November-December 2018 NPJ

www.NuclearPlantJournal.com Product & Service Directory–2019 76 Risk Informed Resolution By Wes Schulz, Drew Richards, Steve Blossom and Rob Engen, STP Nuclear Operating Company. The Nuclear Energy Institute’s Top Innovative Practice Awards highlight the nuclear industry’s most innovative techniques and ideas. This innovation won a 2018 NEI Top Innovative Practice Award. The STP Nuclear Operating Company team members who participated included Steve Blossom, Project Manager; Ernie Kee, Risk Management; Wayne Harrison, Licensing; Drew Richards, Licensing; and Wes Schulz, Design Engineering. Debris from Postulated Pipe Breaks Summary STP Nuclear Operating Company (STPNOC)hascompletedafirst-of-a-kind majorinitiativefortheindustrythatapplies an innovative risk-informedmethodology to resolve an industry-wide safety issue associated with effects of debris from postulated pipe breaks identified in NRC Generic Safety Issue (GSI) -191 andNRC Generic Letter (GL) 2004-02. In 1996, GSI-191 (Assessment of Debris Accumulation on PWR Sump Performance) was identified as an industry-wide safety issue by the Nuclear Regulatory Commission. For nearly 20 years, despite significant efforts, closure of GSI-191 had eluded resolution for plants, such as STP Units 1 and 2, with large amounts of fibrous insulation inside containment. InDecember 2010, the Commission- ersoftheNRCissuedaStaffRequirements Memorandum, SECY-10-0113, directing NRC Staff to take the time needed to consider all options to a risk-informed, safety conscious resolution to GSI-191. In 2011, STPNOC in partnership with industry, academia, and national laboratories developed an industry first- of-a-kind project designed to establish a technical basis to quantify the risk associated with the effects of debris in the very unlikely event of a large pipe break inside the Reactor Containment Building (RCB). STPNOC was the advocate and the pilot for the risk-informed approach to GSI-191 closure. For the next six years, STPNOC led the industry in resolving the GSI-191 challenge by first developing a full risk- informed approach that was followed by an alternative approach, designated as Risk over Deterministic (RoverD), which simplified the review process. TheSTPNOCanalysiswas consistent with Regulatory Guide (RG) 1.174 (An Approach for Using Probabilistic Risk Assessments in Risk-Informed Decisions onPlant SpecificChanges to theLicensing Basis). The results of the analysis showed that the effects of debris in containment following a large pipe break are not a significant contribution to risk as defined by the parameters in RG 1.174 for Core Damage Frequency (CDF) and Large EarlyReleaseFrequency (LERF).The risk results were acceptable for being within RG 1.174 Region III (very small) while maintaining adequate defense-in-depth and safety margin. On July 11, 2017, the NRC formally approved STPNOC’s license amendment request toadopt a risk-informed resolution to GSI-191 that would also close out GL 2004-02. The approved license amendment also included a Technical Specification change that added a debris- specific action statement that gives a 90- day allowedduration to address any issues associatedwithLOCA-induceddebristhat may arise during normal operation. The NRC approval also included exemptions to certain design criteria to allow the use of the risk-informed approach for LOCA- induced debris. Implementation of this methodology avoided the large occupational radiation dose and high cost that would have resulted from the removal/replacement/ modificationof fibrous insulationmaterial inside the reactor containment. The risk-informed process to resolve GSI-191 is easily transferable across the industry. There are plant-specific inputs, but the process is generic such that it can be applied to all plants. Detailed PRA modeling is not required to implement the RoverD process; the principle inputs are the Level 1 (CDF) and Level 2 (LERF) PRA values that are used as inputs to RG 1.174 for determining the applicable risk region. Project Partners The project partners with STPNOC includedTheUniversityofTexasatAustin; Texas A&M University; The University of Illinois at Urbana-Champaign; Alion Science and Technology; and ABS Consulting.Over the course of the project, contributions such as testing, simulation, analysis and oversight were provided by Soteria Consultants; The University of New Mexico; Los Alamos National Laboratory; KnF Consulting Services; CR Grantom PE & Associates Inc; Mike Golay Consulting; and YK.risk, LLC. The following utility participants provided financial contribution to this industry initiative: • Southern Nuclear - Vogtle 1 & 2 • Exelon - Calvert Cliffs 1 & 2 • Pacific Gas & Electric - Diablo Canyon 1 & 2 • Wolf Creek Nuclear Operating Company (WCNOC) - Wolf Creek • Florida Power&Light -TurkeyPoint 3 & 4 and Saint Lucie 1 & 2 • NextEra - Seabrook • UE Ameren - Callaway Innovation Resolution of GSI-191 involves two distinct but related safety concerns: 1. Potential cloggingof the containment emergency sump strainers that can result in Emergency Core Cooling System pump failure and Containment Spray pump failure (upstream effects), and 2. Potential clogging of fuel channels within the reactor vessel because of debris that bypasses the containment emergency sump strainers (in-vessel effects).