ML21057A101
| ML21057A101 | |
| Person / Time | |
|---|---|
| Site: | Clinton  |
| Issue date: | 02/26/2021 |
| From: | Joel Wiebe NRC/NRR/DORL/LPL3 |
| To: | Nicely K Exelon Generation Co |
| References | |
| Download: ML21057A101 (5) | |
Text
From:
Wiebe, Joel Sent:
Friday, February 26, 2021 10:02 AM To:
Nicely, Ken M.:(GenCo-Nuc) (ken.nicely@exeloncorp.com)
Subject:
Clarification of Second Round RAI - Clinton TSTF-505 License Amendment Request Hi Ken, Here is the clarified second round RAI. Let me know if you need another clarification call.
Joel By letter dated April 30, 2020 (ADAMS Accession No. ML20121A178), Exelon Generation Company, LLC (Exelon, the licensee) submitted an application to revise technical specifications (TS) to adopt TSTF-505, Revision 2, "Provide Risk-Informed Extended Completion Times -
RITSTF Initiative 4b, for Facility Operating License No. NPF-62 for Clinton Power Station, Unit 1 (CPS). By letter dated November 24, 2020 (ADAMS Accession No. ML20329A433) Exelon responded to the NRC staff request for additional information dated October 27, 2020 (ADAMS Accession No. ML20307A659). The NRC staff has determined that additional information is needed regarding the treatment of sensitive equipment in the fire probabilistic risk assessment (PRA).
Regulatory Basis The regulation under 10 CFR 50.36(c)(2) requires that TS contain Limiting Conditions for Operations (LCOs), which are the lowest functional capability or performance levels of equipment required for safe operation of the facility. When an LCO of a nuclear reactor is not met, the licensee shall shut down the reactor or follow any remedial action within the completion time permitted by the TS until the LCO can be met. Typically, the TS require restoration of equipment in a timeframe commensurate with its safety significance, along with other engineering considerations. The regulation under 10 CFR 50.36(b) requires that TS be derived from the analyses and evaluation included in the safety analysis report, and amendments thereto.
In its letter dated April 30, 2020, Exelon Generation Company, LLC (EGC) stated that it has reviewed TSTF-505, Revision 2, Provide Risk Informed Extended Completion Times - RITSTF Initiative 4b, dated July 2, 2018 (ADAMS Accession No. ML18183A493) and the model safety evaluation dated November 21, 2018 (ADAMS Accession No. ML18267A259). This review included the information provided to support TSTF-505 and the safety evaluation for NEI 06 A. EGC concluded that the technical basis is applicable to CPS and support incorporation of this amendment in the CPS TS. When implementing the model safety evaluation, the NRC staff must conclude that the PRA models for internal and external events, including fires, used to implement the Risk Informed Completion Time (RICT) Program satisfy the guidance of RG 1.200, An Approach for Determining the Technical Adequacy of Probabilistic Risk Assessment Results for Risk-Informed Activities,. Revision 2 of RG 1.200 describes an acceptable approach for determining whether the PRA acceptability, in total or the parts that are used to support an
application, is sufficient to provide confidence in the results, such that the PRA can be used in regulatory decision making for light water reactors. This RG provides guidance for assessing the technical adequacy of a PRA.
RG 1.200 states NRC reviewers, [will] focus their review on key assumptions and areas identified by peer reviewers as being of concern and relevant to the application. The relatively extensive and detailed reviews of fire PRAs undertaken in support of LARs to transition to NFPA-805 determined that implementation of some of the complex fire PRA methods often used nonconservative and over-simplified assumptions to apply the method to specific plant configurations. Some of these issues were not always identified in F&Os by the peer review teams but are considered potential key assumptions by the NRC staff because using more defensible and less simplified assumptions could substantively affect the fire risk and fire risk profile of the plant. FAQ 13-0004, Clarifications on Treatment of Sensitive Electronics (ADAMS Accession No. ML13322A085) provides supplemental guidance for application of the damage criteria provided in Sections 8.5.1.2 and H.2 of NUREG/CR-6850, Volume 2, for solid-state and sensitive electronics.
DRA/APLB RAI 02.01 - Treatment of Sensitive Electronics in the Fire PRA In order to make a finding on the impact of assumptions made in the fire PRA (FPRA) on the proposed RICT program, the NRC staff, in DRA/APLB RAI 02 - Treatment of Sensitive Electronics, requested that the licensee describe the treatment of sensitive electronics for the FPRA, its consistency with the guidance in FAQ 13-0004, and the impact of any deviations on the proposed RICT program. The NRC staffs review of the licensees response identified multiple assumptions in the modeling of sensitive electronics in the FPRA supporting this application.
The licensees response explains that the caveats cited in FAQ 13-0004 about configurations, such as surface-mounted electronics, that invalidate the FAQ 13-0004 approach (i.e.,
configurations for which damage to the sensitive electronics could happen at a lower thermal threshold than identified in FAQ 13-0004) were not explicitly addressed in its FPRA. The response assumes that consideration of such configurations would have negligible impact on fire risk in fire scenarios where only the ignition source and sensitive electronics are damaged from the fire because of the functions associated with sensitive electronics (e.g., a single logic channel).
The licensee identified a gap in its analysis where the exposed sensitive electronics are at a distance away from the ignition source such that damage to sensitive electronics would occur beyond the zone of influence (ZOI) of the ignition source determined by thermoset failure criteria in FAQ 13-0004 (i.e., the ZOI of the ignition source using the sensitive electronics damage criteria extends beyond the ZOI using the thermoset criteria in FAQ 13-0004). To address this gap, the response indicated that a beyond ZOI scenario was developed and a full physical analysis unit (PAU) burnout was assumed if a fire from an ignition source has the potential to grow beyond the ZOI of the ignition source. It appears to the NRC staff that the beyond ZOI scenario is an assumption made as a modeling simplification. It is unclear to the NRC staff whether this assumption captures the full impact of the gap cited by the licensee. As a result, the NRC staff is unable to evaluate the impact of this assumption on the proposed RICTs.
In its response, the licensee included a limited sampling of sensitive electronics to conclude that the sensitive electronics either 1) have no impact on a PRA credited function, or 2) arent damaged in the fire or their failure is modeled in the FPRA scenarios. The limited sampling is insufficient to determine the impact of the modeling of sensitive electronics in the FPRA on the RICTs.
Based on the observations above, the licensees fire PRA includes assumptions related to modeling of sensitive electronics that are simplifications made for modeling convenience. Based on the available information, the NRC staff is unable to determine the potential impact of these assumptions on the proposed RICTs and therefore, whether these assumptions constitute key assumptions for this application. Therefore, address the following:
Provide justification for the negligible impact on the RICT calculations from the assumptions for modeling sensitive electronics in the licensees FPRA. The justification can include:
A) A sensitivity study showing that guaranteed failure of sensitive electronics has a minimal impact on the RICTs calculated for LCOs that are most likely to be affected by sensitive electronics. As a part of the response, identify those functions that sensitive electronics perform in the plant.
OR B) A detailed explanation of the technical basis for the negligible impact. The explanation should:
- 1) Discuss the functions that sensitive electronics perform in the plant, which of these functions are modelled in the FPRA, and why none of the modeled sensitive electronics can have more than negligible impact on the RICT calculations. Include confirmation that the sampling results provided in Table APLB-02-1 of the response are based on a walkdown of sensitive electronics in the plant and that the dispositions provided can be considered to be representative of those sensitive electronics.
- 2) Summarize your fire PRA modeling approach for evaluating sensitive electronics. In this summary, discuss whether a range of targets sets is assumed in the fire modeling approach for ignition sources, i.e., whether the target sets range from non-propagating fires constrained to the ignition source to scenarios of full PAU burnout.
Also, indicate in general how the scenario frequency is established for a target set.
Discuss whether the fire PRA modeling analysis is complete with respect to the state of practice or whether gaps exist in this analysis.
- 3) If sensitive electronics are incompletely analyzed in your PRA, then provide an assessment (e.g., a sensitivity study) on the impact of modelling these sensitive electronics at a lower heat flux damage threshold of 3 kW/m2 and 65°C for solid state controls components as recommended in Sections 8.5.1.2 and H.2 of NUREG/CR-6850(ADAMS Accession No. ML052580118).
Hearing Identifier:
NRR_DRMA Email Number:
1045 Mail Envelope Properties (MN2PR09MB4971A91BA0F4B2B20AF2B7EB8B9D9)
Subject:
Clarification of Second Round RAI - Clinton TSTF-505 License Amendment Request Sent Date:
2/26/2021 10:02:06 AM Received Date:
2/26/2021 10:02:00 AM From:
Wiebe, Joel Created By:
Joel.Wiebe@nrc.gov Recipients:
"Nicely, Ken M.:(GenCo-Nuc) (ken.nicely@exeloncorp.com)" <ken.nicely@exeloncorp.com>
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