ML21162A031
| ML21162A031 | |
| Person / Time | |
|---|---|
| Issue date: | 06/11/2021 |
| From: | Jerry Dozier, Steve Jones, Shilp Vasavada NRC/NRR/DRA |
| To: | |
| Dozier J | |
| References | |
| Download: ML21162A031 (22) | |
Text
1 Supplemental Guidance for Radiological Consequence Analyses Using Alternative Source Terms DRA-ISG 2021-XX Jerry Dozier, Senior Reliability and Risk Analyst, Division of Risk Assessment Steve Jones, Senior Safety and Plant Systems Engineer, Division of Safety Systems Shilp Vasavada, Branch Chief (Acting), Division of Risk Assessment Office of Nuclear Reactor Regulation (NRR)
2 Outline
- Overview of the Interim Staff Guidance (ISG)
- Background of ISG
- Basis for ISG - Technical Assessment
- Overview
- Details
- Difference between ISG and Regulatory Guide 1.183 Revision
- Takeaways
3 Overview of ISG - Timeline
- Published in Federal Register for public comment day comment period closes on June 21, 2021
- Advisory Committee on Reactor Safeguards (ACRS) subcommittee briefing scheduled for July 23, 2021
- ACRS full committee briefing in September 2021 (tentative)
- OMB approval - November 2021(tentative)
- Final FRN - December 2021(tentative)
4 Overview of ISG - Primary Insight
- High probability that doses will be lower than those estimated strictly using traditional deterministic methods, which include accepted assumptions, that do not credit hold-up and retention of the Main Steam Isolation Valve (MSIV) leakage within the power conversion system (PCS)
5 Overview of ISG - Objective and Expectation
- Objective: Near-term formal regulatory footprint for staffs use of primary insight
- Expectations:
- Used by staff to offset uncertainty in input parameter(s) for deterministic calculations
- Supports reasonable assurance finding during reviews
- Transitioned to Standard Review Plan Chapter 15.0.1
- Caveat: Does not change acceptable methods to demonstrate conformance with 10 CFR 50.67
6 Background of ISG - Genesis
- Commission direction to become a modern, risk-informed regulator (e.g., SRM-SECY-19-0036; ML19183A408)
- Challenges due to uncertainty in input parameter values in dose calculations
- LIC-206 (ML19031C861) invoked for multi-disciplinary risk insights
7 Background of ISG - Genesis (Contd)
- Integrated review team approach following LIC-206 guidance
- Identified that risk insights support consideration of holdup in PCS
- Ability to offset challenges without changing calculation methods and assumptions
- Documented insights in technical assessment
- Internal reviews and deliberations
- Included in all four safety evaluations for the four LARs to increase MSIV leakage (ML20140A070; ML20150A328; ML20241A190; ML20265A240)
8 Basis for ISG - Technical Assessment
- Formal credit for condenser through safety evaluation on BWROG Topical Report
9 Large Holdup Volume in PCS
10 Technical Assessment - Overview Technical Assessment Problem Formulation using Risk Triplet Operational Insights Random Failure Probabilities Seismic Capacity
11 Technical Assessment - Risk Triplet Formulation
Operational Insights 12
- Main Steam System Piping:
- Large internal volume
- Typically designed to B31.1.0, "Power Piping"
- Constructed with augmented quality
- BWR 5 and BWR 6 designed to B&PV Code - safety-related
- Typically, large globe valves that seat with pressure
- Stem leakage from outboard valve considered a small fraction of measured seat leakage
- Passive features provide hold-up volume for MSIV seat leakage
13 Realistic Transport Pathway
- Consideration of piping attached to steam lines
- No alignment of specific leakage path
- Reliability of complete isolation; larger valves leak more
- Functional drain lines flow to main condenser
- Turbine bypass valves also flow to main condenser
- Other leakage, primarily through stop and governor valves to high pressure turbine, provide for less holdup and deposition than main condenser
14 Seismic Capacity: A Primer
- Fragility: Conditional failure probability as a function of seismic acceleration; Analytically determined; Lognormally distributed
- Median fragility (Am): Seismic acceleration at which there is 50%
probability of failure
- Lognormal uncertainty parameters (r for randomness; u for uncertainty): Parameters characterizing the uncertainty in the fragility
- Seismic acceleration: Measure of strength of earthquake in terms of multiples of gravitational acceleration (e.g., 0.1g, 1g)
- Peak ground acceleration: Commonly used acceleration level for seismic analysis; corresponds to acceleration of 100 Hz oscillator
15 Seismic Capacity: A Primer From Electric Power Research Institute Report 1025287 (also known as SPID; ML123330282 Higher Median Capacity Value
16 Approach for Seismic Capacity Evaluation in Assessment Lower Bound Median Fragility to Encompass Seismic Failure Modes Fragility Data
- Multiple and diverse sources
- Recent seismic probabilistic risk assessments (PRAs)
Operating Experience - Walkdowns
- North Anna
- Kashiwazaki-Kariwa
- The Great Tohoku Earthquake of 2011 Representative Risk
- Hazard and fragility convolution
17 Seismic Capacity Insights
- Welded piping, bolted piping, and valves have high median fragilities
- Main condenser is usually a seismic Category II structure
- Anchorage designed to avoid failure at design-basis seismic loads
- Seismic risk from accelerations at and below plants safe shutdown earthquake (SSE) is small
18 Seismic Capacity Insights
- Lower bound median fragility parameters
-Am = 0.4g; r = 0.22; u = 0.22
-Based on fragility of expansion joint connecting circulating water piping to condenser
-Encompasses failure modes of relevant SSCs
-Supports low likelihood of gross failure of SSCs in PCS
19 Seismic Capacity Insights - Representative Risk Calculation
- Convolution of range of hazards with lower bound median fragility parameters
- Provides estimate of risk of gross failure of SSCs in PCS
- Uses latest seismic hazard curves
- Estimates demonstrate low risk of gross failure
- Even lower if contribution only till SSE is considered
20 Uncertainty Consideration
- Uncertainty in median fragility explicitly included
- Conservatisms exist that address uncertainty in selected median fragility
-Use of lower bound median fragility
-Consideration of SSE concurrent with the accident postulated for dose calculations
-Conservatisms in remainder of dose calculation guidance are unchanged
21 Difference between ISG and Regulatory Guide 1.183 Revision
- Regulatory Guide Revision:
- Provides guidance for quantitative credit for holdup and deposition in main condenser
- Provides guidance for establishment and qualification of leakage pathway
- Quantitative credit changes the licensing basis dose calculations
- Interim Staff Guidance:
- Does not provide guidance or assumptions for licensee developed dose calculations
- Provides risk-informed basis supporting acceptance of uncertainties in parameters and assumptions
22 Takeaways
- Offset uncertainty in input parameter(s) for deterministic calculations
- Support reasonable assurance finding during reviews
- Formal condenser holdup credit for licensee's is being considered in revision to Regulatory Guide 1.183