NRC-24-0009, Request for NRC to Suspend Regulatory Audit of License Amendment Request for a Risk Informed Approach to ECCS Strainer Performance for Fermi Unit 2
| ML24037A189 | |
| Person / Time | |
|---|---|
| Site: | Fermi  |
| Issue date: | 02/06/2024 |
| From: | Frank E DTE Electric Company |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| NRC-24-0009 | |
| Download: ML24037A189 (1) | |
Text
DTE ElectricCompany 6400 N. Dixie Highway Newport, MI 48166 DTE
February 6, 2024 10 CFR 50.90 NRC-24-0009
U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555-0001
Fermi 2 Power Plant NRC Docket No. 50-341 NRC License No. NPF-43
Subject:
Request for NRC to Suspend Regulatory Audit of License Amendment Request for a Risk Informed Approach to ECCS Strainer Performance for Fermi Unit 2
References:
1)DTE Electric Letter to NRC License Amendment Request for a Risk Informed Approach to ECCS Strainer Performance, NRC-23-0020, Dated June 13, 2020 (ML23164A232) 2)NRC Letter to DTE Electric Fermi Unit 2 - Regulatory Audit in Support of Review of License Amendment Request for a Risk-Informed Approach for Addressing the Effects of Debris on ECCS Strainer Performance (EPID LL-2023-LLA-0092), Dated September 7, 2023 (ML23248A224)
In Reference 1, DTE Electric Company (DTE) submitted a license amendment request (LAR) for a Risk Informed Approach to ECCS Strainer Performance. The NRC began an audit in support of review of the LAR, per the audit plan in Reference 2. During the audit, the NRC provided discussion topics that require DTE to complete additional analysis to provide complete responses. DTE is requesting that the Regulatory Audit be suspended until June 7, 2024 to allow for completion of the analysis and to provide complete responses to the Audit discussion topics.
The Enclosure tothislettercontains a list of the discussion topics that require additional time for DTE to provide completeresponses.
No newcommitments are being made in this submittal.
Should you have any questions or require additional information, please contact me at (734)586-4772.
Sincerely,
Eric Frank Manager - Nuclear Licensing USNRC NRC-24-0009 Page 2
Enclosure:
Audit Discussion Topics - Fermi ECCS Strainer Risk Informed LAR
cc:NRC Project Manager NRC Resident Office Regional Administrator, Region III Michigan Department of Environment, Great Lakes, and Energy
Enclosure to NRC-24-0009
Fermi 2 NRC Docket No. 50-341 Operating License No. NPF-43
Audit Discussion Topics - Fermi ECCS Strainer Risk Informed LAR
Enclosure to NRC-24-0009 Page 1
Audit Discussion Topics - Fermi ECCS Strainer Risk Informed LAR
Tech Spec. Branch (STSB)
- 6) Provide a basis for the 1/8-inch bed thickness criteria. Strainer acceptance criteria should be based on the plant specific debris loads for Fermi. For example, describe how the design basis testing and analysis that was used to qualify the strainer demonstrates that a fibrous debris amount of 1/8 inch is acceptable. See page 16 of 26 of Attachment 1-1 (PDF pg. 19) that states that breaks that exceed 1/8 inch are already included in the design basis. See page 5 of 94 of the Serco calculation (pg 77) that makes a similar finding. An explanation of these statements may provide the necessary detail. Page 55 of 94 (pg. 127) of the Serco calculation makes the statement that without a fiber bed there is no substrate to capture particulates on the strainer. The staff has observed strainer testing with only microporous insulation, and also vertical loop testing with Cal-Sil, that resulted in high headlosses. No fiber, beyond that in the microporous insulation types was included in these tests. In addition, references in the submittal provide information regarding Min-K headloss with no fiber. See EFA-11-16-004 and DC-5979 which establishes a maximum Min-K load per strainer of 10 lbm. regardless of fiber load. These references are both based on test results. On Page 57 of 94 (pg. 129) the Serco calculation discusses breaks smaller than the design basis maximum that generate and transport debris in the range of 1/8 inch. On page 65 of 94 of the Serco calculation (pg. 137) it is stated that the risk analysis verifies that debris types and quantities are within the design basis limits. Clear explanation of these statements may help the NRC staff with understanding the acceptability of the 1/8-inch acceptance criterion. What is the bed thickness at the design basis fiber limit? If it is less than 1/8 inch, why is it considered an acceptable criterion since the design basis is based on testing and associated analysis? Also discussed on page 79 of 94 (pg. 151).
Initial Response:
A more complete response to each part of the question is in progress. Particular emphasis is placed on explaining that 1/8th-inch of uniform strainer coverage is intended to represent a minimum quantity of equivalent fiber at which debris induced head loss concerns begin to arise. Sensitivity results for 1/16th -inch of uniform fiber will also be presented, keeping in mind that it is very difficult to form perfectly contiguous debris beds over complex strainer geometries at such small fiber loads. The following bullets provide additional perspective:
- The design basis permits up to 21 ft3 of fiber (LAR Table 2-5). When spread over three full strainer areas, the debris thickness is 21 ft3/387.42 ft2 /3 x 12 = 0.22 inches, which is 1.7 times thicker than the assumed failure criterion.
- The maximum quantity of microporous Min-K that can be generated in any single non-isolable break is approximately 0.52 ft3 (see figure in response to STSB Question #12) that, when multiplied by a manufactured density of 11.4 lbm/ft3, gives a Min-K debris mass of 5.9 lbm. Additional information in the response to STSB Question #12 suggests that the Enclosure to NRC-24-0009 Page 2
maximum amount of Min-K alone would form a layer approximately 1/64th inch thick, an amount unlikely to cause or sustain thin-bed headloss.
o Note that the maximum quantity of microporous Min-K that can be generated in any single non-isolable break may increase depending on final resolution of potential erosion of Min-K insulation located inside penetrations by external breaks, but similar conclusions are expected. If cases are found that exceedthe design-basis amount of Min-K (or any other debris type), additional perspective will be provided by reporting the total risk associated with all cases capable of exceeding one or more design-basis debris type limits.
- An examination of head loss induced by debris beds at or near the 1/8th-inch failure criterion would help relate assumed failure to the currently approved strainer capabilities.
Some information to this effect is provided in responses to STSB Questions #12.
- 13) On page 14 of 16 of attachment 1-1 (Pg 17), it is stated that deterministic goals applied to the original strainer design bound all debris loads generated by LOCAs for all inboard non-isolable locations. On page 79 of 94 (pg. 151) the safety margin discussion states that the Fermi strainers are designed to meet the design basis loads with sufficient margins. In addition, it states that the current debris generation estimates for non-isolable breaks are lower than the design basis inventories. Explain these statements. If the new estimates are to become the design basis, it sounds like there is margin over the old analysis that establishes the strainer allowable loads. If this is the case, there would be no need to perform a risk analysis for the non-isolable breaks. However, the additional miscellaneous debris may cause the updated debris loading per area to be higher than the current design basis. Maybe newly discovered tags and microporous debris are not covered by these statements? Discuss whether the updated debris loading is bounded by the current licensing basis debris loads and the resulting impact on the need to assess risk impact for non-isolable welds.
Initial Response: (See General Observation preceding response to STSB Question #6)
- A more complete response is in progress, but it is correct to say that the statements of compliance with all debris limits in the design basis do not include tags and labels.
Justifying the risk of additional tags/labels is a primary objective of the LAR. The FSAR markups will emphasize that a new misc debris limit of 100 ft2 of total strainer obstruction over all active strainers is being established by the risk-informed analysis.
o Note that the maximum quantity of microporous Min-K that can be generated in any single non-isolable break may increase depending on final resolution of potential erosion of Min-K insulation located inside penetrations by external breaks. If cases are found that exceed the design-basis amount of Min-K (or any other debris type),
additional perspective will be provided by reporting the total risk associated with all cases capable of exceeding one or more design-basis debris type limits.
Enclosure to NRC-24-0009 Page 3
- 16) Starting on page 38 of 94 of the Serco calculation (pg. 110) the strainer flow configuration is described. This is also discussed in other sections, e.g., starting on page 54 of 94. It appears that the configuration assumed for the analysis (single train runout) is an unlikely configuration that may result in earlier scenario failure, but also result in lower risk values than the more likely single train suppression pool cooling case. It is also stated that the suppression pool cooling case is the design basis case. Further discussion of the strainer flow rates is provided on page 73 of 94 (page 144), and 80 and 81 (pg. 152 and 153) of the Serco calculation. The submittal states that the Suppression Pool Cooling (SPC) mode (1 RHR strainer and 1 CS strainer) results in higher risk than the baseline.
- c. Provide the basis for the choice of using the runout case for the baseline for the risk-informed analysis instead of the more likely suppression pool cooling (SPC) mode.
Initial Response:
- The baseline case provides a point of comparison for assessing stability of the risk evaluation with respect to the challenges being addressed, and it is not always possible to intuit, a priori, which cases will maximize risk. Choice of the LPCI runout case with one failed ECCS division as the baseline was somewhat historical in that divisional failure with runout flow is common to other risk-informed assessments of sump recirculation performance, so it was the first flow condition fully quantified. At the same time, the most likely configuration of two division operation was evaluated to demonstrate no defined strainer failures and no additional risk.
- Informal evaluations of dual-train SPC mode indicate risk lower than the baseline.
Single-train SPC mode can also have risk lower than the baseline when the 10% full train failure probability is applied. (Baseline risk does not apply the factor of 10 reduction for single train failure). Fermi will perform and summarize the results of alternate flow configuration sensitivity studies to further address this question.
- 18) On page 82 of 94 (pg. 154) the submittal discusses the assumptions for RHR strainers in service. It states that the loads are calculated assuming one RHR strainer and one CS strainer in SPC mode. Bringing the idle strainer back into service is discussed. The discussion does not appear to be consistent with the baseline evaluation that assumes two RHR pumps in runout and one CS strainer operating. Clarify the referenced discussion.
Initial Response:
As described, strainer maximum debris loads were calculated by GEH assuming one RHR strainer and one CS strainer operating in SPC mode, consistent with RHR and CS pump NPSH calculations, and the baseline evaluation assumes two RHR pumps in runout and one CS strainer operating. Although it may be desirable from a point of consistency, there is no requirement that the risk-informed baseline analysis must adopt the same flow conditions used in the strainer design basis. In general, risk-informed analyses permit consideration of a matrix of pump-failure combinations, each with a respective probability weight, that are not identical to the strainer design basis. Fermi will perform and summarize the results of Enclosure to NRC-24-0009 Page 4
alternate flow configuration sensitivity studies, including SPC mode, to further address this question.
- 20) Provide a description of non-RMI insulation installed in the plant at locations other than the penetrations. Page 18 of 94 of the Serco calculation (pg. 90) states that DC-5797 identifies that both Nukon and Min-K are installed at whip restraints. On page 25 of 48 of Ref. 3 (and
- 49) of the Serco calculation (DC-5979, Rev. 0) Min-K on pipe whip restraints is described.
On page 9 of Ref. 41 of the Serco calculation (EFA-E11-16-004) Min-K on the main steam drain lines is described.
- a. On Page 61 of 94 (pg. 133) of the Serco calculation modeling of whip-restraint insulation is described. How were the materials and associated volumes of insulation at these locations determined?
Initial Response:
Non-RMI insulation locations, types, and quantities represented in the CAD model are based on Table 6.1 from DC-5979 Rev. 0. A comparison of this table to the same information presented in Ref. 11 Table 7-2 identified several locations that will be added to the CAD model totaling approximately 4.6 ft3 of fiber (5 locations) and 0.11 ft3of Min-K (1 location). All fiber is substantially above gratings located below RCPs. The blue plane in the figure below denotes the minimum elevation of the additional pipe restraint targets which is 17.5 ft above the gratings. A minor risk increase may result from re-quantification of non-isolable breaks with these additional insulation targets included. Fermi will re-compute debris generation and transport using the debris configuration in the revised CAD model to re-quantify risk associated with non-isolable breaks.
- 21) On Page 59 of 94 (pg. 131) the Serco calculation says that all breaks that occur outside of the penetrations damage Min-K based on line-of-sight. On page 36 of 94 of the Serco calculation it is stated that non-isolable breaks are permitted to damage Min-K in penetrations to the extent that robust barriers do not intervene. Is line-of-sight defined by robust barriers? Verify that all targets within the ZOI are assumed to be damaged unless the ZOI is truncated by a robust barrier. Reference 11 implies that this is the case, but it is not evident from the discussion in the submittal.
Initial Response:
In general, the line of sight, or point-to-point distance, between the break location and any insulation target is truncated by robust barriers (concrete, containment wall, reactor vessel).
Structural steel, piping, and large equipment like reactor coolant pumps are not generally considered to be robust barriers, even though significant jet deflection and energy dissipation is likely to occur. All targets within a ZOI are assumed to be damaged unless the ZOI is truncated by a robust barrier. Targets in the shadow of a robust barrier (defined by ray tracing outward from the break location) are not damaged in CASA Grande debris generation calculations.
Enclosure to NRC-24-0009 Page 5
The subject of damage caused to penetration Min-K by non-isolable breaks occurring outside of the penetrations is complicated by the fact that penetration Min-K resides in a narrow annular gap defined on the outside by a large diameter guard pipe extending into containment approximately 4 inches beyond the insulation and on the inside by the hydraulic pipe passing through the penetration. Significant congestion composed of structural steel, piping, valve bodies, etc. is also generally found in front of penetration entry points into containment.
It is Fermis intention that reported baseline risk results be consistent with the following statement found on page 36 of 94 and on page 59 of 94 in LAR Att-3:
It should be noted that all breaks postulated to occur outside of penetrations are permitted line-of-sight access to damage Min-K insulation present inside penetrations, despite the piping and equipment congestion surrounding most penetration access points inside containment.
However, review of CASA Grande input files revealed that Min-K present in penetrations was not included in the Non-Isolable break calculations. In effect, the present Non-Isolable break calculations inadvertently credit perfect shielding of penetration Min-K by piping and structural congestion around penetration openings.
To remain consistent with the baseline assumption allowing line-of-sight damage of penetration Min-K by breaks occurring outside of penetrations, Fermi is adding fidelity to the containment geometry that exists at penetrations and re-evaluating Min-K debris quantities generated and transported for each break scenario. Fermi will supply CAD illustrations of the refined penetration geometry, a new maximum Min-K debris quantity arriving at the strainers, and updated risk results based on the baseline 1/8th-inch fiber strainer failure criterion. If Non-Isolable break cases are found that exceed the strainer design basis maximum quantity of Min-K debris, the frequency of these breaks will be added to the total risk, consistent with the described LAR methodology.