ML15057A047

Supplemental Information Regarding Response to Generic Letter 2008-01 - Managing Gas Accumulation in Emergency Core Cooling, Decay Heat Removal, and Containment Spray Systems

ML15057A047
Person / Time
Site:	Surry
Issue date:	02/19/2015
From:	Mark D. Sartain Virginia Electric & Power Co (VEPCO)
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
Download: ML15057A047 (9)

Text

VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 February 19, 2015 10CFR50.90 U. S. Nuclear Regulatory Commission Serial No.: 15-038 Attention: Document Control Desk SPS/LIC-CGL: R2 Washington, DC 20555-0001 Docket Nos.: 50-280 50-281 License Nos.: DPR-32 DPR-37 VIRGINIA ELECTRIC AND POWER COMPANY SURRY POWER STATION UNITS 1 AND 2 SUPPLEMENTAL INFORMATION REGARDING RESPONSE TO GENERIC LETTER 2008 MANAGING GAS ACCUMULATION IN EMERGENCY CORE COOLING, DECAY HEAT REMOVAL, AND CONTAINMENT SPRAY SYSTEMS The Nuclear Regulatory Commission (NRC) issued Generic Letter (GL) 2008-01 on January 11, 2008 to request that each licensee evaluate the licensing basis, design, testing, and corrective action programs for the Emergency Core Cooling Systems (ECCS), Residual Heat Removal (RHR) System, and Containment Spray (CS)

System to ensure that gas accumulation is maintained less than the amount that could challenge operability of these systems and that appropriate action is taken when conditions adverse to quality are identified.

Virginia Electric and Power Company (Dominion) provided the nine-month response to GL 2008-01 for Surry Power Station (Surry) in an October 14, 2008 letter (Serial No. 08-0013B). By letter dated January 14, 2015 (Serial No.14-485), Dominion submitted a License Amendment Request (LAR) for Surry based on review and applicability of the NRC-approved TSTF-523, Revision 2, "Generic Letter 2008-01, Managing Gas Accumulation." During preparation of the Surry LAR, it was recognized that two statements in the October 14, 2008 letter regarding the RHR System require update. Thus, an update supplementing the October 14, 2008 GL 2008-01 response is provided in the attachment.

The attached update does not affect the conclusion of the Surry response to GL 2008-01 that "neither the suction nor the discharge piping of the RHR System is detrimentally impacted by gas accumulation." Furthermore, we consider the NRC closeout of GL 2008-01 for Surry is not affected by this update. Specifically, the NRC's April 12, 2011 GL 2008-01 closeout letter for Surry states: "The NRC staff has concluded that the licensee [Dominion] has acceptably demonstrated 'that gas accumulation is maintained less than the amount that challenges operability of these

Serial No.15-038 Docket Nos. 50-280/281 Page 2 of 3 systems, and that appropriate action is taken when conditions adverse to quality are identified,' as stated in GL 2008-01." This NRC conclusion is not affected by the attached supplemental information for the RHR System.

Should you have any questions or require additional information, please contact Mr. Gary D. Miller at (804) 273-2771.

Respectfully, Mark D. Sartain Vice President - Nuclear Engineering ,FVickl L.Wifl NOTARY PUSLIC Commonwoott of Virginia Reg. # 140542 my. ComMission Expires-May 31, 2018 Commitments contained in this letter: None ----------------------------

Attachment:

Supplemental Information Regarding Surry Response to Generic Letter 2008 Managing Gas Accumulation in Emergency Core Cooling, Decay Heat Removal, and Containment Spray Systems COMMONWEALTH OF VIRGINIA )

)

COUNTY OF HENRICO )

The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by Mr. Mark D. Sartain, who is Vice President - Nuclear Engineering, of Virginia Electric and Power Company. He has affirmed before me that he is duly authorized to execute and file the foregoing document in behalf of that company, and that the statements in the document are true to the best of his knowledge and belief.

Acknowledged before me this ?day of A 2015.

.A 'I.. .,,

My Commission Explres:

Notary Public

Serial No.15-038 Docket Nos. 50-280/281 Page 3 of 3 cc: U.S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Avenue, NE Suite 1200 Atlanta, GA 30303-1257 Ms. K. R. Cotton Gross NRC Project Manager - Surry U.S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 Dr. V. Sreenivas NRC Project Manager - North Anna U.S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 NRC Senior Resident Inspector Surry Power Station

Serial No.15-038 Docket Nos. 50-280/281 Attachment Supplemental Information Regarding Surry Response to Generic Letter 2008 Managing Gas Accumulation in Emergency Core Cooling. Decay Heat Removal, and Containment Spray Systems Virginia Electric and Power Company (Dominion)

Surry Power Station Units 1 and 2

Serial No.15-038 Docket Nos. 50-280/281 Attachment Page 1 of 5 Supplemental Information Regarding Surry Response to Generic Letter 2008 Managing Gas Accumulation in Emergency Core Cooling, Decay Heat Removal, and Containment Spray Systems Introduction Generic Letter (GL) 2008-01 was issued on January 11, 2008 by the Nuclear Regulatory Commission (NRC) to request that each licensee evaluate the licensing basis, design, testing, and corrective action programs for the Emergency Core Cooling Systems (ECCS), Residual Heat Removal (RHR) System, and Containment Spray (CS)

System to ensure that gas accumulation is maintained less than the amount that could challenge operability of these systems and that appropriate action is taken when conditions adverse to quality are identified. Dominion provided the nine-month response to GL 2008-01 for Surry Power Station (Surry) in an October 14, 2008 letter (Serial No. 08-0013B). By letter dated January 14, 2015 (Serial No.14-485), Dominion submitted a License Amendment Request (LAR) for Surry based on review and applicability of the NRC-approved TSTF-523, Revision 2, "Generic Letter 2008-01, Managing Gas Accumulation." During preparation of the Surry LAR, it was recognized that two statements in the October 14, 2008 letter regarding the RHR System require update. The following discussion provides supplemental information updating our October 14, 2008 GL 2008-01 response.

Supplemental Information Section D Items 1 through 5 on pages 12 and 13 of 26 in Attachment 1 to letter dated October 14, 2008 discuss the evaluation of the design and operation of the RHR System and its susceptibility to gas accumulation. The following paragraphs are excerpted from the October 14, 2008 letter with annotations in italic print either confirming no change or providing updated information.

"1. The RHR system, which is located entirely inside of containment, has a safety classification of NSQ (Non-safety related with Regulatory Significance) and does not serve the dual function of a low pressure safety injection pump. The RHR system is used during plant startup, shutdown, and refueling (Condition I events) when the RCS temperature is 350'F or less. It is not in service during normal operation or during hot standby operation. As such RHR system function is not assumed in the safety analyses for any Condition II, Ill, or IV events."

This paragraphrequiresno update and remains valid. It is important to note that the Surry RHR System is a non-safety related system with regulatory significance (classified as NSQ), does not serve the dual function of low pressure safety injection, and provides no accident mitigation function.

Serial No.15-038 Docket Nos. 50-280/281 Attachment Page 2 of 5 "2. The RHR system is placed in service during the plant cooldown phase that follows a period of initial heat removal by the Main Steam System following reactor shutdown.

Generally, during startup conditions, residual heat from the reactor core is being removed by the RHR system. The RHR system is placed in operation when the RCS pressure and temperature are reduced to a pressure low enough to prevent lifting the RHR pump suction relief valves and a temperature less than the design temperature of the RHR system. Initially, the flow through the heat exchangers is throttled to control the rate of cooldown of the RCS. The inlet to the RHR system is located in the hot leg of one reactor coolant loop while the return line is connected to the cold leg of the other two loops. If the RCS is drained below the top of the hot leg (e.g., mid-loop operation), the RHR system procedurally operates at a reduced flow rate to remove decay heat. The reduced flow rate is established as a precaution to prevent vortexing in the partially drained hot leg and to prevent air ingestion into the operating RHR pump. The RHR system remains in service throughout the shutdown and startup periods, which may include a refueling operation. The RHR system is shut down and isolated when the temperature of the RCS reaches 350'F."

This paragraphrequires no update and remains valid.

"3. The RHR system is filled and vented by procedure whenever the system is drained for maintenance. System velocities during operation of the RHR system for normal RCS cooldown are sufficient to sweep any small gas voids that might have remained in local piping high points after fill and vent of the piping. Many years of successful operation of the RHR system during RCS cooldown have demonstrated that the fill and vent procedures and operating procedures are effective with no observed detrimental performance in operation of the RHR pumps and system. Thus, control of system operating flow rates with consideration to limiting inlet conditions ensures that the entire RHR piping system is sufficiently full of water during system operation. Two MOVs [motor-operated valves] (part of the RCS pressure boundary) in series isolate the RHR system inlet line from the RCS while the discharge lines are isolated by a check valve (part of the RCS pressure boundary in the SI accumulator discharge line) and an MOV. If the RCS pressure exceeds the normal operating pressure of the RHR system, an interlock between the RCS hot-leg pressure channels and the two inlet isolation valves prevents the valves from opening."

This paragraphrequires an update. In the second sentence in this paragraph,the word "small" is being deleted since it is subjective and could be misleading.

Subsequent to the transmittal of the October 14, 2008 nine-month response to GL 2008-01 for Surry, further evaluation and calculation of gas voids in the RHR System was completed based on evolving industry guidance. The conclusions of this further evaluation and calculation did not result in a change to the October 14, 2008 response since the final acceptable evaluated gas void volume was below the Westinghouse prescribed limit of 5 ft3 at 400 psig once the system is pressurized, which could still be considered small. However, during the preparation of the GL

Serial No.15-038 Docket Nos. 50-280/281 Attachment Page 3 of 5 2008-01-related LAR for Surry (submitted to the NRC by the January 14, 2015 letter), it was determined that the word "small" in this sentence should be deleted since, as stated above, it is subjective and could be misleading.

"4. The RHR system has system boundary interfaces with two systems which could result in inleakage into the RHR system. The system boundaries with the RCS system via the SI accumulator discharge lines and the CH [Chemical and Volume Control] letdown system both are potential sources of inleakage to the RHR system through leaking system boundary isolation valves. The operating pressure in the CH letdown system is less than the RHR system relief valve set pressure and high enough to prevent degassing of the letdown water, so inleakage from the letdown system would not be a source or mechanism for gas intrusion. RCS system leakage into the RHR system could potentially result in a lift of an RHR relief valve if the system boundary with the letdown system is tight, but would not be a source of degassing since the RHR system would be pressurized by the postulated RCS leakage. However, leakage of SI accumulator water into the RHR system would be a source of gas accumulation from degassing of the accumulator water since it is N 2 saturated gas at approximately 600 psig and would initially be at a lower pressure after leaking into the RHR system. The SI accumulator level is monitored, and thus any leakage out of the accumulators is identified and corrective actions are taken.

This condition would only occur with the RHR system in the standby mode of operation. This condition would result in a prompt assessment of its impacts on system operability via the corrective action system."

This paragraphrequiresno update and remains valid.

"5. Therefore, when the RHR system is not in operation, it is isolated with both its suction and discharge piping full of water from the RCS. The temperature of the water in the RHR suction and discharge in the non-flowing shutdown condition does not increase enough to cause degassing of the water. Once isolated during unit normal or hot standby operation, the RHR system piping remains sufficiently full of water as the only mechanism for gas intrusion or accumulation in either the RHR system suction or discharge piping is potential SI accumulator water system inleakage which would be identified and mitigated prior to operating the RHR system."

This paragraph requires an update. During the preparation of the GL 2008-01-related LAR for Surry, it was confirmed that the RHR System is not susceptible to gas intrusion except primarily from SI Accumulator line back leakage through the RHR discharge motor operated valves. It was also concluded that an update to the GL 2008-01 response was appropriateto document the other unlikely and low probabilityflow paths. Thus, the Item 5 paragraphabove is replaced in its entirety with the following paragraphs:

Serial No.15-038 Docket Nos. 50-280/281 Attachment Page 4 of 5 Therefore, when the RHR system is not in operation, it is isolated with both its suction and discharge piping full of water from the RCS. The RHR system is left in an isolated, standby condition for the duration of plant power operations. Prior to placing the system in service, procedural controls quantify the amount of fluid required to re-fill the system to allow accommodations to be made if fill volumes in excess of the analyzed allowable volumes are required. There is no need for a periodic surveillance to monitor for gas voids in the RHR System based on the following considerations:

• the system is entirely inside containment,

• the system remains out of service until procedurallyplaced in service,

• the procedure has adequate controls to preclude the existence of an unacceptablegas void priorto operating the system, and

• the system is a non-safety related system with regulatory significance (classified as NSQ), does not serve the dual function of low pressure safety injection, and provides no accident mitigation function.

The most probable cause of gas accumulation in the RHR system would be directly from the accumulatorthrough the RHR discharge MOVs. For this path to be viable for gas accumulation, there must also be a leakage path out of the RHR System that exceeds the RHR discharge MOV leakage rate to decrease pressure below approximately 600 psig, the nitrogen saturationpressure of the fluid in the accumulators. That leakage path could be either external leakage (e.g., from the RHR pump seals) or into the letdown system through the letdown flow path control valve. Since leakage from the accumulators is monitored and trended by the control room operators, this source of gas accumulation would be identified and mitigatedprior to placing the RHR System in service. This leakage path was considered viable during the initial GL 2008-01 evaluation.

Two other less likely flow paths would require the RHR system to depressurize below nominal volume control tank (VCT) pressure (approximately 30-40 psig),

the hydrogen saturationpressure in the VCT (RCS make-up source). For either of these conditions to occur, the leakage out of the system (e.g., from the pump seals) must exceed the capacity of the inleakage source. One RHR inleakage flow path would be through the letdown flow path control valve from the letdown system. The letdown system operates at approximately 300 psig and could leak into RHR. The other flow path would be through the RHR discharge MOV; however, the RCS isolation check valve on the accumulatorinjection line would also have to leak back into the SI accumulator line, seating the accumulator discharge check valve and pressurizing this pipe section to potentially nominal RCS pressure (2235 psig). These leakage paths were not considered viable during the initial GL 2008-01 evaluation and have a low probability.

The least likely gas accumulation scenario would be for there to be multiple external leaks in the RHR System, one at a lower elevation in the system (e.g., at

Serial No.15-038 Docket Nos. 50-280/281 Attachment Page 5 of 5 the RHR flats) and one at a high elevation, near the top of the RHR system. In this condition, the leak through the lower elevation source would allow a lower pressure to be established in the RHR System and air could leak into the system from the higher elevation leak location. Walkdowns of the RHR System are performed in accordance with the inservice inspection and boric acid corrosion control programs which would identify leakage from the system; therefore, this is considered an unlikely source of gas accumulation. This leakage path was not considered viable during the initial GL 2008-01 evaluation and has a low probability.

As noted, the completely depressurized RHR System requires an inleakage source capability that is less than the external leakage path capability. If the inleakage source is greater than the external leakage path, the RHR System will pressurize to the inleakage source system pressure. However, the RHR System pressure is limited by a high pressure annunciatorthat will direct Operations to open the letdown isolation valve to reduce system pressure with an overpressurization back up provided by the RHR System relief valve. At high pressures, no degassing of RCS fluid will occur. If the system were to completely depressurize due to the extemal leakage rate exceeding the system inleakage rate, procedure controls preclude startup of the system with an unacceptable void.

Conclusion In summary, the RHR System is a non-safety related system with regulatory significance (classified as NSQ), does not serve the dual function of low pressure safety injection, and provides no accident mitigation function. This update is being provided to supplement the October 14, 2008 GL 2008-01 response for completeness with regard to the Surry RHR System; however, this update does not affect the conclusion of the Surry response to GL 2008-01 that "neither the suction nor the discharge piping of the RHR System is detrimentally impacted by gas accumulation." Furthermore, the NRC closeout of GL 2008-01 for Surry is not affected by this update. Specifically, the NRC's April 12, 2011 GL 2008-01 closeout letter for Surry states: "The NRC staff has concluded that the licensee [Dominion] has acceptably demonstrated 'that gas accumulation is maintained less than the amount that challenges operability of these systems, and that appropriate action is taken when conditions adverse to quality are identified,' as stated in GL 2008-01." We conclude that the NRC conclusion is not affected by the supplemental information provided herein for the RHR System.