05000454/LER-2015-004, Regarding Unanalyzed Condition Due to a Design Deficiency with Pressurizer Power Operated Relief Valve Circuitry That Could Prevent Valve Manual Closure to Mitigate Spurious Operation

Regarding Unanalyzed Condition Due to a Design Deficiency with Pressurizer Power Operated Relief Valve Circuitry That Could Prevent Valve Manual Closure to Mitigate Spurious Operation

ML15292A015
Person / Time
Site:	Byron
Issue date:	10/19/2015
From:	Kearney F Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
BYRON 2015-0120 LER 15-004-00
Download: ML15292A015 (5)

LER-2015-004, Regarding Unanalyzed Condition Due to a Design Deficiency with Pressurizer Power Operated Relief Valve Circuitry That Could Prevent Valve Manual Closure to Mitigate Spurious Operation

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(B) 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
4542015004R00 - NRC Website

text

Byron Generating Station

.Z Exe[on Generation Rd www.exeloncorp.com October 19, 2015 LTR:

BYRON 2015-0120 File:

1.10.0101 (1D.101) 2.07.0100 (5A.108)

United States Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Byron Station, Units 1 and 2 Facility Operating License Nos. NPF-37 and NPF-66 NRC Docket Nos. STN 50-454 and STN 50-455

Subject:

Licensee Event Report (LER) 454-2015-004-00, Unanalyzed Condition Due to a Design Deficiency with Pressurizer Power Operated Relief Valve Circuitry That Could Prevent Valve Manual Closure to Mitigate Spurious Operation Enclosed is Byron Station Licensee Event Report (LER) No. 454-2015-004-00 regarding electrical circuit deficiencies with a design basis fire in the Main Control Room or adjacent electrical rooms could prevent the Pressurizer PORV Block Valves from being manually closed from the local control switches in accordance with the Fire Safe Shutdown mitigating strategies. This condition is reportable in accordance with 10 CFR 50.73(a)(2)(ii)(B) for any event or condition that resulted in an unanalyzed condition that significantly degraded plant safety.

There are no regulatory commitments in this report.

Should you have any questions concerning this submittal, please contact Mr. Douglas Spitzer, Regulatory Assurance Manager, at (815) 406-2800.

tt f6erA. ai Site Vice President Byron Generating Station FAK/GC/sg

Enclosure:

LER 454-2015-004-00 cc:

Regional Administrator NRC Region Ill NRC Senior Resident Inspector Byron Generating Station Respectfully,

NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0704 EXPIRES: 01/31/2017 (02-2014)

Estimated burden per response to comply with this mandatoly collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />.

/

Reported lessons learned ate incorporated into the licensing process and ted back to industry.

u Send comments regarding burden estimate to the FOIA, Pdvacy and Information Collections

/

Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to lnfocollects.Resource@nrc,gov, and to the Desk Officer, Otfice ot Information and (See Page 2 for required number of RegulatoryAffairs, NEOB-10202, (3150-0104), Office ot Management and Budget, Washington, DC d

f

/ k

+

I k ki 20503. If a means used to impose an intormation collection does not display a currently valid 0MB igI s1c arac ers or eac Li OCft) control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

3. PAGE Byron Station, Unit 1 05000454 1

OF 4

4. TITLE Unanalyzed Condition due to a Design Deficiency with Pressurizer Power Operated Relief Valve Circuitry that Could Prevent Valve Manual Closure to Mitigate Spurious Operation

5. EVENT DATE

6. LER NUMBER

7. REPORT DATE

8. OTHER FACILITIES INVOLVED FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR SE3JJAL MONTH DAY YEAR Byron Station, Unit 2 05000455 FACILITY NAME DOCKET NUMBER 08 20 2015 2015 004

- 00 10 19 2015 N/A N/A

9. OPERATING MODE 1 1. THIS REPORT IS SUBMITtED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply)

LI 20.2201(b)

LI 20.2203(a)(3)(i)

LI 50.73(a)(2)O)(C)

LI 50.73(a)(2)(vii)

LI 20.2201(d)

LI 20.2203(a)(3)(H)

LI 50.73(a)(2)(ii)(A)

LI 50.73(a)(2)(vHi)(A)

LI 20.2203(a)(1)

LI 20.2203(a)(4) 50.73(a)(2)(ii)(B)

LI 50.73(a)(2)(viii)(B)

LI 20.2203(a)(2)O)

LI 50.36(c)(1)Ø)(A)

LI 50.73(a)(2)(iU)

LI 50.73(a)(2)(ix)(A)

10. POWER LEVEL LI 20.2203(a)(2)(ii)

LI 50.36(c)(1)(ii)(A)

LI 50.73(a)(2)(iv)(A)

LI 50.73(a)(2)(x)

LI 20.2203(a)(2)(iii)

LI 50.36(c)(2)

LI 50.73(a)(2)(v)(A)

LI 73.71 (a)(4) 100 LI 20.2203(a)f2)(iv)

LI 50.46(a)(3)(ii)

LI 50.73(a)(2)(v)(B)

LI 73.71(a)(5)

LI 20.2203(a)(2)(v)

LI 50.73(a)(2)O)(A)

LI 50.73(a)(2)(v)(C)

LI OTHER LI 20.2203(a)(2)(vi)

LI 50.73(a)(2)ç)fB)

LI 50.73(a)(2)(v)(D)

Sit1ract below or in

12. LICENSEE CONTACT FOR THIS LER

13. COMPLETE t

CAUSE

SYSTEM COMPONENT N/A N/A 14: SUPPLEMENTAL REPORT EXPECTED N/A BSTRACT (Limit to 1400 spaces,Le., approximately 15 single-spaced typewritten lines)

On August 20, 2015 at 1755 hours0.0203 days <br />0.488 hours <br />0.0029 weeks <br />6.677775e-4 months <br />, a design deficiency associated with pressurizer power operated relief valve (PORV) block valve control circuitry was confirmed in which a design basis fire in the main control room (MCR) or cable spreading rooms (CSR) could prevent the credited fire safe shutdown action (i.e., locally close pressurizer PORV block valve) mitigating a spurious pressurizer PORV opening.

On September 2, 2015, during an extent of condition review, an additional design deficiency was confirmed in which credited fire safe shutdown action (i.e., removing 125 Vdc control power fuses) mitigating a spurious opening of the pressurizer PORVs during design basis fire does not adequately mitigate design basis fire induced hot short.

The causes of these design deficiencies are legacy design errors made during original construction.

Corrective actions include plant configuration changes correcting the specific design deficiencies.

This condition is being reported in accordance with 10 CFR 50.73(a)(2)(ii)(B) for any event or condition that results in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety.

JCENSEE CONTACT TELEPHONE NUMBER (include Area Code)

Douglas Spitzer Manager, Byron Regulatory Assurance (815) 406-2800 LI YES (If yes, complete 15. EXPECTED SUBMISSION DATE)

NO NRC FORM 366 (02-2014)

A.

Plant Condition Prior to Event Event Date/Time: August 20, 2015 / 1755 hours0.0203 days <br />0.488 hours <br />0.0029 weeks <br />6.677775e-4 months <br /> CST Unit 1

- Mode 1

- Power 100 percent Unit 1

- Mode 2

- Power 100 percent Unit 1 Reactor Coolant System: [ABJ

- Normal operating temperature and pressure Unit 2 Reactor Coolant System:

- Normal operating temperature and pressure No structures, systems or components were inoperable at the start of this event that contributed to the event.

System Background The pressurizer power operated relief valves (PORVs) are DC solenoid controlled air operated valves. These valves provide the primary overpressure protection of the reactor coolant system (RCS) during most modes of operation. Each PORV discharge line has a normally open motor operated block valve immediately upstream of the PORV itself. The pressurizer PORVs and associated block valves are considered a high-low pressure interface between the high pressure RCS and the low pressure Pressurizer Relief Tank (PRT). A remote/local isolation switch and local control switch are provided for each pressurizer PORV block valve at their associated motor control centers (MCC). The block valves can be locally closed by placing their remote/local isolation switch in Local and then closing the valve with the local control switch.

Components forming high-low pressure interface must consider the following credible design basis fire induced circuit failures: multiple open circuits, short circuits, shorts to ground, and multiple hot shorts within the control circuit.

In addition, three-phase AC power circuit cable-to-cable proper phase sequence faults and two-wire ungrounded DC circuit cable-to-cable proper polarity faults are considered to be credible, and must be evaluated.

B.

Description of Event

On August 20, 2015 at 1755 hours0.0203 days <br />0.488 hours <br />0.0029 weeks <br />6.677775e-4 months <br />, while responding to an NRC Triennial Fire Protection Inspection question, a design deficiency associated with the pressurizer PORV block valve control circuitry was confirmed. Specifically, the circuit deficiency for which a design basis fire in the main control room (MCR) or cable spreading rooms (CSRs) could prevent the pressurizer PORV block valves from being locally closed at their local control switch.

The design failed to isolate all energized conductors within the control circuit thereby allowing a short to ground to clear the associated control power fuses. Consequently, a design basis fire induced short to ground in the MCR or CSR could clear the associated control power fuses preventing the block valves from operating at the local control switch.

This identified pressurized PORV block valve circuit deficiency prevents the credited fire safe shutdown action of locally closing the block valves to mitigate the spurious operation of a pressurizer PORV.

Fire watches of the affected MCR and CSR fire zones were immediately implemented.

In addition, the MCR is continuously staffed and the affected CSR fire zones are equipped with fire detection and automatic and manual fire suppression systems.

On September 2, 2015, during an extent of condition review, an additional design deficiency associated with the pressurizer PORV and pressurizer PORV block valves was confirmed. Specifically, the current fire safe shutdown mitigating strategy for removing pressurizer PORV control power fuses does not adequately prevent a pressurizer PORV from spuriously opening due to a fire induced hot short. Furthermore, local actions to close the associated pressurizer PORV block valve at the MCC are not effective because the MCC will not have electrical power during all design basis fires. Therefore, the credited safe shutdown action to remove the pressurizer PORV control power fuses does not prevent the pressurizer PORV from spuriously opening during design basis fires in CSR fire zones. The affected Fire Zones were in the same CSRs previously identified on August 20, 2015, and fire watches of the affected areas remained in place.

This condition is being reported in accordance with 10CFR5O.73(a)(2)(ii)(B), for any event or condition that results in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety. This LER is being submiffed in follow-up to NRC Event Notification51335 made on August 20, 2015, followed by supplemental ENS call on September 2, 2015 under the same ENS 31335.

C. Cause of Event

The cause of this event was legacy design errors introduced during original plant design / construction, and fire safe shutdown strategy changes introduced during resolution of Information Notice (IN) 92-18, Potential for Loss of Remote Shutdown Capability During a Control Room Fire. Several missed opportunities to identify these design errors included: implementation of IN 92-18 actions; during the corrective actions resolving IN 92-18 implementation inadequacies; and most recently during the multiple spurious operation project.

D. Safety Significance

This condition had no actual safety consequences impacting plant or public safety because a fire did not occur and damage credited safe shutdown equipment.

The issue affects compliance with the fire protection licensing requirements and does not affect the PORV or associated block valve from performing their specified safety functions required by Technical Specifications of RCS depressurization. The Fire Protection Program provides pre-evaluated compensatory actions in the event that one of the credited features (i.e., suppression,detection,barrier) is degraded or unavailable. In this case, with the absence of a fire rated barrier, spurious operation or loss of control of the PORVs and block valves is possible if a fire of sufficient size were to occur.

The current circuit configuration would not prohibit control of the PORV block valve from the MCR or locally from the MCCs in the event of any UFSAR Chapter 15 accident; therefore, there was no loss of safety function.

Furthermore, the affected MCR and CSRs are either continuously staffed or have fire detection and automatic and manual fire suppression systems. Therefore considering the Fire Protection Program defense-in-depth strategy, these design errors do not significantly degrade plant safety.

E. Corrective Actions

Immediate action

- Established fire watches in the affected areas

- MCR and affected CSRs.

Corrective Action Completed

- Implemented plant configuration changes rewiring the affected pressurizer PORV block valve control circuits to fully isolate all remote conductors thereby ensuring the local control functions remain available to mitigate consequences of design basis fires in the MCR and CSRs.

Corrective Actions planned

- Implement plant configuration changes to install design features defeating design basis fire induced hot shorts from spuriously opening the pressurizer PORVs.

F. Previous Occurrences

There have been no other occurrences of this nature in the previous three years.

A.

Plant Condition Prior to Event Event Date/Time: August 20, 2015 / 1755 hours0.0203 days <br />0.488 hours <br />0.0029 weeks <br />6.677775e-4 months <br /> CST Unit 1

- Mode 1

- Power 100 percent Unit 1

- Mode 2

- Power 100 percent Unit 1 Reactor Coolant System: [ABJ

- Normal operating temperature and pressure Unit 2 Reactor Coolant System:

- Normal operating temperature and pressure No structures, systems or components were inoperable at the start of this event that contributed to the event.

System Background The pressurizer power operated relief valves (PORVs) are DC solenoid controlled air operated valves. These valves provide the primary overpressure protection of the reactor coolant system (RCS) during most modes of operation. Each PORV discharge line has a normally open motor operated block valve immediately upstream of the PORV itself. The pressurizer PORVs and associated block valves are considered a high-low pressure interface between the high pressure RCS and the low pressure Pressurizer Relief Tank (PRT). A remote/local isolation switch and local control switch are provided for each pressurizer PORV block valve at their associated motor control centers (MCC). The block valves can be locally closed by placing their remote/local isolation switch in Local and then closing the valve with the local control switch.

Components forming high-low pressure interface must consider the following credible design basis fire induced circuit failures: multiple open circuits, short circuits, shorts to ground, and multiple hot shorts within the control circuit.

In addition, three-phase AC power circuit cable-to-cable proper phase sequence faults and two-wire ungrounded DC circuit cable-to-cable proper polarity faults are considered to be credible, and must be evaluated.

B.

Description of Event

On August 20, 2015 at 1755 hours0.0203 days <br />0.488 hours <br />0.0029 weeks <br />6.677775e-4 months <br />, while responding to an NRC Triennial Fire Protection Inspection question, a design deficiency associated with the pressurizer PORV block valve control circuitry was confirmed. Specifically, the circuit deficiency for which a design basis fire in the main control room (MCR) or cable spreading rooms (CSRs) could prevent the pressurizer PORV block valves from being locally closed at their local control switch.

The design failed to isolate all energized conductors within the control circuit thereby allowing a short to ground to clear the associated control power fuses. Consequently, a design basis fire induced short to ground in the MCR or CSR could clear the associated control power fuses preventing the block valves from operating at the local control switch.

This identified pressurized PORV block valve circuit deficiency prevents the credited fire safe shutdown action of locally closing the block valves to mitigate the spurious operation of a pressurizer PORV.

Fire watches of the affected MCR and CSR fire zones were immediately implemented.

In addition, the MCR is continuously staffed and the affected CSR fire zones are equipped with fire detection and automatic and manual fire suppression systems.

On September 2, 2015, during an extent of condition review, an additional design deficiency associated with the pressurizer PORV and pressurizer PORV block valves was confirmed. Specifically, the current fire safe shutdown mitigating strategy for removing pressurizer PORV control power fuses does not adequately prevent a pressurizer PORV from spuriously opening due to a fire induced hot short. Furthermore, local actions to close the associated pressurizer PORV block valve at the MCC are not effective because the MCC will not have electrical power during all design basis fires. Therefore, the credited safe shutdown action to remove the pressurizer PORV control power fuses does not prevent the pressurizer PORV from spuriously opening during design basis fires in CSR fire zones. The affected Fire Zones were in the same CSRs previously identified on August 20, 2015, and fire watches of the affected areas remained in place.

This condition is being reported in accordance with 10CFR5O.73(a)(2)(ii)(B), for any event or condition that results in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety. This LER is being submiffed in follow-up to NRC Event Notification51335 made on August 20, 2015, followed by supplemental ENS call on September 2, 2015 under the same ENS 31335.

C. Cause of Event

The cause of this event was legacy design errors introduced during original plant design / construction, and fire safe shutdown strategy changes introduced during resolution of Information Notice (IN) 92-18, Potential for Loss of Remote Shutdown Capability During a Control Room Fire. Several missed opportunities to identify these design errors included: implementation of IN 92-18 actions; during the corrective actions resolving IN 92-18 implementation inadequacies; and most recently during the multiple spurious operation project.

D. Safety Significance

This condition had no actual safety consequences impacting plant or public safety because a fire did not occur and damage credited safe shutdown equipment.

The issue affects compliance with the fire protection licensing requirements and does not affect the PORV or associated block valve from performing their specified safety functions required by Technical Specifications of RCS depressurization. The Fire Protection Program provides pre-evaluated compensatory actions in the event that one of the credited features (i.e., suppression,detection,barrier) is degraded or unavailable. In this case, with the absence of a fire rated barrier, spurious operation or loss of control of the PORVs and block valves is possible if a fire of sufficient size were to occur.

The current circuit configuration would not prohibit control of the PORV block valve from the MCR or locally from the MCCs in the event of any UFSAR Chapter 15 accident; therefore, there was no loss of safety function.

Furthermore, the affected MCR and CSRs are either continuously staffed or have fire detection and automatic and manual fire suppression systems. Therefore considering the Fire Protection Program defense-in-depth strategy, these design errors do not significantly degrade plant safety.

E. Corrective Actions

Immediate action

- Established fire watches in the affected areas

- MCR and affected CSRs.

Corrective Action Completed

- Implemented plant configuration changes rewiring the affected pressurizer PORV block valve control circuits to fully isolate all remote conductors thereby ensuring the local control functions remain available to mitigate consequences of design basis fires in the MCR and CSRs.

Corrective Actions planned

- Implement plant configuration changes to install design features defeating design basis fire induced hot shorts from spuriously opening the pressurizer PORVs.

F. Previous Occurrences

There have been no other occurrences of this nature in the previous three years.