The availability of a component or system is the probability that it will be able to operate successfully at a random point in time when required. It is a dimensionless quantity.


Best Estimate

The point estimate of a parameter utilized in a computation which is not biased by conservatism or optimism. Generally, the mean value of a parameter is considered to be the best estimate.


Common Cause Failure

A common cause failure is a single, shared event that adversely affects two or more components at the same time. When the consequences of the event include the occurrence of an accident sequence initiating event, the event is called a common cause initiating event.


Containment Bypass

A containment bypass involves a direct release of radioactive material to the environment that bypasses the containment atmosphere. Examples include PWR steam generator tube ruptures (single tube failures or induced tube ruptures), which allow radionuclides to be released through the secondary system, or interfacing systems LOCAs, which allow radionuclides to be released through a breach in a system outside the containment that interfaces with the reactor coolant system.


Containment Isolation Failure

Failure to isolate all lines that penetrate the containment is considered as a potential severe accident containment failure mode. Although multiple redundant valves are provided on each line to effect isolation, there is a quantifiable frequency at which redundant valves on a line fail to isolate. The frequency of containment isolation failure includes the frequency of pre-existing unisolable leaks.



Accident sequence failure combinations.


Core Damage

Uncovery and heatup of the reactor core to the point where prolonged oxidation and severe fuel damage is anticipated.


Decision Criteria

Decision Criteria involve the consideration of the quantitative and qualitative factors which influence a decision. This includes both quantitative screening criteria and the evaluation of other qualitative factors which influence the results of an application.


Dependent Events

Two or more events are dependent when the probability of any event in the set is dependent on the occurrence or nonoccurrence of any other event in the set. The two events A and B are independent events if and only if:

Prob[A and B] = Prob[A]qProb[B]

This requires that:

Prob[B|A] = Prob [B]


Prob[A|B] = Prob[A]


End State

An end state is the set of conditions at the end of an event sequence that characterizes the impact of the sequence on the plant or the environment. In most PSAs, end states typically include: success states, plant damage states for Level 1 sequences, and release categories for Level 2 sequences. Major Level 2 end state groups and subgroups identify groups of release categories with similar potential for offsite consequences.


Event Tree

An event tree is a quantifiable logical network that begins with an initiating event or condition and progresses through a series of branches (usually binary) that represent expected system or operator performance that either succeeds or fails and arrives at either a success or failed condition (e.g. core damage) at the end of the tree.


Event Tree Top Event

Top events are the conditions that are considered at each branch point of an event tree. They may address system behavior or operability, human actions, or phenomenological events. A particular event tree sequence can be described in terms of the status of the plant relative to each top event.


External Event

An external event is an event that initiates outside of the plant systems that can affect the operability of plant systems. An earthquake or a missile generated by a tornado are examples of external events, as well as fires within the plant.


Failure Rate

Failure rate can be demand dependent or time dependent. The failure rate of a component is the conditional probability of failure on the next demand (for a standby component) or in the next hour of operation (for an operating component), given it has not already failed.


Fault Tree

A fault tree is a logic diagram that is used to determine the logical combination of failure or condition causes that will produce an undesired event. Fault trees are generally used to determine and quantify the logical combination of causes that would result in failure or unavailability of a system that was modeled in the event tree models.


Fault Tree Top Event

A fault tree top event is the event at the very top of the fault tree, sometimes referred to as the undesired event, for which the fault tree determines the causes.


Figure of Merit

Figure of Merit is the quantitative value obtained from a PSA analysis used for evaluating the results of an application. Typically these include core damage frequency (CDF) and large, early release frequency (LERF), although other measures can be used as surrogates for these or as supplements to these.



Frequency is the number of occurrences of an event per trial or per unit time.


Fussell-Vesely Importance (F-V)

Fussell-Vesely Importance of a modeled plant feature (usually a component, train, or system) is defined as the fractional decrease in total risk level (usually CDF) when the plant feature is assumed perfectly reliable (failure rate = 0.0). If all the sequences comprising the total risk level (e.g. CDF) are minimal, the F-V also equals the fractional contribution to the total risk level of all sequences containing the (failed) feature of interest. Note that F-V = 1-1/RRW. (See Risk Reduction Worth.)



Hazard is a source of danger or consequence.


Human Reliability Analysis

Human Reliability Analysis is the quantitative evaluation of human performance considered in PSAs.


Independent Events

See dependent events.


Initiating Event

An initiating event is any event that perturbs the steady state operation of the plant, if operating, or the steady state operation of the decay heat removal systems during shutdown operations such that a transient is initiated in the plant. Initiating events trigger sequences of events that challenge the plant control and safety systems.


Interfacing Systems LOCA (ISLOCA)

An ISLOCA is a breach in a system that interfaces with the reactor coolant system (RCS) and could cause a loss of coolant accident, if the breach is not isolated from the RCS. Such a breach could be caused if valves fail to isolate the RCS from an interfacing system not designed for the high RCS pressures. When portions of an interfacing system are located outside the containment, particular concern arises because an unisolated system breach outside containment can result in a release of radionuclides that bypasses the containment. Interfacing systems LOCAs that bypass the containment were recognized in WASH-1400, where they were referred to as a V-sequence.


Large, Early Release

A large, early release is a radioactivity release from the containment which is both large and early. Large is defined as involving the rapid, unscrubbed release of airborne fission products to the environment. Early is defined as occurring before the effective implementation of the off-site emergency response and protective actions.


Level 2 End State Group

See end state.


Mission Time

The mission time is the time that a system or component is required to operate in order to successfully perform its function.


Performance Shaping Factor (PSF)

Performance Shaping Factors (PSF) are those factors which influence human error rates as considered in a PSAs human reliability analysis. Typical PSFs include level of training, quality/availability of procedural guidance, time factors, etc.


Plant Damage State

Plant damage states are collections of accident sequence end states according to plant conditions at the onset of severe core damage. The plant conditions considered are those that determine the capability of the containment to cope with a severe core damage accident. The plant damage states represent the interface between the Level 1 and Level 2 analyses.


Probabilistic Risk Assessment (PRA)

See probabilistic safety assessment.


Probabilistic Safety Assessment (PSA)

PSA is a quantitative assessment of the risk associated with plant operation and maintenance. The risk is measured in terms of the frequency of occurrence of different events, including severe core damage. In general, the scope of a PSA is divided into three categories: Level 1, Level 2, and Level 3. The Level 1 scope maps from initiating events to plant damage states including their aggregate, severe core damage. Level 2 includes Level 1 mapping from initiating events to release categories (source term). Finally, Level 3 includes Level 2 and uses the source term of Level 2 to quantify consequences, the most common of which are health effects and property damage in terms of costs. Of course, the scope of a PSA may vary considerably within each Level depending on the preferences of the plant owners or the regulators.



Probability is a numerical measure of a state of knowledge, a degree of belief, or a state of confidence about the outcome of an event.


Quantitative Screening Criteria

Quantitative screening criteria are the quantitative, best estimate values used to screen application results to determine whether the results indicate that the change evaluated is non-risk significant, unacceptably high risk, or if further evaluation is needed.


Release Category

Release categories characterize major classes of accident sequences in terms of the nature, timing, and magnitude of the release of radioactive material from the plant during a severe core damage accident. The factors addressed in the definition of the release categories include the response of the containment structure, timing, and mode of containment failure; timing, magnitude, and mix of any releases of radioactive material; thermal energy of release; and key factors affecting deposition and filtration of radionuclides. Release categories can be considered the end states of the Level 2 portion of a PSA.



A measure of the expectation (i.e., probability), assuming that the SSC is available, that the SSC will perform its function upon demand at any future instant in time.



Risk encompasses what can happen (scenario), its likelihood (probability), and its level of damage (consequences).


Risk Achievement Worth (RAW)

Risk Achievement Worth (RAW) of a modeled plant feature (usually a component, train, or system) is the increase in risk if the feature is assumed to be failed at all times. It is expressed in terms of the ratio of the risk with the event failed to the baseline risk level.


Risk Reduction Worth (RRW)

Risk Reduction Worth (RRW) of a modeled plant feature is the decrease in risk if the feature is assumed to be perfectly reliable. It is expressed in terms of the ratio of the baseline risk level to the risk with the feature guaranteed to succeed. See Fussell-Vesely Importance.


Screening Criteria

See Quantitative Screening Criteria.


Severe Accident

Severe accidents are those that result in catastrophic fuel rod failure, core degradation and fission product release into the reactor vessel, containment or the environment.


Source Term

The radiological source term for a given accident sequence or release category consists of the release fractions for various radionuclide groups (expressed as fractions of initial core inventory), and the timing, elevation, and energy of the release.


Split Fraction

A split fraction is a unitless parameter (i.e., probability) used in quantifying an event tree. It represents the fraction of the time that each possible outcome, or branch, of a particular top event may be expected to occur. Split fractions are, in general, conditional on precursor events. At any branch point, the sum of all the split fractions representing possible outcomes should be unity. (Popular usage equates "split fraction" with the failure probability at any branch [a node] in the event tree.)


Vessel Breach

A failure of the reactor vessel occurring during core melt (e.g., at a penetration or due to thermal attack of the vessel bottom head or wall by molten core debris).