ML23283A284
| ML23283A284 | |
| Person / Time | |
|---|---|
| Site: | 99902049 |
| Issue date: | 10/25/2023 |
| From: | Mccloskey S Holtec, SMR |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML23283A282 | List: |
| References | |
| 160-USNRC-067 | |
| Download: ML23283A284 (1) | |
Text
www.holtec.com www.smrllc.com NRC Meeting: Risk Significance Methodology Date: 25 October 2023 Presented By: Sean McCloskey SMR, LLC, A Holtec International Company Krishna P. Singh Technology Campus One Holtec Boulevard Camden, NJ 08104, USA
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holtec.com l smrllc.com l Page 2 Meeting Agenda
Introductions
Purpose & Outcome Regulations Guidance Need for Absolute Risk Significance Criteria SMR-160+ Risk Significance Criteria Basis for Risk Significance Criteria Basis for CDF Criteria Basis for LRF Criteria Benefits Open Forum
holtec.com l smrllc.com l Page 3
Introductions
NRC Staff Holtec Staff
holtec.com l smrllc.com l Page 4 Purpose and Outcome Purpose To provide a high-level overview of the SMR-160+ risk significance methodology Outcome To inform the NRC staff for their review of the LTR
holtec.com l smrllc.com l Page 5 Regulations RG 1.200 RAW 2.0 FV 0.005 It is recognized that for those new reactor designs with substantially lower risk profiles (e.g., internal events CDF below 10-6/year) that the quantitative screening value should be adjusted according to the corresponding baseline risk value.
holtec.com l smrllc.com l Page 6 Regulations (Cont.)
RG 1.174 Allows risk-informed decision making with adequate justification to reduce unnecessary burden on licensees Provides a benchmark for an acceptable change in absolute risk Metrics are based on the current fleet with CDF ~1 x 10-5/yr and LRF ~1 x 10-6/yr
holtec.com l smrllc.com l Page 7 Guidance ACRS Guidance on SRP Chapter 19 and Section 17.4 (ML14196A119)
ACRS noted that the RG 1.200 criteria may produce an inappropriately large population of SSCs that are subject to enhanced availability and reliability controls, with commensurate undue burden for both the licensee and regulatory staff ACRS recommended that risk significance criteria be consistent for a broad spectrum of designs and absolute levels of overall plant risk
holtec.com l smrllc.com l Page 8 Guidance (Cont.)
NEI 00-04 and RG 1.201 Recommended to perform system-level SSC categorization to ensure that all functions (which are primarily a system-level attribute) are appropriately considered for risk significance
holtec.com l smrllc.com l Page 9 Need for Absolute Risk Significance Criteria Relative risk criteria artificially raises importance of SSCs for a design with significantly lower CDF than the current fleet RG 1.200 criteria are based on relative risk for current fleet of reactors (CDF of ~1 x 10-5/yr)
SMR-160+ CDF expected to be significantly lower than the current fleet Example For a design with CDF of 1 x 10-5, a RAW of 2 implies a change in CDF of 1 x 10-5 For a design with CDF of 1 x 10-7, a RAW of 2 implies a change in CDF of 1 x 10-7
holtec.com l smrllc.com l Page 10 SMR-160+ Risk Significance Criteria Large Release Criteria for Risk Significance Core Damage Criteria for Risk Significance Parameter Conditional LRF 3 x 10-7/yr Conditional CDF 3 x 10-6/yr Component level Conditional LRF 1 x 10-6/yr Conditional CDF 1 x 10-5/yr System level Total FV 0.20 Basic event/contributor Based on all plant conditions including, operating, low power, and shutdown conditions for internal and external events resulting in core damage and a large radiological release to the environment
holtec.com l smrllc.com l Page 11 Basis for CDF Criteria Basis CDF Criteria for Risk Significance Consistent with RG 1.174 risk-acceptance guidelines, which state that increases in CDF between 10-6 and 10-5 are considered if CDF can reasonably be shown to be < 1 x 10-4/yr The component-level threshold of 3 x 10-6 represents approximately the midpoint (on a log scale) of the Region II range identified in RG 1.174 Order of magnitude below NRC safety goal of CDF < 1 x 10-4/yr, with an extra half-order of magnitude (on a log scale) of margin to account for uncertainties in the PRA model Component level CCDF > 3 x 10-6/yr The system-level value of 1 x 10-5 represents the upper end of the Region II range for CDF identified in RG 1.174 for making permanent changes to a plants licensing basis Order of magnitude below NRC safety goal of CDF < 1 x 10-4/yr System level CCDF > 1 x 10-5/yr Threshold more conservative than criteria for operating plants (i.e., CDF of 1 x 10-5/yr
- FV of 0.2 = 2 x 10-8/yr)
Using FV of 0.2 instead of 0.5 conservatively identifies more risk significant basic events Basic event FV > 0.20
holtec.com l smrllc.com l Page 12 Basis for LRF Criteria Basis LRF Criteria for Risk Significance Consistent with RG 1.174 risk-acceptance guidelines, which state that increases in LRF between 10-7 and 10-6 are considered if LRF can reasonably be shown to be < 1 x 10-5/yr The component-level threshold of 3 x 10-7 represents approximately the midpoint (on a log scale) of the Region II range identified in RG 1.174 Order of magnitude below NRC safety goal of LRF < 1 x 10-5/yr, with an extra half-order of magnitude (on a log scale) of margin to account for uncertainties in the PRA model Component level CLRF > 3 x 10-7/yr The system-level value of 1 x 10-6 represents the upper end of the Region II range for LRF identified in RG 1.174 for making permanent changes to a plants licensing basis Order of magnitude below NRC safety goal of LRF < 1 x 10-5/yr System level CLRF > 1 x 10-6/yr Threshold more conservative than criteria for operating plants (i.e., LRF of 1 x 10-6/yr
- FV of 0.005 = 5 x 10-9/yr is greater than the SMR-160+ LRF of 1 x 10-8/yr
- FV of 0.2 = 2 x 10-9/yr)
Using FV of 0.2 instead of 0.5 conservatively identifies more risk significant basic events Basic event FV > 0.20
holtec.com l smrllc.com l Page 13 Benefits Directly addresses the ratio limitations of traditional importance measures Consistent with the ACRS recommendation that risk significance criteria be consistent for a broad spectrum of designs and absolute levels of overall plant risk Consistent with NEI recommendation to consider risk significance at a functional (system) level Allows the licensee to focus resources on the SSCs important to absolute risk
holtec.com l smrllc.com l Page 14 Open Forum
holtec.com l smrllc.com l Page 15 Risk Importance Measures Fussell-Vesely (FV), commonly known as fraction of total risk
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Risk Achievement Worth (RAW), or risk increase ratio given a SSC fails
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()
Conditional CDF (CCDF), or increased CDF when a SSC fails Conditional LRF (CLRF), or increased LRF when a SSC fails