ML25338A039
| ML25338A039 | |
| Person / Time | |
|---|---|
| Site: | 99900003 |
| Issue date: | 12/04/2025 |
| From: | NRC |
| To: | |
| References | |
| Download: ML25338A039 (0) | |
Text
From:
Getachew Tesfaye Sent:
Thursday, December 4, 2025 7:51 AM To:
GEH-BWRX-300RAIsPEm Resource
Subject:
FW: LTR NEDC-34270, Revision 0, BWRX-300 Stability Analysis" -
Request for Additional Information No. 001 (RAI-10910-R1))
Attachments:
NEDC-34270P (BWRX300 Stability LTR) - RAI-10910-R1-Final.pdf From: Getachew Tesfaye Sent: Tuesday, November 18, 2025 12:41 PM To: Enfinger, Timothy (GE Vernova, GCCH) <timothy.enfinger@gevernova.us>; Karkour, Suzanne (GE Vernova, GCCH) <suzanne.karkour@gevernova.us>
Cc: Stacy Joseph <stacy.joseph@nrc.gov>; Mahmoud -MJ-Jardaneh <Mahmoud.Jardaneh@nrc.gov>
Subject:
LTR NEDC-34270, Revision 0, BWRX-300 Stability Analysis" - Request for Additional Information No. 001 (RAI-10910-R1))
Attached please find NRC staffs request for additional information (RAI) concerning the review of GVH Licensing Topical Report (LTR) NEDC-34270P/NEDO-34270, Revision 0, BWRX-300 Stability Analysis" (Agencywide Documents Access and Management System Accession Nos.
ML25090A106, non-public and ML25090A107-public).
Please submit your technically correct and complete response by the agreed upon date to the NRC Document Control Desk.
If you have any questions, please do not hesitate to contact me.
Thank you, Getachew Tesfaye (He/Him)
Senior Project Manager NRC/NRR/DNRL/NRLB 301-415-8013
Hearing Identifier:
GEH_BWRX300_RAIs_Public Email Number:
22 Mail Envelope Properties (BY5PR09MB56821A59CC164CC9556531518CA6A)
Subject:
FW LTR NEDC-34270, Revision 0, BWRX-300 Stability Analysis - Request for Additional Information No. 001 (RAI-10910-R1))
Sent Date:
12/4/2025 7:51:03 AM Received Date:
12/4/2025 7:51:06 AM From:
Getachew Tesfaye Created By:
Getachew.Tesfaye@nrc.gov Recipients:
"GEH-BWRX-300RAIsPEm Resource" <GEH-BWRX-300RAIsPEm.Resource@usnrc.onmicrosoft.com>
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1 REQUEST FOR ADDITIONAL INFORMATION No. 001 (RAI-10910-R1)
BY THE OFFICE OF NUCLEAR REACTOR REGULATION GVH BWRX-300 REVIEW OF STABILITY ANALYSIS TOPICAL REPORT (NEDC-34270P)
GE VERNOVA HITACHI NUCLEAR ENERGY DOCKET NO. 99900003 ISSUE DATE: 11/18/2025
=
Background===
By letter dated March 31, 2025, GE-Hitachi Nuclear Energy Americas, LLC (GEH) [new name:
General Electric Vernova Hitachi Nuclear Energy Company (GVH)] submitted Licensing Topical Report (LTR) NEDC-34270P/NEDO-34270, Revision 0, BWRX-300 Stability Analysis" (Agencywide Documents Access and Management System Accession Nos. ML25090A106, non-public and ML25090A107-public) for staff review to support BWRX-300 licensing applications and is specific to the BWRX-300 design. By letter dated May 21, 2025, GEH submitted NEDC-34043P/NEDO-34043, Revision 1, BWRX-300 TRACG Application, to Supplement NEDC-34270P, Revision 0 (ADAMS ML25141A241, non-public and ML25141A242, public). The NRC staff has reviewed the information in NEDC-34270P and the supplemental information and determined that additional information is required to complete its review.
Regulatory Basis 10 CFR Part 50 Appendix A, General Design Criteria 10 requires that the reactor core and associated coolant, control, and protection systems shall be designed with appropriate margin to assure that specified acceptable fuel design limits are not exceeded during any condition of normal operation, including the effects of anticipated operational occurrences.
10 CFR Part 50 Appendix A, General Design Criteria 12 requires that the reactor core and associated coolant, control, and protection systems shall be designed to assure that power oscillations which can result in conditions exceeding specified acceptable fuel design limits are not possible or can be reliably and readily detected and suppressed.
Question Stability-LTR-1 Issue description In Section 1.3 of the proposed update to NEDC-34270P, Revision 0, BWRX-300 Stability Analysis, GVH stated Type 1 oscillations are inherent in natural circulation BWRs and characterized by initiation of vapor production in the chimney region leading to a reduction in hydrostatic head in the chimney and a resultant core flow increase, which, in turn, could cause voids to collapse in the chimney.This type of oscillation is unavoidable in a natural circulation reactor.. However in NEDE-33083 Supplement 1P-A, TRACG Application for ESBWR Stability Analysis, (Reference 6-3 In ESBWR FSAR ), GEH indicated in its response to Request for Additional Information (RAI)-1 (ADAMS # ML050060158) that Type 1 oscillations result in minimal change in core moderator density, the effect on power response is minimal, and the Specified Acceptable Fuel Design Limit (SAFDL) is not challenged. Similar to these evaluation results of ESBWR Type 1 instability during startup, BWRX-300 Type 1 instability evaluations for operation during start up, as described in NEDC-34270P, also concluded that there was no impact on SAFDLs. However, the evaluation assumed the maximum core power and did not
2 assess the stability at other power levels, such as 10% of the rated power as was assumed for the ESBWR design and claimed negligible impact of two-phase flow instability on reactor power.
At lower power levels with low system pressure, if core wide oscillations occur, SAFDLs could be challenged.
Information Requested GVH is requested to provide a basis for the conclusion that the impact of two-phase flow instability in the BWRX-300 chimney during a startup has a negligible impact on core power and provide the upper bound reactor power oscillation amplitude and CHF margin during start up. If the TRACG code will be used to quantify the impact on total reactor power due to a Type 1 two-phase flow instability, provide the basis that was used to determine that the TRACG code is qualified to conservatively evaluate the Type 1 two-phase flow instability and its subsequent impact on total reactor power and CHF margin for BWRX-300 specific geometry and design using TRACG VESSEL RING nodes. In addition, provide a description of the anticipated start up process and the potential start-up path to avoid or minimize the two-phase flow instability inside the BWRX-300 chimney.
Question Stability-LTR-2 Issue Description In Section 2.5.3.3 of NEDC-34270P, Revision 0 BWRX-300 Stability Analysis, GVH concluded "The analysis of single channel DRs extracted from the limiting exposure core wide pressure perturbation TRACG case was performed. These results demonstrate that the limiting channels do not have DRs higher than the core wide DR. Therefore, only the core wide DR will be confirmed on a cycle-specific basis." Table 2-9 of this LTR showed the limiting channel decay ratio as 0.72, which is approximately the same as the nominal core wide decay ratio (Table 2-7).
Information Requested Since the single channel DRs could be significantly impacted by the hot channel radial power peaking factor and axial power distribution, and corresponding channel conditions, there could be a possibility that the hot channel two-phase flow become unstable while the core wide stability is still maintained. Therefore, GVH is requested to provide justification that only the core wide decay ratio needs to be evaluated to determine the stability margin against Type 2 instabilities without considering the hot channel decay ratio. The staff notes that during the previous ESBWR review, a stability map was constructed by GEH (ADAMS # ML050060158) in a three-dimensional stability phase space as a function of core wide stability decay ratio (DR),
hot channel decay ratio, and regional instability decay ratio. The type of information provided by a similar stability map for BWRX-300 would be one potential way to address this information request if the channel decay ratio is found to be close to the core wide stability decay ratio.
Question Stability-LTR-3 Issue Description In Section 2.5.2.3 of NEDC-34270P, Revision 0 BWRX-300 Stability Analysis, the standard deviation of core wide decay ratios (DR) is calculated for the BWRX-300 with an equilibrium GNF2 fuel core as determined statistically at the limiting cycle exposure point with the highest nominal core wide DR (Table 3-1). This uncertainty is then used to justify a generic nominal TRACG DR acceptance criterion for all other possible GNF2 fuel cycles with the condition that the maximum nominal decay ratio is less than or equal to 0.80. It is unclear if the DR uncertainty would increase if the nominal DR were higher than the highest nominal core wide DR.
3 Therefore, the most limiting DR uncertainty calculation may be expected to occur at a potential cycle exposure condition where the nominal DR = 0.8.
Second, the enrichment distribution in the GNF2 fuel product line varies and therefore the actual enrichment distribution along the fuel rods may be different from the currently assumed GNF2 fuel enrichment distribution. In response to this question, GVH has proposed an update of NEDC-34270P licensing topical report Section 2.5.2.2. However, supplemental information is needed with justifications that the uncertainty either would not increase with a corresponding higher nominal DR or that the uncertainty can be limited to maintain the maximum DR less than 1.0 for all possible GNF2 fuel cycles. The staff also notes that mixed cores for transitions from GNF2 fuel are addressed in Table 2-7, but a limitation for use of the method for non-GNF2 fuel is not provided in the topical report.
Finally, the staff has not found any detailed description of the BWRX-300 normal power operating condition ranges. Without the normal operating condition ranges defined and the demonstration of stability within the allowable operating domain, it is unclear how the claim is justified that BWRX-300 is stable during normal power operation and AOOs prior to the scram.
The staff notes that information similar to what was provided in Figure 15.1-1 and Figure 15.1-3 for ESBWR (ADAMS ML#14099A532) would be one potential way to define the normal operating condition ranges if the BWRX-300 operating pressure has a narrow range around the nominal condition during the normal power operation.
Information Requested In Section 2.5.2.3, the standard deviation of core wide decay ratios (DR) is calculated for the BWRX-300 with an equilibrium GNF2 fuel core as determined statistically at the limiting cycle exposure point with the highest nominal core wide DR (Table 3-1).
- a. Provide justification that the uncertainty statistically determined with an equilibrium GNF2 fuel core at the limiting cycle exposure point can be universally applied to all other possible GNF2 fuel cycles, under different operating conditions within the allowable operation domain with the condition that the maximum nominal decay ratio is less than or equal to 0.80. It is unclear if the DR uncertainty would increase if the nominal DR were higher than the highest nominal core wide DR. Therefore, the most limiting DR uncertainty calculation may be expected to occur at a potential cycle exposure condition where the nominal DR = 0.8.
- b. The enrichment distribution in the GNF2 fuel product line varies and therefore the actual enrichment distribution along the fuel rods may be different from the currently assumed GNF2 fuel enrichment distribution. Supplement the LTR with justifications that the uncertainty either would not increase with a corresponding higher nominal DR or that the uncertainty can be limited to maintain the maximum DR less than 1.0 for all possible GNF2 fuel cycles. Provide a limitation and condition for different fuel designs and cycle designs regarding the uncertainty determination method.
- c. It appears that a BWRX-300 stability map similar to ESBWR shown in Figure 9.1-1 of NEDE-33083 Supplement 1P-A (ADAMS # ML050060158) has not yet been developed. If the BWRX-300 stability map allows a similar diamond shape of normal power operation region other than a single point of nominal full power operation, a demonstration is needed that a stability analysis performed under rated nominal conditions and the statistically determined uncertainty bound all the conditions within the allowable BWRX-300 normal power operation windows and the limiting AOOs initiated from nominal full power operation can bound all AOOs initiated from allowable power operation conditions. If all these are
4 intended to be completed in the future by an applicant to operate a BWRX-300 reactor, describe the scope and the approaches of these anticipated analyses.
Question Stability-LTR-4 Issue Description Benchmark qualifications, as provided in Section 2.4.2, are necessary to demonstrate that the BWRX-300 TRACG stability methodology produces acceptable decay ratio predictions as compared to measured data. However, in the Nine Mile Point 2 (NMP2) benchmark analysis provided in Section 2.4.2.2, the TRACG predicted core power level prior to oscillations are significantly higher than the actual measured power level. Decay ratio increases strongly with power level, therefore a significant power discrepancy would be expected to cause certain misprediction of instability onset time and oscillation growth. During the audit, GVH has noted that APRM measurements become inaccurate when axial power shapes deviate from the nominal power shapes for which they were calibrated. However, the staff notes that the discrepancy in TRACG core power for NMP2 significantly exceeds the discrepancies observed for other flow runback events, including but not limited to Fermi-2 and events in the TRACG qualification basis (NEDE-32177P, Rev. 3).
Information Requested The staff requests that GVH provide additional information regarding the cause of the deviations found in the NMP2 benchmark. In addition, GVH is requested to assess the impact of this power discrepancy and provide further justification that this NMP2 validation is sufficient to demonstrate that the TRACG implicit integration scheme in the core is applicable to BWRX-300.