|Term or Acronym||Definition|
|Accident sequence||The combination of an initiating event with system failures and successes (defined by an event tree) which results in a definable outcome. For a nuclear power plant PRA, the outcome is generally core damage.|
|Availability||The instantaneous availability is the probability that a system will perform satisfactorily at a designated point in time when used under specified operating conditions. The evaluation of system availability includes operating time, time to test, active repair time, administrative time, and logistics time. The Unavailability is the complement of the availability.|
Basic events represent a fault such as a hardware failure, human error, or an adverse condition. Basic events are used to represent failures within fault trees or top events within event trees.
Within SAPHIRE, an analyst must specify a failure probability for each basic event. Also, if the analyst desires, the uncertainty on the event probability may also be specified.
Event trees are logical representations of system responses to initiating events. From an event tree, accident sequences are obtained.
Each sequence results in either a safe condition (e.g., safe plant shutdown) or an accident condition (e.g., core damage). Event trees provide an end-to-end traceability (from left to right) of accident sequences.
Fault trees are logical representations of the credible failures that can cause an undesired event to occur. The undesired event is stated at the top of the fault tree. The fault tree gates specify the logical combinations of basic events that lead to the top event.
Fault trees can be used to identify system "weaknesses" and can help to recognize interrelationships between fault events.
"Solving" a fault tree in SAPHIRE will yield minimal cut sets which represent the combinations of things (i.e., basic events) that have to fail in order to fail the system represented by the fault tree.
|Fault tree linking||A technique whereby fault tree logic is combined with the event tree logic (i.e., successes and failures) resulting in a Boolean logic expression for each sequence in the event tree.|
|Initiating event||An initiating event is the first question that is asked on an event tree. All accident sequences or undesirable outcomes begin with some type of initiating event. Examples of initiating events include: loss of power, earthquake, shipping tank is lifted, widget is purchased, etc.|
|Minimal cut set||A minimum combination of failures needed to result in the occurrence of the event of interest. For a fault tree, the minimal cut sets represent ways in which a system can fail. For an accident sequence, minimal cut sets represent ways in which the sequence can occur.|
|Probabilistic risk assessment (PRA)||
PRA is an analytical method to identify, characterize, quantify probabilistically, and evaluate hazards.
Hazards could include: ionizing radiation (e.g., a nuclear power plant radiation release), electrical hazard (e.g., electrical shock), thermal hazard (e.g., thermal blast effects from an explosion), chemical hazard (e.g., a release of toxic chemicals)
|Reliability||The probability that a system will perform satisfactorily (i.e., does not fail) for a designated period of time (or number of cycles) and under specified operating conditions. The Unreliability is the complement of the reliability, that is, the probability that the system does fail within a designated period of time and under specified operating conditions.|
|Risk||Risk is the potential of loss or damage resulting from exposure to a hazard.|
|Safety||Safety represents an acceptable level of risk relative to the benefits derived from the hazards-causing activity.|
|Top events||Top events are the condition that is represented by a branch (either up or down) within an event tree. Typically, top events are shown above the branch in the event tree graphic. Within SAPHIRE for Windows, top events can have a name up to 24 characters.|