This section contains short descriptions of the analyses used in probabilistic risk assessment. More information can be found in References, [WASH1400], [FTACOURSE], [FTHB], [PRA], [MEF].

Static Fault Tree Analysis

Fault Tree Analysis is a top-down deductive method to understand the logic leading to the top undesired event or fault [FTA]. Boolean logic is used to combine events that could lead to the undesired top event. This analysis assumes that events are independent and may incorporate Common Cause Failure for the analysis with dependent systems. The analysis generates minimal cut sets or prime implicants, importance factors of events, probabilities of top event and gates.

Static Fault Tree Symbols, Gate and Event Types


  1. Top Event :
    Contains a description of the top-system-level fault or undesired event. The root of a fault tree. Top event only receives input from a logic gate.
  2. Intermediate/Fault Event :
    Contains a description of a lower-level fault in the system. Intermediate events receive input from and provide outputs to a logic gate.
  3. Basic Event :
    Contains a failure at the lowest level of examination, which has a potential to cause a fault in the system. This is a leaf of a fault tree, and it provides input to a logic gate. This event must have its probability defined.
  4. Undeveloped Event :
    Similar to a basic event, but an undeveloped event has its own causes that may be analyzed by a separate fault tree. This event is not further developed either because of insufficient consequence or because information is unavailable. This event must have its probability defined and provide input to a logic gate.
  5. House/Input/External Event :
    Contains a normal system operating input with the capability of causing a fault. This event is expected to occur with probability 0 or 1 only, or it is sometimes used as a switch of True or False. Provides input to a logic gate. May be used to switch off a sub-tree.
  6. Conditional Event :
    Defines a specific condition or restriction that apply to any logic gate. Used mostly with Priority AND and Inhibit gates. The alternative name is a Conditional Qualifier. Conditions may be abnormal cases or environment. The probability of this event might be a conditional probability.


  1. Transfer In :
    This symbol indicates that the tree is expanded further at the occurrence of the corresponding Transfer Out symbol. This expansion may be described in another file or page.
  2. Transfer Out :
    This symbol indicates that this portion of the tree must be attached at the corresponding Transfer In symbol of the main fault tree.


  1. AND :
    Output fault occurs if all the input faults occur.
  2. OR :
    Output fault occurs if at least one of the input faults occur.
  3. Exclusive OR (XOR) :
    Output fault occurs if exactly one of the two input faults occurs. (A XOR B) = (A AND B’) OR (A’ AND B). In other words, the result is one or the other but not both. This gate can only have two inputs. For several inputs, the output fault occurs if odd number of faults occur. This can be simulated by chaining XOR gates if needed.
  4. Inhibit :
    This gate is a special case of the AND gate. Output fault occurs if the single input fault occurs in the presence of an enabling condition (the enabling condition is represented by a Conditioning Event drawn to the right of the gate.). This gate restricts input events to only two events.
  5. Combination/Voting/VOTE/Atleast/K-out-of-N (K/N) :
    Output fault occurs if m out of the n input events occurs. The m input events need not to occur simultaneously. The output occurs if at least m events occur. m is more than 1, and n is more than m. The m can also be called vote number.
  6. NOT :
    Output fault occurs if the input event does not occur. This logic leads to non-coherent trees, for which non-occurrence or success of events may lead to occurrence of the main undesirable event.
  7. NAND :
    NOT AND gate. Indicates that the output occurs when at least one of the input events is absent (does not occur or fail). This may lead to non-coherent trees.
  8. NOR :
    NOT OR gate. Indicates that the output occurs when all the input events are absent. This may also lead to non-coherent trees.
  9. NULL/Pass-Through :
    Non-essential. Only one input is allowed. Used in applications with GUI to allow more description or alignment.


  1. Dormant Failure :
    Failures that are not detected by themselves and need secondary specific actions or failures to occur. This is a special case of a primary event that may fail with no visible external effects. May be treated as a basic event for primitive analysis.

Event Tree Analysis

Event Tree Analysis is a bottom-up approach to quantify the risk resulting from an initiating event [ETA]. The tree is branched into conditionally independent, mutually exclusive cases, which lead to several final scenarios, outcomes, or end states. This analysis is conceptually useful when the system incorporates sequentially occurring events.

Most of the time, there are two branches for success and failure cases, but there may be more as long as the events are mutually exclusive. Probabilities of intermediate cases can be calculated with fault trees or assigned manually, and they must sum to 1 for mutually exclusive and independent branches.

Fault Tree Linking

If the original assumption of independent branches does not hold, an event tree branches can be linked to corresponding gates in fault trees, and the final tree is analyzed as a big fault tree.

Common Cause Failure

If events are not statistically independent, common cause or mode analysis is performed to account for the failure of multiple elements at the same time or within a short period [CCF]. These common mode failures may be due to the same manufacture flaws and design, environment, working conditions, maintenance, quality control, normal wear and tear, and many other factors. Several models are used to quantify the common cause failures. The components in the same common cause group must be described by the same probability. The exact formulas to compute factors are given in NRC [NUREG0492].

Beta System

Beta systems assume that if common cause failure occurs, all components in the group fail. The components can fail independently, but multiple independent failures are ignored.

Multiple Greek Letters (MGL) System

MGL is a generalization of Beta system. MGL describes several conditional factors that quantify the failure of the certain number of components due to common cause, so the number of factors can be up to the number of components. The factor for k number of elements indicates failure of k or more components due to common cause.

Alpha System

This system is similar to MGL, but the factor for k number of elements indicates failure of exactly k number of elements due to common cause.

Phi System

Phi system is the same as MGL and Alpha systems except that the factors indicate direct probability distribution of the common cause. The phi factors must sum to 1.

Uncertainty Analysis

Uncertainty quantification is performed for a top event (gate) with determined minimal cut sets or prime implicants [UA]. If events in the products have their probabilities expressed by a statistical distribution with some uncertainties, these uncertainties propagate to the total probability of the top event. This analysis is performed employing the Monte Carlo Method. The values of probabilities are sampled to calculate the distribution of the total probability.

Sensitivity Analysis

Sensitivity analysis determines how much the variation of each event contributes to the total uncertainty of the top event (gate) [SA]. There are many approaches for this analysis, but in general, the analyst modifies the structure of the problem tree or input values to observe changes in results. Key assumptions and issues can be examined at this stage. However, since this analysis follows the uncertainty analysis, the sensitivity analysis may be expensive.

Importance Analysis

The importance of a component or event provides information about its impact on the system. This analysis is used to filter out components that need most attention to reduce the overall risk.


The following interpretations are valid only for coherent fault trees.


This factor is also called Marginal Importance Factor (MIF). This factor gives the increase in risk due to the failure of the component by measuring the difference between failed-event and non-failed event systems.

\[MIF = P(S|e) - P(S|\overline{e})\]

Critical Importance Factor

This factor is also called Criticality Factor and takes into account the reliability of the component.

\[CIF = \frac{P(e)}{P(S)} \times MIF\]


This factor is also called Diagnosis Importance Factor(DIF). The value provides information about how much the component is contributing to the total risk.

\[DIF = P(e|S) = \frac{P(e) \cdot P(S|e)}{P(S)}\]

Risk Achievement Worth

This factor is also called Risk Increase Factor and measures the increase in risk of the system given that the component has already failed. This factor indicates the importance of maintaining the component at its current level of reliability.

\[RAW = \frac{P(S|e)}{P(S)}\]

Risk Reduction Worth

This factor is also called Risk Decrease Factor and indicates the maximum decrease in risk of the system if the component never failed or increased its reliability. This factor helps select the components to improve first with most effect on risk reduction.

\[RRW = \frac{P(S)}{P(S|\overline{e})}\]

Incorporation of Alignments

The system’s configuration may change over time due to maintenance or substitutions of failed/out-of-service events. This temporary configurations create different analyses and final results.

Dynamic Fault Tree Analysis

This analysis takes into account the order of events’ failures. The information about time dependency is incorporated into a fault tree by using specific gates, such as Priority AND, Sequence.


  1. Priority AND (PAND) :
    Output fault occurs if all the input faults occur in a specific sequence. The sequence may also be from first to last member or left to right. In most packages with static fault tree analysis, this gate is treated just like AND gate without the sequence, so it stays for graphical purposes only.
  2. Functional Dependency (FDEP) :
    This is not a gate with an output but a description that a set of basic events depends on one trigger event. If the trigger event occurs, all the basic events occur immediately and simultaneously (no ordering). To achieve this behavior with existing static gates, each occurrence of a basic event in the set can be replaced with an OR gate with two inputs, the basic event and the trigger.
  3. Sequence Enforcer (SEQ) :
    This is not a gate with an output but a constraint that events can only occur in given order.
  4. Spare Gates :
    A collection of spare parts ready to replace failed components. If there are no more replacements, the gate fails. The spare components can be shared and have a waiting state (hot, warm, cold). For simple analysis with hot spare components (the same failure characteristics as the deployed component), this gate can be approximated with an AND gate.

Reliability Block Diagram

RBD or Dependence Diagram (DD) is another way of showing the system component layout using a diagram with series and parallel configurations [RBD]. In this analysis, the success of the system is shown through the paths that are still available after failure of a component. That is, parallel paths are redundancies in the system. The diagram can be converted to a success tree or fault tree. More complex dependent relationships can be handled by a dynamic RBD.