05000483/LER-2025-001, Refueling Water Storage Tank Lowered Below TS 3.5.4 Limit
| ML25182A246 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 07/01/2025 |
| From: | Ameren Missouri, Union Electric Co |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML25182A244 | List: |
| References | |
| ULNRC-06954 LER 2025-001-00 | |
| Download: ML25182A246 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4832025001R00 - NRC Website | |
text
Abstract
On May 4, 2025, at 1340, with Callaway Plant in Mode 4 ascending, during Refueling Outage (RFO) 27, a Reactor Operator (RO) recognized that the Refueling Water Storage Tank (RWST) borated water level was below the minimum level required by Technical Specification (TS) 3.5.4 of 394,000 gallons (93.7%). Action to raise RWST level commenced at 1405, and at 1427, RWST level was restored to above the TS 3.5.4 minimum level. Review of RWST level data found that tank level had lowered belowthe TS 3.5.4 minimum level at 1230. The minimum RWST level reached during this event was 391,596 gallons (93.1%).
Remedial Actions taken in response to the event include disqualification and remediation of the responsible RO and the Control Room Supervisor (CR5), as well as review of this event and strengthening of expectations regarding communications and oversight. Ongoing corrective actions include strengthening of written instructions.
The cause of the event was the CR5 and RO did not establish a level band for the RWST when it was selected as a source of water for ongoing evolutions. Contributing causes were complacency during lower Modes of Operation and inadequate oversight of this activity.
1. DESCRIPTION OF STRUCTURE(S), SYSTEM(S), AND COMPONENT(S)
The RWST supplies borated water to the Chemical and Volume Control System (CVCS) (EIIS: CB) during abnormal operating conditions, to the refueling pool during refueling, and to the Emergency Core Cooling System (ECCS) (EIIS: JE) and the Containment Spray System (EIIS: BE) during accident conditions. As such, the RWST helps to reduce the containment temperature and pressure following a loss of coolant accident (LOCA) or a main steam line break (MSLB).
The RWST is an austenitic stainless-steel tank containing borated water at a concentration of 2,350-2,500 ppm boron, with an assured water Volume of 394,000 gallons.
Callaway TS 3.5.4, Refueling Water Storage Tank (RWST), requires the RWST to be Operable in Modes 1
, 2, 3, and 4.
With RWST boron concentration not within limits (or RWST borated water temperature not within limits), Condition A applies. Required Action A.I requires restoring the RWST to Operable status within a specified Completion Time of 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. When the RWST is inoperable for reasons other than Condition A, Condition B applies, and Required Action B.I requires restoring the RWST to Operable status within a specified Completion Time of 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />. With the Required Action and associated Completion Time not met for Condition A or Condition B, Condition C applies, and Required Actions C.I and C.2 require the plant to be in Mode 3 within a specified Completion Time of 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5 within a specified Completion Time of 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> respectively.
The Limiting Condition for Operation (LCO) of TS 3.5.4 ensures that a) The RWST contains sufficient borated water to support the ECCS during the injection phase; b) Sufficient water volume exists in the containment sump to support continued operation of the ECCS and Containment Spray System pumps at the time of transfer to the recirculation mode of cooling; and c) The reactor remains subcritical following a LOCA.
Insufficient water in the RWST could result in insufficient cooling capacity when the transfer to the recirculation mode occurs. Improper boron concentrations could result in a reduction of shutdown margin (5DM) or excessive boric acid precipitation in the core following a LOCA, as well as excessive caustic stress corrosion of mechanical components and systems inside the containment.
2. INITIAL PLANT CONDITIONS
At the time of the event, on May 4, 2025, Callaway Plant was in Mode 4 ascending, during RFO 27.
In preparation for entry into Mode 3, the B Chemical and Volume Control (CVCS) demineralizer mixed bed (FBGO3B) was required to be placed into service to add lithium to the reactor coolant system (RCS). The resin in FBGO3B had recently been replaced and required flushing prior to being placed into service in order to remove contaminants in the resin.
According to the procedure used for flushing, flush water may be supplied from the volume control tank (VCT) and be directed back to the VCT, but this can impact VCT chemistry. Flush water may instead be diverted to the Recycle Holdup Tank (RHUT). However, with letdown flow being diverted to the RHUT, a makeup source of water to the VCT is required.
This makeup water can be supplied from the RWST or via blended flow from the Reactor Makeup Water Storage Tank (RMWST) and a Boric Acid Storage Tank (BAST).
The CRS determined that since the RMWST contained a high oxygen concentration, and since utilizing the RWST can provide better control of VCT level and pressure, the RWST was the best source for providing makeup to the VCT. At the beginning of dayshift on May 4, 2025, the RWST level was approximately 95% and Main Control Board Annunciator 47D, RWST Level High or Low, was locked in. The RWST level had been lowered by a previous flush of FBGO3B during the previous nightshift, using the RWST as a makeup water source to the VCT. However, due to the inability of Chemistry to obtain a required sample, another flush was required. A second flush of FBGO3B was therefore initiated at 1140.
3. EVENT DESCRIPTION
On May 4, 2025, at I 140, flushing of FBGO3B with makeup flow to the VCT from the RWST was started. At 1200, an RO recorded an RWST level that was above the lower limit in the shiftly logs, but at 1230, the RWST was inadvertently lowered below the TS limit (however, this was not identified until 1340.) The flush of FBGO3B was completed at 1240.
At 1 340, an RO reviewed the control room logs and recognized that a large volume of water from the RWST was used to flush FBGO3B. The RO then checked RWST level and determined it was below the required minimum TS 3.5.4 level of 394,000 gallons. At 1405, the Control Room initiated action to start filling the RWST, and at 1427, the RWST was raised to above the required minimum TS 3.5.4 level of 394,000 gallons.
It was subsequently determined that TS 3.5.4 Condition B should have been entered at 1230 (due to the lowering of RWST borated water level), thereby requiring RWST level to be restored within one hour. Since this condition was not recognized until approximately 1 340, however, no action was taken by the Control Room to restore level at that time (1230), and consequently, the one-hour Required Action was not satisfied. This resulted in entry into TS 3.5.4 Required Actions C.1 and C.2, requiring the plant to be in Mode 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. (Since the plant was already in Mode 4, only Required Action C.2 applied.) However, all Required Actions were exited at 1427 when RWST level was raised to above the required minimum TS 3.5.4 level of 394,000 gallons, thus restoring compliance with the LCO.
Due to the RWST level lowering below the required minimum TS 3.5.4 level of 394,000 gallons, a Yellow risk condition was entered for Decay Heat Removal, with respect to the shutdown safety assessment performed at the time.
4. ASSESSMENT OF SAFETY CONSEQUENCES
The volume of borated RWST water required to support the injection phase of ECCS operation is bounded by the volume required to support Containment Spray and ECCS net positive suction head (NPSH) requirements during the recirculation phase of ECCS operation.
Callaway calculation M-FL-1 8 documents post-LOCA containment flood levels to support containment spray and RHR NPSH. Although a worst-case DBA LOCA is not expected to occur or required to be postulated in Mode 4, an evaluation was performed to show that the reduced RWST inventory seen on May 4, 2025 would lower sump level by approximately 0.5 inches for the limiting case. This impact is negligible compared to the available NPSH margin. RHR calculation EJ-29 Rev. 2 shows a minimum margin of 4.98 ft. With the lowered sump level during this event, the minimum NPSH margin would be 4.94 ft. Containment Spray calculation EN-13 Rev. 0 shows a minimum margin of 5.5 ft. With the lowered sump level during this event, the minimum NPSH margin would be 5.46 ft.
In Mode 4, with the RCS at elevated boron concentration (on the order of 2500 ppm) the required boron contribution from the RWST to maintain post-LOCA subcriticality is minimal. There would be no adverse impact to the LOCA subcriticality analyses from the documented drop in RWST level.
Therefore, an RWST level of 93.1% in Mode 4 would not preventthe RWST from performing its specified safety functions as credited in the accident analysis to support ECCS injection, ECCS recirculation, and post-LOCA subcriticality.
5. REPORTING REQUIREMENTS
This LER is submitted pursuant to 10 CFR 50.73(a)(2)(v)(D)
The licensee shall report any event or condition that could have prevented fulfillment of a safety function of structures or systems that are needed to mitigate the consequences of an accident.
Reporting the event per this reporting criterion is a conservative determination based on the reporting guidance of NUREG-1022, Rev. 3, Event Report Guidelines 10 CFR 50.72 and 50.73, January 2013.
It notes that structures, systems and components (SSCs) within the scope of the I 0 CFR 50.73(a)(2)(v)(D) reporting criterion include those required by the TS to be Operable and are intended to mitigate the consequences of an accident (as discussed in Chapters 6 and I 5 of the FSAR). The RWST thus falls within the scope of applicable SSCs. Further, for SSCs within the scope ofthe noted reported criterion, the NUREG-1022 guidance states, A report is required when (1)there is a determination that the SSC is inoperable in a required mode in the TS applicability, (2) the inoperability is due to one or more personnel errors,.
. and (3) no redundant equipment in the same system is Operable. As also noted in the NUREG-1022 guidance, The level ofjudgment for reporting an event or condition under this criterion is a reasonable expectation of preventing fulfillment of a safety function. However, as also noted, A SSC that has been declared inoperable (per the Technical Specifications) is one in which the SSC capability is degraded to a point where it cannot perform with reasonable expectation or reliability.
In Mode 4, the RWST is only required to support one train of ECCS (in addition to its support of the Containment Spray System). As such, the RWST is part of a single-train system in Mode 4. For single-train systems, the NUREG-1022, Rev. 3 guidance is that inoperability ofthe single train is reportable even though the TS may allow such a condition to exist for a limited time.
A value for the minimum required RWST borated water volume is clearly specified in SR 3.5.4.2 as a TS acceptance criterion. Failure to meet this limit constitutes failure to meet the LCO, which is consistent with Operators declaring the RWST inoperable on May 4, 2025.
Based on the noted NUREG-1022 (Rev. 3) guidance for reporting criterion 10 CFR 50.73(a)(2)(v)(D), which closely ties preventing fulfillment of a safety function to TS inoperability, the event is reported accordingly. However, it has been determined that the event does not represent a Safety System Functional Failure (SSFF) per the reporting guidelines of NEI 99-02, Revision 8, Regulatory Assessment Performance Indicator Guideline, for the purpose of the NRC Reactor Oversight Process Performance Indicator Program. As discussed in the Assessment of Safety Consequences above, the lowering of RWST level to 93.1% for a limited time did not significantly impactthe volume ofwater required to support Containment Spray and ECCS NPSH requirements during accident conditions that may be postulated in Mode 4.
It should be noted that this condition was reported to the NRC pursuant to 10 CFR 50.72(b)(3)(v)(D) via Event Notification57689. The same NUREG-1022 (Rev. 3) guidance cited above was used in making that determination.
6. CAUSE OF THE EVENT
The direct cause of the event was that the CR5 and RO did not establish a level band for the RWST when it was selected as a source of water for ongoing evolutions.
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A contributing cause was that complacency developed during low Mode operation where there are less strict requirements on the RWST.
A second contributing cause was that the Shift Manager (SM) and CRS not providing adequate oversight of the flush of FBGO3B.
7. CORRECTIVE ACTIONS
Remedial actions (which have already been completed) include:
. The responsible RO and the CR5 were disqualified and then subsequently remediated.
. A Callaway red communication describing the event, including lessons learned, was issued and reviewed with Operations department personnel.
. An expectation for increased SM oversight was communicated to the Control Room staff.
. An expectation for the CR5 to lead a discussion at the beginning of each shift, outlining expected evolutions and the plan for the day, and including a turnover of unresolved concerns, was communicated to the Control Room staff.
. An expectation to include the Shutdown Safety Assessments (performed to address Shutdown risk) in the Shift Outage Manager (SOM) turnover was established.
. An expectation requiring each Operations Manager to perform two observations of the Refuel Control Room Alignment Brief prior to the end of Refuel 27 was established.
Additional Corrective actions (which are currently being addressed) include:
. Operator Aids will be created for credited and TS-required equipment for Modes 3, 4, 5, and 6. The Operator Aids will include alarm setpoints to prevent exceeding minimum volumes for required tanks. Use of these Operator Aids will be required to be utilized during the Refuel Control Room Alignment Briefs specified in ODP-ZZ-00001
, Operations Department Code of Conduct, Addendum 2, Briefs. Utilizing the Operators Aids every shift will prevent complacency by helping the control room staff to more easily recognize the changes in required equipment when changing modes since it will be reviewed every shift.
. Procedure ODP-ZZ-00001 Addendum 2 will be revised to require a beginning-of-shift refuel brief for the control room staff. This brief should be led by the CR5 and include the plan for the shift including expected plant status changes, team activities, and important and TS-required shutdown safety equipment. This will also present an opportunity to develop monitoring plans, determine the acceptable level of control room activity, review Operator Aids, and allow SM oversight for the Control Room Watchstanders. The expectation for performance of this brief will be communicated to the SMs during a weekly shift manager call and to the Operations Supervisors at a beginning-of-shift meeting.
- 8. PREVIOUS SIMILAR EVENTS (OCCURRENCES):
A search of previous Licensee Event Reports (LERs) since 1988 did not yield identification of any similar occurrences at Callaway Plant.Page 5
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