ML25356A494
| ML25356A494 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 12/18/2025 |
| From: | Mahesh Chawla Plant Licensing Branch IV |
| To: | Elwood T Ameren Missouri, Union Electric Co |
| Chawla M | |
| References | |
| EPID L-2025-LLA-0040 | |
| Download: ML25356A494 (0) | |
Text
From:
Mahesh Chawla To:
Elwood, Thomas B Cc:
Tony Nakanishi; Michael Mahoney; Khoi Nguyen; Liliana Ramadan; Wendell Morton; Derek Scully; Milton Valentin; Samir Darbali; Youssef Rokes; Thomas Scarbrough; Stewart Bailey; Ahsan Sallman; Nicholas Difrancesco; Khadijah West; Shivani Mehta; Ami Agrawal; Nathan Brown; Morton, Jeremy W
Subject:
Additional Audit Questions - Callaway - LAR to Revise Allowable Diesel Generator Frequency Range for TS 3.8.1 -
EPID: L-2025-LLA-0040 Date:
Thursday, December 18, 2025 1:59:00 PM Attachments:
NPSH Information on CS RHR Pumps in Supplement.docx Callaway EDG Audit Response Review_EMIB.docx
Dear Mr. Elwood,
By letter dated February 27, 2025 (Agencywide Documents Access and Management System (ADAMS) Package Accession No. ML25058A155), Union Electric Company, dba Ameren Missouri (the licensee), submitted a license amendment request (LAR) for Callaway Plant, Unit No. 1 (Callaway). The proposed LAR would revise the emergency diesel generator frequency acceptance criteria for Technical Specification 3.8.1, AC
[Alternating Current] Sources - Operating, surveillance requirements. On May 27, 2025 (ML25120A119), the U S. Nuclear Regulatory Commission (NRC) staff issued an audit plan which identified the need for a regulatory audit to examine Callaways non-docketed information with the intent to gain understanding, verify information, or identify information that will require docketing to support the basis of the licensing or regulatory decision. The NRC staff generated audit questions, which were included in the audit plan. On July 30, 2025, the NRC staff generated audit follow-up questions, which were transmitted to you via an email (ML25219A301). The NRC staff generated additional audit questions which were transmitted on September 10, 2025 (ML25258A070). The licensee provided draft responses which were uploaded to the electronic portal.
On December 9, 2025, an audit discussion was held to discuss the audit question no.10 submitted by Nuclear Systems Performance Branch (SNSB). The During the call the reviewer requested further clarification regarding the calculation provided to address net positive suction head (NPSH) for Containment Spray (CS) and Residual Heat Removal (RHR) pumps. However, during this audit meeting, Mechanical Engineering and Inservice Testing Branch (EMIB) was not available. The NRC staff has revised the previously submitted question no. 10 (attached), and EMIB has provided their review of the responses to their audit questions (attached), so the licensee can address the requested information.
Please let me know if you have any questions and if you would like to discuss further with the NRC staff. Thanking you, Sincerely, Mahesh(Mac) Chawla, Project Manager Licensing Branch LPL4 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Phone: (301) 415-8371 OFFICE NRR/DORL/LPL4/PM NRR/DORL/LPL4/BC(A)
NAME MChawla MMahoney DATE 12/18/2025 12/18/2025
NRC AUDIT QUESTION ON CS AND RHR PUMPS NET POSITIVE SUCTION HEAD (NPSH)
MARGIN For the pumps that draw water from the sump during the loss-of-coolant accident (LOCA) recirculation phase, the sump temperature response and pump flow affects their NPSH available (NPSHA) and NPSH required (NPSHR) respectively. Provide the revised NPSH analysis, if exists, including containment accident pressure used to confirm positive NPSH margin for those pumps.
Information given below is taken from the Certrec Portal file titled Callaway NPSH from Sump Summary August 21 rev.
Verify the accuracy or correct the information given below Provide the missing information for highlighted items indicated by ?.
Information to be Included in the Supplement in Response to Above Audit Question The containment spray (CS) and residual heat removal (RHR) pumps draw water from the sump during the LOCA recirculation phase. The NPSH calculations for both sets of pumps take into account the sump water temperature and do not take credit for the containment accident pressure (CAP) above the saturation pressure at the sump temperature in the available NPSH calculation.
Containment Spray (CS) Pumps NPSH Margin Assumptions:
Minimum sump fluid level elevation at the swap over time to recirculation Sump fluid at the maximum temperature of 270 deg F obtained from LOCA sump temperature response Credit is not taken for CAP above the saturation pressure at sump fluid temperature NPSHR is 16.6 ft (Reference FSAR Table 6.2.2-7)
Pump flow rate increase linearly with speed Nominal Speed = ? at nominal frequency = ?
Analysis pump speed = ? at the maximum frequency of 60.3 Hz Percent speed change from nominal frequency to maximum frequency = ?
Pump developed head and NPSH required (NPSHR) increases with the square of the pump speed Required NPSH = 16.5 x (1786.78/1770)2 = 16.81 ft NPSH Margin Results CS Pump NPSH Margin Versus Pump Speed Speed
(%
Nominal)
Flow (gpm)
Temp (deg F)
NPSHA (ft)
NPSHR (ft)
CAP Used (ft)
(Note
- 1)
NPSH Margin (ft)
(minimum)
Sump Strainer Loss (ft)
NPSH Margin after Strainer Loss (ft) 100
?
270 22.1 16.6 0
5.6
?
?
?
?
270 22.1 16.8 0
5.3
?
?
Notes:
The limiting NPSH margin for RHR NPSHA occurs during the recirculation phase at the time of the ECCS pump suction swap over from RWST to the containment recirculation sumps.
The sump water elevation is at minimum at swap over 1 ft 10 inch.
Cold leg recirculation is the limiting operation for RHR pump NPSH.
The limiting RHR pump NPSHR configuration occurs when only 1 RHR pump is operating when it takes suction from the sump and delivers directly to two RCS cold legs and feeds two centrifugal charging pumps (CCPs) and two SI pumps (SIPs) both of which are injecting into cold legs.
In hot leg recirculation both CCPs continue to inject water through the common header, but each SIP is now aligned with its own hot leg injection header. The maximum flow rate supplied by each SIP can exceed the flow rate during cold leg injection operation. Therefore, the hot leg recirculation typically results in the most limiting NPSHR since the RHR pump flow rate is typically higher in this mode than the cold leg recirculation phase.
An Applied Flow Technology (AFT) Fathom 6.0 ECCS flow model is used to determine the effects on the RHR pump NPSH of the new sump strainer and screen head loss values in the cold leg and hot leg recirculation phase. Fathom is a fluid dynamic simulation software to calculate pressure drop and pipe flow distribution in liquid and low-velocity gas piping and ducting system.
FATHOM 6.0 version has been verified for safety related use by calculation ARC-587. [State if the FATHOM software complies with 10CFR50, Appendix B and applicable portions of ASME NQA-1].
[Information from CERTREC file on NPSH - item 2 under NPSH Calculation Summary and Table 5]
RHR Pump NPSH Margin - Results Based on FATHOM Runs RHR Pump CLR (Note
- 1)
Speed (%
Nominal)
Flow (gpm)
(Note
- 1)
Temp (deg F)
NPSHA (ft)
NPSHR (ft)
CAP Used (ft)
(Note
- 2)
NPSH Margin (ft)
(minimum)
Sump Strainer Loss (ft)
NPSH Margin after Strainer Loss (ft)
A 100 4800
?
25.75 21.31 0
4.44
?
?
B 100 4800
?
25.74 21.73 0
4.01
?
?
Notes
- 1. During cold leg recirculation (CLR), the minimum sump water level is 1 ft-10 inch prior to swap over the containment spray pumps
[Information from CERTREC file on NPSH - item 5 under NPSH Calculation Summary and Table 9]
RHR Pump NPSH Margin with Increased Pump Speed - Results Based on FATHOM Runs RHR Pump CLR (Note
- 1)
Speed (%
Nominal)
Flow (gpm)
Temp (deg F)
NPSHA (ft)
NPSHR (ft)
CAP Used (ft)
(Note
- 2)
NPSH Margin (ft)
(minimum)
Sump Strainer Loss (ft)
NPSH Margin after Strainer Loss (ft)
A 102.5
?
?
25.99 21.22 0
4.77
?
?
B 104.3
?
?
25.88 21.90 0
3.98
?
?
Notes
- 1. During cold leg recirculation (CLR), the minimum sump fluid level is 1 ft-10 inch prior to swap over the containment spray pumps
T. Scarbrough 12-18-2025 Callaway EDG Audit Question Review Yellow highlighted questions need licensee follow-up for specific questions.
Question 13 (EMIB): Describe the plant-specific evaluation methodology used to assess the impact of the revised EDG frequency steady-state limits. Make available applicable technical analyses, calculation packages, procedures, or test reports used to justify component performance, organized by component class: ECCS pumps, MOVs, fans/blowers, uninterruptible power supply (UPS), fuel oil transfer pumps, ESW strainers, SFP cooling.
Licensee Response: RFR 220278 and Reports M-018-01009, 01010, and 01011.
NRC Follow-up:The licensee is requested to provide on the docket a brief nonproprietary summary of RFR 220278 and M-018-01009, 01010, and 01011 (about a paragraph for each document) that can be referenced in the NRC safety evaluation.
Question 14 (EMIB): Make available any updated load qualification matrix or evaluation verifying that pump and fan performance curves support flow, pressure, and margin assumptions within the revised frequency range.
Licensee Response: RFR 220278 and Reports M-018-01009, 01010, and 01011.
NRC Follow-up:
Callaway M-018-01009, Implementation of WCAP-17308-NP-A for Callaway Safety Related Direct Drive Fans, (Westinghouse LTR-SEE-21-58, Revision 1) in Section 2.0 states the calculated flow rates for the cooling tower fans and containment cooler fans will be provided to Ameren for use in downstream evaluations. What licensee actions have been taken in response to these results?
Ameren M-018-01011, Implementation of WCAP-17308-NP-A for Callaway Safety Related Belt Driven Fans (Westinghouse SCP-HV040-TR-MF-000001, 8-26-2024) in Section 4.6, Acceptance Criteria, states that Ameren will determine acceptability of the variation in fan flow rate due to the allowable variations in voltage and frequency for belt driven fans. What licensee actions have been taken in response to these results?
Question 15 (EMIB): Make available any documentation demonstrating that all safety-related mechanical loads (ECCS pumps, residual heat removal (RHR), safety injection (SI),
containment spray (CS)) remain within their qualified frequency operating range under the proposed change.
Licensee Response: RFR 220278 and Reports M-018-01009, 01010, and 01011.
NRC Follow-up:
RFR 220278 provides M-018-01010 calculation results.
What licensee actions have been taken in response to the following M-018-01010 calculation results?
ESW pumps: ESW flow verification procedures should be revised to ensure that ESW flow to each component is at least 1% above the required value in the post-LOCA lineup.
CCW Pumps: The CCW flow verification procedure should be modified to ensure that the CCW flow to each component is at least 1% above the required value in the post-LOCA line-up.
Motor Driven AFW Pumps: AFW flow verification procedures should be modified to ensure that the AFW flow is at least 1% above the required minimum and maximum values in the required AFW line-up.
RHR Pumps: RHR comprehensive pump test procedures should be modified to adjust the applicable flow channels.
Also, why are the results in M-018-01010 for MDAFW pumps different from the list in RFR 220278?
Question 16 (EMIB): Make available any re-analysis or confirmation that the new frequency limits do not challenge NPSH requirements of safety-related pumps during LOCA/recirculation.
Licensee Response: RFR 220278 and Reports M-018-01009, 01010, and 01011.
NRC Follow-up: NPSH questions provided by Ahsan (SNSB).
Question 17 (EMIB): Make available any updated design basis calculations or vendor analysis verifying that pump motors and valves operate within torque/speed ratings at the proposed frequency range.
Response: RFR 220278 and Reports M-018-01009, 01010, and 01011.
NRC Follow-up: The licensee provided the documents as requested. No additional licensee action is needed for Question 17.
Question 18 (EMIB): Make available any updated evaluation showing that margin classifications for power operated valves are unaffected by the proposed frequency range.
Licensee Response: RFR 220278 and Reports M-018-01009, 01010, and 01011.
NRC Follow-up: RFR 220278 states in Section 4.4 that 24 Rising Stem MOVs were determined to have a potential for more frequent periodic testing. What licensee actions have been taken to in response to these results?
Question 19 (EMIB): Provide any updated evaluations demonstrating the acceptability of EDG operation within the proposed steady-state frequency range, including verification that operation remains within the limits and recommendations specified by the EDG manufacturer.
Licensee Response: RFR 220278 and Reports M-018-01009, 01010, and 01011.
NRC Follow-up: The licensee provided the documents as requested. No additional licensee action is needed for Question 19.