ML18337A151
| ML18337A151 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 11/26/2018 |
| From: | Joel Wiebe Plant Licensing Branch III |
| To: | Mathews M Exelon Generation Co |
| References | |
| Download: ML18337A151 (5) | |
Text
1 NRR-DMPSPEm Resource From:
Wiebe, Joel Sent:
Monday, November 26, 2018 11:45 AM To:
Mitchel Mathews Cc:
Subject:
Supplemental Information Needed for Byron Amendment Request Dated August 10, 2018
- Mitch, Here are our supplemental information needs for the subject amendment.
I believe we discussed the details of these during the public meeting. Let me know if you need a clarification call by November 28, 2018.
If no clarification call is needed, we will send the formal letter with a request to provide the information within 13 working days.
Joel The NRC staff requests the following information in order to complete its detailed review:
- 1. In its letter dated August 10, 2018, the licensee proposed the addition of a note to TS 3.8.1, Required Action A.2, CT, as follows:
For the failure of Unit 2 System Auxiliary Transformer 242-1, restore the required qualified circuit to OPERABLE status within 79 days.
The note does not contain an expiration date for the extended CT or link the extended CT to a specific failure date of System Auxiliary Transformer (SAT) 242-2. Therefore, the proposed TS revision would not limit the extended CT to a one-time, temporary extension associated with the SAT 242-2 failure addressed in the license amendment request.
The NRC staff requests that the application be supplemented to provide an expiration date or to link the extended CT to a specific failure date of SAT 242-2
- 2. In its letter dated August 10, 2018, the licensee states:
In addition, the PRA [probability risk assessment] Model of Record includes an assumption that a unit-to-unit crosstie of the ESF [engineered safety feature] buses will be in place if both parts of the Unit 2 SAT are out-of-service (242-1 and 242-2). Since the proposed configuration does not implement the unit-to-unit crosstie, the PRA model is modified to remove that assumption by setting some gates to FALSE or by inserting logic to require the unit-crosstie alignment if necessary.
The NRC staffs understanding is that the risk analysis associated with the proposed 79-day allowed outage time does not include consideration of the maintenance and operator actions required to implement the unit-to-unit crossties. However, the proposed 79-day allowed outage time appears to begin when the 242-1 SAT fails with the existing current condition of the out-of-service 242-2 SAT.
2 In the current configuration, should SAT 242-1 become inoperable, Emergency Diesel Generator (EDG)-2A and EDG-2B will be the immediate sources of power to 4.16 Kilovolt (kV) ESF buses 241 and 242, respectively. In its letter dated August 10, 2018, the licensee proposes to realign the two 4.16kV safety buses from EDGs to the Unit No. 2 unit auxiliary transformers. However, this realignment requires additional operator actions at transformers and circuit breakers that were not considered in the licensees letter dated August 10, 2018, for the risk assessment. The NRC staff notes that there are human error probabilities associated with operator actions to realign the two 4.16kV ESF buses, and these human error probabilities may significantly impact the risk.
Regulatory Guide (RG) 1.177, Revision 1, An Approach for Plant-Specific, Risk-Informed Decisionmaking: Technical Specifications, dated May 2011 (ADAMS Accession No. ML100910008),
Section A-1.3.1.1, states, in part:
If other components are reconfigured while the component is down, these reconfigurations can be incorporated in estimating R1 or R, using the PRA. If other components are tested before repair or if maintenance is carried out on the downed components, the conduct of these tests and their outcomes also can be modeled.
Therefore, it appears that the risk associated with operator reconfiguration should be included in the risk assessment. The NRC staff requests the application be supplemented as follows:
- 1. Provide an explanation of how the risk associated with the actions to realign the two 4.16kV safety buses from the EDGs to the Unit No. 2 auxiliary transformers are accounted for in the Byron Station PRA models, and how the guidelines in RG 1.177 are met.
- 2. If these actions are not modeled in the Byron Station PRA, provide a sensitivity study that reflects the impact of the operator actions and human error probabilities associated with these actions to realign the 4.16kV ESF busses, and justify how the guidelines in RG.177 are met.
- 3. Provide a summary of operator actions following a failure of SAT 242-1.
- 3. Section 2.2.1 of RG 1.177 states, in part:
Consistency with the defense-in-depth philosophy is maintained under the following circumstances:
A reasonable balance among prevention of core damage, prevention of containment failure, and consequence mitigation is preserved (i.e., the proposed change in a TS has not significantly changed the balance among these principles of prevention and mitigation) to the extent that such balance is needed to meet the acceptance criteria of the specific design-basis accidents and transients.
Section 2.4 of RG 1.177 states, in part, with NRC staff edits in square brackets:
The licensee has demonstrated that implementation of the one-time only TS CT change impact on plant risk is acceptable (Tier 1):
ICCDP [incremental conditional core damage probability] of less than [1.0E-6] and an ICLERP
[incremental conditional large early release probability] (of less than [1.0E-7], or ICCDP of less than [1.0E-5] and an ICLERP of less than [1.0E-6] with effective compensatory measures implemented to reduce the sources of increased risk.
3 In its letter dated August 10, 2018, the licensee described compensatory actions in Section 4.3.2 for Tier 2 actions and summarized them in Attachment 6. These actions include providing alternate firewater for centrifugal charging pumps cooling, various protective measures to the operation of the all site EDGs, and protection to the Unit No. 2 diesel-driven auxiliary feedwater pump. There are various procedural briefings and just-in-time training to operational staff.
Section 2.3.6 of RG 1.177 states, in part:
When compensatory measures are part of the TS change evaluation, the risk impact of these measures should be considered and presented, either quantitatively or qualitatively. When a quantitative evaluation is used, the total impact of these measures should be evaluated by comparison to the small guideline (Principle 4, as described in Part B of this regulatory guide[1]). This includes (1) evaluation of the proposed TS changes without the compensatory measures, (2) evaluation of the proposed TS changes with the compensatory measures, and (3) specific discussion of how each of the compensatory measures is credited in the PRA model or during the evaluation process.
In its letter dated August 10, 2018, the licensee presented the relative risk contributors. However, there is no qualitative or quantitative evaluation of risk reduction for each of these compensatory measures. It is unclear how consideration of mitigation as instructed in RG 1.177, and described above, is addressed.
The NRC staff requests the application be supplemented to provide an explanation of how the compensatory measures provided quantitatively and/or qualitatively impact the risk metrics and the guidelines of RG 1.177.
- 4. RG 1.177, Section A-1.3, states, in part:
Contributions from common-cause failures (CCFs) need special attention when calculating the increased risk level R1. If the component is down because of a failure, the common-cause contributions involving the component should be divided by the probability of the component being down because of failure since the component is given to be down. If the component is down because it is being brought down for maintenance, the CCF contributions involving the component should be modified to remove the component and to only include failures of the remaining components (also see Regulatory Position 2.3.1 of Regulatory Guide 1.177).
In its letter dated August 10, 2018, the licensee proposes to incorporate changes to the model in order to assess the impact of the cross-tie of Unit No. 2 to the Unit No. 1 4.16 kV safety buses (bus 241 tied to bus 141, and bus 242 tied to bus 142). In this configuration, all 4.16kV safety buses will be ultimately fed from Unit No. 1 SATs 142-1 and 142-2.
The NRC staff requests the application be supplemented to provide an explanation of how the CCF probabilities have been adjusted accordingly to account for assumed failures of SATs 242-1 and 242-2.
- 5. In its letter dated August 10, 2018, the licensee proposes, in the event of a failure of the SAT 242-1, to operate for 79 days with both Unit No. 2 SATs failed.
The NRC staff requests the application be supplemented to provide a summary of needed operator actions if a Unit No. 1 SAT fails during the 79 days,, including an evaluation of loading on the remaining Unit No. 1 SAT, which could be called upon to provide power to Byron Station, Unit Nos. 1 and 2, ESF buses.
4
- 6. In its letter dated September 13, 2018 (ADAMS Accession No. ML18256A392), the licensee provided a list of PRA action items required to be completed prior to implementation of the 10 CFR 50.69 risk categorization process.
The NRC staff requests that the application be supplemented to describe the impact of these items on the risk assessment provided in the licensees letter dated August 10, 2018.
[1] When proposed changes result in an increase in core damage frequency or risk, the increases should be small and consistent with the intent of the Commissions Safety Goal Policy Statement.
Hearing Identifier:
NRR_DMPS Email Number:
695 Mail Envelope Properties (SN6PR0901MB24473C5FC86185F709258FA08BD70)
Subject:
Supplemental Information Needed for Byron Amendment Request Dated August 10, 2018 Sent Date:
11/26/2018 11:44:30 AM Received Date:
11/26/2018 11:44:00 AM From:
Wiebe, Joel Created By:
Joel.Wiebe@nrc.gov Recipients:
"David Gullott" <David.Gullott@ExelonCorp.com>
Tracking Status: None "Mitchel Mathews" <Mitchel.Mathews@exeloncorp.com>
Tracking Status: None Post Office:
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