ML24283A144
| ML24283A144 | |
| Person / Time | |
|---|---|
| Site: | Surry, North Anna  |
| Issue date: | 10/16/2024 |
| From: | Dominion Energy |
| To: | Geoffrey Miller NRC/NRR/DORL/LPL2-1 |
| Miller G | |
| References | |
| EPID L-2024-LRM-0106 | |
| Download: ML24283A144 (24) | |
Text
North Anna and Surry Power Stations Updated Main Steam Line Break Dose Consequence Analyses to Address Extended Cooldown Timelines Pre-Submittal Meeting October 16th, 2024 1
Overview
2 Background
Submittal Scope Analytical Approach Key Analysis Changes Results Conclusions Schedule
ANC = Asymmetric Natural Circulation ANCC = Asymmetric Natural Circulation Cooldown AOR = Analysis of Record ASG = Affected Steam Generator AST = Alternate Source Term CSD = Cold Shutdown, subcritical, Tavg 200oF DE = Dominion Energy DEGB = Double Ended Guillotine Break EAB = Exclusion Area Boundary EV = Emergency Ventilation HSD = Hot Shutdown (SPS), subcritical, Tavg 547oF ISG = Intact Steam Generator(s)
LAR = License Amendment Request LER = Licensee Event Report LOOP = Loss of Offsite Power LPZ = Low Population Zone MCR = Main Control Room MODE 3 = (NAPS), subcritical, Tavg 350oF MSLB = Main Steam Line Break MSSV = Main Steam Safety Valve NAPS = North Anna Power Station OE = Operating Experience OPA = Offsite Power Available RCP = Reactor Coolant Pump RCS = Reactor Coolant System RHR = Residual Heat Removal SB = Split Break SG = Steam Generator SPS = Surry Power Station TCOA = Time Critical Operator Action TDAFW = Turbine Driven Auxiliary Feedwater Pump TEDE = Total Effective Dose Equivalent TS = Technical Specification T/H = Thermal Hydraulics Acronyms 3
Industry Operating Experience (OE) revealed that, following a Main Steam Line Break (MSLB),
stagnation of affected Reactor Coolant System (RCS) loop flow may occur.
The postulated loop flow stagnation condition arises when a natural circulation cooldown is initiated on the remaining active loops and can be exacerbated by high cooldown rates and low decay heat.
A natural circulation cooldown on active loops with no cooling on inactive loops is known as asymmetric natural circulation (ANC).
Should loop flow stagnation occur during an ANC cooldown (ANCC), the time to cool down and depressurize the RCS is extended.
Background - ANCC 4
Term Definition Active Loop Any RCS loop with Steam Generator capable of being fed and steamed normally.
Inactive Loop Any RCS loop with Steam Generator NOT capable of being fed and/or steamed normally.
Stagnant Loop Any RCS loop in which flow through the loop has ceased.
Dominion Energys MSLB Alternate Source Term (AST) dose consequence Analyses of Record (AOR) assume the following cooldown timelines:
North Anna Power Station (NAPS): 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> Surry Power Station (SPS):
12.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> - Offsite Power Available (OPA) 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br /> - Loss of Offsite Power (LOOP)
Based on Industry OE (2018 Callaway LER 2018-002-00), Dominion Energy initiated a review of the MSLB analyses and determined the AORs for NAPS and SPS were affected.
New analyses were performed to bound the postulated stagnant loop condition and provide Plant Operations the flexibility to either:
Perform an ANCC, or Stabilize and hold the unit at safe shutdown conditions until restoration of a reactor coolant pump (RCP) or restoration of the faulted SG to an intact state, at which point a forced cooldown or symmetric cooldown can proceed (which eliminates the potential for a stagnant loop).
Results of the updated NAPS and SPS MSLB dose consequence analyses require NRC review and approval for the following reasons:
NAPS - Method change, unreviewed X/Qs that credit Main Control Room (MCR) Emergency Ventilation (EV),
SPS - Predicted MCR results exceed the more than minimal threshold for 50.59 applicability.
Background - Licensing Basis 5
Topic
6 Background
Submittal Scope Analytical Approach Key Analysis Changes Results Conclusions Schedule
License Amendment Requests (LARs) will be submitted for approval of the updated NAPS and SPS MSLB dose consequence analyses using extended cooldown timelines.
The LARs will retain as much of the existing analytical design basis as possible while addressing the impact of extended cooldown timelines on the MSLB event.
The updated NAPS and SPS MSLB AST dose consequence analyses do not require any TS changes.
Submittal Scope 7
Topic
8 Background
Submittal Scope Analytical Approach Key Analysis Changes Results Conclusions Schedule
Thermal-hydraulic (T/H) calculations were performed to generate MSLB transient steam releases (first 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />) including protection circuitry timing Evaluated scenarios (Limiting scenarios submitted for review)
Main Steam Line Doubled-Ended Guillotine Break (DEGB) and Split Break (SB) inside the Turbine Building with OPA / LOOP Main Steam Safety Valve (MSSV) failed open with LOOP Primary release paths Faulted or Affected Steam Generator (ASG) steam line break MSSV failed open Secondary release paths Intact Steam Generators (ISG) MSSVs Turbine-Driven Auxiliary Feedwater (TDAFW) pump steam exhaust Analytical Approach - MSLB Steam Releases 9
Analytical Approach - NAPS Releases / Dose Timelines 10 Integrated Mass Release Time 0.5h 4h 12h 10d T/H Calc Dose Releases OPA Dose Releases LOOP Analyzed Expected 7d RCS Temperature Time 4h 10d 1.5h 0.5h TR ST TR = Transient Phase ST = Stabilization Phase Hold at MODE 3 M3 to RHR RHR to CSD 7d
= Break Isolation
= Restore Offsite Power
= Start Intact Loop RCP 8d 9d CSD Notes:
- 1. Plots are for illustration purposes only and do not reflect exact code predictions.
- 2. The analysis does not preclude earlier cooldowns; instead, it bounds any cooldown that could occur prior to or at 10 days.
Analytical Approach - Cooldown Timelines 11 Station Current Licensing Basis Evaluated OPA LOOP NAPS 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> 12 hours1 10 days2 SPS 12.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> (OPA) 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br /> (LOOP) 12.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> 10 days2 Notes:
- 1. Increased to allow for operational flexibility
- 2. Increased to address ANCC extended cooldown timeline
The steam release input to the dose analyses represent 110% of the T/H calculation steam releases, or even greater in the ASG when necessary to remove all of its associated curies.
For the DEGB and SB evaluations, ASG steam release is terminated at 0.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> via manual operator action, consistent with existing time-critical operator action (TCOA).
ISG steam release rates between 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> and 10 days are modeled as constant and equal to the ISG steam release rates corresponding to the core decay heat at 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> and terminated after 10 days.
From day 7 to day 9, the Hot Shutdown (HSD, Note 1) to Residual Heat Removal (RHR) entry conditions cooldown and the RHR to Cold Shutdown (CSD) cooldown, which are not modeled, will limit and subsequently terminate steam releases.
Analytical Approach - Steam Releases Conservatism Note 1: Mode definitions are different between NAPS and SPS. The use of HSD above is using the SPS definition, which is equivalent to MODE 3 at NAPS. Both are referring to post-trip, no-load temperature and pressure.
12
Topic
13 Background
Submittal Scope Analytical Approach Key Analysis Changes Results Conclusions Schedule
Key Analysis Changes - North Anna 14 Item Current Licensing Basis Proposed Licensing Basis Limiting Release DEGB Failed open MSSV w/LOOP and rapid ASG dry-out Steam Releases ISGs - Steam releases for 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> ASG - Steam releases for 30
- minutes, Primary-to-Secondary leakage until steam releases terminate ISGs - Steam releases for 240 hours0.00278 days <br />0.0667 hours <br />3.968254e-4 weeks <br />9.132e-5 months <br /> ASG - Steam releases for up to 30 minutes TDAFW - Steam releases for 240 hours0.00278 days <br />0.0667 hours <br />3.968254e-4 weeks <br />9.132e-5 months <br /> Primary-to-Secondary leakage for 240 hours0.00278 days <br />0.0667 hours <br />3.968254e-4 weeks <br />9.132e-5 months <br />
Key Analysis Changes - North Anna 15 Item Current Licensing Basis Proposed Licensing Basis MCR Ventilation Unfiltered Normal Ventilation Unfiltered Normal Ventilation until MCR Isolation Filtered Emergency Ventilation at 60 minutes (unreviewed X/Qs will be submitted)1 MCR Unfiltered Inleakage 500 cfm 250 cfm Note 1: The meteorological data used as design input has been previously reviewed.
Key Analysis Changes - Surry 16 Item Current Licensing Basis Proposed Licensing Basis Limiting Release DEGB w/LOOP Failed open MSSV w/LOOP and rapid ASG dry-out (EAB and LPZ)
LOOP Steam Releases ISGs - Steam releases for 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br /> ASG - Steam releases for 30 minutes TDAFW - Steam releases for 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br /> Primary-to-Secondary leakage until steam releases terminate ISGs - Steam releases for 240 hours0.00278 days <br />0.0667 hours <br />3.968254e-4 weeks <br />9.132e-5 months <br /> ASG - Steam releases for up to 30 minutes TDAFW - Steam releases for 240 hours0.00278 days <br />0.0667 hours <br />3.968254e-4 weeks <br />9.132e-5 months <br /> Primary-to-Secondary leakage for 240 hours0.00278 days <br />0.0667 hours <br />3.968254e-4 weeks <br />9.132e-5 months <br />
Topic
17 Background
Submittal Scope Analytical Approach Key Analysis Changes Results Conclusions Schedule
Results - North Anna 18 Location Reg Guide 1.183 Rev. 0 Limit Current Licensing Basis Updated Analysis Pre-Incident Coincident Pre-Incident Coincident Pre-Incident Coincident MCR 5
5 3.96 3.33 1.5 2.6 EAB 25 2.5 0.0325 0.03 0.1 0.5 LPZ 25 2.5 0.00121 0.00131 0.1 0.2 Calculated Dose [Rem TEDE]
Results - Surry 19 Location Reg Guide 1.183 Rev. 0 Limit Current Licensing Basis Updated Analysis Pre-Incident Coincident Pre-Incident Coincident Pre-Incident Coincident MCR 5
5 1.4 1.6 2.5 3.2 EAB 25 2.5 0.4 0.6 0.3 1.3 LPZ 25 2.5 0.1 0.1 0.1 0.7 Calculated Dose [Rem TEDE]
Topic
20 Background
Submittal Scope Analytical Approach Key Analysis Changes Results Conclusions Schedule
The updated MSLB AST dose consequence analyses for extended cooldown timelines retain as much of the original AST licensing basis assumptions as possible.
The updated analyses include appropriate conservatisms and meet the RG 1.183 acceptance criteria.
Conclusions 21
Topic
22 Background
Submittal Scope Analytical Approach Key Analysis Changes Results Conclusions Schedule
10/16/2024 - Pre-submittal Meeting 1st Quarter 2025 - License Amendment Request Submittal 1st Quarter 2026 - Requested Approval Date Schedule 23
Questions/Comments?
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