ML16188A108
| ML16188A108 | |
| Person / Time | |
|---|---|
| Site: | Nuclear Energy Institute |
| Issue date: | 07/07/2016 |
| From: | Nuclear Energy Institute |
| To: | Office of Nuclear Regulatory Research |
| Gilbertson A | |
| Shared Package | |
| ML16188A100 | List: |
| References | |
| Download: ML16188A108 (9) | |
Text
Industry Feedback on Proposed RG 1.174 Revisions NRC Public Meeting July 7, 2016
Overview
- Feedback on defense-in-depth (DID) white paper
- Examples for discussion
- Inclusion of aggregation and treatment of uncertainty in upcoming revision
Feedback on DID White Paper
- Maintaining integrated decision making process is critical
- First factor is a high level objective, which is what has to be demonstrated
- Guidelines should describe how to demonstrate that DID philosophy has not been degraded
- Overall criteria must be considered in an integrated manner
- Any single criterion should not be reason to reject a risk-informed change
Feedback on DID White Paper
- Discussion on temporary changes should be a stand-alone paragraph so that it is clear that it is applicable to all criteria
- Evaluation of a risk-informed application should involve review of the change in DID, not absolute DID
- Discussion of Layers of DID is potentially misleading
- First layer recognizes change is acceptable as long as the effect is not significant
- For the next 3 layers use of the term maintain could be interpreted as no change is acceptable rather than no significant change
- Factor 7 is unnecessary and should be deleted
- Some change in DID implementation is acceptable
Suggested Approach to DID in RG 1.174 Describe the philosophy of DID and how it is implemented Clearly state presumption that the as-built, as-operated plant, prior to the change, is consistent with the DID philosophy Give intermediate level acceptance guidelines, such as what is included in the standard review plan
- LAR should demonstrate that the proposed change maintains appropriate safety within the DID philosophy:
- The change does not result in a significant increase in the existing challenges to the integrity of the barriers
- The proposal does not significantly change the failure probability of any individual barrier
- The proposal does not introduce new or additional failure dependencies among barriers that significantly increase the likelihood of failure compared to the existing conditions Include examples regarding the application of DID and safety margin in RG 1.174 in an attachment
- Examples should involve a complete treatment of a risk-informed decision related to a licensing action
Example 1: BTP 8-8 BTP 8-8: Onsite (Emergency Diesel Generators) and Offsite Power Sources Allowed Outage Time Extensions The purpose of this Branch Technical Position (BTP) is to provide guidance from a deterministic perspective in reviewing such amendment requests.
A supplemental power source should be available as a backup to the inoperable EDG or offsite power source, to maintain the defense-in-depth design philosophy of the electrical system to meet its intended safety function. The supplemental source must have capacity to bring a unit to safe shutdown (cold shutdown)1 in case of a loss of offsite power (LOOP) concurrent with a single failure during plant operation (Mode 1).
According to NUREG-1784 [Reference 4], considering the changes in electric grid performance post-deregulation, the duration of LOOP events has increased and the probability of a LOOP as a consequence of a reactor trip has increased.
In summary, in light of the recent experiences in grid outages, it is the staffs position that the availability of an additional power source is a condition for approval of the extended EDG or offsite power AOT. Therefore, a supplemental power source must be available when extending the current AOT allowed by the plant TS for a single inoperable EDG or offsite power source up to 14 days provided the extended AOT is also supported by a risk-informed evaluation.
Example 2: SNC TSTF-500 (DC Electrical Re-Write) Application TSTF-500 allows for the extension of completion times (CTs) using risk-informed methods.
Southern Nuclear submitted TSTF-500 and included a change in the CT from two to 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
Reviewer believed in order to request a change to the two hour CT, licensee would need to take an additional single failure (e.g. have another battery available)
NRC did not approve of the CT change and SNC retracted in June 2016 Per RG 1.177, System redundancy, independence, and diversity are maintained commensurate with the expected frequency and consequences of challenges to the system None of these features of the DC system were affected by the proposed increase to the CT Needs to be stated clearly under Factor 3 in Section 2.1.1.2 of the draft RG Additional clarity could be provided if the draft RG explained that not all seven factors apply, or are even impacted by, a specific type of risk-informed change
Example 3: PSEG CFCU AOT Extension Proposed Application
- Proposal to extend allowed outage time for one or two inoperable containment fan coil units (CFCUs) from seven to 14 days
- Adequate defense-in-depth maintained
- Success criteria is three of five CFCU or one of two trains of Containment Spray
- Safety margins not affected
- Continued monitoring under the maintenance rule program and PSEG performance and predictive monitoring programs
- Reduce risk associated with shutting down to respond to emergent issues without significant change in at-power risk
Treatment of Aggregation and Uncertainty
- Licensees are developing more external event PRA models, while continuing to address the known conservatisms in Fire PRA models
- Issue of aggregation should be addressed in Sections 2.2.5 and 2.2.6 with this revision
- Focus on aspects of aggregation related to
- Decomposing PRA results to understand their level of realism/conservatism
- Effects of uncertainty to give more guidance on how to use these insights in an integrated decision-making framework
- Margin to QHOs
- Recognize the difference between making assumptions for convenience (e.g., as approximations) that introduce biases into the risk assessments, and true model uncertainties
- More completely reference NUREG 1855 and companion EPRI documents