels of the entire plant for all initiating events or states. The calculation formula is as follows: in, k represents the possible initiating events or states of a nuclear power plant, ranging from 1 to n.

2. The method for quantitatively evaluating independence of defense-in-depth layers of a nuclear power plant according to claim 1, characterized in that: The defense-in-depth levels include the third level, which assumes that an accident will occur and returns the nuclear power plant to a safe state through safety systems and procedures; and the fourth level, which mitigates the consequences of an accident caused by the failure of the third level of defense-in-depth.

3. The method for quantitatively evaluating independence of defense-in-depth layers of a nuclear power plant according to claim 1, characterized in that: In step (3), if no countermeasures are set for the defense-in-depth level, it is considered that the correlation between this defense-in-depth level and the previous level is 1.

4. The method for quantitatively evaluating independence of defense-in-depth layers of a nuclear power plant according to claim 1, characterized in that: The method also includes step (4), which provides optimization feedback to the security system based on the quantitative evaluation results of the independence of the defense-in-depth levels.

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