Initiating_Event.txt

Purpose

Initiating_Event.txt specifies the time-dependent initiating events to be included in a simulation. Initiating events are plant faults — such as a loss of offsite power or a steam generator tube rupture — that start accident sequences. Each event is activated at a prescribed simulation time.

Single failure branch

Unlike conditional failures in SystemReliability.txt, an initiating event generates only a single failure branch — no success branch is created. As a result, the sum of all sequence probabilities across a simulation run can be no greater than the largest initiating event probability included.

When the operators attempt to recover an initiating event failure, two additional branches are generated: one for successful recovery and one for permanent (unrecoverable) failure.

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Initiating Event vs. Conditional Failure

Characteristic	Initiating Event	Conditional Failure
Input file	Initiating_Event.txt	SystemReliability.txt
Activation	Simulation time	Component state change
Initial branches generated	One (failure only)	Two (success and failure)
Recovery possible	Yes	Yes

Use initiating events to model time-dependent failures. Use conditional failures (SystemReliability.txt) when modeling a demand failure or a failure triggered by a change in system state.

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Probability Distribution

Failure and recovery probabilities are sampled from a Beta distribution using Equations 17 and 18 respectively:

$$p(x)=\frac{\mathrm{\Gamma }(\alpha +\beta )}{\mathrm{\Gamma }(\alpha )\mathrm{\Gamma }(\beta )}{x}^{(\alpha -1)}(1-x{)}^{(\beta -1)}$$$$\overline{p}=\frac{\alpha }{\alpha +\beta }\text{(mean value)}$$

Two parameters must be provided for each distribution: alpha (α) and beta (β). To improve reproducibility of simulation results, select α and β to minimize variance. For example, p(x, α=100, β=100) and p(x, α=1, β=1) both have mean 0.5, but the former has more than an order of magnitude lower variance.

Dummy recovery values

When recovery from a failure is physically unrealistic (e.g., a steam generator tube rupture cannot be "turned off"), use recognizable dummy values such as alpha_rec = 1.0, beta_rec = 1.0, or set an effectively zero recovery probability (e.g., alpha_rec = 1.0, beta_rec = 1e6).

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Input File Format

Each event is one line of eight fields (this matches the shipped reference file tests/BV4/Input/System/Initiating_Event.txt; the parser is token-based, so line breaks and other whitespace are interchangeable):

Initiating_Event    number_of_initiating_events
event_name_1    control_value    time    control_name    alpha_init    beta_init    alpha_rec    beta_rec
.
.
event_name_n    control_value    time    control_name    alpha_init    beta_init    alpha_rec    beta_rec
obligatory_text
obligatory_text

The file must not end after the last event — two trailer tokens are required

After the events, the parser reads a second section (support-system initiating events: a header word and a count). If the file simply ends, the count is left uninitialized and startup can loop without end, printing SupportSystemReliabilityModule::getSupportSystemReliabilityUnit NONE can not be found. The BV4 file ends with two obligatory_text tokens: the second token does not parse as an integer, which sets the count to 0 and disables the section. Any pair of words whose second word is not a number works the same way; keep the BV4 convention.

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Parameter Reference

Parameter	Format	Range	Description
number_of_initiating_events	Integer	> 0	Number of initiating events to read. Events beyond this count are ignored. Set to 0 to disable all events without deleting them.
event_name	String	any	Descriptive name for the event. No spaces allowed; use _ instead.
control_value	Double	any	Value applied to control_name when the event fires.
time	Double	any	Simulation time (seconds) at which the event activates. Due to RELAP time-step variation, activation occurs at the first step with elapsed time greater than this value. Set to any value < 0.0 to disable the event without removing it from the file.
control_name	String	must match ControlPanel.txt	Controller associated with the event. No spaces; use _.
alpha_init	Double	> 0.0	α parameter of the failure Beta distribution (Equation 17).
beta_init	Double	> 0.0	β parameter of the failure Beta distribution.
alpha_rec	Double	> 0.0	α parameter of the recovery Beta distribution (Equation 18).
beta_rec	Double	> 0.0	β parameter of the recovery Beta distribution.

Enabling and disabling events

To run only a single event, move it to the top of the list and set number_of_initiating_events = 1. All other events are skipped without modification. To disable all events, set number_of_initiating_events = 0.

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Sample Input

The following example defines three initiating events. Only SG_B_MSLB is active (positive time = 60 seconds). The other two events have negative times and will not fire.

Initiating_Event          3
Loss_of_Offsite_Power     1.0    -25.0    X_LOOP         1.0     100.0   4.0   1.0
SG_A_SGTR                 1.0    -60.0    X_SGTR_SG_A    99.0    6.0     1.0   1.0
SG_B_MSLB                 1.0     60.0    X_SG_B_MSLB    1.0     100.0   6.0   4.0
obligatory_text
obligatory_text

What this sample does:

• Loss_of_Offsite_Power — time = −25.0 s → disabled
• SG_A_SGTR — time = −60.0 s → disabled
• SG_B_MSLB — time = 60.0 s → active; sets controller X_SG_B_MSLB to 1.0 at t ≥ 60 s with mean failure probability α/(α+β) = 1/101 ≈ 0.0099 (see note below)

INFO

The recovery parameters for SG_B_MSLB (alpha=6.0, beta=4.0) in this sample are dummy values and do not represent a realistic recovery probability, as a main steam line break cannot be reversed by resetting a controller.