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Chris Hunter Office of Nuclear Regulatory Research Division of Risk Analysis Performance and Reliability Branch
Chris Hunter Office of Nuclear Regulatory Research Division of Risk Analysis Performance and Reliability Branch


On August 10, 2020, a derecho moved through Iowa and other parts of the Midwest.
Event Overview On August 10, 2020, a derecho moved through Iowa and other parts of the Midwest.
                    - The most extreme winds were estimated to be near 110 mph, wind gusts of 80-100 mph were common.
- The most extreme winds were estimated to be near 110 mph, wind gusts of 80-100 mph were common.
Duane Arnold experienced a grid perturbation that caused the emergency diesel generators (EDGs) to automatically start, but initially ran unloaded.
Duane Arnold experienced a grid perturbation that caused the emergency diesel generators (EDGs) to automatically start, but initially ran unloaded.
Event              Approximately 15 minutes later, the main Overview            generator tripped resulting in a loss of offsite power (LOOP) and subsequent reactor trip.
Approximately 15 minutes later, the main generator tripped resulting in a loss of offsite power (LOOP) and subsequent reactor trip.
                    - The two EDGs automatically loaded to their respective safety buses.
- The two EDGs automatically loaded to their respective safety buses.
The licensee declared a Notice of an Unusual Event.
The licensee declared a Notice of an Unusual Event.
February 18, 2022          7th Annual NRC PFHA Research Workshop           2
2 7th Annual NRC PFHA Research Workshop February 18, 2022


Prior to the event, the licensee was loading fuel into a spent fuel canister.
Additional Event Details Prior to the event, the licensee was loading fuel into a spent fuel canister.
North FLEX building was damaged and equipment within was declared inoperable.
North FLEX building was damaged and equipment within was declared inoperable.
The main steam isolation valves remained open, allowing operator to align main steam-line drains.
The main steam isolation valves remained open, allowing operator to align main steam-line drains.
Additional Event Details      Approximately 10 hours into the event, the essential service water (ESW) strainers started to get plugged due an ingress of debris.
Approximately 10 hours into the event, the essential service water (ESW) strainers started to get plugged due an ingress of debris.
A small tear was discovered in the reactor building resulted in secondary containment being declared inoperable.
A small tear was discovered in the reactor building resulted in secondary containment being declared inoperable.
February 18, 2022          7th Annual NRC PFHA Research Workshop 3
3 7th Annual NRC PFHA Research Workshop February 18, 2022


Accident Sequence Precursor (ASP)
Accident Sequence Precursor (ASP)
Evaluation February 18, 2022 7th Annual NRC PFHA Research Workshop 4
Evaluation February 18, 2022 7th Annual NRC PFHA Research Workshop 4


Initial ASP analysis started within a few days of the event and showed that the event was potentially a significant precursor.
Initial ASP Information Initial ASP analysis started within a few days of the event and showed that the event was potentially a significant precursor.
Focused on early sequence results to determine which modeling/event assumptions that needed to be Initial ASP          evaluated further.
Focused on early sequence results to determine which modeling/event assumptions that needed to be evaluated further.
Information          - FLEX modeling
- FLEX modeling
                    - Stuck-open SRV scenario modeling
- Stuck-open SRV scenario modeling
                    - ESW strainer challenge Provided some initial high-level information during ASP presentation to international precursor community.
- ESW strainer challenge Provided some initial high-level information during ASP presentation to international precursor community.
February 18, 2022        7th Annual NRC PFHA Research Workshop 5
5 7th Annual NRC PFHA Research Workshop February 18, 2022


MELCOR calculations were performed to determine timing information for postulated stuck-open SRV scenarios.
SPAR Model Changes MELCOR calculations were performed to determine timing information for postulated stuck-open SRV scenarios.
                    - Based on these calculations and discussions with the licensee revealed that operators would have enough time to connect and initiate either firewater or FLEX reactor makeup.
- Based on these calculations and discussions with the licensee revealed that operators would have enough time to connect and initiate either firewater or FLEX reactor makeup.
Credit for FLEX mitigation strategies was applied.
Credit for FLEX mitigation strategies was applied.
                    - Modified FLEX hardware reliability data based on initial data evaluation (3x multiplier was used).
- Modified FLEX hardware reliability data based on initial data evaluation (3x multiplier was used).
SPAR Model          Modified ESW Strainer common-cause failure (CCF)
Modified ESW Strainer common-cause failure (CCF) parameters to use environmental causal alpha factors.
Changes              parameters to use environmental causal alpha factors.
- Added operator action to bypass clogged strainers to SPAR model, which largely mitigates significant CCF potential.
                    - Added operator action to bypass clogged strainers to SPAR model, which largely mitigates significant CCF potential.
Additional Changes
Additional Changes
                    - Eliminated some assumptions deemed needlessly conservative (72-hour AC power recovery requirement).
- Eliminated some assumptions deemed needlessly conservative (72-hour AC power recovery requirement).
                    - Removed EDG repair for scenarios where ELAP is declared due to potential for load shed activity precluding recovery.
- Removed EDG repair for scenarios where ELAP is declared due to potential for load shed activity precluding recovery.
February 18, 2022            7th Annual NRC PFHA Research Workshop                   6
6 7th Annual NRC PFHA Research Workshop February 18, 2022


The preliminary ASP analysis resulted in a mean conditional core damage probability (CCDP) of 1x10-3.
Preliminary ASP Analysis The preliminary ASP analysis resulted in a mean conditional core damage probability (CCDP) of 1x10-3.
                    - Risk dominated by station blackout (SBO) scenarios.
- Risk dominated by station blackout (SBO) scenarios.
                    - Offsite power recovery credit not provided within 24 hours.
- Offsite power recovery credit not provided within 24 hours.
Preliminary ASP Analysis        Identified several key uncertainties.
Identified several key uncertainties.
                    - Performed sensitivities to evaluate the impact of these uncertainties.
- Performed sensitivities to evaluate the impact of these uncertainties.
Sent the preliminary ASP analysis to the licensee for a 60-day review per Regulatory Issue Summary 2006-24.
Sent the preliminary ASP analysis to the licensee for a 60-day review per Regulatory Issue Summary 2006-24.
February 18, 2022          7th Annual NRC PFHA Research Workshop     7
7 7th Annual NRC PFHA Research Workshop February 18, 2022


NextEra provided comments on the preliminary ASP analysis on February 9, 2021 (ADAMS Accession No. ML21042A079).
Industry Comments NextEra provided comments on the preliminary ASP analysis on February 9, 2021 (ADAMS Accession No. ML21042A079).
                    - In addition, the Pressurized-Water Reactor Owners Group provided comments.
- In addition, the Pressurized-Water Reactor Owners Group provided comments.
Offsite power was available to be restored to the safety buses approximately 22.6 hours after the event started.
Offsite power was available to be restored to the safety buses approximately 22.6 hours after the event started.
                    - This change resulted in mean CCDP decreasing to 8x10-4.
- This change resulted in mean CCDP decreasing to 8x10-4.
Industry            The PWROG showed that the rate for EDG failures to Comments            run (FTR) could be reduced from 1.4x10-3 per hour to 8.4x10-4 per hour.
The PWROG showed that the rate for EDG failures to run (FTR) could be reduced from 1.4x10-3 per hour to 8.4x10-4 per hour.
                    - Idaho National Laboratory performed an updated calculation that was only slightly smaller than current estimate and, therefore, no changes were made.
- Idaho National Laboratory performed an updated calculation that was only slightly smaller than current estimate and, therefore, no changes were made.
A significant conservatism is that FTR events are assumed to occur at the start of the event.
A significant conservatism is that FTR events are assumed to occur at the start of the event.
                    - The treatment of FTR events in the SPAR models is consistent with the current state-of-practice.
- The treatment of FTR events in the SPAR models is consistent with the current state-of-practice.
February 18, 2022          7th Annual NRC PFHA Research Workshop                       8
8 7th Annual NRC PFHA Research Workshop February 18, 2022


ASP Comparison February 18, 2022 7th Annual NRC PFHA Research Workshop 9
ASP Comparison February 18, 2022 7th Annual NRC PFHA Research Workshop 9


Storm-generated debris resulted in a LOOP to Unit 1 in August 2020.
Brunswick LOOP during Hurricane Isaias Storm-generated debris resulted in a LOOP to Unit 1 in August 2020.
                      - The LOOP lasted approximately 14 hours.
- The LOOP lasted approximately 14 hours.
Brunswick            Electrical Design Elements LOOP during          - Each unit has two EDGs and share an SBO Hurricane              EDG that can crosstied to the other unit.
Electrical Design Elements
Isaias              The mean CCDP was 2x10-5.
- Each unit has two EDGs and share an SBO EDG that can crosstied to the other unit.
                      - LOOP transient scenarios dominated risk; SBO risk was minimal.
The mean CCDP was 2x10-5.
                      - FLEX credit provided minimal risk reduction.
- LOOP transient scenarios dominated risk; SBO risk was minimal.
February 18, 2022        7th Annual NRC PFHA Research Workshop   10
- FLEX credit provided minimal risk reduction.
10 7th Annual NRC PFHA Research Workshop February 18, 2022


LOOP caused by flashover of the switchyard insulators as a result of snowpack and salt spray in January 2015.
Pilgrim LOOP during Winter Storm Juno LOOP caused by flashover of the switchyard insulators as a result of snowpack and salt spray in January 2015.
                    - Offsite power was restored in ~60 hours.
- Offsite power was restored in ~60 hours.
Pilgrim LOOP        Electrical Design Elements
Electrical Design Elements
                    - Unit has two EDGs and an SBO diesel.
- Unit has two EDGs and an SBO diesel.
during Winter
- A separate 23kV offsite power source remained available.
                    - A separate 23kV offsite power source remained Storm Juno            available.
The mean CCDP was 4x10-5.
The mean CCDP was 4x10-5.
                    - Risk equally distributed between transient LOOP sequences and postulated SBO scenarios.
- Risk equally distributed between transient LOOP sequences and postulated SBO scenarios.
                    - FLEX mitigation strategies not credited.
- FLEX mitigation strategies not credited.
February 18, 2022          7th Annual NRC PFHA Research Workshop     11}}
11 7th Annual NRC PFHA Research Workshop February 18, 2022}}

Latest revision as of 17:53, 27 November 2024

4A-3_of_4-Hunter-Duane_Arnold_ASP_Analysis-SLIDES
ML22061A105
Person / Time
Site: Duane Arnold NextEra Energy icon.png
Issue date: 02/18/2022
From: Christopher Hunter
NRC/RES/DRA/PRAB
To: Thomas Aird
NRC/RES/DRA
Aird, Thomas - 301 415 2442
Shared Package
ML22061A095 List:
References
Download: ML22061A105 (11)
v • d • e


Text

Why the Risk of the Extended Loss of Offsite Power Was Almost a Significant Precursor?

Chris Hunter Office of Nuclear Regulatory Research Division of Risk Analysis Performance and Reliability Branch

Event Overview On August 10, 2020, a derecho moved through Iowa and other parts of the Midwest.

- The most extreme winds were estimated to be near 110 mph, wind gusts of 80-100 mph were common.

Duane Arnold experienced a grid perturbation that caused the emergency diesel generators (EDGs) to automatically start, but initially ran unloaded.

Approximately 15 minutes later, the main generator tripped resulting in a loss of offsite power (LOOP) and subsequent reactor trip.

- The two EDGs automatically loaded to their respective safety buses.

The licensee declared a Notice of an Unusual Event.

2 7th Annual NRC PFHA Research Workshop February 18, 2022

Additional Event Details Prior to the event, the licensee was loading fuel into a spent fuel canister.

North FLEX building was damaged and equipment within was declared inoperable.

The main steam isolation valves remained open, allowing operator to align main steam-line drains.

Approximately 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> into the event, the essential service water (ESW) strainers started to get plugged due an ingress of debris.

A small tear was discovered in the reactor building resulted in secondary containment being declared inoperable.

3 7th Annual NRC PFHA Research Workshop February 18, 2022

Accident Sequence Precursor (ASP)

Evaluation February 18, 2022 7th Annual NRC PFHA Research Workshop 4

Initial ASP Information Initial ASP analysis started within a few days of the event and showed that the event was potentially a significant precursor.

Focused on early sequence results to determine which modeling/event assumptions that needed to be evaluated further.

- FLEX modeling

- Stuck-open SRV scenario modeling

- ESW strainer challenge Provided some initial high-level information during ASP presentation to international precursor community.

5 7th Annual NRC PFHA Research Workshop February 18, 2022

SPAR Model Changes MELCOR calculations were performed to determine timing information for postulated stuck-open SRV scenarios.

- Based on these calculations and discussions with the licensee revealed that operators would have enough time to connect and initiate either firewater or FLEX reactor makeup.

Credit for FLEX mitigation strategies was applied.

- Modified FLEX hardware reliability data based on initial data evaluation (3x multiplier was used).

Modified ESW Strainer common-cause failure (CCF) parameters to use environmental causal alpha factors.

- Added operator action to bypass clogged strainers to SPAR model, which largely mitigates significant CCF potential.

Additional Changes

- Eliminated some assumptions deemed needlessly conservative (72-hour AC power recovery requirement).

- Removed EDG repair for scenarios where ELAP is declared due to potential for load shed activity precluding recovery.

6 7th Annual NRC PFHA Research Workshop February 18, 2022

Preliminary ASP Analysis The preliminary ASP analysis resulted in a mean conditional core damage probability (CCDP) of 1x10-3.

- Risk dominated by station blackout (SBO) scenarios.

- Offsite power recovery credit not provided within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.

Identified several key uncertainties.

- Performed sensitivities to evaluate the impact of these uncertainties.

Sent the preliminary ASP analysis to the licensee for a 60-day review per Regulatory Issue Summary 2006-24.

7 7th Annual NRC PFHA Research Workshop February 18, 2022

Industry Comments NextEra provided comments on the preliminary ASP analysis on February 9, 2021 (ADAMS Accession No. ML21042A079).

- In addition, the Pressurized-Water Reactor Owners Group provided comments.

Offsite power was available to be restored to the safety buses approximately 22.6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> after the event started.

- This change resulted in mean CCDP decreasing to 8x10-4.

The PWROG showed that the rate for EDG failures to run (FTR) could be reduced from 1.4x10-3 per hour to 8.4x10-4 per hour.

- Idaho National Laboratory performed an updated calculation that was only slightly smaller than current estimate and, therefore, no changes were made.

A significant conservatism is that FTR events are assumed to occur at the start of the event.

- The treatment of FTR events in the SPAR models is consistent with the current state-of-practice.

8 7th Annual NRC PFHA Research Workshop February 18, 2022

ASP Comparison February 18, 2022 7th Annual NRC PFHA Research Workshop 9

Brunswick LOOP during Hurricane Isaias Storm-generated debris resulted in a LOOP to Unit 1 in August 2020.

- The LOOP lasted approximately 14 hours1.62037e-4 days <br />0.00389 hours <br />2.314815e-5 weeks <br />5.327e-6 months <br />.

Electrical Design Elements

- Each unit has two EDGs and share an SBO EDG that can crosstied to the other unit.

The mean CCDP was 2x10-5.

- LOOP transient scenarios dominated risk; SBO risk was minimal.

- FLEX credit provided minimal risk reduction.

10 7th Annual NRC PFHA Research Workshop February 18, 2022

Pilgrim LOOP during Winter Storm Juno LOOP caused by flashover of the switchyard insulators as a result of snowpack and salt spray in January 2015.

- Offsite power was restored in ~60 hours.

Electrical Design Elements

- Unit has two EDGs and an SBO diesel.

- A separate 23kV offsite power source remained available.

The mean CCDP was 4x10-5.

- Risk equally distributed between transient LOOP sequences and postulated SBO scenarios.

- FLEX mitigation strategies not credited.

11 7th Annual NRC PFHA Research Workshop February 18, 2022

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