JP7561638B2 - RISK IMPACT ASSESSMENT METHOD, RISK IMPACT ASSESSMENT DEVICE, AND PROGRAM - Google Patents

Description

This disclosure relates to a risk impact assessment method, a risk impact assessment device, and a program for identifying risk-critical equipment.

The top events that can occur as a result of combinations of equipment failures are sometimes modeled using a fault tree (FT) to perform risk assessment. When a system that includes a large number of pieces of equipment, such as a nuclear plant, is modeled using FT, many combinations of equipment failures that can lead to top events are generated. The combination of base events (such as equipment failures) that cause the top events defined in FT is called a cutset. It requires a great deal of effort to analyze (detailed analysis) the probability that equipment failures contained in each cutset will occur simultaneously for all of the many cutsets that are generated, and to consider measures on the facility side to prevent equipment failures from occurring simultaneously.

Patent Document 1 describes a risk assessment method in which risk items are classified into multiple groups based on their importance and relevance, and a risk assessment value for each group is calculated. The method of Patent Document 1 makes it possible to assess risk by reflecting the influence relationships between risk items.

JP 2005-190402 A

When there are many base events, if it is possible to extract the base events or groups of base events that have a large impact on risk, detailed analysis and consideration of countermeasures can be carried out efficiently. There is a need for a method to identify base events that are effective for risk assessment from among a huge number of base events.

The present disclosure provides a risk impact assessment method, a risk impact assessment device, and a program that can solve the above problems.

The risk impact assessment method disclosed herein is a risk impact assessment method executed by a computer, the risk impact assessment method including the steps of: setting, by the computer, a probability of failure at random for each of base events constituting a fault tree; calculating, by the computer, a RAW value for each of the base events based on the fault tree and the probability; grouping, by the computer, the base events whose RAW values are within a predetermined range; and calculating , by the computer , a FV importance for each of the grouped groups. In the step of calculating the RAW value for each of the base events, In the step of calculating the FV importance, when the RAW value of the base event i is RAWi, P(TOP/i=1) is the occurrence probability of a top event when the occurrence probability of the base event i in the fault tree becomes 1, and P(TOP) is the occurrence probability of the top event in the fault tree, the RAW value is calculated by RAWi=P(TOP/i=1)÷P(TOP), and in the step of calculating the FV importance, when the FV importance of the group i is FVi, and Pi(TOP) is the difference between the occurrence probability of the top event in the case where none of the base events belonging to the group i occur and P(TOP), the FV value is calculated by FVi=Pi(TOP)÷P(TOP).

The risk impact assessment device disclosed herein further comprises a probability setting unit which randomly sets a probability of failure for each of base events constituting a fault tree, a RAW value calculation unit which calculates a RAW value for each of the base events based on the fault tree and the probability, a grouping unit which groups the base events whose RAW values are within a predetermined range, and an FV importance calculation unit which calculates an FV importance for each of the grouped groups , wherein the RAW value calculation unit sets the RAW value of the base event i constituting the fault tree as RAWi, and calculates P(TOP/i=1) as the FV importance for each of the grouped groups. In the step of calculating the FV importance, if FVi is the FV importance of the group i, and Pi(TOP) is the difference between the occurrence probability of the top event in the case where none of the base events belonging to the group i occur and P(TOP), the FV value is calculated by FVi=Pi(TOP)÷P(TOP), where P(TOP) is the probability of occurrence of the top event in the fault tree when the occurrence probability of the base event i in the tree becomes 1, and P(TOP) is the probability of occurrence of the top event in the fault tree.

The program of the present disclosure includes a step of setting a probability of failure at random for each of base events constituting a fault tree, a step of calculating a RAW value for each of the base events based on the fault tree and the probability, a step of grouping the base events whose RAW values are within a predetermined range, and a step of calculating an FV importance for each of the grouped groups, wherein in the step of calculating the RAW value for each of the base events, the RAW value of the base event i constituting the fault tree is set to RAWi, and P(TOP/i=1) is set to the FV importance of the base event i constituting the fault tree. In the step of calculating the FV importance, the FV value is calculated by: RAWi=P(TOP/i=1)÷P(TOP), where P(TOP) is the probability of occurrence of the top event in the fault tree when the occurrence probability of the base event i in the group i becomes 1; and P(TOP) is the probability of occurrence of the top event in the fault tree. In the step of calculating the FV importance, the FV value is calculated by: FVi=P(TOP)÷P(TOP), where FVi is the FV importance of the group i; and Pi(TOP) is the difference between the probability of occurrence of the top event in the case where none of the base events belonging to the group i occur and P(TOP) .

The above-mentioned risk impact assessment method, risk impact assessment device, and program make it possible to identify risk-critical equipment and key events.

1 is a block diagram showing an example of a risk influence assessment device according to an embodiment. FIG. FIG. 1 is a diagram illustrating an example of a fault tree. FIG. 2B shows a cut set for the fault tree of FIG. 2A. 11A and 11B are diagrams illustrating the setting of probabilities for base events and the calculation of RAW values according to an embodiment. FIG. 11 is a diagram illustrating grouping according to an embodiment. FIG. 13 is a diagram showing an example of a risk assessment model using groups according to an embodiment. FIG. 11 is a diagram illustrating an example of a grouping process according to the embodiment. FIG. 11 is a diagram illustrating an example of a group integration process according to the embodiment. 11 is a flowchart illustrating an example of a risk impact assessment process according to the embodiment. FIG. 2 is a diagram illustrating an example of a hardware configuration of a risk effect assessment apparatus according to an embodiment.

<Embodiment>
The risk influence assessment method of the present disclosure will be described below with reference to FIGS.
(composition)
FIG. 1 is a block diagram showing an example of a risk influence assessment device according to an embodiment.
The risk impact assessment device 10 comprises an FT information acquisition unit 11, a probability setting unit 12, a RAW value calculation unit 13, a grouping unit 14, an FV importance calculation unit 15, a group selection unit 16, an output unit 17, and a memory unit 18.

The FT information acquisition unit 11 acquires information on FT (fault tree). FT is a risk assessment model that expresses the relationship of the combination of basic events that cause a top event by a tree structure. A top event is, for example, a serious accident event in a nuclear power plant. A basic event is, for example, an individual accident event that leads to a top event, such as a pump failure to start, a check valve failure to open, or a fire in a component in a control panel. Here, reference is made to Figures 2A and 2B. An example of FT is shown in Figure 2A. In the FT of Figure 2A, basic event A1 is a failure of equipment A, basic event B1 is a failure of equipment B, basic event C1 is a failure of equipment C, and basic event D1 is a failure of equipment D. The FT of Figure 2A shows that a top event occurs when basic event A1 and any of basic events B1 and C1 occur simultaneously, or when basic event D1 and any of basic events B1 and C1 occur simultaneously. Figure 2B shows a cut set of the FT of Figure 2A. A cutset is a combination of one or more base events that cause a top event. In this example, there are four cutsets: (1) a combination of base event A1 and base event B1, (2) a combination of base event A1 and base event C1, (3) a combination of base event D1 and base event B1, and (4) a combination of base event D1 and base event C1. For example, in the case of cutset (1), when base event A1 and base event B1 occur simultaneously, a top event TE occurs. The FT information acquisition unit 11 acquires, as FT information, structural information of the FT as exemplified in FIG. 2A. This structural information includes information that defines a tree structure, such as the fact that base event B1 and base event C1 are connected by an OR gate in the FT, and base event A1 and base events B1 and C1 are connected by an AND gate. In addition, the FT information acquisition unit 11 acquires the occurrence probability (actual occurrence probability) of base events A1 to D1 as FT information. The occurrence probabilities of base events A1 to D1 are set in advance based on the analysis results of knowledgeable engineers. The following explanation uses the FT shown in Figure 2A as an example.

The probability setting unit 12 randomly sets a probability for each of the base events A1 to D1 of the FT acquired by the FT information acquisition unit 11. Generally, in PRA (probabilistic risk assessment), risk assessment is performed based on the actual occurrence probability of the cut set. The actual occurrence probability of the cut set is often set based on the actual value of the occurrence probability of the base events that constitute the cut set and the failure rate of the equipment related to the base events. For this reason, for example, when equipment A and equipment D are of the same type, the occurrence probability of base event A1 and base event D1 are often set to approximately the same value. In contrast, the probability setting unit 12 sets a probability of an arbitrary value for each cut set, for example, by generating random numbers, regardless of the contents of the base events that constitute the cut set. Here, refer to FIG. 3. The upper row of the table 100 shown in FIG. 3 shows the failure probabilities of equipment A to D. The failure probabilities of equipment A to D are the occurrence probabilities of base events A1 to D1, respectively. The probability setting unit 12 generates random numbers to calculate the failure probabilities of equipment A to D. For example, the probability setting unit 12 calculates a random number "2.10E-05" and sets it as the failure probability of device A, and calculates a random number "7.40E-06" and sets it as the failure probability of device B. If the generated random number is an excessively low value (if the random number is equal to or less than the threshold value) so that the cut set including each base event is not cut off, the probability setting unit 12 generates a random number again and sets the value as the failure probability of the device (probability of occurrence of the base event).

The RAW value calculation unit 13 calculates the RAW values (Risk Achievement Worth) of the base events A1 to D1. The RAW value is also called risk increase worth, and can be calculated by the following formula (1).
RAWi=P(TOP/i=1)÷P(TOP)...(1)
Here, P(TOP) represents the unavailability of the system (probability of a top event occurring). Also, P(TOP/i=1) represents the unavailability of the system when the failure probability of element i becomes 1 (i.e., when element i fails). RAWi is an index indicating the increase in the unavailability of the system when element i of the system fails. Element i is, for example, any of the devices A to D. RAWi represents the increase in risk when element i fails, and is therefore effective in grasping the impact of risk mitigation of the devices. The RAW value calculation unit 13 calculates the RAW value for each of the devices A to D when each device fails. The lower part of the table in FIG. 3 shows the RAW value when it is assumed that devices A to D fail with the probability in the upper part of the table in FIG. 3 in the FT in FIG. 2A. For example, in calculating the RAW value of base event A1, P(TOP/i=1) is the probability that device B or device C will fail based on the FT structure in FIG. 2A. Also, P(TOP) is the probability that any of devices A to D will fail. The RAW value calculation unit 13 calculates the RAW value of base event A1, "3.52E+04", using the probabilities randomly set by the probability setting unit 12 for base events A1 to D1, based on FT and formula (1). Similarly, the RAW value calculation unit 13 calculates the RAW values of base events B1 to D1. As shown in the lower row of table 100, the RAW values of base events B1 to D1 are "3.52E+04", "1.76E+03", and "1.76E+03", respectively.

Based on the RAW values calculated by the RAW value calculation unit 13, the grouping unit 14 groups the base events A1 to D1 into groups with close RAW values. In the case of table 100 in FIG. 3, the grouping unit 14 groups base events A1 and D1 into the same group because the RAW values match at "3.52E+04". Also, the grouping unit 14 groups base events B1 and C1 into the same group because the RAW values match at "1.76E+03". The result of grouping by the grouping unit 14 is shown in FIG. 4A. For convenience, the group of base events A1 and D1 is group 1, and the group of base events B1 and C1 is group 2. By grouping the base events, the original FT can be rewritten with a simpler structure. FIG. 4B shows a simple FT in which the FT in FIG. 2A is rewritten using groups 1 and 2 in this example. According to the FT in Figure 4B, if an equipment failure in group 1 (failure of either equipment A or D) and an equipment failure in group 2 (failure of either equipment B or C) occur simultaneously, it is easy to understand that a top event will occur.

For example, in the case of FT in FIG. 2A, theoretically, even if the probability setting unit 12 sets the probability for each base event completely randomly, the RAW values of base events A1 and D1 will match, and the RAW values of base events B1 and C1 will match. However, due to the influence of random numbers, there is a possibility that errors will occur in the RAW values of base events A1 and D1, and the RAW values of base events B1 and C1. In addition, due to the influence of random numbers, there is a possibility that the RAW values of base events that should belong to another group will be close by chance. Therefore, in this embodiment, the probability setting unit 12 attempts to calculate the probability for one base event multiple times, and the RAW value calculation unit 13 calculates the RAW value for each of the multiple probabilities. In other words, multiple RAW values are calculated for one base event. The grouping unit 14 groups base events whose RAW values are close each time into the same group based on the multiple RAW values, for example.

An example of grouping processing is shown in Figure 5. INST_001, 002, CABLE_001, 002, 100, 101, 102, 200, 201, 202, 310, 311, 312, and VALVE_100, 200 in table 101 in Figure 5 are each base events. In the example in Figure 5, a probability is set for each base event and a RAW value is calculated based on that probability five times. For example, in the first trial, the raw values of INST_001 and CABLE_001 match, the raw values of INST_002 and CABLE_002 match, the raw values of VALVE_100 and CABLE_100, 101, and 102 match, and the raw values of VALVE_200 and CABLE_200, 201, and 202 match. The grouping unit 14 groups these into one provisional group, respectively. These are called provisional groups G1, G2, G3, and G4, in that order. For example, if the above basic events are classified into provisional groups G1, G2, G3, and G4 in the same way over five trials, the grouping unit 14 finally sets each of these as one group. These are called groups G_001, G_002, G_003, and G_004, in that order.

In addition, in the first and fifth trials, the RAW value of CABLE_301 is different from the others, and the RAW values of CABLE_310, 311, and 312 match. As a result, the grouping unit 14, for example, classifies CABLE_301 into provisional group G5 and classifies CABLE_310, 311, and 312 into provisional group G6. In the second trial, the RAW values of CABLE_301, 310, 311, and 312 match. The grouping unit 14 classifies CABLE_301, 310, 311, and 312 into provisional group G5. Finally, the grouping unit 14 sets CABLE_310, 311, and 312, which have matching RAW values over five rounds, as one group G_006, and sets CABLE_301 as one group G_005.

Logically, the RAW values of base events that have similar effects on risk are the same. However, when there are a huge number of base events, it is costly to analyze the FT structure and group the base events. In contrast, in this embodiment, base events with similar RAW values are grouped together using RAW values calculated based on the randomly set probability of occurrence of the base events. This makes it possible to group a large number of base events that have similar effects on risk by simple calculation without analyzing the FT structure. In addition, by randomly setting probabilities for base events, different arbitrary probabilities can be set for failures of the same type of equipment, for example, and the RAW value is calculated based on that value. This makes it possible to group together events that can be treated similarly in terms of risk impact in terms of the fault tree structure, regardless of the type of equipment.

The FV importance calculation unit 15 calculates FV (Fussell-Vesely) importance for each group created by the grouping unit 14. The FV importance can be calculated by the following formula (2).
FVi=Pi(TOP)÷P(TOP)...(2)
Here, P(TOP) represents the unavailability of the system, and Pi(TOP) represents the difference between the unavailability of the system when the probability of failure of element i becomes 0 (i.e., when element i is not broken) and the unavailability of the system (=P(TOP)). The FV importance indicates the proportion of the probability of the occurrence of a top event that is caused by the failure of element i, and an element i with a high FV importance has a greater impact on risk than an element i with a low FV importance. The FV importance is used to grasp the effectiveness of measures taken by devices in the system. In addition, the FV importance calculation unit 15 does not calculate the FV importance for devices A to D, but calculates the FV importance for all devices grouped by the grouping unit 14. For example, the FV importance calculation unit 15 calculates the FV importance for group 1 by taking the difference between the unavailability of the system and P(TOP) when both devices A and D are not broken as Pi(TOP) in formula (2). That is, the FV importance calculation unit 15 calculates the degree of risk reduction when both devices A and D are not broken (when neither of the basic events A1 and D1 occurs) using formula (2) compared to the other case. Unlike the case of grouping processing, the FV importance calculation unit 15 calculates the FV importance of group 1 based on the actual failure rates of devices A and D (the actual occurrence probability of basic events A1 and D1). For example, the FV importance calculation unit 15 calculates the FV importance of group 1 based on the actual occurrence probability of each basic event included in the FT information acquired by the FT information acquisition unit 11. Similarly, the FV importance calculation unit 15 calculates the degree of risk reduction (FV importance) for group 2 when both devices B and C are not broken. Note that if the calculated FV importance is not a significant value, the FV importance calculation unit 15 merges the groups and calculates the FV importance for the larger group after the merge. An example of merging groups will be described with reference to FIG. 6.

Table 102 in FIG. 6 shows the FV importance calculated for each group G_001 to 006 created by the grouping process described in FIG. 5. The FV importance of G_003 is "1.19E-01", the FV importance of G_004 is "2.22E-16", and 0 for the other groups. For example, the FV importance calculation unit 15 compares the FV importance of each group with a predetermined threshold, and if the calculated FV importance is equal to or less than the threshold, it is determined that a significant value has not been obtained, and the groups are integrated, and the FV importance of the integrated group is calculated in the same manner. An example of the integrated group and FV importance is shown in Table 103 in FIG. 6. For example, the FV importance calculation unit 15 creates all patterns of combinations of two of the groups G_001 to 006, and calculates the FV importance for each of them. The FV importance calculation unit 15 compares all the FV importance including the integrated group with the threshold to determine whether a significant value has been obtained. For example, the FV importance calculation unit 15 compares the FV importance of the group G_003+G_004, "6.87E-01", with the threshold value. It is assumed that all the FV importances shown in Table 103 are equal to or less than the threshold value. Then, the FV importance calculation unit 15 further integrates the groups and calculates the FV importance again. For example, the FV importance calculation unit 15 may create all combinations of the groups that have already been created, as in the previous example, and calculate the FV importance. Alternatively, since the amount of calculation increases as the number of groups increases, the integration of the groups may be performed by focusing on the components of the group with the highest FV importance (for example, the components of the group G_003+G_004 with the highest FV importance are G_003 and G_004). For example, the components of a predetermined number of groups with the highest FV importance may be combined, or the components of groups with FV importance values equal to or greater than a predetermined value may be combined. Table 104 shows an example of calculating FV importance by additionally combining components of the top-ranked groups. For example, the FV importance calculation unit 15 calculates the degree of risk reduction (FV importance) for the integrated group G_003+G_004+G_001, assuming that there is no equipment failure in this group. The FV importance calculation unit 15 integrates groups and calculates FV importance until a significant value is obtained.

The group selection unit 16 sorts the groups in descending order of FV importance value and selects the top-ranked groups. Groups with large FV importance values are groups of base events that have a large impact on risk (are important in terms of risk). The group selection unit 16 may select a predetermined number of groups in descending order of FV importance, or may select groups whose FV importance is equal to or greater than a predetermined threshold. In the example of FIG. 4A, the group selection unit 16 selects, for example, groups 1 and 2. In the case of table 104 in FIG. 6, the group selection unit 16 selects, for example, the top two groups.

The output unit 17 outputs the grouping results by the grouping unit 14, the FV importance calculated for each group, the group selected by the group selection unit 16, etc. to a display device, an electronic file, or the like.
The storage unit 18 stores information necessary for selecting a group of base events that are significant in terms of risk. For example, the storage unit 18 stores the FT information acquired by the FT information acquisition unit 11.

(Operation)
Next, the operation of the risk influence assessment device 10 will be described with reference to FIG.
FIG. 7 is a flowchart showing an example of a risk influence assessment process according to the embodiment.
First, the FT information acquisition unit 11 acquires information on the FT to be evaluated (step S10). The FT information acquisition unit 11 records the FT information in the storage unit 18.

Next, the probability setting unit 12 randomly sets a probability for each base event of the FT acquired in step S10 (step S11). The probability setting unit 12 analyzes the information of the FT stored in the memory unit 18, extracts base events, randomly generates random numbers for each base event, and sets the random numbers as the occurrence probability of the base event. If the random number is less than a predetermined reference value, the probability setting unit 12 generates a random number again. In addition, the probability setting unit 12 sets a probability multiple times for one base event. For example, the probability setting unit 12 sets five probabilities for each of the base events A1 to D1. The probability setting unit 12 associates the set probabilities with the target base event and records them in the memory unit 18 by the number of trials (for example, the probability set for the first trial to the probability set for the fifth trial separately).

The RAW value calculation unit 13 calculates the RAW value of each base event (step S12). For example, the RAW value calculation unit 13 calculates five RAW values using each of the five probabilities set for base event A1 and formula (1) based on the FT acquired in step S10. The RAW value calculation unit 13 also calculates five RAW values for each of base events B1 to D1. The RAW value calculation unit 13 associates the calculated RAW values with the base events in question, and records them in the storage unit 18 by the number of trials (for example, separately for the RAW value based on the probability set in the first trial to the RAW value based on the probability set in the fifth trial).

Next, the grouping unit 14 groups base events with close RAW values together (step S13). For example, the grouping unit 14 groups base events whose differences in RAW values at the same trial number are all within a predetermined range over five trials into the same group. There is no limit to the number of base events that can belong to one group, and there may be only one, or three or more. There is also no limit to the number of groups that can be created.

The grouping unit 14 then determines whether consistent grouping results have been obtained (step S14). For example, if the number of groups created in each of the first to fifth trials and the basic events that make up each group are all different, the grouping unit 14 determines that consistent grouping results have not been obtained. For example, as in the example shown in FIG. 5, if the grouping results are the same four out of five times, the grouping unit 14 determines that consistent grouping results have been obtained. If consistent grouping results are not obtained (step S14; No), the processing of steps S11 to S14 is performed again.

When a consistent grouping result is obtained (step S14; Yes), the FV importance calculation unit 15 calculates the FV importance for each group indicated by the consistent grouping result (step S15). The FV importance calculation unit 15 uses the actual occurrence probability of the base events belonging to each group to calculate the sum of the contributions to risk made by the base events belonging to the group (the degree of risk reduction when all the base events in the group do not occur) by FT and formula (2). The FV importance calculation unit 15 records the calculated FV importance in the memory unit 18.

The FV importance calculation unit 15 compares the FV importance with a predetermined threshold value and determines whether a significant result has been obtained (step S16). If a significant result has not been obtained (step S16; No), the groups are merged as described in FIG. 6 (step S17). For example, the FV importance calculation unit 15 selects the group with the highest FV importance, and then creates all possible combinations by selecting two groups from the selected groups and merging them. The FV importance calculation unit 15 then calculates the FV importance for the merged group (step S15) and determines whether a significant result has been obtained (step S16). The FV importance calculation unit 15 repeats the processes of steps S17, S15, and S16 until a significant result is obtained.

If a significant result is obtained (step S16; Yes), the group selection unit 16 selects the group with the highest FV importance as the group with the highest risk importance (step S18). Next, the output unit 17 outputs the group selected by the group selection unit 16 (step S19). For example, in the case of FIG. 6, the group selection unit 16 outputs "G_003+G_004+G_001" and "G_003+G_004+G_002" and the basic events belonging to each group. In addition to the groups selected in step S18, the output unit 17 may also output, for example, the finally created groups and their FV importance, and a list of the basic events that make up each group.

As described above, according to this embodiment, by using the indices (RAW, FV importance) used in importance analysis in PRA, it is possible to group a large number of base events into those that can be treated similarly in terms of their impact on risk. In addition, it is possible to extract groups that are important in terms of risk from multiple groups. This enables efficient risk assessment by prioritizing detailed analysis and consideration of countermeasures for base events that belong to groups that are important in terms of risk.

In addition, by grouping basic events that have the same impact on risk, it is possible to simplify an FT consisting of many basic events into a risk assessment model with groups as units (for example, the FT in Figure 4B), and then determine which groups are important in terms of risk. In addition, the simplified risk assessment model makes it possible to grasp the combination of groups that will have a large impact on risk if they fail simultaneously. This makes it possible to efficiently analyze which equipment will lead to a peak event if it fails simultaneously, and to efficiently consider equipment responses that will prevent a peak event from occurring. For example, when probabilistically evaluating the risk of fire in a nuclear plant (fire PRA), all combinations of equipment A to D are created, such as when a fire occurs in both equipment A and equipment B, when a fire occurs in equipment A and equipment C, when a fire occurs in equipment A to C, etc., and detailed analysis and equipment responses are considered for each combination. It is sufficient if the number of equipment is about four, but the number of equipment equipped in a nuclear plant is enormous, and it is difficult to perform detailed analysis, etc. for all combinations of equipment. According to this embodiment, a group of equipment (base events) with a large risk impact can be identified, and detailed analysis can be limited to combinations that include equipment (base events) that belong to the identified group, thereby making the detailed analysis more efficient. Also, as illustrated in FIG. 4B, according to this embodiment, the original FT can be simplified and modeled. For example, the simplified risk assessment model makes it easy to understand that it is important to prevent simultaneous failures in group 1 and group 2. By identifying groups that would cause a top event if they were to fail simultaneously and considering measures to prevent simultaneous failures of equipment belonging to these groups, effective equipment responses can be efficiently implemented.

Furthermore, in conventional fire risk assessments, nuclear plants are often divided into compartments, and when a fire occurs in one piece of equipment within a compartment, the risk assessment is often performed assuming that the fire has occurred in the entire compartment, which often results in overly conservative assessment results. In contrast, according to this embodiment, risk assessment can be effectively performed by focusing on equipment that belongs to a group that is significant in terms of risk, allowing for more precise risk assessment and consideration of countermeasures. Note that the application of the risk impact assessment of this embodiment is not limited to fire risk assessment.

While both the RAW value and FV importance are indices used in risk assessment, these values are generally compared with a certain reference value to assess the magnitude of risk. In contrast, in this embodiment, base events are grouped based on the RAW value, which is an index indicating an increase in risk when equipment breaks down. In addition, for each group of grouped base events, the FV importance, which is an index indicating a reduction in risk when the equipment does not break down, is calculated, and a group with a large FV importance value is selected. In this way, in this embodiment, by using a combination of general indices, it is possible to simplify an FT consisting of a large number of base events into a model based on groups, and then determine which groups are important in terms of risk.

FIG. 8 is a diagram illustrating an example of a hardware configuration of a risk effect assessment apparatus according to the embodiment.
The computer 900 includes a CPU 901 , a main memory device 902 , an auxiliary memory device 903 , an input/output interface 904 , and a communication interface 905 .
The risk effect assessment device 10 described above is implemented in a computer 900. Each of the above-mentioned functions is stored in the auxiliary storage device 903 in the form of a program. The CPU 901 reads the program from the auxiliary storage device 903, loads it in the main storage device 902, and executes the above-mentioned processing in accordance with the program. The CPU 901 also reserves a memory area in the main storage device 902 in accordance with the program. The CPU 901 also reserves a memory area in the auxiliary storage device 903 for storing data being processed in accordance with the program.

A program for realizing all or part of the functions of the risk impact assessment device 10 may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read into a computer system and executed to perform processing by each functional unit. The term "computer system" here includes hardware such as an OS and peripheral devices. In addition, if a WWW system is used, the term "computer system" also includes a home page providing environment (or display environment). In addition, the term "computer-readable recording medium" refers to portable media such as CDs, DVDs, and USBs, and storage devices such as hard disks built into a computer system. In addition, if the program is distributed to a computer 900 via a communication line, the computer 900 that receives the program may expand the program into the main storage device 902 and execute the above processing. In addition, the above program may be for realizing part of the functions described above, or may be capable of realizing the functions described above in combination with a program already recorded in the computer system. The risk impact assessment device 10 may be composed of multiple computers 900 that can communicate with each other.

As described above, several embodiments of the present disclosure have been described, but all of these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope of the invention and its equivalents as described in the claims, as well as in the scope and gist of the invention.

<Additional Notes>
The risk and effect assessment method, risk and effect assessment apparatus, and program described in each embodiment can be understood, for example, as follows.

(1) A risk impact assessment method according to a first aspect includes the steps of randomly setting a probability of failure for each of basic events A1 to D1 constituting a fault tree (FIG. 2A) (S11), calculating a RAW value for each of the basic events based on the fault tree and the probability (S12), grouping basic events whose RAW values are within a predetermined range (S13), and calculating an FV importance for each of the grouped groups (S15).
As a result, even when there are many pieces of equipment or base events to be evaluated, those that can be treated similarly from the perspective of risk impact can be grouped together. By calculating the FV demand for each group, the impact on risk of each group can be evaluated. By referring to the impact on risk of each group, it is possible to grasp base events that are effective for risk assessment. By grouping base events, the original FT can be modeled into a simple format. This makes it easy to identify the combination that leads to the top event from among the many combinations of equipment failures that exist. As a result, risk assessment based on FT can be performed efficiently.

(2) A risk impact assessment method according to a second aspect is the risk impact assessment method of (1), further comprising the step (S18) of selecting a group whose FV importance is equal to or greater than a predetermined value.
This makes it possible to identify groups of base events that have a large impact on risk from among a huge number of combinations of base events. By understanding the base events that have a large impact on risk, detailed analysis can be performed with little effort, and failure countermeasures can be considered with little effort.

(3) A risk impact assessment method according to a third aspect is a risk impact assessment method according to (1) to (2), in which, if the FV importance is less than a predetermined threshold, the groups are merged and the FV importance is calculated again.
If the FV importance is not a significant value, the groups are integrated and the FV importance is calculated again, making it possible to create and extract groups that are important in terms of risk.

(4) A risk impact assessment method according to a fourth aspect is a risk impact assessment method according to any one of (1) to (3), wherein in the step of setting the probability, a process of generating random numbers and setting the probability for the base event is executed multiple times, in the step of calculating a RAW value of the base event, the RAW value is calculated for each of the multiple probabilities set by executing the process multiple times, and in the step of grouping, grouping is performed based on the multiple calculated RAW values.
By setting the probability multiple times and calculating the RAW value, it is possible to eliminate the influence of setting the probability randomly (for example, the RAW values being close to each other by chance).

(5) A risk impact assessment method according to a fifth aspect is a risk impact assessment method according to any one of (1) to (4), wherein in the step of calculating the FV importance, the degree of risk reduction is calculated in the case where none of the base events classified into the groups occurs.
By calculating the FV importance based on the difference in the probability of reaching a top event when the failure probabilities of the base events belonging to the group are all 0 and when they are not, the impact on the risk of the entire group can be evaluated.

(6) A risk impact assessment method according to a sixth aspect is a risk impact assessment method according to any one of (1) to (5), wherein the fault tree is a fault tree that models a top event that occurs when a fire occurs in equipment provided in a nuclear power plant.
The risk impact assessment method of this embodiment is effective in a situation where a risk assessment is performed for each combination of a large number of devices equipped in a nuclear power plant, and a risk assessment of a fire is performed for each combination of devices equipped in the nuclear power plant. For example, by performing the process shown in FIG. 7 on a fault tree for assessing the risk of a fire occurring in each device equipped in the nuclear power plant, a combination of devices that has a large impact on the fire risk can be extracted.

(7) The risk impact assessment device 10 according to the seventh aspect includes a probability setting unit 12 that randomly sets a probability of failure for each of the base events constituting a fault tree, a RAW value calculation unit 13 that calculates a RAW value for each of the base events based on the fault tree and the probability, a grouping unit 14 that groups base events whose RAW values are within a predetermined range, and an FV importance calculation unit 15 that calculates the FV importance for each of the grouped groups.

(8) The program according to the eighth aspect causes the computer 900 to execute the steps of randomly setting a probability of failure for each of the base events constituting a fault tree, calculating a RAW value for each of the base events based on the fault tree and the probability, grouping base events whose RAW values are within a predetermined range, and calculating the FV importance for each of the grouped groups.

10: Risk influence assessment device 11: FT information acquisition unit 12: Probability setting unit 13: RAW value calculation unit 14: Grouping unit 15: FV importance calculation unit 16: Group selection unit 17: Output unit 18: Memory unit 900: Computer 901: CPU,
902...Main storage device,
903... Auxiliary storage device,
904: Input/output interface 905: Communication interface

Claims (8)

1. A computer implemented method for risk impact assessment, comprising:
A step of randomly setting a probability of failure for each of the basic events constituting the fault tree by the computer ;
calculating , by the computer, a Risk Achievement Worth (RAW) value for each of the base events based on the fault tree and the probability;
grouping , by the computer, the base events whose RAW values are within a predetermined range;
calculating , by the computer, a Fussell-Vesely (FV) importance for each of the groups;
having
In the step of calculating a RAW value for each base event, the RAW value of the base event i constituting the fault tree is RAWi, P(TOP/i=1) is the occurrence probability of a top event when the occurrence probability of the base event i in the fault tree becomes 1, and P(TOP) is the occurrence probability of the top event in the fault tree, and the RAW value is calculated by RAWi=P(TOP/i=1)÷P(TOP);
In the step of calculating the FV importance, the FV importance of the group i is FVi, and Pi(TOP) is the difference between the occurrence probability of the top event in the case where none of the base events belonging to the group i occurs and P(TOP), and the FV value is calculated by FVi=Pi(TOP)÷P(TOP).
Risk impact assessment methods. The risk influence assessment method according to claim 1 , further comprising the step of: selecting , by the computer, the group in which the FV importance is equal to or greater than a predetermined value. If the FV importance is less than a predetermined threshold, the groups are integrated and the FV importance is calculated again.
3. The risk impact assessment method according to claim 2. In the step of setting the probability, a process of generating a random number and setting the probability for the base event is executed a plurality of times;
In the step of calculating a RAW value for each of the base events, the RAW value is calculated for each of a plurality of the probabilities set by a plurality of executions,
In the grouping step, grouping is performed based on the plurality of calculated RAW values.
A risk effect assessment method according to any one of claims 1 to 3. In the step of calculating the FV importance, a degree of risk reduction in a case where none of the basic events belonging to the group occurs is calculated.
A risk influence assessment method according to any one of claims 1 to 4. The fault tree is a fault tree that models a top event that occurs when a fire occurs in equipment included in a nuclear power plant.
A risk influence assessment method according to any one of claims 1 to 5. a probability setting unit that randomly sets a probability of failure for each of the basic events that constitute the fault tree;
a RAW value calculation unit that calculates a RAW (Risk Achievement Worth) value for each of the base events based on the fault tree and the probability;
a grouping unit that groups the basic events whose RAW values are within a predetermined range;
an FV importance calculation unit that calculates an FV importance for each of the grouped groups;
Equipped with
the RAW value calculation unit calculates the RAW value by RAWi=P(TOP/i=1)÷P(TOP), where RAW value of the base event i constituting the fault tree is RAWi, P(TOP/i=1) is the occurrence probability of a top event when the occurrence probability of the base event i in the fault tree becomes 1, and P(TOP) is the occurrence probability of the top event in the fault tree;
In the step of calculating the FV importance, the FV importance of the group i is FVi, and Pi(TOP) is the difference between the occurrence probability of the top event in the case where none of the base events belonging to the group i occurs and P(TOP), and the FV value is calculated by FVi=Pi(TOP)÷P(TOP).
Risk impact assessment instrument. On the computer,
A step of randomly setting a probability of failure for each of the basic events constituting the fault tree;
calculating a RAW (Risk Achievement Worth) value for each of the base events based on the fault tree and the probability;
A step of grouping the base events whose RAW values are within a predetermined range;
Calculating FV importance for each of the groups;
having
In the step of calculating a RAW value for each base event, the RAW value of the base event i constituting the fault tree is RAWi, P(TOP/i=1) is the occurrence probability of a top event when the occurrence probability of the base event i in the fault tree becomes 1, and P(TOP) is the occurrence probability of the top event in the fault tree, and the RAW value is calculated by RAWi=P(TOP/i=1)÷P(TOP);
In the step of calculating the FV importance, the FV importance of the group i is FVi, and Pi(TOP) is the difference between the occurrence probability of the top event in the case where none of the base events belonging to the group i occurs and P(TOP), the FV value is calculated by FVi=Pi(TOP)÷P(TOP);
A program that executes the following.

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