Seismic PRA Outputs (Task 14)

Task Overview

Objective

The objective is to provide final and intermediate results to provide insights gleaned from the SPRA analysis.

Purpose

This section describes the following expected outputs for an SPRA.

• Typical quantitative results
• Conclusions and insights
• Uncertainty and sensitivity analysis guidance

Guidance

Typical Quantitative Results

Typical results from an SPRA include the following:

• Important seismic initiators

- CDF contribution by seismic ground motion bin

- Conditional core damage probability values for the seismic interval initiators

• Importance contributors to CDF

- FV importance identifies the fractional contribution to the CDF risk metric for the cutsets containing the given basic event. Typical SSCs with high FV importance include structural failures of buildings and events related to loss of station power (that is, offsite power).

- RAW characterizes risk by comparing the increase in risk if a given component is assumed to be completely unavailable (that is, failed). Typical SSCs with high RAW importance include structural failures of buildings and multi-train, common-cause failure events. RAW values are typically lower for SPRA results than they would be for other PRA hazard assessments.

• Cutset evaluation

- By identifying sequences of events in cutsets with high frequencies that lead to core damage, it becomes possible to take preemptive mitigating actions to reduce operational risk.

• Dominant accident sequences

- The summary of the top accident sequences provides insights to the significant CDF contributors.

• Accident classes

- The CDF distribution among the contributing functional accident sequence classes should be summarized to identify the accident sequence progression and timing characteristics for the dominant CDF scenarios. The dominant CDF accident classes may also provide insights to the Level 2 (LERF) characterization.

Conclusions and Insights

Typical conclusions from a seismic PRA model may include the following:

• Develop an appreciation of severe accident behavior under seismic challenges.
• Understand the most likely seismically induced severe accident sequences that could occur at the plant.
• Gain a more quantitative understanding of the overall frequency of core damage due to seismic events.
• Identify insights to reduce the overall probabilities of core damage and fission product releases by modifying hardware and procedures that would help prevent or mitigate severe accidents.

Uncertainty Analysis

There are two types of uncertainty in probabilistic risk models: aleatory and epistemic.

• Aleatory uncertainty is associated with the random nature of events. These include initiating events (seismic initiating events) and component failures.
• Epistemic uncertainty is associated with lack of knowledge of information. This includes statistical inferences from data.

Epistemic uncertainties may be classified into three different categories:

• Completeness uncertainty is associated with risk contributors that are not included in the PRA model.
• Parameter uncertainty is associated with the input parameter values used to quantify the failure frequencies and probabilities of other events in the PRA (e.g. initiating events, human error probabilities)

- The parametric uncertainty propagation for a typical SPRA can be performed using the commercially available software UNCERT Version 3.0 (part of the EPRI Phoenix Architect software suite) or an alternative software.

- A Monte Carlo (or Latin Hypercube) evaluation of PRA logic can be performed using correlated or uncorrelated probability distributions to represent the inputs for the basic events. The probability density distribution describing the uncertainty in a component failure probability is characterized as a state of knowledge about an assumed fixed value.

- The range factor for the sampled CDF (or LERF) can be calculated as the CDF (or LERF) at the 95^th percentile divided by the median CDF (or LERF) at the 50^th percentile (that is, 95%/50%).

• Model uncertainty is associated with the aspects of the PRA model which may be represented by different modeling approaches.

Specific areas of uncertainty identified for seismic PRA model include HRA, earthquake caused correlations or dependencies, relay chatter and recovery analysis. More details regarding uncertainty approaches are found in the guidance listed below.

Sensitivity Analysis

Example sensitivity cases to address SPRA modeling uncertainty include:

• Seismic hazard curve sensitivity
• Fragility parameters sensitivity
• Correlation assumption sensitivity
• Number of hazard curve intervals sensitivity
• HRA sensitivities
• Treatment of SSLOCA sensitivity
• Impact on LERF sequence sensitivity

Supplemental Guidance

Related Element of ASME/PRA Standard

Part 5, Seismic Plant-Response (SPR)

EPRI Guidance

Practical Guidance on the Use of Probabilistic Risk Assessment in Risk-Informed Applications with a Focus on the Treatment of Uncertainty (1026511). Section 5.10.2 provides a summary of generic sources of seismic PRA modeling uncertainty

Seismic Probabilistic Risk Assessment Implementation Guide (3002000709) provides general guidance to identify sources of uncertainty associated with the seismic hazards, seismic fragilities, and seismic PRA model throughout the document. Example sensitivity cases are provided regarding correlation and SPRA HEPs.

Surry Seismic Probabilistic Risk Assessment Pilot Plant Review (1020756) identifies sources of uncertainty consistent with EPRI and ASME/ANS guidance. The report documents the parametric uncertainty analysis. In addition, a set of sensitivity cases were performed based on a review of the dominant contributors to seismic risk.

Phoenix Architect v2.0 Suite (3002023512)

Other Guidance

NUREG-1855, Rev1, Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decision making, provides an industry-recognized methodology for identifying sources of plant-specific or generic uncertainty including external events.

IAEA Safety Guide SSG-3, Development and Application of Level 1 Probabilistic Safety Assessment for Nuclear Power Plants, provides general guidance on the identification of sources of uncertainty consistent with EPRI and ASME/ANS guidance. IAEA SSG-3 does not provide any specific guidance on the performance of parametric uncertainty analysis or sensitivity studies.