05000483/LER-2020-002, Reactor Trip and AFW Actuation Following Spurious MFRV Closure
| ML20155K873 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 06/03/2020 |
| From: | Ameren Missouri |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20155K871 | List: |
| References | |
| ULNRC-06585 LER 2020-002-00 | |
| Download: ML20155K873 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(iv)(B), System Actuation |
| 4832020002R00 - NRC Website | |
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NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3750-0104 EXPIRES: 03/31/2020 (04-201 8)
- 1. Facility Name
. Docket Number
- 3. Page CallawayPlantUniti 05000483 i 0F4
- 5. Event Date LER Number
- 7. Report Date
- 8. Other Facilities Involved Sequential Rev Facility Name Docket Number Month Day Year YEAR Number No.
Month Day Year 5000 04 04 2020 2020
- - 002
- - 0 06 03 2020 FacilityName DocketNumber
- 9. OperatingMOde
- 11. Th Report is Submitted Rirsuantto the Requirements of 10 CFR §: (Check all that appy)
D 202201(b)
D 20.2203(a)(3)(i)
Q 5073(a)(2)(ii)(A)
Q 50.73(a)(2)(viii)(A) 1 20.2201(d) i: 20.2203(a)(3)(ii)
D 50.73(a)(2)(ii)(B)
D 50.73(a)(2)(viii)(B)
D 20.2203(a)(1)
D 20.2203(a)(4)
D 50.73(a)(2)(iii)
D 50.73(a)(2)(ix)(A) i: 20.2203(a)(2)(i)
D 50.36(c)(1)(i)(A) 0 50.73(a)(2)(iv)(A)
Q 50.73(a)(2)(x)
- 10. PowerLevel Q 20.2203(a)(2)(ii)
D 50.36(c)(1)(ii)(A) 0 50.73(a)(2)(v)(A)
D 73.71(a)(4)
C 20.2203(a)(2)(iii)
D 50.36(c)(2)
Q 50.73(a)(2)(v)(B)
D 73.71(a)(5)
D 20.2203(a)(2)(iv) 0 50.46(a)(3)(ii)
Q 50.73(a)(2)(v)(C)
D 73.77(a)(1) 1 00 i: 20.2203(a)(2)(v)
Q 50.73,ja)(2)(i)(A)
D 50.73(a)(2)(v)(D)
D 73.77(a)(2)(i)
D 20.2203(a)(2)(vi)
Q 50.73(a)(2)(i)(B)
Q 50.73(a)(2)(vii)
D 73.77(a)(2)(ii)
D 50.73(a)(2)(i)(C)
D OTHER SpecifyinAbstractbeloworin NRC Form 366A
- 12. Licensee Contact for this LER Licensee Contact teiephone Number (Inciude Area Code(
TB. Elwood, Supervising Engineer, Regulatory Affairs and Licensing 314-225-1 905
- 13. COMPLErE ONE UNE FOR EACH COMPONENT FAILURE DESCRIBED INTH1S REPORT
Cause
System Component Manufacturer Reportable to ICES
Cause
System Component Manufacturer Reportable to ICES
- 14. Supplemental Report Expected Month Day Year Yes (II yes, complete 1 5. EXPECTED SUBMISSION DATE)
No Expected Submission Date BSTRACT (Limitto 1400 spaces, i.e., apprmdmately 14 single-spaced typewritten lines)
On April 4, 2020 at 0 1 15, a reactor trip and auxiliary feedwater actuation occurred at Callaway Plant (Callaway) following malfunction of the C main feedwater regulating valve (MFRV). Following the reactor trip, the plant safety systems responded per design.
The malfunction of the C MFRV was caused by a failure of the primary valve positioner. The primary positioners for the A, B, C, and D MFRVs were replaced prior to plant startup.
[he backup positioner for the C MFRV was verified to be functioning properly during troubleshooting activities.
Investigation into the cause and development of corrective actions to prevent recurrence of the positioner nalfunction are ongoing, and have been hampered by the COVID-19 pandemic. A supplemental LER will be submitted following completion of the root cause investigation.
NRC FORM 366 (04-2020)
1.
DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The main feedwater regulating valves (MFRVs) function to control feedwater flow to the steam generators and
)rOvide backup isolation of main feedwater (MFW) (EIIS : $J) flow in the event that a main feedwater isolation valve (MFIV) fails to close. One MFIV (EIIS: ISV) and one MFRV (ElIS: FCV) are located on each MFW line, outside of containment. If the single active failure postulated for a secondary pipe break is the failure of a safety grade MFIV to close, then credit is taken for closing the non-safety grade MFRVs.
Closure of the MFIVs or MFRVs terminates flow to the steam generators, in the event of a feedwater line break (FWLB) occurring upstream of the MFIVs or MFRVs. Since the MFIVs are located upstream of the point where the auxiliary feedwater lines connect to the main feedwater lines, which is in turn upstream of the main feedwater check valves (located in containment), closure of the MFIVs or the MFRVs ensures delivery of auxiliary feedwater to the steam generators for support of the auxiliary feedwater function in the event of a main edwater line break in the turbine building (i.e., upstream of the MFIVs and MFRVs).
Similarly, the consequences of events occurring in the main steam lines or in the MFW lines downstream from he MFIVs will be mitigated by MFIV or MFRV valve closure. Closure of the MFIVs or MFRVs effectively erminates the addition of feedwater to an affected steam generator, limiting the mass and energy release for a steam line breaks ($LB) or FWLB inside containment, and reducing the cooldown effects for a SLB.
The MFIVs and MFRVs close on receipt of any safety injection signal, a Tavg
- - Low coincident with reactor trip (P-4), a low-low steam generator level, or steam generator water level
- - high high signal.
The MFIVs and MFRVs provide the primary success path for events requiring feedwater isolation and isolation of non-safety-related portions from the safety-related portion of the system, so as to provide for auxiliary feedwater addition. Each MFRV actuator (skid-mounted at the valve) has two associated redundant actuation rains. The MFRV positioners serve no function to close a MFRV in response to a feedwater isolation signal.
[hus, the malfunction of the C MFRV positioner described in this LER had no impact on the capability of the C MFRV to perform its specified safety function.
The specified safety function of the MFRVs is to provide a diverse backup function to the MFIVs for the
)otential failure of an MFIV to close, even though the MFRVs are located in the non-safety-related portion of he feedwater system.
2.
INITIAL PLANT CONDITIONS
Callaway was initially in MODE 1 at 100% rated thermal power at the time of this event.Page 2 of 4U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 04/30/2020 (04-2020)
1. FACILITY NAME 2 DOCKEt NUMBER [ER NUMBER YEAR SEQUENTIAL REV Callaway Plant Unit 1 05000-483 NUMBER NO.
2020
- - 002
- - 0 3.
EVENT DESCRIPTION
Dn April 4, 2020, at 01 15 a reactor trip, a Feedwater Isolation Signal, and an Auxiliary Feedwater Actuation signal to the motor-driven AFW pumps (MDAFAS) occurred due to a C Steam Generator Water Level Low-ow signal. Callaway was initially in MODE 1 at 100% rated thermal power. Annunicator 126F, Digital eedwater Trouble, alarmed in the main control room at approximately 01 13. The Balance of Plant Operator ioted the C steam generator level was lowering with full open demand on the C MFRV. The Balance of Plant Dperator took manual control of the C MFRV and attempted to restore C steam generator level. The inability 0 control level in the C steam generator level was caused by a malfunction ofthe primary C MFRV
)ositioner. At approximately 01 15, with C steam generator level at approximately 30% narrow range (NR), the 2ontrol Room Supervisor directed the Reactor Operators to trip the reactor due to the inability to maintain C steam Generator Level. However, a reactor trip signal was automatically generated from Steam Generator evel Low-Low on C Steam Generator level at approximately 17% NR level before the Reactor Operators were ible to manually trip the reactor. Following the reactor trip the crew performed E-O, Reactor Trip and Safety [njection, and transitioned to ES-O. 1, Reactor Trip Response, to stabilize the plant.
Steam Generator Water Level Low-Low signal generates the following signals:
1.
Reactor trip signal 2.
Feedwater isolation signal (fWIS) 3.
Motor-Driven Auxiliary Feedwater Actuation Signal (MDAFAS) (Low-Low Level signal present on one-out-of-four steam generators) 4.
Turbine-Driven Auxiliary Feedwater Actuation Signal (TDAFAS) (Low-Low Level signal present on two-out-of-four steam generators)
The logic for the first three signals was satisfied, and subsequently, a reactor trip signal, FWIS, and MDAFAS were all generated. Subsequent to the start of the motor driven auxiliary feedwater pumps, (MDAFPs) level in the other steam generators lowered such that the start logic for the turbine driven auxiliary feedwater pump was satisfied. All auxiliary feedwater pumps started and ran in response to valid signals.
After the reactor trip, the plant safety functions responded as designed. One unexpected component response, which had no bearing on the AFW safety function, was an unexpected opening of ALHVO22O, Hardened Condensate Storage Tank Isolation Valve to AFW, when both MDAFPs started on the AFAS. The unexpected opening of ALHVO22O is being evaluated under the stations corrective action program.
The primary valve positioners on all MFRVs were replaced following the reactor trip and noted actuations.
Proper functioning of the backup positioner on the C MFRV was verified during troubleshooting activities. All valves were diagnostically tested prior to being released for operation. The station was then restarted in accordance with station procedures. It is noted that the MFRV backup positioners do not assume control automatically upon failure of the primary positioner and the transient event could not be mitigated manually in time to prevent the event reported in this LER.Page 3 of 4U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0704 EXPIRES: 04/30/2020 (04-2020)
- 3. LER N1RIIBER YEAR SEQUENTIAL REV Callaway Plant Unit 1 05000-483 NUMBER NO.
2020
- - 002
- - 0 4.
ASSESSMENT OF SAFETY CONSEQUENCES
The event reported in this LER was a reactor trip and auxiliary feedwater actuation following a malfunction of a MFRV. The plant responded as designed with the exception of the opening of ALHVO22O described previously, and all safety functions were fulfilled in a manner consistent with the plants safety analysis. The event reported in this LER does not represent an event that significantly degraded the plants safety.
5.
REPORTING REQUIREMENTS
hd event reported in this LER was an event that resulted in automatic actuation of the reactor protection system rnd the auxiliary feedwater system. This event is reportable as a Licensee Event Report per 10 CFR 50.73(a)(2)(iv)(A), which requires reporting any event or condition that resulted in manual or automatic ctuation of any of the systems listed in 10CFR5O.73(a)(2)(iv)(B). The reactor protection system and auxiliary [edwater system are both specified in 10 CFR 50.73(a)(2)(iv)(B).
6.
CAUSE OF THE EVENT
flhe root cause investigation associated with the positioner failure for this event has been hampered by the 2OVID-19 pandemic and is ongoing. Cause(s) and corrective actions will be provided in a supplement to this
£R once the root cause process is complete. The cause(s) of the event will be identified as part of the root ause analysis process.
7.
CORRECTIVE ACTIONS
The root cause investigation for this event and development of corrective actions to prevent recurrence of the MFRV positioner malfunction are ongoing, and have been hampered by the COVID-19 pandemic. Following the reactor trip, all primary MFRV positioners were replaced. Proper functioning of the backup positioner for the C MFRV was verified during troubleshooting. All MFRVs were also diagnostically tested before being released to operations for plant startup. Investigation into the event is ongoing, cause(s) and corrective actions will be provided in a supplement to this LER once the root cause process is complete. The cause(s) of the event and corrective actions to prevent recurrence will be identified as part of the root cause analysis process.
Although root cause investigation and development of corrective actions are not complete at this time, it should be noted that the MFRVs are capable of performing their specified safety function of providing a diverse and redundant isolation of main feedwater flow following a postulated failure of a MFIV to close in response to a feedwater isolation signal.
8.
PREVIOUS SIMILAR EVENTS
)uring the previous three years, there have been no similar events involving spurious closure of a MFRV resulting in a eactor trip and auxiliary feethvater actuation.Page 4 of 4