Passive Safety System Reliability

Purpose

Passive Safety Systems will play a crucial role in the mitigation of risk for many different advanced reactor designs. Advanced reactor designs may rely more heavily on Passive Safety Systems than the current fleet of water reactors which is why this area of research is deemed a high priority.

Scope

Not only is it important to understand how passive safety systems operate, but also how different factors (both internal and external) can influence the actuation of the safety system and how to determine the associated uncertainty. Uncertainty will be a major focus in this topic, as it is likely that passive safety systems designed for advanced reactors will have a larger range of uncertainty than those designed for the current fleet of water reactors.

Passive Safety Systems

New Research

After evaluating the different approaches in EPRI 3002032218 [67] an integrated approach was identified in EPRI 3002032223 [68] that provides a structured method to evaluate PSS reliability. Such a framework serves as a useful structure to support licensing and operational decision-making, as well as regulatory reviews in an effective and efficient manner.

Once the phenomenological impacts on PSS reliability are evaluated, the integrated approach provides a straightforward way to combine results with hardware related failures to obtain a comprehensive estimate of PSS reliability over the various identified transient and accident sequences.

Several key areas presented in EPRI 3002032218 [67] that showcase the challenges to the estimation of passive safety system reliability, include:

• Reliance on expert judgment in defining and assessing the critical parameters (CPs) that affect the reliability of PSSs.
  • Expert judgement can be relied upon, if necessary, when experimental and/or operational data is lacking.
  • A critical element of the integrated approach is to characterize and evaluate the parameters that can contribute to the phenomenological failure of the system.
• Treatment of uncertainty, including in the selection of CP, the uncertainties within those CP, and the mathematical approaches representing those uncertainties.
• Treatment of time-dependent effects on passive system performance, including both short-term impacts due to accident-induced conditions and long-term impacts due to aging.
• Understanding of the fidelity and uncertainties of thermal-hydraulic codes that are relied upon to model passive system performance.

Another aspect of Passive Safety Systems in advanced reactors is a response surface:

• Passive Safety System reliability not only includes assessing system and component reliability, but also phenomenological reliability.
• To obtain meaningful results related to phenomenological reliability, thousands of case runs may need to be performed which is computationally expensive.
• A response surface solves this problem by using a multivariate regression to estimate the reliability code.

An example of a response surface that uses a cubic spline for multiple variables is shown below:

Historical

EPRI documents the initial research of passive safety system in EPRI-1015101 [31] and EPRI-1016474 [32], however more recent research can be found in references 33-36 on the References page. Since passive safety systems operate through natural phenomena, factors such as operating conditions, environmental conditions (seismic events, high winds, external flooding), corrosion, aging effects, and fouling of heat transfer surfaces can have a much larger impact on system reliability of a passive safety system compared to active safety systems. Since passive SSCs generally do not fail catastrophically but are more prone to gradual degradation over time implementation of effective monitoring and diagnostics to detect and address these mechanisms will be more important than they have been for the existing fleet of water reactors.

Additionally, lack of data on some phenomena and missing operating experience need to be considered when analyzing the performance of passive systems. Natural circulation-based passive systems also have unique challenges due to the relatively low magnitude of the driving forces involved. Review of reliability literature [37], [38], and [39] suggests that the technical methods can address the uncertainties associated with passive safety systems and existing methods can be categorized as:

• reliability evaluation of passive safety systems
• reliability methods for passive safety functions
• analysis of passive systems reliability

Another challenge that has been considered is that the current methodology passive safety systems are viewed as a binary, success, or failure, where in reality passive safety systems can fail in intermediate conditions during operation. In order to overcome this, the passive safety system reliability methodology will need to consider dynamic variations of independent process parameters, such as atmospheric temperature [40].

The ultimate goal in the development of passive safety systems in ARs is to be able to define the uncertainty well enough that risk-informed decisions can be made and modeled with a high level of confidence.

Research Roadmap (EPRI 3002026495) actions supported for passive system reliability include:

• Develop and Qualify Analytical Tools for Advanced Reactor Designs
• Develop Enhancements to Licensing Process
• Demonstrate Risk-Informed and Performance Based Approach
• Reduce Operating and Maintenance Costs to a Level Similar to Other Thermal Plants

A higher level of detail for these actions can be found in the report.