ML24312A143

SDP for Nextera RUG Meeting - 11.7.2024 - Rev. 0

ML24312A143
Person / Time
Site:	Watts Bar
Issue date:	11/07/2024
From:	Matthew Leech NRC/NRR/DRA/APOB
To:
References
Download: ML24312A143 (14)

Text

Significance Determination Process Risk Evaluation for Watts Bar Unit 2 Open Penetration While Shutdown November 7, 2024 Matt Leech Risk Analyst Office of Nuclear Reactor Regulation Division of Risk Assessment PRA Oversight Branch

2 Event Summary

• Day 5 of a refueling outage, workers opened a 24 containment penetration without tracking it for containment closure

• If Residual Heat Removal (RHR) cooling had been lost and containment closure would have to be established, operations would not know about the open penetration

• Introduced a potential release path from the containment through the Reactor Auxiliary Building (RAB) to the environment Large Early Release Frequency (LERF concern)

• NRC procedure, IMC 0609, App H for Containment Significance Determination Process (SDP) screened the issue as preliminary White finding

• To assess the risk for LERF, the base shutdown CDF is needed

• Exposure period of about 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />

3 Complicated SDP

• Shutdown SDPs are infrequent and often complex

• In order to obtain the base shutdown CDF risk, multiple Event Trees needed to be created in multiple plant configurations

• The NRC doesnt have many shutdown models

• Required 10 different Event Trees to be developed and analyzed

• The plant conditions were dynamic.changing RCS level

• The default shutdown models need plant specific adjustments and event specific adjustments

5 Complicated LERF Considerations

• LERF is the metric of concern

• Procedure IMC 0609, App H for Containment Integrity SDP doesnt have detailed guidance

• How to account for LERF going through the RAB vs direct path to the environment?

• Other aspects like crediting evacuation?

6 Detailed Risk Evaluation

• Idaho National Labs (INL) created a shutdown SPAR model for Watts Bar for this risk analysis

• Latest shutdown initiating event frequencies from EPRI were incorporated

• Loss of Offsite Power (LOOP) initiating event frequency provided by INL

• CDF was the sum of risk from

- Loss of RHR events

- Loss of Inventory events

- Loss of Offsite Power (LOOP) / Station Blackout (SBO) events

• 2 main plant configurations had to be analyzed

- 1st exposure period water level started in the pressurizer

- 2nd exposure period the RCS head was removed until refueling cavity was full

• LOOP and SBO events were dominant

7 Risk Significant Aspects

• Risk significance for this event was significantly improved by the licensee having both trains of RHR in service.

• Exposure time relatively short which helped risk

• Refueling Water Storage Tank (RWST) tank level was full -

provides source for forced injection, which provides time for recovery of RHR pumps

- Should RHR be lost and not quickly recovered forced injection from a high-pressure pump can provide temporary cooling

• Permanently-installed, large FLEX diesels, with quick deployment capability

• High contribution from LOOP / SBO

8 Initial Risk Results

• Risk for LERF was low in the White range

• Forced feed and high RWST level was helpful for lowering risk Some un-resolved items remained

• Should FLEX equipment and actions be credited?

• How to account for LERF passing through the RAB before releasing to the environment.

9 FLEX

• Watts Bar has permanently-installed large FLEX diesels

• Quick deployment

• Did not have clear procedures for implementing FLEX during shutdown

• NRC Region 2 Senior Reactor Analyst travelled to the site and walked down procedures and equipment with licensee

10 Crediting FLEX

• Based on exchange of information between the NRC and Watts Bar, the decision was made to credit FLEX into this shutdown SDP

• This led to a significant improvement in risk!

11 LERF and the Level 3 PRA Project The NRC has recently completed work on an extensive Level 3 PRA project The project had a useful sensitivity that compared the release between 2 different SBO events

- One SBO event causing core damage, breaching containment and releasing direct to the environment

- Another SBO event causing core damage, breaching containment, but the release passed through the RAB first The sensitivity showed a significant reduction in the release by passing through the RAB first NRC Level 3 Probabilistic Risk Assessment (PRA) Project volume 3c: Reactor, At-Power, Level 2 PRA for Internal Events and Floods (ML22067A214)

12 Final Significance

• Credit for FLEX applied

• A risk reduction for LERF was applied

• Risk for this SDP was concluded to be GREEN

13 Important Take-Aways By maintaining both trains of RHR in service and operating, even after target coolant temperature had been reached, Watts Bar achieved a significant risk reduction due to potential loss of RHR events Watts Bar had a full RWST level which would have provided significant inventory and time for forced injection Good exchange of information between the NRC and the licensee led to crediting FLEX for this specific shutdown SDP even without specific procedures for the situation The recent work performed by the NRC Office of Research for the PRA Level 3 project provided a useful sensitivity on how to factor for LERF reduction passing through the RAB vs direct to the environment

Questions Matt Leech Risk Analyst US NRC (301) 415-8312 Matthew.leech@nrc.gov