Version: 1.2

Risk Spectrum PSA (Output)

Overview

FRI3D provides a comprehensive 3D fire and hazard modeling environment that integrates directly with RiskSpectrum PSA, a leading probabilistic safety analysis (PSA/PRA) software used for risk evaluation in the nuclear, energy, and industrial safety domains.

This integration bridges physics-based fire simulations with probabilistic risk analysis, allowing users to quantify the impact of fire scenarios on plant systems and components, evaluate failure propagation, and compute resulting changes in Core Damage Frequency (CDF) or Large Early Release Frequency (LERF).

Concept of Fire Modeling in PSA and FRI3D

There are multiple approaches to integrating fire events into a PSA/PRA model. Fire scenarios may be:

Added directly into fault trees and quantified as new initiating events.
Incorporated into fire-specific event trees to define conditional sequences.
Linked as modified initiating events within existing event trees.

A fire model typically consists of a collection of “Full room burnup” scenarios or sets of individual fire source scenarios for each compartment or area.
Each fire scenario is treated as an initiating event characterized by its frequency, severity, and non-suppression probability.

During quantification, the components that fail within each scenario are represented as Basic Events (BEs) set to a probability of 1.0.
The difference in total CDF between the base case and the fire-augmented case indicates the incremental fire-induced risk.

FRI3D ↔ RiskSpectrum PSA Integration Workflow

Step	Description
1. Scenario Definition	Fire scenarios are created in FRI3D by the user or imported from an existing RiskSpectrum project.
2. Simulation Execution	FRI3D simulates the scenario and determines affected cable trays, damaged components, and failed basic events.
3. Scenario Analysis	User reviews simulation results and defines ignition frequency, severity factors, and non-suppression probabilities.
4. Export to RiskSpectrum	FRI3D prepares and exports the modified PRA data for import into RiskSpectrum.
5. Quantification	RiskSpectrum executes the event/fault tree analysis for all fire scenarios, calculating overall risk contributions.

In view of the above steps the following detailed steps are outlined to run a fire PRA/PSA in RiskSpectrum via FRI3D.

Model Preparation

Nothing has to be built by hand in RiskSpectrum before fire modeling. FRI3D adds the fire exchange event and the fire analysis case group itself on every export (see the next section). The RiskSpectrum model only needs what an internal-events PSA already has:

A consequence analysis case that solves the event tree(s) the fire should be run through. This becomes the base analysis case in the FRI3D preferences.
The initiating event of that event tree, modeled as a basic event with a Frequency reliability model. This becomes the base initiating event.

Both are explained under FRI3D/Risk Spectrum Preferences below.

Integration with RiskSpectrum model (Automatically done by FRI3D)

To create or add scenarios from FRI3D to the RiskSpectrum model the following steps are performed automatically by FRI3D.

Use the Risk Spectrum Macro commands to export the base project that the user has specified links to the FRI3D model to a temporary work location using the excel output feature in Risk Spectrum.
Add the fire exchange event FIRE=1, a basic event with a probability of 1.0, if the model does not already have it. It is substituted for every basic event that fails due to fire, which raises the probability of the cut sets using those events when the fire scenario is quantified.
Create the fire analysis case group FIRE_CURRENT_SET so that all the fire scenarios can be solved at the same time. It is rebuilt on each export, so it always holds the scenarios that were just sent.
For each FRI3D fire scenario the user wants to calculate do the following:
1. Create a house event for the scenario.
2. Link the house event and the fire exchange event for each basic event in the scenario.
3. Copy the base analysis case as a new fire analysis case, including its event tree assignments.
4. Add the fire analysis case to the fire analysis case group.
5. Create a boundary condition set that turns the scenario's house event on. If the base analysis case has a boundary condition set, it is included as well.
6. Create a fire initiating event FI_IE-<scenario> with a frequency of ignition frequency × severity factor × non-suppression probability, and exchange the base initiating event for it when the scenario's house event is on.
Save the modified Excel sheets
Run the Risk Spectrum Macro commands to import the modified PRA Excel sheets
Open RiskSpectrum to solve and execute the fire analysis case group

FRI3D/Risk Spectrum Preferences (Manually done by the user in FRI3D)

Appropriate modifications to the FRI3D interface was made to accommodate Risk Spectrum output, these included specification of the Risk Spectrum master PSA file, the user credentials, the base analysis case and the base initiating event.

FRI3D does not build a fire event tree. Each fire scenario reuses an event tree that already exists in the RiskSpectrum model, and the two base selections tell FRI3D which one:

Base analysis case (RS BASE ANALYSIS CASE) is the template for every fire analysis case. FRI3D copies its setup and event tree assignments, so each fire scenario is solved through the same event tree(s) as the base case, and keeps its boundary conditions. Only consequence analysis cases are offered, and fire analysis cases created earlier by FRI3D (FI_ prefix) are left out.
Base initiating event (RS BASE INIT EVENT) is the initiating event of that event tree. In each fire analysis case it is exchanged for the scenario's fire initiating event, so the event tree starts from "this fire broke out" at the fire's frequency, while the fire-failed components are set to fail through the fire exchange event. Only basic events with a Frequency reliability model are offered. Pick the initiating event of the base analysis case's event tree; if a different one is picked, the event tree keeps its original initiating event and the fire frequency is not used.

The base analysis case itself is not changed. Because the fire initiating event carries the fire's frequency, the result of each fire analysis case is that scenario's contribution to CDF. Changing the RiskSpectrum model clears both selections, because they name entities in the previous model.

RS Setup Preferences

RiskSpectrum Fire PSA Model

In Risk Spectrum PSA, all the fire related events corresponding to FRI3D are created with a prefix FI. The house events are created for each fire scenario run. For example: FI_FSOURCE_1_CFAST and FI_SGR-A_FULLROOM are created which are exchanged with the corresponding basic events. could be examples of a fire scenario with a fire at one section of a room and a full room burnout.

The house events from the scenario are exchanged with these basic events via Exchange Events in RiskSpectrum.

RiskSpectrum Fire PSA Model Structure

Within RiskSpectrum, FRI3D-generated fire events are named and structured consistently to allow automatic linking and analysis.

Type	Naming Convention	Description
Fire Event Prefix	`FI_`	Identifies fire-originated scenarios (e.g., `FI_FSOURCE_1_CFAST`, `FI_ROOM_FULLBURN`).
House Events	`FI_<ScenarioName>`	Scenario-specific events activated when the scenario is quantified.
Exchange Events	`EX_<ComponentID>`	Used to replace failed components in fire cases.

For example:

FI_FSOURCE_1_CFAST → represents a localized fire in one zone.
FI_SGR-A_FULLROOM → represents a full room burnout in Sub-Generator Room A.

These house events are exchanged with the corresponding basic events via Exchange Events in RiskSpectrum.
This mechanism enables the model to activate appropriate failure combinations during quantification.

Example Workflow Illustration

flowchart LR
A[FRI3D Fire Scenario Simulation] --> B[Identify Failed Components & Zones]
B --> C[Generate House and Exchange Events]
C --> D[Export Modified PRA Data (Excel/XML)]
D --> E[Import into RiskSpectrum PSA]
E --> F[Quantify Fire Analysis Case Group]
F --> G[Compute Fire-Induced ΔCDF and ΔLERF]

Key Outputs

File	Description
FRI3D_RiskSpectrum_FireScenarios.xlsx	Contains generated house and exchange events for each scenario.

Notes on Frequency and Severity Inputs

Each scenario exported from FRI3D includes parameters defined by the user:

Parameter	Description	Example
Ignition Frequency (λ)	Frequency of the initiating fire event.	`1.2E-4` per year
Severity Factor (SF)	Represents the proportion of fires reaching the modeled severity.	`0.3`
Non-Suppression Probability (NSP)	Probability that suppression fails.	`0.1`
Conditional Core Damage Probability (CCDP)	Output from RiskSpectrum quantification.	Computed post-analysis

The combined product of these parameters defines the Fire CDF contribution:

Fire_CDF = λ × SF × NSP × CCDP

Reliability and Validation

Integration tested against **RiskSpectrum PSA 1.6 **.
Uses standard Excel interface macros for safe model import/export.
Supports multi-scenario batch processing.
Validation recommended after import to confirm correct event linkages.
All exported events are traceable back to their originating FRI3D zone and simulation timestamp.

Summary

FRI3D exports fire scenario data, equipment failures, and probability modifiers directly into RiskSpectrum-compatible Excel workbooks.
Each scenario can be quantified as a unique analysis case in RiskSpectrum.
The integration provides a seamless bridge between physical simulation and probabilistic safety modeling.
Results can be used to derive fire risk metrics, prioritize mitigation actions, and refine safety case justifications.

References

RiskSpectrum PSA User Manual (v2.2) — RiskSpectrum AB
https://riskspectrum.com
FRI3D Fire PRA Export Guide — Internal Technical Note, Centroid LAB, 2025.
Describes event mapping, macro automation, and validation workflow.
NUREG/CR-6850 — Fire PRA Methodology for Nuclear Power Facilities, U.S. NRC / EPRI.
Standard reference for fire PRA quantification.

⚠️ Disclaimer:
RiskSpectrum export functionality in FRI3D automates scenario and event generation for fire PSA.
Since user-defined event trees and macros can vary between installations, exported cases should always be verified within RiskSpectrum prior to quantification or regulatory submission.