Fire PRA Research Articles

Maximum likelihood estimation of probabilistic non-suppression model for OECD NPP electrical fire applying non-negative continuous distribution

The securement and improvement of realism in a probabilistic fire risk assessment (Fire PRA) are important in risk-informed performance-based regulations and decision support. In this study, a probabilistic fire brigade non-suppression model is developed for electrical fires using the Organization for Economic Cooperation and Development fire incident data on operating nuclear power plants collected from various countries by applying a non-negative continuous probability distribution with the maximum likelihood estimation method. The result of fitting 15 types of a non-negative continuous probability distributions shows that the log-normal probability model is the best fitting and most adequate model, and can best represent the actual fire suppression time by a fire brigade. The selected log-normal probability model was compared with the exponential probability model being used in an existing Fire PRA, which shows that the level of adequacy of the log-normal probability model is improved with a decrease in the bayesian information criteria by 7.9%, residual sum of squares by 100.0%, and mean squared error by 57.6%. The log-normal probability model selected from this study is expected to contribute to an enhancement of the Fire PRA realism in support of risk-informed decision making by reflecting actual fire suppression experience.

Probabilistic Non-Suppression Model of Electrical Fire in Nuclear Power Plant Using Maximum Likelihood Estimation Method

Electrical fire is the most significant fire-induced risk in nuclear power plants. In this study, we developed a probabilistic non-suppression model of electric fire by estimating the relevant parameters using the Maximum Likelihood Estimation (MLE) method to secure and improve the realism in Fire PRA in application to the Risk Informed Fire Protection Program. Based on the results of fitting 32 continuous probability distributions into fire incident data, 3-Parameter Weibull probability model was selected as the best fitted and adequate model and was compared with the existing exponential probability model. The comparison shows that the level of model adequacy is improved with decreases in BIC by 17.7%, in RSS by 24.4%, and in MSE by 58.2%. The 3-Parameter Weibull probability model drawn in this study is expected to contribute to the enhancement of the Fire PRA realism for supporting risk-based decision by reflecting real fire event experiences.

Decision tree for FSSA rooms in CANDU

This paper presents a new decision tree that identifies fire compartments that are screened out from further analysis according to pre-determined criteria. The paper identifies the most common fire zones and uses Computational Fluid Dynamics (CFD) model Fire Dynamics Simulator to calculate the consequences. This is a critical and very significant step in the Fire PRA. It screens out fire zones that do not affect the integrity of Fire Safe Shutdown Analysis (FSSA) cables during worst-case fire scenarios.

An integrated methodology for spatio-temporal incorporation of underlying failure mechanisms into fire probabilistic risk assessment of nuclear power plants

In this research, an Integrated probabilistic risk assessment (I-PRA) methodological framework for Fire PRA is developed to provide a unified multi-level probabilistic integration, beginning with spatio-temporal simulation-based models of underlying failure mechanisms (i.e., physical phenomena and human actions), connecting to component-level failures, and then linking to system-level risk scenarios in classical PRA. The simulation-based module, called the fire simulation module (FSM), includes state-of-the-art models of fire initiation, fire progression, post-fire failure damage propagation, fire brigade response, and scenario-based damage. Fire progression is simulated using a CFD code, fire dynamics simulator (FDS), which solves Navier–Stokes equations governing the turbulent flow field. Uncertainty quantification is conducted to address parameter uncertainties. The I-PRA paves the way for reducing excessive conservatisms derived from the modeling of (i) fire progression and damage and (ii) the interactions between fire progression and manual suppression. Global importance measure analysis is used to rank the risk-contributing factors. A case study demonstrates the implementation of I-PRA for a regulatory-documented fire scenario.

Application of the CFAST zone model to the Fire PSA

The integrity of the cables located in the target room is very important in the Fire PSA, because the CDF and CCDP are changed according to the results of a cable integrity that depends on the surrounding gas temperature. The conservative assumptions used in the Fire PSA typically specify that all of the equipment and cables of a room would fail when a fire happens in the room. But the realistic assessment of a fire risk by using a fire simulation tool has become necessary in the Fire PSA as described in the ANS Fire PRA Standard. This paper evaluates the cable integrity of eight pump rooms in the nuclear power plant by using the CFAST zone fire model. The upper layer gas temperature of each room is estimated, and an analysis based on the results of model simulations is used to judge the cable integrity. According to the analysis results, the integrity of the cable located in the upper layer in the pump rooms is maintained without any thermal damage, and the CCDP is reduced to about the half of the original CCDP. A fire safety assessment for dominant pump rooms by using a fire modeling tool is seen capable of evaluating the consequences of a fire scenario, of reducing the uncertainty, and of developing a more realistic Fire PSA model. It is considered that the fire modeling can satisfy the requirements of the ANS Fire PRA standard related to a fire modeling, and improve the quality of the Fire PSA.

An evaluation of risk methods for prioritizing fire protection features: a procedure for fire barrier penetration seals

This paper generally evaluates risk methods available for prioritizing fire protection features. Risk methods involving both the use of qualitative insights, and quantitative results from a fire probabilistic risk analysis are reviewed. The applicability of these methods to develop a prioritized list of fire barrier penetration seals in a plant based on risk significance is presented as a procedure to illustrate the benefits of the methods. The paper concludes that current fire risk assessment methods can be confidently used to prioritize plant fire protection features, specifically fire barrier penetration seals. Simple prioritization schemes, using qualitative assessments and insights from fire PRA methodology may be implemented without the need for quantitative results. More elaborate prioritization schemes that allow further refinements to the categorization process may be implemented using the quantitative results of the screening processes in good fire PRAs. The use of the quantitative results from good fire PRAs provide several benefits for risk prioritization of fire protection features at plants, mainly from the plant systems analyses conducted for a fire PRA.