NRC-24-0033, DTE Electric Company - Final Response to Request for Additional Information for License Amendment Request Regarding Risk Informed Approach to Performance ECCS Strainer Performance

DTE Electric Company - Final Response to Request for Additional Information for License Amendment Request Regarding Risk Informed Approach to Performance ECCS Strainer Performance

ML24151A173
Person / Time
Site:	Fermi
Issue date:	05/30/2024
From:	Domingos C DTE Electric Company
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
NRC-24-0033
Download: ML24151A173 (1)

Text

Christopher P. Domingos Site Vice President DTE Electric Company 6400 N. Dixie Highway, Newport, MI 48166 Tel: 734.586.5025 Fax: 734.586.5295 Email: christopher.domingos@dteenergy.com 10 CFR 50.90 May 30, 2024 NRC-24-0033 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:

Final Response to Request for Additional Information for License Amendment Request Regarding Risk Informed Approach to Performance ECCS Strainer Performance In Reference 1, DTE Electric Company (DTE) submitted a License Amendment Request (LAR) for a Risk Informed Approach to ECCS Strainer Performance. In Reference 2, an email from Mr. Surinder Arora to Mr. Eric Frank dated March 20, 2024, the NRC sent DTE a Request for Additional Information (RAI) regarding the LAR. In Reference 3, a partial response to the RAI was provided. Enclosure 1 of this letter provides a response to the RAIs that were not provided in Reference 3. Attachment 2 contains revised versions of Fermi LAR Attachment 2-1 and -2, and unchanged versions of Fermi LAR Attachment 2-3 and Attachment 2-4.

References:

1) DTE Letter NRC-23-0020, License Amendment Request for a Risk Informed Approach to ECCS Strainer Performance, dated June 13, 2023 (ML23164A232)

2) NRC E-mail Capture, Fermi 2 - Request for Additional Information for License Amendment Request Regarding Risk-Informed ECCS Strainer Performance Evaluation (L-2023-LLA-0092)", dated March 20, 2024 (ML24080A391)

3) DTE Letter NRC-24-0027, Partial Response to Request for Additional Information for License Amendment Request Regarding Risk Informed Approach to Performance ECCS Strainer Performance dated April 24, 2024 (ML24115A095)

DTE

to NRC-24-0033 Fermi 2 NRC Docket No. 50-341 Operating License No. NPF-43 Response to Requests for Additional Information to NRC-24-0033 Page 1 STSB-RAI-1 Provide revised exemption requests that are consistent with regulatory precedent. The exemptions should request exemption to 10 CFR 50.46(a)(1) which points to other properties sufficient to provide assurance that the most severe postulated loss-of-coolant accidents are calculated. The NRC considers the deterministic evaluation of sump performance to be one of the other properties. A quote from a previously approved exemption indicates NRC acceptance of this precedent - The NRC staff interprets the section 50.46(a)(1) requirement to calculate ECCS performance for other properties as requiring licensees to consider the impacts of debris generation and transport in containment. Consider discussing whether testing or further deterministic analysis had been evaluated as a potential way to resolve the issue instead of using the exemption process when evaluating special circumstances.

DTE Response:

Fermi acknowledges and accepts NRCs observation.

Revised exemption requests related to 10CFR50.46 are provided in attached revised versions of Fermi LAR Attachments 2-1 and 2-2. The revised versions wholly replace the original Fermi LAR Attachments 2-1 and 2-2. Fermi LAR Attachments 2-3 and 2-4 are unaffected by the change, but Attachments 2-3 and 2-4 are also provided for completeness.

Markup and clean copies are provided to aid review of the limited specific changes.

References in Fermi LAR Attachment 1 to 10CFR50.46(d) in section 1,

SUMMARY

DESCRIPTION - Methodology Change, and section 2.3, Description of the Proposed Change, are superseded by the changes to Attachments 2-1 and 2-2 in this submittal, and are now understood to refer to 10CFR50.46(a)(1).

The Fermi exemption requests are motivated solely by the departure from traditional, bounding, deterministic methods to implement a risk-informed approach that is allowed by NRC as an elective of the licensee to resolve technical issues. In general, risk-informed methods acknowledge that conditions may exist that exceed the strainer design bases, and require a quantification of additional risk contributed by those conditions. While considerations of testing and further deterministic analyses factored strongly in Fermis choice to pursue the risk-informed LAR format, those considerations are not pertinent as special circumstances of the exemptions.

The exemptions are requested only because accepted risk-informed methodologies do not strictly satisfy traditional deterministic requirements.

to NRC-24-0033 Page 2 STSB-RAI-6 Provide a justification for the 1/8-inch bed thickness criteria used in the risk-informed analysis.

The justification should be based on plant-specific debris loads and design basis strainer testing and evaluation for Fermi. For example, describe how the design basis testing and analysis that was used to qualify the strainer demonstrates that a fibrous debris bed 1/8-inch thick is acceptable. On page 82 of 94 of Attachment 3 of the LAR, SERCO-REP-DTE-22609-02, Revision 1 (Serco calculation) (PDF 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. This was also stated in the audit responses. In response to audit questions regarding whether the 1/8-inch debris bed limit for the risk-informed analysis was bounded by the design basis, a bed thickness was calculated assuming that 3 strainers were in service. Based on the NRC understanding, the GE strainer loading limits based on testing and analysis assumed two strainers in service. To maintain consistency with the design basis loading for the strainer it appears that the discussion regarding the risk-informed 1/8-inch debris bed limit should be based on debris collecting on two strainers. Note that this should result in greater margin than reported during the audit. However, this NRC observation is a simplification based on an equal distribution of debris across two strainers, while actual debris distribution in the risk-informed analysis is based on the flow through each strainer. It is unclear to the NRC what the assumptions were for debris distribution when the limits were established by GE. Provide a comparison between the risk-informed debris bed limit and the maximum fiber bed thickness resulting from the GE limiting strainer tests and evaluation. Discuss why 3 strainers were assumed in the risk-informed baseline case when the design basis case appears to credit only two strainers.

DTE Response:

The 1/8th-in debris thickness criterion is specifically chosen as a threshold of concern for the onset of appreciable head loss, above which, all debris accumulation scenarios are conservatively assigned to Emergency Core Cooling System (ECCS) failure, and below which, there is no appreciable head loss that can challenge NPSH limits. The 1/8th-in failure criterion is never used in the LAR to substantiate acceptable head loss. However, acceptable head loss at the assumed threshold failure limit is preserved by:

1. Concurrent compliance with strainer Design Basis (DB) debris constituent limits, including Nukon' and Min-K',

2. Recognition that 1/8th-in (and less) debris bed scenarios do not pose risks that are not already included in the range of strainer performance challenges,

3. Descriptions of changes in areal density and flow velocity presented in responses to STSB-RAI-9 (misc. debris area) and STSB-RAI-10 (areal density and velocity increase) show that changes to hydraulic flow conditions at the low debris threshold cannot induce more head loss than analyzed in strainer qualification calculations performed for fully circumscribed debris loads.

to NRC-24-0033 Page 3

4. Parameter sensitivity results showing that risk, as measured by violations of the 1/8th-in threshold, is relatively insensitive to miscellaneous debris obstruction in the range from 65 ft2 to 100 ft2 (LAR Attachment 4 Ref. 3 - SERCO-REP-DTE-22929-02, Rev. 0, Sensitivity Study of Fermi Unit 2 Risk-Informed Core Damage Frequency, May 2024).

5. Parameter sensitivity results show that a 1/16th-inch failure limit increases risk from assumed bed thickness failure by 10% to 40% depending on the flow history examined, but does not induce risk equal to contributions from exceeding the DB Min-K' limit (LAR Attachment 4, Ref. 3).

Comparisons between the risk-informed debris bed limit and the maximum fiber bed thickness resulting from the GE limiting strainer tests and evaluation are provided in responses to STSB-RAI-9 (misc. debris area) and STSB-RAI-10 (areal density and velocity increase).

As stated in the question, GE strainer qualification calculations confirm that pump Net Positive Suction Head (NPSH) requirements are met with DB debris loads for one Residual Heat Removal (RHR) strainer and one Core Spray (CS) strainer under flow rates typical of suppression pool cooling. The ratio of individual flow rates to the combined total flow rate determines the proportion of debris collected by each strainer (i.e., load factors). The GE load factor for RHR strainers is 11,000 (11,000 + 6100) = 0.643

. Total DB Min-k' debris is 0.52 ft3, so the Min-K' limit applied to each RHR strainer under every break and flow history scenario is 0.643 x 0.52 3 = 0.334 3. A similar limit is derived and applied for CS strainers, but the RHR limit is bounding.

While two-strainer pool cooling is the desired long-term end state for Loss of Cooling Accident (LOCA) response, a strict application of the two-strainer configuration as the risk analysis baseline would imply that flow is dedicated to one specific RHR strainer and one specific CS strainer until they either violate DB debris limits and fail, or do not violate DB debris limits and successfully manage the LOCA. Operators are not bound by the analysis assumptions in this manner. They have the flexibility to utilize all available equipment and the training necessary to diagnose adverse signs of excessive debris accumulation. The number of opportunities for rotating flow to idle equipment is greatly increased for two-division response where operators have previously secured multiple systems after meeting Reactor Pressure Vessel (RPV) level and pressure requirements.

Rigid enforcement of Suppression Pool Cooling (SPC) on only two strainers could only occur in practice if 1) an entire division plus one additional RHR pump fails, in which case the probability of incurring the tabulated risk would be even lower; or 2) operators fail to diagnose symptoms of excessive debris accumulation, which is accounted for in the modified baseline risk discussed in LAR Attachment 4 - Technical Supplement. Thus, three-strainer flow scenarios are better indicators of actual risks associated with single-division response.

Table STSB-RAI-11.1 illustrates risks for 8 different flow histories, including several versions of SPC. All of the three-strainer flow cases have remarkably similar total risk because flow rates can only change the timing and proportion of debris arriving at the operating strainers.

Exceedance of either failure criterion (1/8th-in thickness or DB debris limits) is dominantly controlled by debris quantity and active strainer surface area. Case 3B and Case 4A yield higher to NRC-24-0033 Page 4 risk estimates because the flow histories incorporate elective actions to implement two-strainer SPC. The three-strainer flow histories do not acknowledge the idle RHR strainer that was turned off and is still available to prevent an analyzed failure caused by exceeding a DB strainer limit.

The exact flow histories that are assumed can affect final total risk as measured by the two failure criteria, but within the range of RHR and CS flow rates typical for accident response, risks are relatively stable.

Additional discussion of the suppression pool cooling flow histories is included in the responses to STSB-RAI-11 and STSB-RAI-12. A further summary is provided in LAR Attachment 4 -

Technical Supplement, and technical development is presented in LAR Attachment 4 Reference 3 - SERCO-REP-DTE-22929-02, Rev. 0).

to NRC-24-0033 Page 5 STSB-RAI-11 Describe the pump operating combinations and flow rates over time as they were considered in the risk-informed analysis.

DTE Response:

Table STSB-RAI-11.1 and Table STSB-RAI-11.2, displayed below, define and describe the pump operating combinations and flow rates over time that are considered in the risk-informed analysis. The two tables have common headers to align the information specific to each flow condition denoted as Case 1A, Case 1B, Case 2A, Case 2B, Case 3A, Case 3B, Case 4A, and Case 5A. The same set of eight flow conditions are evaluated for every parameter sensitivity variation (Parameter sensitivity cases are discussed in SERCO-REP-DTE-22929-02, Rev. 0, Sensitivity Study of Fermi Unit 2 Risk-Informed Core Damage Frequency, May 2024). The baseline flow condition (Case 2A) is highlighted in blue.

Risk results reported in Table STSB-RAI-11.1 are specific to each defined flow history and they correspond to baseline values for all sensitivity parameters. All risk results include the effects of penetration Min-K' as described by baseline debris generation assumptions (prior to application of modified baseline assumptions). Only variations in flow history are shown in this table. Risk results in Table STSB-RAI-11.1 have not been modified by additional factors such as plant-state or operator success probabilities, so direct comparisons between case study risks indicate the effect of flow history alone. Table STSB-RAI-11.2 provides footnotes to explain the purpose of each flow history case study.

Case 1A and Case 2A align with conditions analyzed in RHR and CS hydraulic calculations for full two-division system response. These cases are important to analyze for potential failures (defined by 1/8th-in bed thickness and DB debris load criteria) because debris is accumulated rapidly over the maximum active strainer surface area. Any identified failures would occur earlier for these cases than for any other flow histories. Because strainer DB Min-K' loads are very small, this is the dominant risk contributor for all analyzed cases. For two-division response (all strainers active), no break scenarios are found that are capable of exceeding 1/8th-in of fiber on any single strainer.

Case 2A defines the baseline flow configuration, which represents maximum flow rates anticipated for single-division Low Pressure Coolant Injection (LPCI), as analyzed in plant hydraulic calculations. This flow history minimizes the time for analyzed failures to occur across three operating strainers and establishes a minimum bound on risk that could be achieved by successfully rotating flow to idle strainers in a suppression pool cooling configuration. Case 2B examines a minor variation in flow rate that maximizes the difference between RHR and CS flow rates. Only times to failure are affected.

Case 3A examines a generic scenario that transitions to a pool cooling configuration. This case was originally conceived as a representation of single-division response with pump flow throttling, but it is equally useful to describe conservatively the minimum risk achievable under to NRC-24-0033 Page 6 full two-division response when operators elect to shut off an entire division based on early indications of acceptable RPV level and pressure. Case 3B is a plausible variation of Case 3A that proceeds to torus cooling through only two strainers. Risk is increased because more debris collects on less active strainer surface area, but the Case 3B risk is not dramatically higher and it does not acknowledge that operators can rotate flow to any pump that has been previously secured or throttled.

Case 4A provides a hypothetical, but plausible, scenario where pumps are sequentially secured to diagnose break size and eventually reach two-strainer suppression pool cooling. Case 4A provides the highest time resolution of the eight cases analyzed to illustrate that operators are given the flexibility and training to monitor and respond to the symptoms of each accident scenario.

The most important finding of comparing Case 3A, 3B, and 4A for suppression pool cooling is that risks are remarkably uniform, despite variations in flow rates and timing. Factors applied to combine one-division and two-division risks, and mitigate the effects of penetration Min-K' debris are discussed elsewhere. Risk results presented in Table STSB-RAI-11.1 have not been modified. Additional insights are provided in SERCO-REP-DTE-22929-02, Rev. 0, Sensitivity Study of Fermi Unit 2 Risk-Informed Core Damage Frequency, May 2024.

to NRC-24-0033 Page 9 STSB-RAI-12 The baseline configuration assumed for the risk-informed analysis is the single train runout case.

This case appears to represent an unlikely scenario that may result in earlier scenario failure but also result in lower risk values than the more likely single train suppression pool cooling (SPC) case. The submittal states that the SPC case is the design basis case and that it results in higher risk than the baseline (single train runout) case. Justify using the runout case as the baseline in the risk-informed analysis instead of the more likely SPC mode. Justify using the risk values from the single train runout case for comparison against the regulatory acceptance criteria or provide CDF values for the SPC and baseline cases including pump failure probabilities.

DTE Response:

The LAR Supplement provided with DTE RAI responses examines and incorporates risk contributed by the SPC mode. Comparative risk results presented in Table STSB-RAI-11.1 (see response to STSB-RAI-11) show that baseline risk from flow history Case 2A (one-division LPCI runout) and flow history Case 3A (initial three-strainer pool cooling) are nearly identical.

Elective operator actions needed to enter two-strainer pool cooling mode are examined in flow history Case 3B and Case 4A. Although two-strainer SPC is the desired end-state of a LOCA response, risks shown for flow history Case 3B and Case 4A will not be incurred automatically because operators monitoring pump status have the option to turn secured equipment back on to manage the effects of debris accumulation.

As explained in the LAR, the 10% probability of losing an entire division of emergency recirculation equipment is a conservative estimate that accounts for concurrent on-demand failure of multiple pumps.

to NRC-24-0033 Page 10 STSB-RAI-14 On Page 75 of 94 (PDF pg. 147) of the Serco calculation a discussion regarding the defense-in-depth-philosophy is provided. The discussion does not include the information specified in the guidance of RG 1.174. Provide a defense-in-depth discussion that follows the guidance of RG 1.174 or an equivalent alternate discussion.

DTE Response:

Revision 3 of Regulatory Guide 1.174 (p.4/53) presents up-to-date defense-in-depth guidance using precise language to assure consistent interpretation and implementation of the defense-in-depth philosophy. Revision 3 contains significant changes including expansion of the guidance on the meaning of, and the process for, assessing defense-in-depth considerations.

Assessments of the nature and impact of proposed licensing basis changes should consider defense-in-depth attributes, including success criteria and equipment functionality, reliability, and availability to assure that maintenance of sufficient defense-in-depth is reasonably assured (p. 6/53).

In risk-informed decision-making, licensing basis changes are expected to meet a set of five key principles, including Principle 2: The proposed licensing basis change is consistent with the defense-in-depth philosophy (p. 8/53).

Section 2.1.1 of RG 1.174 explains that the intent of the key defense-in-depth principle of risk-informed decision-making is to ensure that any impact of the proposed licensing basis change on defense in depth is fully understood and addressed and that consistency with the defense-in-depth philosophy is maintained. The proposed licensing basis change requests an amendment allowing Fermi-2 to operate with an acceptable risk of generating and transporting more Min-Ktm insulation debris and more miscellaneous debris than currently specified by strainer design basis maximum loads. With respect to the Integrated Evaluation of the Defense-In-Depth Considerations (RG 1.174, Section 2.1.1.4, p. 22/53), the Fermi-2 LAR concludes that no changes are made to the defense-in-depth philosophy maintained at the plant and no changes are made to the functionality, reliability, or availability of any systems defined as providing defense in depth under the current licensing conditions. The Fermi-2 LAR is, in every substantive respect, analogous to risk-informed LARs previously approved for high-fiber pressurized water reactors having an acceptable risk of generating and transporting more debris than specified in their strainer design basis maximum loads.

Fermi acknowledges that the seven considerations for evaluating how a proposed licensing basis change impacts defense in depth, defined in Sub-Section 2.1.1.2 of Section C, Staff Regulatory Guidance, of RG 1.174, Rev. 3 (p. 14/53), are not explicitly discussed in Section 6.2, Consistent with Defense-in-Depth Philosophy, of the Serco calculation, which is Attachment 3 of the LAR for the DTE Fermi-2 Risk Informed ECCS Evaluation. However, each of the seven considerations is identified within the Fermi-2 LAR in Attachment 1-1, Section 4.2, Regulatory to NRC-24-0033 Page 11 Guidance, Sub-Section 2 as necessary elements of the Defense in Depth evaluation. To supplement the statements provided in Section 4.2 of Attachment 1-1 of the Fermi-2 LAR, Table STSB-RAI-14.1 (below) provides additional discussion on the seven (7) defense-in-depth considerations that are to be evaluated for potential impact by the proposed change, consistent with the guidance in Section 2.1.1 of RG 1.174 Rev. 3. Table STSB-RAI-14.1 includes the following columns:

1. Defense in Depth Consideration Number and Description

2. Discussion from Fermi-2 LAR found in Attachment 1-1, Section 4.2

3. Supplemental Defense-In-Depth Discussion In reading Table STSB-RAI-14.1, it is understood that generic references to the system apply to all of the defense-in-depth systems described in Section 6.2 of the Serco calculation (p. 75 of 94). No identified defense-in-depth systems are vulnerable to the additional quantities of design-basis debris sources addressed by the LAR (the licensing change) because no new accident initiators, hazards, phenomenology, or sequences are introduced by the excess debris, and because the identified defense-in-depth systems do not draw water from the suppression pool that may contain post-LOCA debris. Residual Heat Removal strainers and Core Spray strainers are not evaluated as defense-in-depth systems, because they are challenged by additional quantities of the design-basis debris sources, and their risk of failure is the subject of the Regulatory Guide 1.174, Rev. 3 risk quantification. The generic evaluation that The defense-in-depth system is not affected by additional quantities of the design-basis debris sources addressed by the licensing change. applies to many of the seven defense-in-depth considerations and their subparts itemized in Table STSB-RAI-14.1.

Itemized descriptions of Fermi-2 systems providing defense-in-depth, found in Section 6.2 of the Serco calculation (p. 75 of 94), and the information provided in Table STSB-RAI-14.1 below demonstrate and support the conclusion that The proposed change is consistent with the Fermi-2 defense-in-depth philosophy, which is stated in Section 4.2, Regulatory Guidance, Sub-Section 2, The proposed change is consistent with a defense-in-depth philosophy, of LAR Attachment 1-1.

to NRC-24-0033 Page 12 Table STSB-RAI-14.1. Fermi-2 Supplemental Defense-In-Depth Discussion.

DEFENSE-IN-DEPTH CONSIDERATION DESCRIPTION (REF.

SECTION 2.1.1.2 OF RG 1.174)

DISCUSSION FROM FERMI-2 LAR IN ATTACHMENT 1-1, SECTION 4.2 (PDF P. 23 OF 274)

SUPPLEMENTAL DEFENSE-IN-DEPTH DISCUSSION (E.G., FROM SERCO CALCULATION IN ATTACHMENT 3 OF FERMI-2 LAR)

1. Preserve a reasonable balance among the layers of defense Because of the limited scope of the proposed change, a reasonable balance is preserved among prevention of core damage, prevention of containment failure, and consequence mitigation. Although some scenarios are assigned by analysis to core damage, the proposed change does not adversely affect prevention of containment failure.

The following discuss the four (4) topics associated with preserving a reasonable balance among the layers of defense for Consideration #1:

1. Robust plant design to survive hazards and minimize challenges that could result in the occurrence of an event. Allowing the additional identified insulation and miscellaneous debris (tags/labels) to remain installed in containment does not result in an increased likelihood of initiating events or create new significant initiating events. The presence of additional debris does not change the phenomenology or the maximum severity of events for which multiple plant systems are designed to mitigate and operators are trained to respond to.

2. Prevention of a severe accident (core damage) if an event occurs. The proposed change does not significantly impact the overall availability and reliability of SSCs providing the safety functions that prevent plant challenges from progressing to core damage. Given the low likelihood for unavailability of one or more ECCS suction strainers, Section 6.2 of Attachment 3 of the Fermi-2 LAR discusses proceduralized methods to maintain the Inventory Control function (e.g., multiple proceduralized options to align alternate RPV injection from clean water suction sources) (LAR PDF p. 148 of 274). In addition, given the low likelihood for loss of the Suppression Pool Cooling (SPC) mode of RHR caused by loss of RHR pump suction from the suppression pool, the Shutdown Cooling (SDC) mode of RHR and Primary Containment Venting remain available as proceduralized backup systems for the Containment Heat Removal function as credited in the Fermi-2 PRA. SDC and Primary Containment Venting are not impacted by post-LOCA debris because they are not dependent on the suppression pool suction strainers (LAR PDF p. 124 of 274). The availability of backup proceduralized mitigation methods, supported by operator training, help prevent plant challenges from progressing to core damage.

to NRC-24-0033 Page 13 DEFENSE-IN-DEPTH CONSIDERATION DESCRIPTION (REF.

SECTION 2.1.1.2 OF RG 1.174)

DISCUSSION FROM FERMI-2 LAR IN ATTACHMENT 1-1, SECTION 4.2 (PDF P. 23 OF 274)

SUPPLEMENTAL DEFENSE-IN-DEPTH DISCUSSION (E.G., FROM SERCO CALCULATION IN ATTACHMENT 3 OF FERMI-2 LAR)

(continued)

3. Containment of the source term if a severe accident occurs. The proposed change does not impact the overall containment function with respect to mitigating the release of source terms.

As mentioned above, SDC and Primary Containment Venting remain available as proceduralized backup systems for the Containment Heat Removal function (e.g., prevention of catastrophic containment failure) as credited in the Fermi-2 PRA. For scenarios with successful Primary Containment Venting (e.g., via the wetwell vent path), any radionuclide source terms will be "scrubbed" though the suppression pool water.

4. Protection of the public from any releases of radioactive material. The proposed change does not reduce the effectiveness of the emergency preparedness program, including the ability to detect and measure releases of radioactivity, notify offsite agencies and the public, and shelter or evacuate the public as necessary.

2. Preserve adequate capability of design features without an overreliance on programmatic activities as compensatory measures Over-reliance on programmatic activities as compensatory measures associated with the change in the licensing basis is avoided. The Defense-in-Depth discussion in Attachment 3 identifies numerous operator actions that could be implemented, but which are not applied in the risk quantification.

These actions (response measures) are based on existing EOPs, operator training, and physical plant design.

Supplemental programmatic activities such as inspections and monitoring are discussed in as Performance Monitoring mechanisms.

The proposed change preserves adequate capability of design features without an overreliance on programmatic activities as compensatory measures. Based on the low calculated risk impact, the proposed change does not require any compensatory measures.

The plant mitigation features and strategies discussed in Section 6.2 and Table 6-1 of of the Fermi-2 LAR represent additional layers of defense-in-depth that are not based on programmatic activities. These defense-in-depth options are not considered explicit compensatory measures because they are not credited numerically to reduce analyzed risk.

to NRC-24-0033 Page 14

3. Preserve system redundancy, independence, and diversity commensurate with the expected frequency and consequences of challenges to the system, including consideration of uncertainty Because of the limited scope of the proposed change, system redundancy, independence, and diversity are preserved commensurate with the expected frequency and uncertainties, and consequences of challenges to the system. No new risk contributors are introduced by the proposed change.

System redundancy, independence, and diversity are preserved based on the following evaluations for this consideration:

(1) Is the proposed licensing basis change consistent with the assumptions in the plants safety analysis?. The presence of additional debris analyzed in the LAR does not affect the plants safety analysis. No new debris types or untested debris combinations are introduced by the licensing change. The Fermi-2 LAR supports the conclusion that post-LOCA mitigation response effectiveness is not reduced by the additional debris and remains aligned with the plant's design criteria based on the discussion below.

(2) Does the proposed licensing basis change increase the frequency of challenges to the plant resulting from failure of the defense-in-depth system? The presence of additional debris analyzed in the LAR does not increase the frequency of challenges to the plant resulting from failure of the defense-in-depth system, because the reliability of the defense-in-depth system is not affected by the licensing change (see next item). Given no postulated change in the frequency of challenges, the risk impact is not significant based on the discussion below.

(3) Does the proposed licensing basis change decrease the reliability or availability of the system to perform its intended functions? The presence of additional debris analyzed in the LAR does not decrease the reliability or availability of the system to perform its intended functions, because the reliability and availability of the system are not affected by additional debris based on the discussion below.

Similar to the discussion for Consideration #1, given the low likelihood for unavailability of one or more ECCS suction strainers due to the presence of additional debris analyzed in the LAR, Section 6.2 of Attachment 3 of the Fermi-2 LAR discusses proceduralized methods to maintain the Inventory Control function (e.g., multiple proceduralized options to align alternate RPV injection from clean water suction sources) (LAR PDF p. 148 of 274). In addition, given the low likelihood for loss of the Suppression Pool Cooling (SPC) mode of RHR due to loss of RHR pump suction from the suppression pool, the Shutdown Cooling (SDC) mode of RHR and Primary Containment Venting remain as proceduralized backup systems for the Containment Heat Removal function as credited in the Fermi-2 PRA. SDC and Primary Containment Venting are not impacted by post-LOCA debris because they are not dependent on the suppression pool suction strainers (LAR PDF p. 124 of 274).

The availability of backup proceduralized mitigation methods, supported by operator training, help preserve system redundancy, independence, and diversity commensurate with the expected frequency and consequences of challenges to the system, even when including consideration of uncertainty.

to NRC-24-0033 Page 15

4. Preserve adequate defense against potential CCFs The limited scope of the proposed change does not reduce defenses against potential common-cause failures. No potential new common-cause failure mechanisms are introduced by the proposed change.

The proposed change preserves adequate defense against potential common cause failures (CCFs) based on evaluating if the proposed change could do any of the following:

(1) Introduce a new potential CCF cause or event for which a defense is not in place. The presence of additional debris analyzed in the LAR does not introduce a new potential CCF cause or event for which a defense is not in place based on the discussion below.

(2) Increase the probability or frequency of a cause or event that could cause simultaneous multiple component failures. The presence of additional debris analyzed in the LAR does not result in a realistic increase in the probability or frequency of a cause or event that could cause simultaneous multiple component failures based on the discussion below.

(3) Introduce a new coupling factor for which a defense is not in place. The presence of additional debris analyzed in the LAR does not introduce a new coupling factor for which a defense is not in place based on the discussion below.

(4) Weaken or defeat an existing defense against a cause, event, or coupling factor. The presence of additional debris analyzed in the LAR does not introduce a new coupling factor for which a defense is not in place based on the discussion below.

The analytic assumption of common cause, immediate, ECCS suction failure induced by 1/8 of fiber on any single strainer is not considered a realistic outcome of debris accumulation.

System redundancy (multiple strainers and pumps for independent divisions of RHR and Core Spray) provide defense against blockage of multiple strainers causing an actual CCF of all ECCS systems that rely on torus suction. In addition, as discussed in Considerations #1 above, the availability of backup proceduralized methods to maintain the Inventory Control and Containment Heat Removal functions preserves adequate defense in depth against the low likelihood for any potential CCF of the ECCS suction strainers.

to NRC-24-0033 Page 16

5. Maintain multiple fission product barriers Because of the limited scope of the proposed change, the independence of existing barriers is not degraded.

The proposed change maintains multiple fission product barriers (i.e., the fuel cladding, reactor coolant system pressure boundary, and containment).

The evaluation of the proposed licensing basis change should demonstrate that the change does not:

(1) Create a significant increase in the likelihood or consequence of an event that simultaneously challenges multiple barriers. The presence of additional debris analyzed in the LAR does not create a significant increase in the likelihood or consequence of an event that simultaneously challenges multiple barriers, because the performance of fission product barriers is not affected by additional debris addressed by the licensing change.

(2) Introduce a new event that would simultaneously impact multiple barriers. The presence of additional debris analyzed in the LAR does not introduce a new event that would simultaneously impact multiple barriers. All debris types and strainer failure modes are previously analyzed by the strainer design basis.

To evaluate this consideration, the licensee should consider achieving the following objectives to ensure that the proposed licensing basis change remains consistent with the defense-in-depth philosophy.

(a) The change does not result in a significant increase in the frequency of existing challenges to the integrity of the barriers. The presence of additional debris analyzed in the LAR does not result in a significant increase in the frequency of existing challenges to the integrity of the barriers (e.g., no increase in the frequency of interfacing system loss of coolant accidents), because the risk of increased core damage (which is determined by initiating event frequency) is analyzed to be low to very low (RG1.174 Risk Region II to III).

(b) The change does not significantly increase the failure probability of any individual barrier.

The presence of additional debris analyzed in the LAR does not significantly increase the failure probability of any individual barrier (e.g., no postulated or calculated increase in the failure probability of the RCS or containment), because the failure probability of barriers and defense-in-depth systems does not depend on the presence of the additional debris.

(c) The change does not introduce new or additional failure dependencies among barriers that significantly increase the likelihood of failure compared to the existing conditions.

The presence of additional debris analyzed in the LAR does not introduce new or additional failure dependencies among barriers that significantly increase the likelihood of failure compared to the existing conditions (e.g., no new failure modes that could to NRC-24-0033 Page 17 cause a failure of the RCS pressure boundary that results in an increased likelihood for consequential failure of the containment due to overpressure), because no previously unanalyzed conditions are introduced by the additional debris.

6. Preserve sufficient defense against human errors Because of the limited scope of the proposed change, new sources of human error are not introduced. As discussed in LAR, multiple operator actions exist that are not applied in the quantification of incremental risk that remain as viable mitigation resources.

The proposed change preserves sufficient defense against human errors.

The proposed change does not signifiantly increase the potential for or create new human errors that might adversely impact one or more layers of defense. As discussed in Section 6.2 of Attachment 3 of the Fermi-2 LAR, The Fermi-2 operators are trained to identify subsequent indications of ECCS suction strainer blockage (e.g., ECCS pump flow rate or discharge pressure lower than expected for present plant conditions) and implement appropriate mitigation actions (e.g., align alternate RPV injection as discussed in the next paragraph) in a timely manner. (LAR PDF p. 150 of 274).

Given that the Fermi-2 operators are trained to recognize and diagnose symptoms for potential ECCS suction strainer clogging events and implement appropriate mitigation actions, the proposed change is evaluated to not (1) create new human actions that are important to preserving any of the layers of defense for which a high reliability cannot be demonstrated, or (2) significantly increase the probability of existing human errors by significantly affecting performance shaping factors, including mental and physical demands and level of training.

7. Continue to meet the intent of the plants design criteria The intent of the plant's design criteria is maintained. Other than addressing a portion of the potential Min-K insulation debris and miscellaneous debris using a risk-informed method, which defines the limited scope of the proposed change, post-LOCA mitigation response is not affected by the additional debris and remains aligned with the plant's design criteria.

The proposed change continues to meet the intent of the plants design criteria.

The Fermi-2 strainer design includes both Min-K insulation debris and miscellaneous debris (tags/labels). The proposed change acknowledges and quantifies the risk posed by increased debris loadings but does not introduce new debris types or untested hazards that are inconsistent with the strainer design criteria.

Similarly, Fermi-2 design features, EOPs, and operator training anticipate potentially degraded torus suction caused by debris accumulation on strainers. The licensing change does not introduce new, novel, or unanticipated hazards or accident sequences that are inconsistent with the plant design criteria.

The Fermi-2 LAR supports the conclusion that post-LOCA mitigation response effectiveness is not reduced by the additional debris and remains aligned with the plant's design criteria.

to NRC-24-0033 Fermi 2 NRC Docket No. 50-341 Operating License No. NPF-43 Revisions to LAR Attachment 2

to NRC-23-0020 Fermi 2 NRC Docket No. 50-341 Operating License No. NPF-43 Requests for Exemptions for DTE Fermi Risk-Informed ECCS Evaluation 2-1 General 2-2 Request for Exemption from 10 CFR 50.46(a)(1) 10CFR50.46(d) 2-3 Request for Exemption from GDC-35*

2-4 Request for Exemption from GDC-38*

• NOTE: Attachment 2-3, Request for Exemption from GDC-35, and Attachment 2-4, Request for Exemption from GDC-38, are not affected by NRC RAIs and consequently are not revised.

-1 to NRC-24-0033 Page 1 2-1 General Introduction In support of the DTE Electric Company (DTE) risk-informed approach to addressing the impact of isolated debris sources on the Emergency Core Cooling System (ECCS) suppression pool strainer performance at Fermi Unit 2 (Fermi 2), Attachment 2 Sections 2-2 through 2-4 provide DTE requests for exemptions under 10CFR50.12 from certain requirements in 10CFR50.46 and 10CFR50 Appendix A, General Design Criteria (GDC). The exemption requests complement a proposed license amendment request (LAR) provided in Attachment 1 to this letter, proposing methodology changes that will be incorporated in the Fermi 2 Updated Final Safety Analysis Report (UFSAR) based on NRC acceptance of the risk-informed method and results. No changes are requested for the Technical Specifications under this proposed LAR. Note that acronyms used in this Attachment are defined in Attachment 1.

Specifically, "the proposed change addressed by the LAR and the accompanying exemptions is to apply a risk-informed method rather than a strictly deterministic method to quantify the risk associated with additional debris sources identified in Attachment 1 and to establish a high probability of success for performance of ECCS in accordance with the ECCS cooling performance design addressed in 10CFR50.46(d).based on guidance in Regulatory Guide (RG) 1.174.

It may be inferred by regulatory precedent that licensees are required to demonstrate compliance with the regulations cited below using a bounding calculation or other deterministic method.

DTE requests, in Attachment 1, relief from a solely deterministic method in order to enable the use of the risk-informed method to demonstrate acceptable ECCS and containment heat removal performance with regard to the effects of LOCA generated debris. While demonstrating very low risk, as defined by applicable guidance, the risk-informed analysis does not strictly satisfy all elements of the cited regulations, e.g., 10 CFR 50.46(a)(1)(i), because the NRC has interpreted these regulations as requiring a deterministic approach and bounding calculation to show compliance with ECCS and CSS performance criteria. Thus, the exemptions are deemed necessary to implement the risk-informed analysis method in the Fermi 2 licensing basis.

The exemptions from regulation are requested only for the scope of debris effects that exceed the quantities that have been shown to be acceptable using deterministic design basis analysis, which documents successful ECCS performance on a deterministic basis. Note that in the balance of this document, debris quantities that exceed those shown to be acceptable based on deterministic analysis are referred to as debris effects, additional debris, or similar.

Specific exemption requests, pertaining to requirements that concern the Emergency Core Cooling System (ECCS) and the containment cooling mode of the ECCS function for core

-1 to NRC-24-0033 Page 2 cooling and containment heat removal following a postulated loss of cooling accident (LOCA),

are provided as follows:

• Attachment 2-2, Request for Exemption from 10CFR50.46(a)(1) 10CFR50.46(d)

• Attachment 2-3, Request for Exemption from GDC 35

• Attachment 2-4, Request for Exemption from GDC 38 Approval of the exemptions will allow use of a risk-informed method accounting for the probabilities and uncertainties associated with mitigation of the effects of debris following postulated LOCAs. The method evaluates the effects on strainer blockage and RHR/CS pump NPSH resulting from post-LOCA debris. In order to confirm acceptable performance, the risk associated with additional debris sources is evaluated to include the failure mechanisms associated with loss of core cooling and strainer blockage.

Each separate Attachment 2-2 through 2-4 identifies the applicable rule from which exemption is requested, the regulatory requirements involved, the purpose of the request, and the technical basis and justification for the exemption request, including the presence of special circumstances pursuant to 10CFR50.12(a). The requested exemptions are part of a risk-informed approach to address the impact of retaining the additional debris sources in the licensing basis. The risk informed approach is designed to be consistent with the guidance in Regulatory Guide (RG) 1.174, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis."

The scope of the exemptions applies for the limited debris effects addressed in the risk-informed element of the Fermi RoverD methodology as described in Attachment 3 (SERCO-REP-DTE-22609-02, Fermi-2 Risk Informed ECCS Strainer Performance Evaluation) that was used to respond to a series of corrective actions regarding identified debris sources exceeding the existing Fermi strainer design basis. Fermi 2 retains the deterministic methodology in the current design basis for the existing debris loads but modifies the license to accept the identified debris sources exceeding the current design basis using a risk informed methodology following the guidance in RG 1.174.

The DTE risk-informed approach addresses the five key principles in RG 1.174 for risk informed decision-making. The resulting risk metrics, i.e. changes in Core Damage Frequency (CDF) and Large Early Release Frequency (LERF), associated with these debris source concerns are used to determine whether plant modifications are warranted to ensure acceptable strainer performance. The requested exemptions support this approach.

The DTE risk approach is similar in method to the STP pilot study (ML17038A223) that suggests additional debris loads, or variations in debris loads, identified at Fermi 2 may be dispositioned by demonstrating low or very-low risk significance, as defined by RG 1.174 in

-1 to NRC-24-0033 Page 3 terms of incremental change to core damage frequency (CDF) and incremental change to large early release frequency (LERF).

Based on the results of analyses for Fermi 2, documented in Attachment 3, the risk from the effects of additional debris is within Region III, "Very Small Changes," of RG 1.174. Thus, no additional physical changes to the facility or changes to the operation of the facility are proposed.

Background and Overview Fermi LAR Attachment 1 is "License Amendment Request for DTE Fermi Risk Informed ECCS Evaluation". Attachment 1 is designated as Attachment 1-1.

Fermi LAR Attachment 1 has an Attachment 1-2, "UFSAR Page Markups", designated as "Attachment 1-2".

Fermi LAR Attachment 2 (this document) is "Requests for Exemptions for DTE Fermi Risk-Informed ECCS Evaluation". Attachment 2 is designated as "Attachment 2-1".

Fermi LAR Attachment 2 (this document) includes Attachments 2-2, 2-3, and 2-4 that address the deterministic requirements in 10CFR50.46, GDC 35 and 38 for which exemptions are proposed. Specifically:

• -2, Request for Exemption from 10CFR50.46(a)(1)10CFR50.46(d)

• -3, Request for Exemption from GDC 35

• -4, Request for Exemption from GDC 38 is SERCO-REP-DTE-22609-02, R0, SERCO-REP-DTE-22609-02, Fermi-2 Risk Informed ECCS Strainer Performance Evaluation.

Special Circumstances Common to Proposed Exemptions to 10CFR50.46(a)(1) 10CFR50.46(d), GDC 35 and GDC 38 10CFR50.12(a)(2)(ii) applies:

Application of the regulation in the particular circumstances would not serve the underlying purpose of the rule or is not necessary to achieve the underlying purpose of the rule.

An objective of each of the regulations (10CFR50.46(a)(1)10CFR50.46(d), GDC 35 and GDC 38) for which an exemption is proposed is to maintain low risk to the public health and safety through functions that are supported by the ECCS, including the suppression pool as a water source. By regulatory precedent, licensees are required to demonstrate this capability by the use of a bounding calculation or other deterministic method. The supporting analysis

-1 to NRC-24-0033 Page 4 demonstrates that a risk-informed approach to ECCS performance is consistent with the Commission's Safety Goals for nuclear power plants and supports operation of those functions with a high degree of reliability.

Consequently, the special circumstances described in 10CFR50.12(a)(2)(ii) apply to each of the exemptions proposed by DTE.

10CFR50.12(a)(2)(iii) applies:

Compliance would result in undue hardship or other costs that are significantly in excess of those contemplated when the regulation was adopted or that are significantly in excess of those incurred by others similarly situated.

In order to meet a deterministic strainer design limit for debris loading, Min-K insulation in the penetrations at Fermi 2 would need to be removed and replaced. In terms of potential radiation expose to personnel, the affected penetrations are in very challenging drywell locations that would require significant time and personnel protections to access. The total dose estimated to be expended for Fermi 2 in support of insulation replacement in the penetrations is expected to be well in excess of industry norms and would not be consistent with plant ALARA objectives.

Any materials removed from containment must be disposed of as potentially contaminated waste. If a suitable insulation could be found for replacing Min-K in very small gaps, that material would also be treated as waste at the end of plant life, thus, doubling the amount of penetration-specific waste to achieve a very small risk reduction (as defined by RG 1.174).

The dose considerations discussed above demonstrate that compliance would result in substantial personnel exposure that is not commensurate with the expected safety benefit based on the results showing that the risk associated with the potential debris concerns is in Region III in RG 1.174. Consequently, the special circumstances described in 10CFR50.12(a)(2)(iii) apply to each of the exemptions proposed by DTE.

Environmental Consideration Pursuant to the requirements of 10CFR51.41 and 10CFR51.21, "Criteria for and identification of licensing and regulatory actions requiring environmental assessments," the discussion in the sections beginning with Identification of the Proposed Action, below, is provided. As demonstrated in the discussion below, the requested exemptions qualify for a categorical exclusion of 10CFR51.22. However, if the NRC determines that an environmental assessment is necessary, the information that follows will support a finding of no significant impact. The assessment applies to all of the proposed exemptions.

Identification of the Proposed Action The proposed exemptions allow for use of a risk-informed approach to evaluate the residual risk associated with potential debris sources greater than those currently evaluated within the strainer

-1 to NRC-24-0033 Page 5 design basis, i.e., those concerns that have not been fully addressed using deterministic methods.

The proposed exemptions are for the purpose of amending the license basis for acceptable mitigation of the effects of debris during low-pressure or containment cooling modes of the ECCS following postulated LOCAs. Approval of the proposed exemptions would complement approval of the methodology addition to be incorporated in the UFSAR, as provided in to this letter, for implementation of the risk-informed method at DTE Fermi 2.

Need for the Proposed Action In the Commission's Policy Statement on "Use of Probabilistic Risk Assessment Methods in Nuclear Regulatory Activities", the Commission stated that "the use of PRA technology in NRG regulatory activities should be increased to the extent supported by the state-of-the-art in PRA methods and data and in a manner that complements the NRC's deterministic approach" and that is consistent with traditional defense-in-depth concepts.

The intent of the Commission's Policy Statement is to use the PRA to further understand the risk associated with a proposed change for the purpose of removing unnecessary conservatism associated with regulatory requirements in order to focus attention and allocation of resources to areas of true safety significance.

To implement the Commission Policy Statement, the NRC issued RG 1.174 to provide guidance on an acceptable approach to risk-informed decision-making, based on a set of five key principles. The proposed action (approval of stated exemptions) is needed to allow DTE to use a riskinformed method to address the potential for insulation and other debris generated in the event of a postulated LOCA within the containment to impact acceptable operation for ECCS and challenge the ability of ECCS to provide adequate long-term core cooling. The proposed exemptions are consistent with the key principle in RG 1.174 requiring the proposed change to meet current regulations unless explicitly related to a requested exemption.

Environmental Impacts Consideration The proposed exemptions have been evaluated and determined to result in no significant radiological environmental impacts associated with the implementation of the change. This conclusion is based on the following:

The proposed exemptions allow a risk-informed method for demonstrating that the design and licensing bases for the ECCS are not significantly impacted by debris effects. No physical modifications or changes to operating requirements are proposed for the site or facility, including any systems, structures and components relied upon to mitigate the consequences of a LOCA.

The intent of the proposed change is to quantify the risk associated with identified potential debris sources. This quantification, provided in the form of risk metrics using the guidance in RG 1.174, demonstrates that the risk is in Region III, "Very Small Changes," in RG 1.174.

Therefore, the proposed exemptions support a change that represents a very small increase in

-1 to NRC-24-0033 Page 6 Large Early Release Frequency (LERF) that corresponds to an insignificant impact on the environment.

Based on the results of the risk-informed method demonstrating that the increases in risk are very small, the proposed exemptions have a negligible effect on accident probability, and adequate assurance of public health and safety is maintained. The proposed exemption does not involve any changes to the facility or facility operations that could create a new or significantly affect a previously analyzed accident or release path, and therefore would result in no significant changes in the types or quantities of radiological effluents that may be released offsite. The proposed change does not affect the generation of any radioactive effluents and does not affect any of the permitted effluent release paths.

The proposed exemptions have no impact on requirements related to the integrity of the reactor coolant system piping or any other aspect related to the initiation of a LOCA. No physical modifications or changes to operating requirements are proposed for the facility, including any systems, structures and components relied upon to mitigate the consequences of a LOCA.

Therefore, the proposed exemption does not affect the probability of an accident initiator.

The proposed exemptions do not significantly impact a release of radiological effluents during and following a postulated LOCA. The design-basis LOCA radiological consequence analysis in the current licensing basis is a deterministic evaluation based on the assumption of a major rupture of the reactor coolant system piping and a significant amount of core damage as specified in RG 1.183. The current licensing basis analysis shows the resulting doses to the public and to control room and technical support center personnel are acceptable. The proposed change validates and does not change the input parameter values used in the radiological analysis.

Therefore, the proposed exemption does not affect the amount of radiation exposure resulting from a postulated LOCA.

The proposed exemptions do not involve any changes to the site property, physical changes to the facility, or changes to the operation of the facility. Therefore, there are no irreversible and irretrievable commitments of resources which would be involved in the proposed action should it be implemented. The risk-informed method requires a determination that the risk associated with the proposed change meets the Commission's safety goals. Therefore, the proposed action would not result in a significant increase in any radiological hazard beyond those events previously analyzed in the UFSAR. There will be no change to radioactive effluents that affect radiation exposures to plant workers and members of the public. Therefore, no significant changes or different types of radiological impacts are expected as a result of the proposed action. The proposed exemptions do not change the input parameter value used in the radiological analysis.

Therefore, the proposed change would not significantly increase the probability or consequences of an accident, and there will be no significant offsite impact to the public from approval of the proposed exemptions.

-1 to NRC-24-0033 Page 7 No additional physical modifications or changes to operating requirements are proposed for the facility, including any systems, structures and components relied upon to mitigate the consequences of a LOCA. Therefore, the proposed exemptions do not result in a significant increase in individual or cumulative occupational radiation exposure and will not cause radiological exposure in excess of the dose criteria for restricted and unrestricted access specified in 10 CFR Part 20.

The proposed exemptions do not involve any changes to non-radiological plant effluents or any activities that would adversely affect the environment. The proposed exemptions do not affect any procedures used to operate the facility, or any physical characteristics of the facility, systems, structures, and components. The proposed change only pertains to the licensing basis for components located within the restricted area of the facility, to which access is limited to authorized personnel. Therefore, the proposed exemptions do not create any significant non-radiological impacts on the environment in the vicinity of the plant.

Since implementation of the exemption requests, if approved, would result in no physical changes to the facility, there is no possibility of irreversible or irretrievable commitments of resources. Similarly, the proposed exemptions do not involve the use of any resources not previously considered by the NRC in its past environmental statements for issuance of the facility operating licenses or other licensing actions for the facility. As a result, the proposed exemptions do not involve any unresolved conflicts concerning alternative uses of available resources.

Alternatives The alternative to approval of these exemptions is compliance with the existing provisions in 10CFR50.46(a)(1)10CFR50.46(d) and the relevant GDCs. Compliance with 10CFR50.46(a)(1) 10CFR50.46(d) and the relevant GDCs would entail removal and disposal of significant amounts of insulation and installation of new insulation less likely to impact strainer performance in the event of a LOCA. As discussed below, the alternative would not be environmentally preferable, or cost justified.

The exemptions entail a very small risk, and correspondingly, an environmental impact which is so small that it is remote and speculative for environmental assessment purposes.

Removal and reinstallation of insulation would entail significant appreciable radiation exposures to workers. This option results in extensive modifications to the facility and significant occupational dose. Under the alternative action (strict compliance with applicable regulation),

replacement of insulation in penetrations would double the volume of radiologically contaminated waste associated with these reactor locations, because the waste would be handled first in compliance with regulation and then a second time during decommissioning. As such, the alternative is not environmentally preferable. Additionally, the anticipated very high cost of the

-1 to NRC-24-0033 Page 8 installation replacement is not justified in light of the very small risk associated with the risk-informed exemptions.

Categorical Exclusion Consideration DTE has evaluated the proposed exemptions against the criteria for identification of licensing and regulatory actions requiring environmental assessments in accordance with 10CFR51.21 and determined that the proposed exemptions meet the criteria and is eligible for categorical exclusion as set forth in 10CFR51.22, "Criterion for categorical exclusion; identification of licensing and regulatory actions eligible for categorical exclusion or otherwise not requiring environmental review," paragraph (c)(9).

This determination is based on the fact that these exemption requests arise from requirements under 10CFR50 with respect to the installation or use of a facility component located within the restricted area, as defined in 10CFR20, specifically, to authorize a change to the licensing basis for the ECCS as it relates to acceptable suppression pool strainer performance following a postulated LOCA. The proposed exemptions have been evaluated to meet the following criteria under 10CFR51.22(c)(9).

(i) The exemption involves no significant hazards consideration.

An evaluation of the three criteria set forth in 10CFR50.92(c) as applied to the exemptions is provided below. The evaluation is consistent with the no significant hazards consideration determination provided in Attachment 1 in support of the LAR.

(1)

The proposed change does not involve a significant increase in the probability or consequences of an accident previously evaluated.

Approval of the proposed exemptions and accompanying license amendment request would allow the results of a risk-informed evaluation to be included in the UFSAR that concludes the ECCS will operate with a high probability following a LOCA when considering the impacts and effects of debris accumulation on the ECCS strainers following loss of coolant accidents (LOCAs).

The proposed change does not implement any physical changes to the facility or any structures, systems and components (SSCs), and does not implement any changes in plant operation. The proposed change confirms that required SSCs supported by the ECCS suction strainers will perform their safety functions with a high probability, and does not alter or prevent the ability of SSCs to perform their intended function to mitigate the consequences of an accident previously evaluated within the acceptance limits. The safety analysis acceptance criteria in the UFSAR continue to be met for the proposed change. The proposed change does not affect initiating events because it addresses existing initiating events, i.e., loss of coolant accidents. The proposed

-1 to NRC-24-0033 Page 9 change does not significantly affect the operation of the containment systems to ensure that there is a large margin between the temperature and pressure conditions reached in the containment and those that would lead to failure so that there is a high degree of confidence that damage of the containment cannot occur.

The calculated risk associated with the proposed change is very small and in Region III as defined by RG 1.174, for both CDF and LERF. As required in the guidance of RG 1.174, there is substantial safety margin and defense in depth that provide additional confidence that the design basis functions are maintained.

Therefore, the proposed change does not involve a significant increase in the probability or consequences of any accident previously evaluated in the UFSAR.

(2)

The proposed change does not create the possibility of a new or different kind of accident from any accident previously evaluated.

The proposed change is a risk-informed analysis of debris effects from accidents that are already evaluated in the DTE UFSAR; no new or different kind of accident is being evaluated. The change neither installs nor removes any plant equipment, nor alters the design, physical configuration, or mode of operation of any plant structure, system, or component. The proposed change does not introduce any new failure mechanisms or malfunctions that can initiate an accident. The proposed change does not introduce failure modes, accident initiators, or equipment malfunctions that would cause a new or different kind of accident.

Therefore, the proposed change does not create the possibility for a new or different kind of accident from any accident previously evaluated.

(3)

The proposed change does not involve a significant reduction in a margin of safety.

The proposed change does not involve a change in any functional requirements, the configuration, or method of performing functions of plant SSCs. The effects from a full spectrum of LOCAs, including double-ended guillotine breaks for all piping sizes up to and including the largest pipe in the reactor coolant system, are analyzed. Appropriate redundancy and consideration of loss of offsite power and worst-case single failure are retained, such that defense-in-depth is maintained.

Application of the risk-informed methodology showed that the increase in risk from the contribution of debris effects is very small as defined by RG 1.174 and that there is adequate defense in depth and safety margin. Consequently, DTE determined that the suppression pool ECCS strainers will continue to support the ability of safety related components to perform their design functions when the effects of debris are considered. The proposed change does not alter the manner in which safety limits are determined or deterministic acceptance criteria associated

-1 to NRC-24-0033 Page 10 with a safety limit are fulfilled. The proposed change does not implement any changes to plant operation and does not significantly affect SSCs that respond to safely shutdown the plant and to maintain the plant in a safe shutdown condition. The proposed change does not significantly affect the existing safety margins in the barriers for the release of radioactivity. There are no changes to any of the safety analyses in the UFSAR. Therefore, the proposed change does not involve a significant reduction in a margin of safety.

(ii)

The proposed exemptions involve no significant change in the types or significant increase in the amounts of any effluents that may be released offsite.

No physical modifications or changes to operating requirements are proposed for the facility, including any systems, structures and components relied upon to mitigate the consequences of a LOCA. Approval of the exemptions requires the calculated risk associated with debris effects to meet the acceptance guidelines in RG 1.174, thereby maintaining public health and safety.

Therefore, there is no significant change in the types or significant increase in the amounts of any effluents that may be released offsite.

(iii)

The proposed exemptions involve no significant increase in individual or cumulative occupational radiation exposure.

No physical modifications or changes to operating requirements are proposed for the facility, including any systems, structures and components relied upon to mitigate the consequences of a LOCA. Therefore, with respect to installation or use of a facility component located within the restricted area there is no significant increase in individual or cumulative occupational radiation exposure as a result of granting the exemption requests. Approval of the exemption requests avoids significant occupational dose exposure and other occupational hazards associated with insulation removal, disposal, and replacement that would be performed to achieve compliance with existing regulations.

Based on the above, DTE concludes that the proposed exemptions meet the eligibility criteria for categorical exclusion set forth in 10CFR51.22(c)(9). Additional technical justification for this conclusion is provided on the basis that the guidance and acceptance criteria provided in RG 1.174 are satisfied as described in Attachment 3.

-2 Request for Exemption from 10CFR50.46(a)(1)10CFR50.46(d)

-2 to NRC-24-0033 Page 1 Request for Exemption from Certain Requirements of 10CFR50.46(a)(1)10CFR50.46(d)

1.

Exemption Request Pursuant to 10CFR50.12, DTE Electric Company (DTE) is submitting, this request for exemption from certain requirements of 10CFR50.46(a)(1)10CFR50.46(d), "..other requirements," as it relates to using specific deterministic methodology to evaluate the effects of debris on long-term core cooling.as specified in 10CFR50.46, "Acceptance criteria for emergency core cooling systems for light-water nuclear power reactors." 10CFR50.46(d) states:

The requirements of this section are in addition to any other requirements applicable to ECCS set forth in this part. The criteria set forth in paragraph (b), with cooling performance calculated in accordance with an acceptable evaluation model, are in implementation of the general requirements with respect to ECCS cooling performance design set forth in this part, including in particular Criterion 35 of appendix A.

A portion of 10 CFR 50.46(a)(1) is shown below with the "other properties" portion (for which exemption is requested) in bold italics.

(a)(1)(i) Each boiling or pressurized light-water nuclear power reactor fueled with uranium oxide pellets within cylindrical zircaloy or ZIRLO cladding must be provided with an emergency core cooling system (ECCS) that must be designed so that its calculated cooling performance following postulated loss-of-coolant accidents conforms to the criteria set forth in paragraph (b) of this section. ECCS cooling performance must be calculated in accordance with an acceptable evaluation model and must be calculated for a number of postulated loss-of-coolant accidents of different sizes, locations, and other properties sufficient to provide assurance that the most severe postulated loss-of-coolant accidents are calculated. Except as provided in paragraph (a)(1)(ii) of this section, the evaluation model must include sufficient supporting justification to show that the analytical technique realistically describes the behavior of the reactor system during a loss-of-coolant accident. Comparisons to applicable experimental data must be made and uncertainties in the analysis method and inputs must be identified and assessed so that the uncertainty in the calculated results can be estimated. This uncertainty must be accounted for, so that, when the calculated ECCS cooling performance is compared to the criteria set forth in paragraph (b) of this section, there is a high level of probability that the criteria would not be exceeded. Appendix K, Part II Required Documentation, sets forth the documentation requirements for each evaluation model. This section does not apply to a nuclear power reactor facility for which the certifications required under §50.82(a)(1) have been submitted.

The DTE risk-informed approach to addressing potential debris sources and adequate ECCS NPSH is consistent with the NRC staff safety evaluation of NEI 04-07 that discussed the modeling of strainer performance as follows:

-2 to NRC-24-0033 Page 2 While not a component of the 10CFR50.46 ECCS evaluation model, the calculation of sump (strainer) performance is necessary to determine if the sump and the residual heat removal system are configured properly to provide enough flow to ensure long-term cooling, which is an acceptance criterion of 10CFR50.46. Therefore, the staff considers the modeling of sump performance as the validation of assumptions made in the ECCS evaluation model. Since the modeling of sump performance is a boundary calculation for the ECCS evaluation model, and acceptable sump performance is necessary for demonstrating long-term core cooling capability (10CFR50.46(b)(5)), the requirements of 10CFR50.46 are applicable.

By the reference to the General Design Criteria, 10CFR50.46(d) incorporates their requirements into 10CFR50.46. For consistency with the proposed exemption to GDC 35, DTE proposes exemption to parts of 50.46(d) that require strict compliance to GDC 35. The result will be that the risk-informed methodology will be allowed rather than the currently required demonstration of mitigation capability by use of a bounding calculation or other deterministic method to model LOCA debris effects. DTE requests an exemption from that requirement in order to enable the use of a risk-informed method to demonstrate acceptable ECCS strainer performance and LOCA debris mitigation and to validate assumptions in the Emergency Core Cooling System (ECCS) evaluation model.

The scope of the requested exemption applies for limited debris effects addressed in the DTE RoverD methodology described in Attachment 3 that was used to respond to a corrective action associated with Min-K found in the drywell penetrations at Fermi 2. The LOCA break sizes and locations potentially generating an amount of Min-K debris exceeding the quantity established by the DTE original design basis testing and analysis are described in Attachment 3.

The key elements of the exemption request are:

1. It applies only to the effects of excess debris as described in Attachment 1.

2. It applies only for LOCA breaks that can generate and transport fiber debris that is not bounded by the DTE strainer failure criteria established in Attachment 1.

3. It applies to any LOCA break that can generate and transport fiber debris that is not bounded by strainer failure criteria and provided that the CDF and LERF associated with the break size remain in Region III of RG 1.174.

This exemption request is complemented by the accompanying License Amendment Request (LAR) (Attachment 1) seeking NRC approval of the changes to the Fermi 2 Updated Final Safety Analysis Report (UFSAR), to amend the licensing basis based on acceptable design of the ECCS suction strainers. The risk-informed method provides high probability assurance that, as calculated in the ECCS evaluation model (Attachment 3), ECCS operation, including strainer performance and debris mitigation, will be acceptable.

-2 to NRC-24-0033 Page 3

2. Regulatory Requirements Involved By regulatory precedent, licensees are required to demonstrate compliance with the relevant regulations by the use of a bounding calculation or other deterministic method. DTE seeks exemption to the extent that 10CFR50.46(d) invokes requirements for deterministic-only calculations or other analyses to address the debris concerns related to acceptable plant performance during core and containment cooling modes following a LOCA. The proposed changes to the licensing basis, submitted for NRC approval with Attachment 1, address the additional debris loads at the ECCS strainers for Fermi 2 on the basis that the associated risk is shown to meet the acceptance guidelines in Regulatory Guide (RG) 1.174 and that, in conjunction with the existing licensing basis, adequate safety is demonstrated.

This exemption request is for the purpose of allowing the use of a risk-informed method to demonstrate acceptable mitigation of the effects of debris following postulated loss of coolant accidents (LOCAs). The effects of LOCA debris have been evaluated, using deterministic methods, to meet the current licensing basis assumptions for analyzing the effects of post-LOCA debris blockage at the suction strainers; however, these evaluations have not been shown to fully address debris effects for the as-built, as-operated plant. The risk informed approach evaluates the risk associated with additional potential debris sources on ECCS strainer performance beyond those currently established in the strainer design basis, which employed deterministic methods. Based on confirmation of acceptable ECCS design as determined by the resulting risk meeting the acceptance guidelines in RG 1.174, the licensing basis for ECCS compliance with 10CFR50.46(d) is amended by approval of this exemption.

2.1 Evaluation of Impacts on the Balance of 10CFR50.46 and Appendix K to 10CFR50 The exemption request is intended to address ECCS cooling performance design as invoked by the need for documentation of other properties sufficient to provide assurance that the most severe postulated loss-of-coolant accidents are calculated in 10 CFR 50.46(a)(1)10CFR50.46(d) in reference to imposing the requirements of the General Design Criteria.

For the purposes of demonstrating the balance of the acceptance criteria of 10CFR50.46, the design and licensing basis descriptions of accidents requiring ECCS operation, including analysis methods, assumptions, and results, which are provided in Fermi 2 UFSAR Chapters 6 and 15 remain unchanged. The performance evaluations for accidents requiring ECCS operation described in UFSAR Chapters 6 and 15, based on the Appendix K Large-Break Loss-of-Coolant Accident (LBLOCA) analysis, demonstrate that for breaks up to and including the double-ended severance of a recirculation loop pipe, the ECCS will limit the clad temperature to below the limit specified in 10CFR50.46 and assure that the core will remain in place and substantially intact with its essential heat transfer geometry preserved.

-2 to NRC-24-0033 Page 4 The requirements of 10CFR50.46(a)(1) remain applicable to the model of record that meets the required features of Appendix K. Approval of the requested exemption does not impact the current ECCS evaluation. The current ECCS evaluation model remains the licensing basis for demonstrating that the ECCS calculated cooling performance following postulated LOCAs fulfills the acceptance criteria.

The DTE risk-informed approach uses the break frequencies from NUREG 1829 to quantify the residual risk associated with the additional debris loads for those LOCAs which have not been resolved using deterministic methods and shows that it meets the acceptance guidelines defined in RG 1.174. The exemption request is specific to the requirement for demonstrating ECCS cooling performance design as required by 10 CFR 50.46(a)(1), which has traditionally been interpreted as requiring deterministic-only evaluation 10CFR50.46(d) as it pertains to the application of the General Design Criteria, and to provide regulatory consistency between the requirements of 10CFR50.46(d) and the GDC. It is not intended to be applicable to other requirements provided in 10CFR50.46 or Appendix K to 10CFR50.

As noted above, the NRC staff considers the modeling of strainer performance (pressure drop) as an input to the ECCS evaluation model, and therefore the requirements of 10CFR50.46 are applicable. Consistent with this, the requirements and attributes for the proposed DTE risk informed method evaluates a full spectrum of postulated, double-ended guillotine breaks up to and including the largest piping in containment.

Engineering analyses and evaluations to address the deterministic scope of the Fermi 2 ECCS performance analysis are consistent with the Utility Resolution Guidance (URG) and related NRC SER for evaluation of strainer performance. The proposed exemption does not affect any of the 10CFR50.46 (a)(1) or Appendix K requirements for an acceptable ECCS evaluation model and does not change the ECCS acceptance criteria in 50.46(b) as it applies to the calculated results. Application of the exemption request allows use of a risk-informed approach to evaluate the effects of LOCA debris beyond a deterministic strainer failure limit. The results of the risk-informed method demonstrate that the risk associated with additional debris sources beyond those deemed acceptable by deterministic means meet the acceptance guidelines of RG 1.174.

The current licensing basis for addressing the adequacy of ECCS to meet the criteria of 10CFR50.46, including the Appendix K Large-Break LOCA analysis and the associated Chapter 15 accident analysis for LOCA, remain in place.

2.2 Evaluation of Impacts on other Regulatory Requirements - Conclusion The proposed exemption does not result in any physical changes to the facility or changes to the operation of the plant and does not change any of the programmatic requirements. In, the ECCS suction strainer design and hence ECCS design are shown to be acceptable using a risk-informed approach, i.e., with an exemption from 10CFR50.46(a)(1)10CFR50.46(d) requirements to use of deterministic evaluation methods.

-2 to NRC-24-0033 Page 5 Therefore, compliance with other regulatory requirements that rely on acceptable design for these systems and components continue to be met in the current licensing basis.

3. Basis for the Exemption Request Under 10CFR50.12, a licensee may request and the NRC may grant exemptions from those requirements of 10CFR50 which are authorized by law, will not present an undue risk to the public health and safety, are consistent with the common defense and security, and when special circumstances are present.

The exemption request ensures compliance with a key principle of RG 1.174, which states "The proposed change meets the current regulations unless it is explicitly related to a requested exemption." This exemption request is provided in conjunction with the proposed License Amendment Request in Attachment 1.

3.1 Justification for the Exemption Request As required by 10CFR50.12(a)(2), the Commission will not consider granting an exemption unless special circumstances are present. Special circumstances are present whenever one of the listed items (i) through (vi) under 10CFR50.12(a)(2) are applicable. DTE has evaluated the proposed exemption against the conditions specified in 10CFR50.12(a) and determined that this proposed exemption meets the requirements for granting an exemption from the regulation, and that special circumstances are present. The information supporting the determination is provided below.

Pursuant to 10CFR50.12, "Specific exemptions," the NRC may grant exemptions from the requirements of this part provided the following three conditions are met as required by 10CFR50.12(a)(1).

The exemption is authorized by law.

The NRC has authority under the Atomic Energy Act of 1954, as amended, to grant exemptions from its regulations if doing so does not violate the requirements of law. This exemption is authorized by law as is provided by 10CFR50.12 which provides the NRC authority to grant exemptions from 10CFR50 requirements with provision of proper justification. Approval of the exemption from applicable parts of 10CFR50.46(a)(1)10CFR50.46(d) does not conflict with any provisions of the Atomic Energy Act of 1954, as amended, any of the Commission's regulations, or any other law.

The exemption does not present an undue risk to the public health and safety.

The purpose of 10CFR50.46 is to establish acceptance criteria for ECCS performance, and together with GDC 35, to provide high confidence that the systems will perform their required functions. The proposed exemption does not involve any modifications to the plant that could

-2 to NRC-24-0033 Page 6 introduce a new accident precursor or affect the probability of postulated accidents, and therefore the probability of postulated initiating events is not increased. The PRA and engineering analysis demonstrate that the calculated risk is very small and consistent with the intent of the Commission's Safety Goal Policy Statement, which defines an acceptable level of risk that is a small fraction of other risks to which the public is exposed.

As discussed in previous 10CFR50.46 rulemaking, the probability of a large break LOCA is sufficiently low that the application of a risk-informed approach to evaluate the ability of the ECCS to meet 10CFR50.46 and relevant GDCs with high probability and with low uncertainty, rather than using a calculational model using deterministic methods to achieve similar understanding, would have little effect on public risk. This is applicable to evaluating acceptable strainer performance in support of ECCS in both core and containment cooling modes.

The proposed change is to apply a risk-informed method rather than a strictly deterministic method to quantify the risk associated with additional debris sources and to establish a high probability of success for performance of ECCS in accordance with the ECCS cooling performance design addressed in 10CFR50.46(d). The risk-informed approach involves a complete evaluation of the spectrum of LOCA breaks, including double-ended guillotine breaks, up to and including the largest pipe in the reactor coolant system. The riskinformed approach analyzes LOCAs, regardless of break size, using the same methods, assumptions, and criteria in order to quantify uncertainties and overall risk metrics. This ensures that large break LOCAs with relatively small contribution to CDF due to the low probability of such a break as well as smaller break LOCAs with higher probabilities of occurrence are considered in the results. Since the design basis requirement for consideration of double-ended guillotine breaks of the largest pipe in the reactor coolant system is retained and since no physical changes to the facility or changes to the operation of the facility are being made, the existing defense-in-depth and safety margin established for the design of the facility is not reduced.

This exemption only affects 10CFR50.46(a)(1)10CFR50.46(d) and does not impact the acceptance criteria for cladding performance that is important to maintain adequate safety margins.

The exemption is consistent with the common defense and security.

The exemption involves a change to the licensing basis for the plant that has no relation to the control of licensed material or any security requirements that apply to DTE Fermi 2. Therefore, the exemption is consistent with the common defense and security.

3.2 Special Circumstances This section discusses the presence of special circumstances as related to 10CFR50.12(a).

10CFR50.12(a)(2) states that NRC will not consider granting an exemption to the regulations

-2 to NRC-24-0033 Page 7 unless special circumstances are present. Special circumstances are present whenever one of the listed items (i) through (vi) under 10CFR50.12(a)(2) are applicable.

Such special circumstances are present in this instance to warrant exemption from the implicit requirement in 10CFR50.46(a)(1)10CFR50.46(d) to reference GDCs, which with respect to use of a deterministic calculational method as the design basis for demonstrating acceptable strainer performance and to validate that the results of the ECCS evaluation model demonstrating long-term cooling criterion is met. Approval of this exemption request would allow the use a risk-informed method to amend the design basis for acceptable performance of the ECCS strainer, for validation of inputs used in the ECCS evaluation model, and to support the existing licensing bases for compliance with all other parts of 10CFR50.46.

Specifically, 10CFR50.12(a)(2)(ii) applies:

Application of the regulation in the particular circumstances would not serve the underlying purpose of the rule or is not necessary to achieve the underlying purpose of the rule.

The intent of 10CFR50.46(a)(1)10CFR50.46(d) is to ensure ECCS cooling performance design requirements imposed by 10CFR50.46 are complemented by the requirements of the relevant GDCare fulfilled. This exemption request is consistent with that purpose in that use of the proposed riskinformed approach accounts for the effect of debris on the ECCS cooling performance and supports a high probability of successful ECCS performance, based on the risk results meeting the acceptance guidelines of RG 1.174.

As discussed in the Commission's Policy Statement on "Use of Probabilistic Risk Assessment Methods in Nuclear Regulatory Activities", NRC regulations and their implementation are generally based on deterministic approaches that consider a set of challenges to safety and determine how those challenges should be mitigated.

This exemption is requested to align requirements in the regulations for using deterministic methods to demonstrate acceptable design with risk informed regulation guidance that ensure high probability of system performance and acceptable risk. Regulatory requirements are largely based on a deterministic framework, and are established for design basis accidents, such as the LOCA, with specific acceptance criteria that must be satisfied. Licensed facilities must be provided with safety systems capable of preventing and mitigating the consequences of design basis accidents to protect public health and safety. The deterministic regulatory requirements were designed to ensure that these systems are highly reliable. The LOCA analysis and the General Design Criteria (GDC) were established as part of this deterministic regulatory framework.

In comparison, the risk-informed approach considers nuclear safety in a complementary way by examining the likelihood of a broad spectrum of initiating events and potential challenges,

-2 to NRC-24-0033 Page 8 considering the frequencies of a wide range of credible events, and assessing the risk based on the reliability of mitigating systems.

An objective of 10CFR50.46 is to maintain low risk to the public health and safety through a reliable ECCS. The supporting analysis demonstrates that a risk-informed approach to strainer performance is consistent with the Commissions Safety Goals for nuclear power plants, including ECCS operation with a high degree of reliability. Consequently, the special circumstances described in 10CFRS0.12(a)(2)(ii) apply.

Specifically, 10CFR50.12(a)(2)(iii) applies:

Compliance would result in undue hardship or other costs that are significantly in excess of those contemplated when the regulation was adopted, or that are significantly in excess of those incurred by others similarly situated.

The specific hardship is the excessive occupational radiological dose that is expected to be incurred for plant modifications to remove and replace insulation. This was discussed in -1, above.

In conclusion, special circumstances in 10CFR50.12(a)(2)(ii) and 10CFR50.12(a)(2)(iii) are present as required by 10CFR50.12(a)(2) for consideration of the request for exemption to applicable parts of 10CFR50.46(a)(1)10CFR50.46(d).

4. Technical Justification for the Exemption Technical justification for the risk-informed method is provided in Attachment 1 and Attachment

3.

The proposed risk-informed approach meets the key principles in RG 1.174 in that it is consistent with the defense-in-depth philosophy, maintains sufficient safety margins, results in small increase in risk, and is monitored using performance measurement strategies that are discussed in. The proposed exemption from references in 10CFR50.46(a)(1)10CFR50.46(d) that cite GDC 35 allow enables use of the risk-informed method and is consistent with the key principle in RG 1.174 that requires the proposed change to meet current regulations unless explicitly related to a requested exemption.

The results of analyses in Attachment 3 show that risks associated with the additional debris sources are in Region III, "Very Small Changes," of RG 1.174, and therefore are consistent with the Commission's Safety Goals for public health and safety. *

5. Conclusion Approval of an exemption to allow the use of the risk-informed approach is authorized by law, will not present an undue risk to the public health and safety, and is consistent with the common defense and security requirements of 10CFR50.12(a)(1). Furthermore, special circumstances

-2 to NRC-24-0033 Page 9 required by 10CFR50.12(a)(2) are present for item 10CFR50.12(a)(2)(ii) in that application of the regulation in the particular circumstances is not necessary to achieve the underlying purpose of the rule.

Based on the determination that the risk of the exemption meets the acceptance guidelines of RG 1.174; the results demonstrate there is reasonable assurance that the ECCS will function in the recirculation mode and that the public health and safety will be protected.

6.

Implementation DTE requests that this exemption request be approved for implementation 1 year of the submittal of this License Amendment Request, and concurrent with review and acceptance of Attachment 1, the LAR modifying the licensing basis of ECCS strainers for select debris from a deterministic basis to a risk-informed basis.

-3 Request for Exemption from General Design Criterion 35

-3 to NRC-24-0033 Page 1 Request for Exemption from Certain Requirements of General Design Criterion 35

1.

Exemption Request Pursuant to 10CFR50.12, DTE is submitting this request for exemption from certain requirements of 10CFR50 Appendix A, General Design Criterion (GDC) 35, which states:

Criterion 35 - Emergency core cooling. A system to provide abundant emergency core cooling shall be provided. The system safety function shall be to transfer heat from the reactor core following any loss of reactor coolant at a rate such that (1) fuel and clad damage that could interfere with continued effective core cooling is prevented and (2) clad metal-water reaction is limited to negligible amounts.

Suitable redundancy in components and features, and suitable interconnections, leak detection, isolation, and containment capabilities shall be provided to assure that for onsite electric power system operation (assuming offsite power is not available) and for offsite electric power system operation (assuming onsite power is not available) the system safety function can be accomplished, assuming a single failure.

By regulatory precedent, licensees are required to demonstrate this capability by the use of a bounding calculation or other deterministic method. DTE requests an exemption from the need to use only deterministic methods, in order to enable the use of a risk-informed method to demonstrate acceptable strainer design and ECCS performance with regard to the effects of LOCA debris.

Approval of this exemption will allow use of a risk-informed method to account for the probabilities and uncertainties associated with mitigation of the effects of debris following postulated LOCAs. The method evaluates the effects on strainer blockage resulting from debris concerns identified within the DTE corrective action program. In order to confirm acceptable ECCS strainer design, the risk associated with post-LOCA debris sources are evaluated to include the failure mechanisms associated with loss of core cooling and strainer blockage.

The scope of the exemption applies for limited debris effects addressed in the DTE RoverD methodology described in Attachment 3 that was used to respond to a corrective action associated with Min-K found in the drywell penetrations at Fermi 2 and unqualified tags/labels found throughout containment. The LOCA break sizes and locations that potentially generate an amount of Min-K that exceeds the quantity established in DTE original design basis testing and analysis are described in Attachment 3. The key elements of the exemption request are:

1. It applies only to the effects of debris as described in Attachment 3.

2. It applies only for LOCA breaks that can generate and transport fiber debris that is not bounded by the DTE strainer failure criteria established in Attachment 3.

-3 to NRC-24-0033 Page 2

3. It applies to any LOCA break that can generate and transport fiber debris that is not bounded by strainer failure criteria, provided that the cumulative CDF and LERF associated with these breaks remain in Region III of RG 1.174.

This exemption request is complemented by the accompanying License Amendment Request (LAR) (Attachment 1), which seeks NRC approval to change the licensing basis in the Fermi 2 Updated Final Safety Analysis Report (UFSAR) to allow the use of a risk-informed approach for demonstrating acceptable design of the ECCS suction strainers. The risk-informed method provides high probability assurance that, as calculated in the ECCS evaluation model (Attachment 3), ECCS operation, including strainer performance and debris mitigation, will be acceptable.

2.

Regulatory Requirements Involved DTE seeks exemption to the extent that GDC 35 requires deterministic calculations or other deterministic analyses to address the debris concerns related to acceptable plant performance during core and containment cooling mode following a LOCA. The proposed changes to the licensing basis, submitted for NRC approval with Attachment 1, address the additional debris loads at the ECCS strainers for Fermi 2 on the basis that the associated risk is shown to meet the acceptance guidelines in Regulatory Guide (RG) 1.174 and that, in conjunction with the existing licensing basis, adequate safety is demonstrated.

This exemption request is for the purpose of allowing the use of a risk-informed method to demonstrate acceptable mitigation of the effects of debris following postulated loss of coolant accidents (LOCAs). The effects of LOCA debris have been evaluated, using deterministic methods, to meet the current licensing basis assumptions for analyzing the effects of post-LOCA debris blockage at the suction strainers; however, these evaluations have not been shown to fully address debris effects for the as-built, as-operated plant. The risk informed approach evaluates the risk associated with additional potential debris sources on ECCS strainer performance beyond those currently established in the strainer design basis, which employed deterministic methods. Based on confirmation of acceptable ECCS design as determined by the resulting risk meeting the acceptance guidelines in RG 1.174, the licensing basis for ECCS compliance with GDC 35 expectation of solely deterministic evaluations will be changed to permit use of a complementary risk-informed methodology.

2.1 Evaluation of Impacts on other Regulatory Requirements - Conclusion The proposed exemption does not result in any physical changes to the facility or changes to the operation of the plant and does not change any of the programmatic requirements. In Attachment 3, the ECCS suction strainer design and hence ECCS design are shown to be acceptable using a risk-informed approach, i.e., with an exemption from GDC 35 requirements to use only deterministic evaluation methods. Therefore, compliance with other regulatory requirements that

-3 to NRC-24-0033 Page 3 rely on acceptable design for these systems and components continue to be met in the current licensing basis.

3.

Basis for the Exemption Request Under 10CFR50.12, a licensee may request and the NRC may grant exemptions from those requirements of 10CFR50 that are authorized for exemption by law, will not present an undue risk to the public health and safety, are consistent with the common defense and security, and when special circumstances are present.

The exemption request meets a key principle of RG 1.174 that states "The proposed change meets the current regulations unless it is explicitly related to a requested exemption." This exemption request is provided in support of the proposed change provided in the License Amendment Request provide in Attachment 1.

3.1 Justification for the Exemption Request As required by 10CFR50.12(a)(2), the Commission will not consider granting an exemption unless special circumstances are present. Special circumstances are present whenever one of the listed items (i) through (vi) under 10CFR50.12(a)(2) are applicable. DTE has evaluated the proposed exemption against the conditions specified in 10CFR50.12(a) and determined that this proposed exemption meets the requirements for granting an exemption from the regulation, and that special circumstances are present. The information supporting the determination is provided below.

Pursuant to 10CFR50.12, "Specific exemptions," the NRC may grant exemptions from the requirements of this part provided the following three conditions are met as required by 10CFR50.12(a)(1):

The exemption is authorized by law.

The NRC has authority under the Atomic Energy Act of 1954, as amended, to grant exemptions from its regulations if doing so does not violate the requirements of law. This exemption is authorized by law as is provided by 10CFR50.12, which provides the NRC authority to grant exemptions from 10CFR50 requirements with provision of proper justification. Approval of the exemption does not conflict with any provisions of the Atomic Energy Act of 1954, as amended, the Commission's regulations, or any other law.

The exemption does not present an undue risk to the public health and safety.

The proposed change is to apply a risk-informed method rather than a strictly traditional deterministic method in order to quantify the residual risk associated with additional debris loads and to establish a high confidence of acceptable ECCS design. The purpose of GDC 35 is to establish an acceptable design for the ECCS, and together with the acceptance criteria of

-3 to NRC-24-0033 Page 4 10CFR50.46, to provide high probability that the systems will perform the required functions.

The proposed exemption does not involve any modifications to the plant that could introduce a new accident precursor or affect the probability of postulated accidents, and therefore, the probability of postulated initiating events is not increased. The PRA and engineering analysis demonstrate that the calculated risk is very small and consistent with the intent of the Commission's Safety Goal Policy Statement, which defines an acceptable level of risk that is a small fraction of other risks to which the public is exposed As discussed in previous 10CFR50.46 rulemaking, the probability of a large break LOCA is sufficiently low that application of a risk-informed approach to evaluate the ability of the ECCS to meet its design requirements with high probability and with low uncertainty, rather than using a calculational model using deterministic methods to achieve similar understanding, would have little effect on public risk. This is applicable to evaluating the effects of debris on the acceptability of ECCS design during the recirculation modes.

The risk-informed approach involves a complete evaluation of the spectrum of LOCA breaks, including double-ended guillotine breaks, up to and including the largest pipe in the reactor coolant system. The risk-informed approach analyzes LOCAs, regardless of break size, using the same methods, assumptions, and criteria in order to quantify uncertainties and overall risk metrics. This ensures that large break LOCAs with relatively small contribution to CDF due to the low probability of such a break as well as smaller break LOCAs with higher probabilities of occurrence are considered in the results. Since the design basis requirement for consideration of a double-ended guillotine break of the largest pipe in the reactor coolant system is retained and since no physical changes to the facility or changes to the operation of the facility are being made, the existing defense-in-depth and safety margin established for the design of the facility is not reduced.

The exemption is consistent with the common defense and security.

The exemption involves a change to the licensing basis for the plant that has no relation to the possession of licensed material or any security requirements that apply to DTE Fermi 2.

Therefore, the exemption is consistent with the common defense and security.

3.2 Special Circumstances This section discusses the presence of special circumstances as related to 10CFR50.12(a).

10CFR50.12(a)(2) states that NRC will not consider granting an exemption to the regulations unless special circumstances are present. Special circumstances are present whenever one of the listed items (i) through (vi) under 10CFR50.12(a)(2) are applicable.

Such special circumstances are present in this instance to warrant exemption from the implicit requirement in GDC 35 to use a deterministic method to demonstrate acceptable ECCS suction

-3 to NRC-24-0033 Page 5 strainer performance. Approval of the exemption request would allow use of a risk-informed method to amend the licensing basis for acceptable ECCS strainer design to demonstrate ECCS design compliance with GDC 35. Specifically, 10CFR50.12(a)(2)(ii) applies:

Application of the regulation in the particular circumstances would not serve the underlying purpose of the rule or is not necessary to achieve the underlying purpose of the rule.

The intent of GDC 35 is to ensure ECCS design provides abundant core cooling to mitigate fuel and clad damage and clad metal-water reaction following any loss of reactor coolant. GDC 35 sets forth the general ECCS cooling performance design requirements, which are in addition to the requirements of 10CFR50.46. This exemption request is consistent with the stated purpose in that use of the proposed risk-informed approach demonstrates a high probability of successful ECCS performance, which includes realistically available long-term cooling, based on the risk results meeting the acceptance guidelines of RG 1.174. The risk-informed approach assesses ECCS design for a full spectrum of breaks and assesses equipment failures that include loss of offsite power and worst-case single failure, consistent with the GDC 35 requirements.

Since the proposed exemption does not involve any physical changes to the plant, there is no effect on the GDC 35 requirements for ECCS design for redundancy in components and features, interconnections, leak detection, isolation, and containment capabilities. The current licensing basis evaluations for ECCS compliance with GDC 35 for these aspects continue to be met.

As discussed in the Commission's Policy Statement on "Use of Probabilistic Risk Assessment Methods in Nuclear Regulatory Activities", NRC regulations and their implementation are generally based on deterministic approaches that consider a set of challenges to safety and determine how those challenges should be mitigated.

This request does not seek exemption from any explicit language in the regulatory requirements.

Rather, the exemption request addresses the implicit requirements in the regulations for using deterministic methods to demonstrate acceptable design. Regulatory requirements are largely based on a deterministic framework, and are established for design basis accidents, such as the LOCA, with specific acceptance criteria that must be satisfied. Licensed facilities must maintain safety systems capable of preventing and mitigating the consequences of design basis accidents to protect public health and safety. The deterministic regulatory requirements were designed to ensure that these systems are highly reliable. The LOCA analysis and the General Design Criteria (GDC) were established as part of this deterministic regulatory framework.

In comparison, the probabilistic approach considers nuclear safety in a complementary way by examining the likelihood of a broad spectrum of initiating events and potential challenges, considering a wide range of credible events, and assessing the risk based on the reliability of mitigating systems.

-3 to NRC-24-0033 Page 6 An objective of GDC 35 is to maintain low risk to the public health and safety through a reliable ECCS. The supporting analysis provided in Attachment 3 demonstrates that a risk-informed approach to suction strainer performance is consistent with the Commissions Safety Goals for nuclear power plants, including ECCS operation with a high degree of reliability. Consequently, the special circumstances described in 10CFR50.12(a)(2)(ii) apply.

Specifically, 10CFR50.12(a)(2)(iii) applies:

Compliance would result in undue hardship or other costs that are significantly in excess of those contemplated when the regulation was adopted, or that are significantly in excess of those incurred by others similarly situated.

The specific hardship is the excessive occupational radiological dose that is expected to be incurred for plant modifications to remove and replace insulation. This was discussed in -1.

In conclusion, special circumstances in 10CFR50:12(a)(2)(ii) and 10CFR50.12(a)(2)(iii) are present as required by 10CFR50.12(a)(2) for consideration of the request for exemption.

4.

Technical Justification for the Exemption Technical justification for the risk-informed method is provided in Attachment 3 and Attachment

1.

The proposed risk-informed approach meets the key principles in RG 1.174 in that it is consistent with the defense-in-depth philosophy, maintains sufficient safety margins, results in small increase in risk, and is monitored using performance measurement strategies that are discussed in. The proposed change to the licensing basis to allow use of the risk-informed method is consistent with the key principle in RG 1.174 that requires the proposed change to meet current regulations unless explicitly related to a requested exemption.

The results of analyses in Attachment 3 show that the risk associated with GSI-191 concerns is in Region III, "Very Small Changes," of RG 1.174, and therefore are consistent with the Commission's Safety Goals for public health and safety.

5.

Conclusion Approval of this exemption that is needed to allow the use of the risk-informed approach is authorized by law, will not present an undue risk to the public health and safety, and is consistent with the common defense and security requirements of 10CFR50.12(a)(1). Furthermore, special circumstances required by 10CFR50.12(a)(2) are present for item 10CFR50.12(a)(2)(ii) in that application of the regulation in the particular circumstances is not necessary to achieve the underlying purpose of the rule.

-3 to NRC-24-0033 Page 7 Based on the determination that the risk of the exemption meets the acceptance guidelines of RG 1.174, the results demonstrate there is reasonable assurance that the ECCS will function in the recirculation mode and that the public health and safety will be protected.

6.

Implementation DTE requests that this exemption request be approved concurrently with the LAR (Attachment

1) for implementation within 1 year of the submittal of this License Amendment Request.

-4 Request for Exemption from General Design Criterion 38

-4 to NRC-24-0033 Page 1 Request for Exemption from Certain Requirements of General Design Criterion 38

1.

Exemption Request Pursuant to 10CFR50.12, DTE is submitting this request for exemption from certain requirements of 10CFR50 Appendix A, General Design Criterion (GDC) 38, which states:

Criterion 38 - Containment heat removal. A system to remove heat from the reactor containment shall be provided. The system safety function shall be to reduce rapidly, consistent with the functioning of other associated systems, the containment pressure and temperature following any loss-of-coolant accident and maintain them at acceptably low levels.

Suitable redundancy in components and features, and suitable interconnections, leak detection, isolation, and containment capabilities shall be provided to assure that for onsite electric power system operation (assuming offsite power is not available) and for offsite electric power system operation (assuming onsite power is not available) the system safety function can be accomplished, assuming a single failure.

By regulatory precedent, licensees are required to demonstrate this capability by the use of a bounding calculation or other deterministic method. DTE requests an exemption from the need to use only deterministic methods, in order to enable the use of a risk-informed method to demonstrate acceptable strainer design and ECCS performance with regard to the effects of LOCA debris.

Approval of this exemption will allow use of a risk-informed method to account for the probabilities and uncertainties associated with mitigation of the effects of debris following postulated LOCAs. The method evaluates the effects on strainer blockage resulting from debris concerns identified within the DTE corrective action program. In order to confirm acceptable ECCS strainer design, the risk associated with post-LOCA debris sources are evaluated to include the failure mechanisms associated with loss of core cooling and strainer blockage.

The scope of the exemption applies for limited debris effects addressed in the DTE RoverD methodology described in Attachment 3 that was used to respond to a corrective action associated with Min-K found in the drywell penetrations at Fermi 2 and unqualified tags/labels found throughout containment. The LOCA break sizes and locations that potentially generate an amount of Min-K that exceeds the quantity established in DTE original design basis testing and analysis are described in Attachment 3. The key elements of the exemption request are:

1.

It applies only to the effects of debris as described in Attachment 3.

2.

It applies only for LOCA breaks that can generate and transport fiber debris that is not bounded by the DTE strainer failure criteria established in Attachment 3.

-4 to NRC-24-0033 Page 2

3.

It applies to any LOCA break that can generate and transport fiber debris that is not bounded by strainer failure criteria, provided that the cumulative CDF and LERF associated with these breaks remain in Region III of RG 1.174.

This exemption request is complemented by the accompanying License Amendment Request (LAR) (Attachment 1), which seeks NRC approval to change the licensing basis in the Fermi 2 Updated Final Safety Analysis Report (UFSAR) to allow the use of a risk-informed approach for demonstrating acceptable design of the ECCS suction strainers The risk-informed method provides high probability assurance that, as calculated in the ECCS evaluation model (Attachment 3), ECCS operation, including strainer performance and debris mitigation, will be acceptable.

2.

Regulatory Requirements Involved DTE seeks exemption to the extent that GDC 38 requires deterministic calculations or other deterministic analyses to address the debris concerns related to acceptable plant performance during core and containment cooling modes following a LOCA. The proposed changes to the licensing basis, submitted for NRC approval with Attachment 1, address the additional debris loads at the ECCS strainers for Fermi 2 on the basis that the associated risk is shown to meet the acceptance guidelines in Regulatory Guide (RG) 1.174 and that, in conjunction with the existing licensing basis, adequate safety is demonstrated.

This exemption is needed for the purpose of allowing the use of a risk-informed method to demonstrate acceptable mitigation of the effects of debris following postulated loss of coolant accidents (LOCAs). The effects of LOCA debris have been evaluated, using deterministic methods, to meet the current licensing basis assumptions for analyzing the effects of post-LOCA debris blockage at the suction strainers; however, these evaluations have not been shown to fully address debris effects for the as-built, as-operated plant. The risk informed approach evaluates the risk associated with additional potential debris sources on ECCS strainer performance beyond those currently established in the strainer design basis. Based on confirmation of acceptable ECCS design as determined by the resulting risk meeting the acceptance guidelines in RG 1.174 and approval of this exemption, the licensing basis for ECCS compliance with GDC 38 requirements requiring use of deterministic methods will be amended to a risk-informed methodology.

2.1 Evaluation of Impacts on other Regulatory Requirements - Conclusion The proposed exemption does not result in any physical changes to the facility or changes to the operation of the plant and does not change any of the programmatic requirements. In Attachment 3, the ECCS suction strainer design and hence ECCS design are shown to be acceptable using a risk-informed approach, (with an exemption from GDC 38 requirements to use deterministic evaluation methods). Therefore, compliance with other regulatory requirements that rely on

-4 to NRC-24-0033 Page 3 acceptable design for these systems and components continue to be met in the current licensing basis.

3.

Basis for the Exemption Request Under 10CFR50.12, a licensee may request and the NRC may grant exemptions from those requirements of 10CFR50 for which exemptions are authorized by law, will not present an undue risk to the public health and safety, are consistent with the common defense and security, and when special circumstances are present.

The exemption request meets a key principle of RG 1.174, which states "The proposed change meets the current regulations unless it is explicitly related to a requested exemption." This exemption request is provided in support of the proposed change provided in the License Amendment Request provided in Attachment 3.

3.1 Justification for the Exemption Request As required by 10CFR50.12(a)(2), the Commission will not consider granting an exemption unless special circumstances are present. Special circumstances are present whenever one of the listed items (i) through (vi) under 10CFR50.12(a)(2) are applicable. DTE has evaluated the proposed exemption against the conditions specified in 10CFR50.12(a) and determined that this proposed exemption meets the requirements for granting an exemption from the regulation, and that special circumstances are present. The information supporting the determination is provided below.

Pursuant to 10CFR50.12, "Specific exemptions," the NRC may grant exemptions from the requirements of this part provided the following three conditions are met as required by 10CFR50.12(a)(1):

The exemption is authorized by law.

The NRC has authority under the Atomic Energy Act of 1954, as amended, to grant exemptions from its regulations if doing so does not violate the requirements of law. This exemption is authorized by law as is provided by 10CFR50.12 which provides the NRC authority to grant exemptions from 10CFR50 requirements with provision of proper justification. Approval of the exemption does not conflict with any provisions of the Atomic Energy Act of 1954, as amended, the Commission's regulations, or any other law.

The exemption does not present an undue risk to the public health and safety.

The proposed change is to apply a risk-informed method rather than a strictly traditional deterministic method in order to quantify the residual risk associated with the additional debris loads and to establish a high confidence of acceptable ECCS design. The purpose of GDC 38 is to establish acceptable design for containment heat removal, which includes the heat removal

-4 to NRC-24-0033 Page 4 functions of the ECCS, together provide high probability that the systems will perform the required functions. The proposed exemption does not involve any modifications to the plant that could introduce a new accident precursor or affect the probability of postulated accidents, and therefore the probability of postulated initiating events is not increased. The PRA and engineering analysis demonstrate that the calculated risk is very small and consistent with the intent of the Commission's Safety Goal Policy Statement, which defines an acceptable level of risk that is a small fraction of other risks to which the public is exposed As discussed in previous 10CFR50.46 rulemaking, the probability of a large break LOCA is sufficiently low that the application of a risk-informed approach to evaluate the ability of the ECCS to meet its design requirements with high probability and with low uncertainty, rather than using a calculational model using deterministic methods to achieve similar understanding, would have little effect on public risk. This conclusion is consistent with evaluation of the effects of debris on the acceptability of ECCS design during the recirculation modes provided in.

The risk-informed approach involves a complete evaluation of the spectrum of LOCA breaks, including double-ended guillotine breaks, up to and including the largest pipe in the reactor coolant system. The risk-informed approach analyzes LOCAs, regardless of break size, using the same methods, assumptions, and criteria in order to quantify uncertainties and overall risk metrics. This ensures that large break LOCAs with relatively small contribution to CDF due to the low probability of such a break as well as smaller break LOCAs with higher probabilities of occurrence are considered in the results. Since the design basis requirement for consideration of a double-ended guillotine break of the largest pipe in the reactor coolant system is retained and since no physical changes to the facility or changes to the operation of the facility are being made, the existing defense-in depth and safety margin established for the design of the facility is not reduced.

The exemption is consistent with the common defense and security.

The exemption involves a change to the licensing basis for the plant that has no relation to the possession of licensed material or any security requirements that apply to DTE Fermi 2.

Therefore, the exemption is consistent with the common defense and security.

3.2 Special Circumstances This section discusses the presence of special circumstances as related to 10CFR50.12(a).

10CFR50.12(a)(2) states that NRC will not consider granting an exemption to the regulations unless special circumstances are present. Special circumstances are present whenever one of the listed items (i) through (vi) under 10CFR50.12(a)(2) are applicable.

-4 to NRC-24-0033 Page 5 Such special circumstances are present in this instance to warrant exemption from the implicit requirement in GDC 38 to use a deterministic method to demonstrate acceptable ECCS suction strainer design. Approval of the exemption request would allow use of a risk-informed method to amend the licensing basis for acceptable ECCS strainer performance to demonstrate compliance with GDC 38. Specifically, 10CFR50.12(a)(2)(ii) applies:

Application of the regulation in the particular circumstances would not serve the underlying purpose of the rule or is not necessary to achieve the underlying purpose of the rule.

The intent of GDC 38 is to ensure the ECCS design provides abundant containment cooling to rapidly reduce containment pressure and temperature following any LOCA and maintain them at acceptably low levels. This exemption request is consistent with that purpose in that use of the proposed risk-informed approach demonstrates a high probability of successful ECCS performance, which includes realistically available long-term containment cooling and suppression pool cooling, based on the risk results meeting the acceptance guidelines of RG 1.174. The risk-informed approach assesses ECCS design for a full spectrum of breaks and assesses equipment failures that include loss of offsite power and worst-case single failure, consistent with the GDC 38 requirements.

Since the proposed exemption does not involve any physical changes to the plant, there is no effect on the GDC 38 requirements for ECCS design for redundancy in components and features, interconnections, leak detection, isolation, and containment capabilities. The current licensing basis evaluations for containment heat removal with GDC 38 for these aspects continue to be met.

As discussed in the Commission's Policy Statement on "Use of Probabilistic Risk Assessment Methods in Nuclear Regulatory Activities", NRC regulations and their implementation are generally based on deterministic approaches that consider a set of challenges to safety and determine how those challenges should be mitigated.

This request does not seek exemption from any explicit language in the regulatory requirements.

Rather, the exemption request addresses the implicit requirements in the regulations for using deterministic methods to demonstrate acceptable design. Regulatory requirements are largely based on a deterministic framework, and are established for design basis accidents, such as the LOCA, with specific acceptance criteria that must be satisfied. Licensed facilities must have safety systems capable of preventing and mitigating the consequences of design basis accidents to protect public health and safety. The deterministic regulatory requirements were designed to ensure that these systems are highly reliable. The LOCA analysis and the General Design Criteria (GDC) were established as part of this deterministic regulatory framework.

In comparison, the probabilistic approach considers nuclear safety in a complementary way by examining the likelihood of a broad spectrum of initiating events and potential challenges,

-4 to NRC-24-0033 Page 6 considering a wide range of credible events, and assessing the risk based on the reliability of mitigating systems.

An objective of GDC 38 is to maintain low risk to the public health and safety through a reliable ECCS. The supporting analysis provided in Attachment 3 demonstrates that a risk-informed approach to strainer performance is consistent with the Commission's Safety Goals for nuclear power plants, including ECCS operation with a high degree of reliability. Consequently, the special circumstances described in 10CFR50.12(a)(2)(ii) apply.

Specifically, 10CFR50.12(a)(2)(iii) applies:

Compliance would result in undue hardship or other costs that are significantly in excess of those contemplated when the regulation was adopted, or that are significantly in excess of those incurred by others similarly situated.

The specific hardship is the excessive occupational radiological dose that is expected to be incurred for plant modifications to remove and replace insulation. This was discussed in -1.

In conclusion, special circumstances in 10CFR50:12(a)(2)(ii) and 10CFR50.12(a)(2)(iii) are present as required by 10CFR50.12(a)(2) for consideration of the request for exemption.

4.

Technical Justification for the Exemption Technical justification for the risk-informed method is provided in Attachment 3 and in the LAR (Attachment 1).

The proposed risk-informed approach meets the key principles in RG 1.174 in that it is consistent with the defense-in-depth philosophy, maintains sufficient safety margins, results in small increase in risk, and is monitored using performance measurement strategies that are discussed in. The proposed exemption is needed to allow use of the risk-informed method in a manner consistent with the key principle in RG 1.174 that requires the proposed change to meet current regulations unless explicitly related to a requested exemption.

The results of analyses in Attachment 3 show that the risks associated with additional debris concerns is in Region III, "Very Small Changes," of RG 1.174, and therefore are consistent with the Commission's Safety Goals for public health and safety.

5.

Conclusion Approval of an exemption that is needed to allow the use of the risk-informed approach is authorized by law, will not present an undue risk to the public health and safety, and is consistent with the common defense and security requirements of 10CFR50.12(a)(1). Furthermore, special circumstances required by 10CFR50.12(a)(2) are present for item 10CFR50.12(a)(2)(ii) in that

-4 to NRC-24-0033 Page 7 application of the regulation in the particular circumstances is not necessary to achieve the underlying purpose of the rule.

Based on the determination that the risk of the proposed change to a RI methodology meets the acceptance guidelines of RG 1.174, the results demonstrate there is reasonable assurance that the ECCS will function in the recirculation mode and that the public health and safety will be protected

6.

Implementation DTE requests that this exemption request be approved concurrently with the LAR (Attachment

1) for implementation within 1 year of the submittal of this License Amendment Request.

to NRC-24-0033 Fermi 2 NRC Docket No. 50-341 Operating License No. NPF-43 LAR Attachment 4 - Technical Supplement to NRC-24-0033 Page 1

1. Introduction to LAR Attachment 4 - Technical Supplement Various audit topics and RAIs question the amount, treatment, and risk implications of Min-K' insulation present in containment penetrations. LAR Att-3, consistent with the strainer design basis (DB), does not include penetration Min-K' as a potential debris source for non-isolable breaks occurring between the reactor vessel and the isolation valves. To provide a technical basis for responding to these topics related to penetration Min-K', this supplement 1) explains how the CAD model Min-K' inventory was changed, 2) explains how penetration Min-K' can be damaged to form debris, and 3) presents revised baseline risk with penetration Min-K' included as a debris source for non-isolable breaks. These changes do not affect risk contributions from isolable breaks that occur between isolation valves and potentially inside of a penetration. As stated in the LAR (Table 7-1), risk contributed from isolable breaks is 2.12E-07/yr. This supplement also provides technical information supporting responses to RAI questions related to sensitivity cases involving suppression pool cooling, 1/8th-inch debris thickness failure criterion, 100 ft2 of strainer obstruction by miscellaneous debris, and other parameter variations.

To aid review and comparison of results, all baseline risk assumptions defined in LAR Att-3 are applied in this supplement. Principal baseline risk assumptions include: 1) single-division Low Pressure Coolant Injection (LPCI) run-out flow with 2) no quantitative risk reduction for single-division pump-state probability, and 3) Emergency Core Cooling System (ECCS) failure defined by a 1/8th-inch debris thickness accumulation on any single strainer, or, 4) exceedance of any strainer DB debris constituent quantity. (If either condition 3 or 4 are true on any single strainer, the break case is assumed to cause ECCS failure and contributes to risk of core damage).

Because the strainer DB includes only small amounts of Min-K' debris, exceedance of the DB Min-K' quantity can become a significant risk contributor. This supplement defines modified baseline risk quantification assumptions that apply 1) a 10% single-division pump-state probability, 2) a factor of 0.2 to account for operator failure to diagnose debris accumulation, and

3) a 25% reduction in penetration Min-K' debris to credit obstruction of external Zone of Influence (ZOI) by internal hydraulic pipes. A necessary component of the modified baseline definition is the addition of risk arising from two-division suppression pool cooling, which is expected to occur with a 90% complement pump-state probability.

This document contains four sections: 1) Introduction, 2) CAD Model Revisions, 3) Baseline and Modified Baseline Risk With Penetration Min-K', and 4) Sensitivity Case Summary. Technical information provided in this supplement and in the cited references supersedes similar technical information provided in LAR Att-1 and Att-3. In particular, previous statements of the numerical baseline total risk (5.95E-07/yr) are now replaced with a modified baseline risk (6.88E-07/yr) that is defined here in Section 3.

Summaries of methods and results presented in this document are fully developed and explained in the following references.

1. MEMO-9045-AVR-2018-01, Verification of Fermi CAD Model, Rev. 3, March 2024.

2. ALION-CAL-SI-9045-132, DTE Energy Fermi-2 CASA Grande Debris Generation and Risk Quantification, Rev. 2, May 2024.

to NRC-24-0033 Page 2

3. SERCO-REP-DTE-22929-02, Rev. 0, Sensitivity Study of Fermi Unit 2 Risk-Informed Core Damage Frequency, May 2024.

2. CAD Model Revisions Three changes made to the Fermi-2 CAD model can affect the risk quantification.

CAD Change 1: Additional whip restraint insulation Table 7-2 in LAR Att-3 Ref. 11 ("Estimation of Debris Sources for ECCS Suction Strainers, DC-5979," Rev A) provides a list of all non-Reflective Metal Insulation (RMI) present in containment that is not located in penetrations. The CAD model used to generate debris supporting the LAR risk quantification was based on LAR Att-3 Ref. 3 (DC-5979, Rev 0) Table 6.1 that does not include the last 6 rows of Table 7-2 (DC-5979, Rev A). See excerpt of Table 7-2 provided below in Figure. 1.

After revision (Ref. 1 above) the CAD model now includes the six whip-restraint insulation sources found at the bottom of DC-5979 Rev A, Table 7-2. The new points increase total containment fiber by approximately 4.6 ft3 and total containment Min-K' (not including penetrations) by 0.113 ft3.

Additionally, one potentially conservative discrepancy was found in the amount of insulation reported for whip restraint SR1B. DC-5979 Rev 0 Table 6.1 lists 3.7 ft3 of NUKON and DC-5979 Rev A Table 7-2 lists 2.73 ft3 of NUKON at that location. The CAD model continues to use the larger amount specified in Rev 0.

The additional whip-restraint insulation introduced to the CAD model causes a small increase in baseline risk because a few additional breaks are able to exceed the baseline failure criterion of 1/8th-in of fiber accumulating on any single active strainer. No risk-informed conclusions are changed as a result of this small increase in containment insulation.

to NRC-24-0033 Page 6 CAD Change 3, inclusion of guard pipes housing precisely dimensioned Min-K' insulation sleeves, permits a significant number of postulated breaks to exceed the DB limit for Min-K'.

When Min-K' debris volume is counted as fiber, some cases also exceed the 21-ft3 DB limit for fiber, but recall that including Min-K' debris in the fiber debris inventory is a safety margin applied to increase the prevalence of bed thickness failures, not a requirement of the DB, which tracks fiber and Min-K' as separate debris constituents. Therefore, no break cases have been found that exceed the DB fiber limit. The combined effect of all CAD changes 1 - 3 allows baseline CDF from non-isolable breaks to increase from 3.83E-7/yr to 3.19E-6/yr. This revised result does not include any modifications to baseline assumptions defined in the LAR.

In order to address the additional risk increase due to the penetration Min-K' insulation, the following changes are made:

1. Apply a 10% single-division plant-state probability to reduce baseline risk with penetration Min-K' by a factor of 10 (3.19E-06 / 10 = 3.19E-07/yr).

2. Add risk contributed by the planned, most likely, condition of two-division suppression pool cooling weighted by the complement 90% plant-state probability.

As described below, two additional modifications must be applied to risk contributed by two-division suppression pool cooling in order to justify total risk in Risk Region III. The modified baseline assumptions to reduce analyzed risk for two-division suppression pool cooling apply numerical credit for both of the following:

a. Credit operator ability to recognize debris induced pump labor and rotate flow to existing available pumps and strainers (assumed 20% failure rate), and

b. Credit limited damage to penetration Min-K' afforded by the presence of hydraulic pipes inside every penetration (25% damage reduction).

Risk from two-division suppression pool cooling is calculated in the following manner.

First, it is expected that all pumps will respond at their maximum, injection-limited, flow rates to initial indications of a LOCA. If all pumps in both divisions were allowed to operate at maximum flow, supporting calculations (Ref. 3 - SERCO-REP-DTE-22929-02, Rev. 0) show that no strainers would exceed the 1/8th-inch debris thickness criterion for any postulated breaks.

This case demonstrates that it is always beneficial to share debris between as many operating strainers as possible.

Second, ignore the benefit of initial two-division automatic debris accumulation on all active strainers and assume operators quickly diagnose reactor pressure and flood level and elect to turn off all Division-II pumps and establish the suppression pool cooling flow rates defined in Table

1.

to NRC-24-0033 Page 7 Table 1. Assumed Suppression Pool Cooling Flow Rate Assignments.

Suppression Pool Cooling (Strainer Flow Rate (gpm))

Division Pump 0 - 20 min

>20 min I

RHR-A 15540 10500 II RHR-B 0

0 I

RHR-C 14550 10500 II RHR-D 0

0 I

CS-A 6350 6350 I

CS-C II CS-B 0

0 II CS-D Supporting calculations for the Table 1 flow configuration (from Ref. 3 - SERCO-REP-DTE-22929-02, Rev. 0, Table 5-1, Case 3A - See Table 5 below) estimate the risk contributions identified in column 2 of Table 2 as Direct Risk, which have not yet been reduced by baseline modifications 2a or 2b.

Table 2. Non-Isolable Risk Contributions for Two-Division Suppression Pool Cooling.

Contributor Direct Risk (per year)

Two-Div Plant-State Probability Conditional Risk (per year)

Bed Thickness Exceedance (alone) 3.11E-07 0.9 2.80E-07 Bed Thickness and DB Exceedance (both) 5.53E-07 0.9 4.98E-07 DB Exceedance (alone) 2.34E-06 0.9 2.11E-06 Total Non-Isolable Risk 3.20E-06 0.9 2.88E-06 Similar results (from Ref. 3 - SERCO-REP-DTE-22929-02, Rev. 0, Table 5-1, Case 2A - see Table 5 below) are shown in Table 3 for non-isolable risk contributions for baseline one-division LPCI response.

Table 3. Non-Isolable Risk Contributions for One-Division LPCI response.

Contributor Direct Risk (per year)

One-Div Plant-State Probability Conditional Risk (per year)

Bed Thickness Exceedance (alone) 3.10E-07 0.1 3.10E-08 Bed Thickness and DB Exceedance (both) 5.46E-07 0.1 5.46E-08 DB Exceedance (alone) 2.34E-06 0.1 2.34E-07 Total Non-Isolable Risk 3.19E-06 0.1 3.19E-07 to NRC-24-0033 Page 8 Baseline Modification 2a - Credit Operator Action Direct application of Table 2 Conditional Risks would ignore the fact that two idle RHR strainers and one idle CS strainer are available to draw necessary cooling flow. Operators are trained to recognize symptoms of debris-induced strainer issues, including increasing pump current, loss of level, loss of pressure, and mechanical vibration in association with active backflush procedure 20.000.29 (Fermi, "Plant Mechanical Procedure - Fermi 2 Abnormal Operating Procedure: LPCI Suppression Pool Suction Strainer Clogging, 20.000.29, Rev. 5," November 5, 2019). In addition, operators have great flexibility to reduce flow on any given system and rotate systems as needed to respond to any LOCA scenario. Recognition of debris blockage symptoms does not imply that a backflush will be performed. Operators must only manage flow using common Emergency Operating Procedures (EOP) and equipment available under successful, automatic, two-division LOCA response to avoid ECCS failure under the proposed modified baseline assumption.

Given the availability of idle equipment under automatic, two-division response, and operator awareness of potential debris accumulation challenges in suppression pool cooling mode, a strong argument can be made that nearly all of the Table 2 Conditional Risk (last column) represents analytic safety margin. It would be very unusual for the normal two-division response with all pumps operable to contribute more risk (2.88E-06/yr from Table 2) than loss of a full division (3.19E-07/yr from Table 3). Table 2 Conditional Risks are inordinately high because the Table 1 pump flow histories do not acknowledge the ability to rotate flow to idle equipment.

Assuming a conservative 1 in 5 chance (20%) that operators do not recognize symptoms of strainer blockage and/or cannot prevent ECCS failure using existing operable resources (i.e.,

must resort to some form of alternate injection that is presently reserved as defense in depth), the total modified baseline risk is (3.19E-06)(0.1) + (3.20E-06)(0.9)(0.20) + 2.12E-07 = 1.11E-06/yr Single-Div NonIso + Two-Div NonIso + Isolable = Total Risk A formal Human Reliability Assessment (HRA) could reduce the failure probability of the provide additional credit for operator action to further reduce the risk from Two-Division Suppression Pool Cooling scenarios, given that no new accident progressions are added and the fact that operators normally monitor and adjust pump flow rates.

Baseline Modification 2b - Credit Reduced Penetration Min-K' Damage Supporting calculations (Ref. 3 - SERCO-REP-DTE-22929-02, Rev. 0) describe how even a small reduction in assumed penetration Min-K' damage can significantly reduce risk from all non-isolable breaks (both One and Two-Division Response). It is evident in Figure 4 that a 25%

reduction in penetration Min-K' source terms would induce more than a factor of 2 reduction in risk contributed from DB Min-K' violations (the dominant risk contributor) for One-Division baseline flow rates. To recreate this observation, first note that each curve in Figure 4 displays an exceedance function of cumulative risk for DB Min-K' violations (y axis) as functions of the amount of Min-K' delivered to the suppression pool. The upper-right, light-blue function

to NRC-24-0033 Page 10 K' failure are assumed not to change, even though the amount of Min-K' debris counted as fiber is being reduced. The second term in each square bracket (1.39E-06), the reduced risk of exceeding the DB Min-K' limit after a 25% reduction in penetration Min-K' damage, replaces the sum of Table 2 and Table 3 rows labeled Bed Thickness and DB Exceedance (both) and DB Exceedance (alone). The sum of these two contributions represents the total risk from exceeding the DB Min-K' limit before the 25% reduction in penetration Min-K' damage is applied.

Note that the modified baseline risk of 6.88E-07/yr presented in this supplement is lower than the modified baseline risk reported in Ref. 3 - SERCO-REP-DTE-22929-02, Rev. 0 because additional risk reduction is applied here for limited Min-K' debris generation. Modified baseline risk would vary if different reduction factors were chosen for Operator-Failure-to-Diagnose-Debris-Accumulation and Reduced-Penetration-Min-K'-Damage. For example, if a detailed HRA demonstrated that operators are much more effective than the assumed 20% failure rate, then either total modified baseline risk would decrease, or the credit for Reduced-Penetration-Min-K'-Damage could be reduced to maintain the same modified baseline risk (6.88E-07/yr). This example explains the total modified baseline risk reported in this LAR - Technical Supplement and illustrates the potential tradeoff between two independent analysis assumptions.

Risk Analysis Insight Supporting analyses (Ref. 3 - SERCO-REP-DTE-22929-02, Rev. 0) also show that a single, relatively small, 3.63-inch diameter (and therefore relatively high-frequency) break contributes 46% of the non-isolable break risk for the suppression pool cooling flow histories defined in Table 1. Proactive monitoring of this weld to substantiate lower break frequency, or removal of Min-K' at a whip restraint within its ZOI would result in modified baseline risks well within RG 1.174 Risk Region III.

Comparison of Complementary Strainer Failure Criteria The LAR baseline analysis explains that two strainer failure criteria are applied to debris accumulation for every postulated break. ECCS failure and subsequent core damage are assumed to occur if any single strainer either 1) accumulates 1/8th-inch of fiber debris, or 2) exceeds the DB limit for any debris constituent. LAR Att-3 presents results that do not include penetration Min-K' as a debris target, effectively assuming complete protection of the insulation in confined annular gaps from non-isolable breaks occurring outside of the penetrations, so no failures were found that exceed DB strainer limits. These results placed a critical focus on the 1/8th-in. bed thickness limit (first failure criterion) as the only measure of strainer failure and the only means of risk contribution. Revised calculations presented in this Supplement (LAR Att-4),

which now include penetration Min-K', show that exceeding strainer DB limits for Min-K' (second failure criterion) is the dominant risk contributor.

to NRC-24-0033 Page 11 The two failure metrics serve separate complementary purposes.

1. The fiber thickness failure criterion is intended to establish a minimum fiber load below which no appreciable head loss can occur and provide a measure of strainer performance that is sensitive to miscellaneous debris obstruction. A fiber thickness of 1/8th-inch has traditionally been used as a threshold for testing whether a strainer having complex geometry can form a contiguous layer of fiber capable of filtering particulate (a thin bed).

In recognition of concerns that Min-K', a microporous material, can aid in formation of a thin bed with less than 1/8th-inch of equivalent fiber thickness, a sensitivity case was evaluated using a 1/16th-inch failure limit (see Section 4 below). Risk contributions from bed thickness failure increase by 10% to 40% depending on the flow history examined, but do not equal risk contributions from exceeding the DB Min-K' limit. It is difficult to induce and sustain appreciable head loss with so little debris (1/16th to 1/8th-in of fiber),

but many cases tallied as failures from DB Min-K' exceedance have even less total fiber thickness. Thus, the fiber thickness failure criterion also adds perspective on the conservatism of strictly applying the second criteria to very small debris inventories.

2. The DB debris limits are intended to confirm that all scenarios recorded as successful strainer performance have debris loads that do not exceed any of the debris constituent quantities they are qualified for, and thus, do not pose new risks that contribute to CDF.

No cases are found that exceed DB fiber loads, but exceedance of DB Min-K' limits is now the dominant risk contributor. Adherence to DB debris loads for most break scenarios provides an important check to ensure strainer performance, but for very small debris loads, less than the bed-thickness failure criterion that defines the onset of appreciable head loss, relevance of the DB Min-K' limit is questionable. Small, higher frequency breaks that dont have enough total debris to induce and sustain head loss are being assigned to failure for exceeding the DB Min-K' limit that was established as a constituent important to a maximum debris load mixture. These observations draw attention to the conservatism inherent to the DB Min-K' limit as currently applied.

However, no adjustments have been made to compensate for this conservatism in the modified baseline risk quantification, so it is considered safety margin inherent to the analysis.

4. Sensitivity Case Summary Reference 3 (SERCO-REP-DTE-22929-02, Rev. 0) reproduces the revised baseline risk (with penetration Min-K') from Reference 2 (ALION-CAL-SI-9045-132, Rev. 2) and analyzes a suite of alternate assumptions representing variations to individual baseline parameters.

Examination of alternate assumptions provides perspective on the sensitivity of Core Damage Frequency (CDF) to key input choices. All results include the CAD model changes and ECCS failure metrics described above. Seven parameter variations are examined:

to NRC-24-0033 Page 12

1. three choices of total strainer face area obstruction by miscellaneous debris (65 ft2, 100 ft2, and 120 ft2);

2. three fiber failure thicknesses (1/16th in., 1/8th in., and 1 in.);

3. two methods of aggregating expert opinion for break frequency quantification (Arithmetic Mean and Geometric Mean);

4. three debris transport periods (0.1 min, 1 min, and 10 min),

5. two design basis Min-K' debris volume limits (0.334 ft3 and 0.536 ft3 on RHR strainers and 0.186 ft3 and 0.297 ft3 on CS strainers),

6. application of 10% single-division and 90% dual-division plant-state probabilities; and

7. eight strainer flow conditions, including dual and single-division suppression pool cooling, applied to every sensitivity case.

Table 4 itemizes all baseline and sensitivity parameter values analyzed.

Table 4. Baseline and Sensitivity Parameter Values.

Parameter Baseline Value Sensitivity Cases a

b 1

Obstructed strainer face area 100 ft2 65 ft2 120 ft2 2

Fiber thickness failure threshold 1/8th inch 1/16th 1 inch*

3 Break frequency aggregation method Geometric Mean (GM)

Arithmetic Mean (AM) 4 Debris introduction time 0.1 min (6 sec) 1 min 10 min 5

Design basis debris limits Min-K' on RHR strainers 0.334 ft3 0.536 ft3**

Min-K' on CS strainers 0.186 ft3 0.297 ft3**

Fiber on RHR strainers 13.5 ft3 Fiber on CS strainers 7.5 ft3 6

Plant-state probabilities 0.1 Single-Div 0.9 Dual-Div applied as needed 7

Isolable break risk 2.12E-7/yr applied to all 8

System response One Division, Run-out multiple

• No break exceeds 1-in. of fiber. This case turns off the thickness failure risk criterion.

• • Additional capacity arises from an identified change in Min-K' product density.

to NRC-24-0033 Page 13 Risk results are tabulated by running identical debris accumulation simulations for all 887 non-isolable weld breaks described in the LAR and summing the frequencies for all events that exceed the defined failure criteria. Table 5 reproduces Table 5-1 from Ref. 3 - SERCO-REP-DTE-22929-02, Rev. 0 and presents risk results and times to failure for the baseline assumptions (Case 2A) and for seven alternative pump flow configurations, holding all other baseline parameters constant.

Note that Non-Isolable Break Risk (third row from bottom) equals the sum of 3 separate failure conditions listed in previous rows: 1) Thickness Exceedance Risk (alone) where a break violates ONLY the fixed thickness criterion, 2) Thickness AND Design Basis Exceedance where a break exceeds BOTH the fixed thickness AND the DB limit for either Min-K' OR fiber, and 3) DB Exceedance Risk (alone) where a break violates ONLY a DB debris limit. It has been determined that there are no violations of DB fiber limits, so all risks entered in row DB Exceedance Risk (alone) are contributed solely by exceedance of DB Min-K' limits. Independently estimated risk from isolable breaks (2.12E-07/yr from Ref. 2) appears as a fixed value in all tables. Also, total risk results exceeding the 1E-06/yr upper limit of RG 1.174 Risk Region III are highlighted in light red for further discussion. Table 5 results do not include any modifications to baseline assumptions, except for presenting risks for alternate flow rates. Definitions and rationale for each analyzed flow configuration are provided in Table 6, which provides a common set of footnotes for all risk results.

Comparison of the Table 5 row labeled DB Exceedance Risk (alone) to the Table 5 row titled Total Risk (1/yr) serves to emphasize that exceedance of the DB Min-K' limit is dominating total calculated risk. It is also important to understand that all break scenarios contributing risk in this row have less than 1/8th-in. of equivalent bed thickness, which implies that smaller breaks with relatively high frequency generate enough Min-K' debris to exceed the per strainer DB limit, but not enough debris to form a classical thin bed. Given the importance of the DB Min-K' failure criterion, it is prudent to examine the potential effect that source term uncertainty may introduce to the risk quantification.

Figure 5 illustrates how smaller breaks at Fermi dominate risk of exceeding the single-strainer DB Min-K' limit. The top panel shows Min-K' debris in the suppression pool (y axis) for every postulated break size (x axis). Lines of stacked points represent breaks having common discrete weld diameters. Marked on the dashed line is the smallest debris source generated that can exceed the DB Min-K' limit on at least one strainer. All cases generating Min-K' debris equal to or greater than (above) this limit will contribute to DB failure risk, which is shown in the lower panel of Figure 4 as an exceedance function. The first blue dot in the upper center denotes a 3.63-in.

diameter break that causes approximately 50% of the baseline risk caused by DB Min-K' limit violations. DEGB on welds larger than 20-in. diameter make up 10% of the total baseline risk caused by Min-K' limit violations.

to NRC-24-0033 Page 17 Results parallel to those described for the baseline evaluation presented in Table 5 were generated for all sensitivity cases defined in Table 4. Sensitivity parameters were varied one at a time, while all other parameters remained fixed at baseline values. Table 7 summarizes raw sensitivity study results without modified baseline assumptions (i.e., plant-state probabilities, operator diagnosis of debris accumulation, and reduced penetration Min-K' debris). Results are presented as if each flow history and sensitivity combination stands alone as an independent total risk estimate. On this basis, risk from each sensitivity case can be compared to baseline results to help judge the importance of each parameter variation. Summation of select one-division and two-division cases is discussed in Section 3 - Baseline and Modified Baseline Risk with Penetration Min-K' to arrive at the total modified baseline risk (6.88E-07/yr).

Trends and comparisons observed between the sensitivity studies are summarized in Ref. 3 -

SERCO-REP-DTE-22929-02, Rev. 0, Section 5.3 - Parameter Sensitivity Studies, p. 31 of 84.

Note that differing flow conditions are denoted by upper case letters (Case 2A, Case 2B, etc.)

and parameter variations are distinguished by lower case letters (Case 2a, Case 3a, etc.). The more important findings include:

1. With exceptions noted for Case 2a (1/16th inch fiber thickness failure) and Case 3a (AM break frequencies), the most important finding in the Table 7 risk sensitivity summary is the fact that risks for Suppression Pool Cooling (SPC) flow histories (Case 3A and 3B) are relatively stable for all sensitivity parameter variations. Risk tabulated for SPC depend on the exact flow histories that are followed, and it is possible that flow combinations like Case 3A and 3B, which are labeled as One-Division flow responses, can be established by operator actions even for Two-Division system response. None of the results in Table 7 credit operator ability to rotate flow across all functional strainer/pump systems.

2. Case 2a risks (1/16th-inch fiber thickness failure criterion) are not dramatically higher than baseline risks, ranging from a 10% to a 39% increase between bold entries in the column labeled Baseline and the column labeled 2a, indicating that the 1/8th-inch baseline failure threshold captures the dominant portion of risk contributed by this failure mode. The Case 2a failure threshold of only 1/16th-inch of fiber is very stringent. It is not realistic to imagine that such a delicate debris layer can form contiguously and simultaneously and induce unacceptably high head loss on six strainers operating at high velocity.

3. Sensitivity Case 2b (1-in. thickness failure) effectively turns off any risk contributions from the fiber thickness failure criterion. Comparison of Non-Isolable break risk between Case 2a (1/16th-inch failure thickness) to Case 2b (1-in failure thickness) shows that risk added by monitoring fiber thickness failure at a very stringent 1/16th-in. thickness (5.27E-07 - 2.88E-07 = 2.39E-07/yr) contributes less risk than from exceeding DB Min-K' limits (compare 2.39E-07/yr from thickness failure vs. 2.88E-07/yr from DB Min-K' exceedance). This is a startling observation. Debris beds formed with only 1/16th-inch of fiber are filmy, tenuous, noncontiguous, do not filter particulates efficiently, and cannot sustain high head loss. Yet, the combined risk of ALL events delivering more 1/16th-in.

to NRC-24-0033 Page 18 or more of fiber on any single strainer is less than the risk of exceeding the DB Min-K' limit on any single strainer. This comparison illustrates how stringent the DB Min-K' limit is compared to the bed thickness limit, but both conditions are applied as assumed failure modes in all risk calculations.

4. Risks tabulated for Case 1a (65 ft2 of miscellaneous debris) and Case 1b (120 ft2 of miscellaneous debris) are nearly identical to the baseline risk assuming 100 ft2 of miscellaneous debris, showing that total miscellaneous debris area has little effect on risk within the debris area and bed thickness ranges of interest.

5. Sensitivity Case 5a (Alternate DB loads) illustrates a fairly significant reduction of risk from Non-Isolable breaks between baseline DB debris loads that retain identified safety margin and increased DB debris loads that explicitly assign the margin for the purpose of risk reduction (3.19E 1.44E-07 = 1.75E-07/yr, a 55% risk reduction). The identified margin arises because of a reduction in Min-K' density from 16 lbm/ft3 to 11.4 lbm/ft3 that effectively reduces Min-K' debris mass associated with the qualified Min-K' debris volume. Baseline assumptions are not modified to explicitly apply the risk reduction, leaving the safety margin available for use in other plant calculations.