Palo Verde Generating Station NEI Lessons Learned. Michael Wittas September 28, 2017

Transcription

1 Palo Verde Generating Station NEI Lessons Learned Michael Wittas September 28, 2017

2 Overview Background & History Pre-FLEX Temporary Equipment Fukushima Daiichi Response NRC RIS Palo Verde Generating Station FLEX Plus Strategies 4160 Volt Alternating Current Power Steam Generator Makeup Application of NEI at Palo Verde Lessons Learned NRC Presentation and Input Staging of FLEX Plus Equipment Deployment Strategy Performance Monitoring Implementation for 10 CFR (a)(4) FLEX Plus Engineering Bases

3 3 Background & History

4 Pre-FLEX: Temporary Equipment for Configuration Risk Management PVGS Shutdown Risk Reduction Site-wide effort beginning in early 2009 Multidisciplinary effort including leadership and frontline from PRA, Operations, Work Management, and Engineering Purpose was to assess existing defense in depth shutdown risk assessment process and identify means to reduce risk Updates implemented include programmatic changes, procedure revisions, and use of temporary equipment Project was subsequently expanded to include at-power, probabilistic configuration risk management These efforts primed the pump for eventually considering FLEX for configuration risk management at PVGS

5 Fukushima Daiichi Response March 2011 Fukushima Daiichi Disaster March 2012 Nuclear Regulatory Commission Order EA Leadership behind the risk reduction initiatives were chosen to lead the PVGS response to NRC Flexible and Diverse Coping Strategies (i.e. FLEX) requirements Carried forward the risk management mindset when deciding how PVGS would implement NRC FLEX requirements 5

6 FLEX Plus at PVGS PVGS FLEX responses that go beyond base FLEX requirements are colloquially known as FLEX Plus FLEX Plus was planned and implemented as part of the PVGS FLEX response, with the explicit intent of use in configuration risk management Examples include: Robust exterior connection points (electrical and mechanical) On-site 4160 volt AC generation N+1 FLEX success paths 6

7 NRC Regulatory Issue Summary Prior to issuance of NEI 16-06, no specific industry guidance on crediting temporary equipment for configuration risk management Pieces of information available across various industry documents, but none specifically for this purpose PVGS identified NRC RIS as providing guidance that could be applied to configuration risk management NRC RIS was originally written to provide guidance crediting mitigating strategies for B.5.b NEI lists NRC RIS as a developmental reference 7

8 PVGS FLEX Plus Strategies implemented in the PVGS probabilistic configuration risk management program 8

9 4160V AC Electrical Power PVGS on-site generator skids 2MW capacity per skid (4160V AC output) Minimum of 2 deployed to a given unit when credited for configuration risk management Meets same PRA mission as PVGS Station Blackout Generator Provides 4160V AC power to a single safety bus Locally operated Operation driven by plant procedures, hard card instructions, and once-per-shift Operator briefings Staged and connected to the unit in order to receive credit in the configuration risk management program

10 2MW 4160V AC Generator Skids 10

11 Permanent 4160V Bus Connection Box 11

12 4160V Manual Transfer Switch 12

13 Palo Verde AC Power System 13

14 Steam Generator Makeup Single diesel-driven portable pump skid Takes suction from Condensate Storage Tank Capable of feeding either Steam Generator, but not both The Palo Verde Combustion Engineering System 80 plant is a 2-loop design Same PRA success criteria as a single permanent Auxiliary Feedwater pump Flow to one Steam Generator with one Atmospheric Dump Valve provides adequate secondary cooling Locally operated Operation driven by plant procedures, hard card instructions, and once-per-shift Operator briefings Staged (but not connected) at the unit in order to receive credit in the configuration risk management program 14

15 FLEX Steam Generator Makeup Pump Skid 15

16 Permanent Exterior Connection Points Discharge Connections Alternate Suction Primary Suction 16

17 Application of NEI at PVGS Overview of methodology and lessons learned 17

18 NEI Overview History of NRC Orders and industry guidance that have driven use of temporary equipment Examples include NRC Order EA (FLEX), NRC Order EA (B.5.b), NUMARC Three-tiered assessment approach Tier 1 Qualitative Assessment Tier 2 Semi-Quantitative Streamlined Assessment Tier 3 Full Probabilistic Risk Assessment Example evaluations for each Tier Example human failure event analysis 18

19 PVGS Assessments Palo Verde performed Tier 1 and 3 assessments Tier 1 approach (NEI section 5) Each subsection treated as its own specific criterion requiring assessment and documentation Criterion focused on general feasibility and availability assessments (as opposed to true PRA assessment) Served as foundation for further tiers of assessment Tier 2 assessment (NEI section 6) Not performed by PVGS Deemed of minimal benefit given the nature of FLEX Plus deployment at PVGS and performance of Tier 3 PRA assessment 19

20 PVGS Assessments Tier 3 assessment (NEI section 7) Full PRA analysis, under the presumption of attempting to meet Regulatory Guide criteria Guidance structured to follow ASME/ANS PRA Standard high-level requirements common to all Level 1 hazards PVGS assessment did not attempt to meet RG criteria for modeling of FLEX Plus, given the limited use of the modeling for 10 CFR (a)(4) configuration risk management 20

21 Lessons Learned NRC Interaction NRC performed a site visit to assess PVGS use of NEI 16-06, specifically for the FLEX Plus 4160V AC power success path Regional and national NRC staff toured PVGS, had briefings with subject matter experts, and participated in a full-day meeting with PVGS leadership to discuss the implementation of NEI PVGS fulfillment of each criteria for Tier 1 and 3 assessments (per NEI sections 5 and 7) were clearly itemized in table format for the NRC This was an effective communications tool that received positive feedback after its presentation Simple items were addressed directly in the table, but others required reference to supporting documents and engineering evaluations 21

22 Example Tier 1 Assessment Table Entry Criterion Description of how each criterion is met Criterion Met (YES/NO) NEI , Scenario Assessment Engineering Evaluation (EE) (Ref. 5) evaluated the PRA mission for the 2MW 4160V portable generators and determined that they could fulfill the same mission as a single SBOG. Hence they may be credited in any scenario where a single SBOG successfully supplies power to a Class 1E 4160V bus. YES See Attachment A for the evaluation detailing the adequacy of the scenario timeline. The human reliability analysis (HRA) assessment performed in Attachment A demonstrates that the available time is adequate to support employing the 2MW 4160V portable generators. Equipment necessary to employ the 2MW 4160V portable generators includes the generators themselves (AEBDNG04A*FLEX*GENERA and AEBDNG04B*FLEX*GENERA), the cables connecting the generators to the plant, permanently installed transfer switches NBN-U10 (connection to PBA-S03) and NBN-U12 (connection to NBN-U12), and supply breakers PBA- S03L and PBB-S04K. Operators are cued to employ the 2MW 4160V portable generators by alarm response procedure 40AL-9RK1A (Ref. 7), response section 1A06A or alarm response procedure 40AL- 9RK1C (Ref. 8), response section 1C10B. Both alarm response procedures direct the operator to enter procedure 40MT-9ZZ01 (Ref. 4), which contains instructions on how to restore power to the appropriate Class 1E bus. 22 Deployment and installation of the 2MW 4160V portable generators has been demonstrated. Successful deployment and installation was demonstrated in Unit 1 in support of an emergent emergency diesel generator outage starting 8/1/16.

23 Example Tier 3 Assessment Table Entry Criterion Description of how each criterion is met Criterion Met (YES/NO) NEI , System Analysis Equipment necessary to employ the 2MW 4160V portable generators includes the generators themselves (AEBDNG04A*FLEX*GENERA and AEBDNG04B*FLEX*GENERA), the cables connecting the generators to the plant, transfer switches NBN-U10 (connection to PBA-S03) and NBN-U12 (connection to NBN-U12), and supply breakers PBA-S03L and PBB-S04K. Existing data and system modeling for supply breakers PBA-S03L and PBB-S04K was incorporated into the FLEX+ 4160V generator modeling, which includes random faults, DC control power faults, and logic circuit faults. Per study 13-NS-B063 (Ref. 12), PVNGS At-Power PRA Study for Generic and Bayesian Updated Reliability Data Analysis, the failure rate for a manual switch to fail to remain closed (MS-RC) is 1.7E-7/hour. Per Attachment B, the random failure likelihoods used for the 2MW 4160V portable generators are 2.94E-2/demand (fail to start) and 1.44E-3/hour (fail to run). The HRA to start and align the 2MW 4160V portable generators has a value of as derived in Attachment A. As the random failure rates are dominated by the magnitude of the HRA, it was decided for modeling simplicity to group the generators, transfer switches, and human action in a single undeveloped event with a value of 0.5. The 0.5 value includes margin to account for the uncertainty associated with both the random failures and the HRA. YES 23 Status flags were used to ensure 1) that the 2MW 4160V portable generators would only be turned on in the model when they were staged and 2) to ensure that only one Class 1E bus could be powered by the portable generators at a time.

24 Staging of FLEX Equipment FLEX strategies are designed for extended losses of offsite power, which have long system time windows This challenges typical PRA success criteria and human reliability analysis methods The apparent answer to this is to stage equipment However, this comes with its own challenges and considerations Environment, security, industrial safety are all factors that determine feasibility of staging equipment PVGS considered all of these both during FLEX response planning and subsequently when deploying FLEX Plus for configuration risk management 24

25 PVGS FLEX Plus Deployment Strategy PVGS does not permanently stage FLEX Plus equipment No FLEX Plus success paths are credited in the baseline zero maintenance PRA model FLEX Plus equipment is deployed in response to specific evolutions, such as: Staging a FLEX Plus steam generator makeup pump during maintenance on an Auxiliary Feedwater pump Staging FLEX 4160V AC generators during an emergency diesel generator super outage 25

26 FLEX Equipment Performance Monitoring Responses as dictated in FLEX response procedures do not fall under the scope of Maintenance Rule performance monitoring However, use of FLEX equipment in roles beyond FLEX raises the potential of inclusion in Maintenance Rule performance monitoring via inclusion in emergency operating procedures This would result in more resource intensive monitoring for FLEX equipment than originally intended by most stations PVGS addressed this concern by using a combination of Alarm Response procedures and an Operations Maintenance Activities procedure Alarm response allows operator flexibility in response to an event Operations Maintenance Activities procedure houses all FLEX Plus actions Neither are Emergency Operating Procedures NRC reviewed the procedures as implemented by PVGS No negative feedback, but NRC is expected to offer guidance in the future 26

27 RG and 10 CFR (a)(4) NEI Tier 3 assessments follow the ASME/ANS PRA Standard with the intent of allowing users to credit temporary equipment in RG applications PVGS performed PRA analysis per Tier 3, but explicitly chose not to meet ASME/ANS PRA Standard requirements 27 The only intended application for FLEX Plus was 10 CFR (a)(4) configuration risk management, which does not require meeting RG criteria Further effort beyond demonstrating compliance with NEI is necessary is necessary to credit temporary equipment in risk-informed applications per RG and RG 1.174

28 Review of Engineering Bases Review of existing FLEX engineering basis information is necessary to support NEI assessments PVGS found instances where evaluations performed in support of FLEX configurations were not applicable to FLEX Plus, requiring further evaluation to justify physical performance of equipment 28

29 Conclusions PVGS has seen tangible benefit to safety, operational flexibility, and cost from implementing temporary equipment for configuration risk management NEI provides a robust framework for both qualitative and quantitative assessment of temporary equipment used for risk reduction The use of FLEX equipment for risk reduction is still in its nascent phase There are still open items in question or under development, such as data for temporary equipment, HRA methodology, and performance monitoring requirements 29

30 30 Questions?