HIGH WIND PRA DEVELOPMENT AND LESSONS LEARNED FROM IMPLEMENTATION Artur Mironenko and Nicholas Lovelace April 27, 2015
Overview Introduction High Wind PRA Development Tasks High Wind PRA Lessons Learned Conclusion & Questions 2
Introduction Over the last couple of years a number of insights and lessons learned have occurred in High Wind Probabilistic Risk Assessment (PRA) model development. The Duke Energy fleet has undergone several successful High Wind Peer reviews recently. The objective is to characterize and understand the high wind risk for each of the plant sites for CDF and LERF The risks associated with thunderstorms, extra-tropical storms, tornadoes, and hurricanes were evaluated for each site. Several improvements and vulnerabilities were discovered during the development of the High Wind PRA Model 3
HWPRA Development Tasks The following six tasks were performed to complete the high wind PRA models: Site Specific High Wind hazard analysis High Wind Analysis Walkdowns and equipment list High Wind Fragility Analysis Integrate High Wind Impacts into PRA Model Quantify High Wind PRA Model Complete Documentation 4
HWPRA Work Flowchart Wind Fragility Analysis Equipment List for PRA Site Walkdown Equipment Screening Wind & Missile Combined Fragilities Missile Fragility Analysis HWPRA Tornado Hazard High Wind Hazard Conditional Loop High Wind and Fragility Analysis 5
Site Specific High Wind Hazard Analysis This task is critical to defining initiating events in the PRA model for the frequency of exceedance for high winds that can impact plant structures, systems, and components (SSCs). Four types of extreme winds are considered: Thunderstorm winds Extra-tropical storms Tornadoes Hurricanes These events also have different durations and occur in different seasons of the year. Duration may be important to operator actions during and following an event. Modeling and random uncertainties were considered in developing the wind hazard risk estimates and are propagated through the hazard models. 6
High Wind Analysis Walkdowns and Equipment List The walkdowns are performed to support High Wind Hazard Analysis, High Wind Fragility Analysis, and the development of the High Wind PRA model. The Internal Events PRA (Core damage and LERF) was used to develop the initial list of equipment to be considered for the High Winds PRA. Plant drawings (P&IDs, single line electrical, CWDs, etc.) were reviewed to determine additional equipment to be added to the equipment list. Data gathered can include elements of the equipment not listed on the high wind equipment list such as a connected pipe attached to a valve, or a valve actuator, which if damage will render the component inoperable. 7
High Wind Analysis Walkdowns Observation DG Fuel Oil vent scored to prevent crimping from a wind borne missile interaction. 8
High Wind Analysis Walkdowns Observation 9
High Wind Fragility Analysis Fragilities for high wind-impacted SSCs consist of wind pressure hazard fragilities and missile fragilities. Wind fragility is defined as the conditional probability of failure for a given value of peak gust wind speed. The peak gust wind speed is the basic open terrain 3 second gust wind speed at 10m above grade. Class I reinforced concrete structures are not generally vulnerable to wind failure modes. The following general failure modes are applicable to the target SSCs. Building interaction failure modes for targets inside non-class I structures Wind Pressure and Atmospheric Pressure Change (APC) Wind-borne missiles 10
Integrate High Wind Impacts into PRA Model Changes to the Internal Events PRA model were implemented to adequately capture postulated high wind-induced initiating events, impacted human failure events, and high windinduced failure modes including wind-borne missile damage. 10 discrete intervals were used to produce the hazard frequencies and fragility calculation points shown below: 11
Refining the interval for lower wind speeds To be more numerically accurate, lower wind speed intervals were sub-divided further. 12
Integrate High Wind Impacts into PRA Model The Figure below provides an example of the high wind missile fault tree logic development for a service building electrical room at an F4 wind speed. The pressure fragilities are generally built separately in the fault tree but with the same structure. 13
Integrate High Wind Impacts into PRA Model The Figure below provides an example of the high wind pressure fault tree logic development for a service building wall/frame failure at an F4 wind speed. 14
Integrate High Wind Impacts into PRA Model When building the high wind logic generally the wind speeds for each failure are grouped before inserting the gate into the internal events model as shown in the Figure below. 15
Integrate High Wind Impacts into PRA Model The missile and pressure failures are then combined which includes the other wind speeds underneath an OR gate and inserted into the internal events model as seen in the Figure below. 16
Integrate High Wind Impacts into PRA Model - HRA The post-initiator human failure events (HFEs) were reviewed to determine the impact of high winds on human reliability. The following key factors were considered in the assessment of operator actions following high wind events: Time after the high wind event Location of the operator action High wind severity (based on high wind speed) One of the key insights that has been determined through operator interviews is that even at the lower wind speeds the operator may not perform certain actions no matter the condition of the plant if the action must be performed outside the main control room. Currently there is no detailed industry guidance for high wind HRA. High wind HRA has unique aspects and complexity when compared to other external events due to having multiple hazards to consider (Hurricane, straight wind, and tornado). 17
Quantify High Wind PRA Model This task required multiple cutset reviews for CDF and LERF. The output of the quantification included cutsets, initiating event contributions, importance measures, uncertainty distributions, and sensitivity results. The Figure below shows the contribution from the high wind initiators. As can be seen the straight winds contribute to over 50% of the contribution and are dominated by the F1 and F2 wind speeds. 18
Quantify High Wind PRA Model Based on the analyses currently done loss of offsite power is a dominant contributor to high wind models at lower wind speeds due to the design of the offsite power lines, switchyard, and Turbine Building siding. Typically, the design wind speeds for the site transmission lines are in the range of 80-90 mph peak gust wind speeds. This leads to high conditional probabilities of a LOOP at relatively low wind speeds. The figure below gives a representation of the conditional LOOP probability versus the wind speed. 19
Documentation For this task documentation of the HWPRA model and inputs was developed. It was documented in accordance with the ASME PRA standard. This included a roadmap developed to direct the peer reviewers to the sections of the High Wind report that address specific supporting requirements. One of the key insights was to make certain that the walkdowns and equipment list were thoroughly documented including pictures if feasible for traceability. 20
Lessons Learned from Implementation Straight-line winds were the dominant hazard groups at the Duke plants analyzed. The Human Reliability Analysis (HRA) can be challenging due to the unknown amount of recovery credit to be applied such as recovery of offsite power. Based on operator interviews and simulations it has become apparent that even at the lower wind speeds the operator may not perform certain actions no matter the condition of the plant if it s outside the control room. Due to CAFTAs limitation of dealing with the point estimate at least 10 intervals are needed to obtain numerically accurate results. Loss of Offsite power is a dominant contributor at lower wind speeds due to the design of the offsite power lines, switchyard, and Turbine Building siding. Vulnerability of Turbine building siding can have a significant missile impact, especially to offsite power. HW PRAs are complicated by multiple hazards with different wind characteristics and effects. This is not the case for other external events such as seismic. 21
CONCLUSION SUMMARY OF PRESENTATION Several improvements and vulnerabilities were discovered during the development of the High Wind PRA Model. This generated insights and equipment/operator action importances that were previously not well understood. Recent experience shows that high wind PRAs may contribute more to the core damage and large early release frequency than previously thought. 22
QUESTIONS? Contact Nicholas Lovelace +1 402-540-4783 nlovelace@jensenhughes.com Artur Mironenko +1 980-373-7460 Artur.Mironenko@duke-energy.com 23