ML081360101
| ML081360101 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry, Watts Bar, Sequoyah  |
| Issue date: | 05/09/2008 |
| From: | Purcell M Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| GL-08-001 | |
| Download: ML081360101 (8) | |
Text
Tennessee Valley Authority, 1101 Market Street, Chattanooga, Tennessee 37402-2801 May 9, 2008 10 CFR 50.54(f)
U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555 Gentlemen:
In the Matter of
)
Docket Nos. 50-259 50-327 Tennessee Valley Authority
)
50-296 50-390 50-328 50-391 50-260 BROWNS FERRY NUCLEAR PLANT (BFN) UNITS 1, 2 AND 3, SEQUOYAH NUCLEAR PLANT (SQN) UNITS 1 AND 2, AND WATTS BAR NUCLEAR PLANT (WBN) UNITS 1 AND 2 - INITIAL RESPONSE TO NRC GENERIC LETTER (GL) 2008-01: MANAGING GAS ACCUMULATION IN EMERGENCY CORE COOLING, DECAY HEAT REMOVAL, AND CONTAINMENT SPRAY SYSTEMS DATED JANUARY 11, 2008
Reference:
INPO Significant Operating Experience Report (SOER) 97-01, "Potential Loss of High Pressure Injection and Charging Capability from Gas Intrusion," dated December 6, 1997.
This letter provides TVA's response to GL 2008-01.
The Nuclear Regulatory Commission (NRC) issued GL 2008-01 to request that each licensee evaluate the licensing basis, design, testing, and corrective actions for the Emergency Core Cooling (ECCS), Decay Heat Removal (DHR), Residual Heat Removal (RHR), and Containment Spray System (CSS) to ensure 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.
Printed on recycled paper
U.S. Nuclear Regulatory Commission Page 2 May 9, 2008 GL 2008-01 directed each licensee to submit a written response in accordance with 10 CFR 50.54(f) within 9 months of the date of the GL to provide the following information:
(a) A description of the results of evaluations that were performed pursuant to the requested actions of the GL. This description should provide sufficient information to demonstrate that you are or will be in compliance with the quality assurance criteria in Sections III, V, XI, XVI, and XVII of Appendix B to 10 CFR Part 50 and the licensing basis and operating license as those requirements apply to the subject systems of the GL; (b) A description of all corrective actions, including plant, programmatic, procedure, and licensing basis modifications that you determined were necessary to assure compliance with these regulations; and, (c) A statement regarding which corrective actions were completed, the schedule for completing the remaining corrective actions, and the basis for that schedule.
Additionally, the NRC requested that if a licensee cannot meet the requested response date, the licensee "shall provide a response within 3 months of the date of the GL." In the 3-month response, the licensee was requested to describe the alternative course of action that it proposes to take, including the basis for the acceptability of the proposed alternative course of action.
By telephone call on April 8, 2008, NRC verbally granted TVA a 1 month extension to the initial 3-month GL response. This extension is discussed in TVA's letter to NRC dated April 10, 2008. provides TVA's response to GL 2008-01. Enclosure 2 provides a brief summary of plant-specific characteristics and corrective actions taken relative to TVA's on-going management of the issues in the subject GL and SOER 97-01. lists the new commitment made by this letter.
Please direct any questions to Rob Brown at (423) 751-7228.
I declar under penalty of perjury that the foregoing is true and correct. Executed on the _d/I~lay of f/y,2008.
Sincerely, Michael A. Purcell Senior Licensing Manager Nuclear Power Group Enclosure cc: See page 3
U.S. Nuclear Regulatory Commission Page 3 May 9, 2008 Enclosure cc (Enclosure):
Eva A. Brown, Senior Project Manager U.S. Nuclear Regulatory Commission MS 08G9A One White Flint, North 11555 Rockville Pike Rockville, Maryland 20852-2738 Thomas H. Boyce, Senior Project Manager U.S. Nuclear Regulatory Commission One White Flint North MS 08G9A 11555 Rockville Pike Rockville, Maryland 20852-2738 Patrick D. Milano, Senior Project Manager U.S. Nuclear Regulatory Commission One White Flint North MS 08H4 11555 Rockville Pike Rockville, Maryland 20852-2738 Regional Administrator U.S. Nuclear Regulatory Commission Region II Sam Nunn Atlanta Federal Center 61 Forsyth St., SW, Suite 23T85 Atlanta, Georgia 30303-8931 NRC Senior Resident Inspector Browns Ferry Nuclear Plant 10833 Shaw Road Athens, Alabama 35611-6970 NRC Senior Resident Inspector Sequoyah Nuclear Plant 2600 Igou Ferry Road Soddy Daisy, Tennessee 37379-3624 NRC Senior Resident Inspector Watts Bar Nuclear Plant 1260 Nuclear Plant Road Spring City, Tennessee 37381-2000
ENCLOSURE1 BROWNS FERRY NUCLEAR PLANT (BFN) UNITS 1, 2 AND 3, SEQUOYAH NUCLEAR PLANT (SQN) UNITS 1 AND 2, AND WATTS BAR NUCLEAR PLANT (WBN) UNIT 1 AND 2 - THREE MONTH RESPONSE TO NRC GENERIC LETTER (GL) 2008-01: MANAGING GAS ACCUMULATION IN EMERGENCY CORE COOLING, DECAY HEAT REMOVAL, AND CONTAINMENT SPRAY SYSTEMS
SUMMARY
FROM GL 2008-01
... if a licensee cannot meet the requested response date, the licensee shall provide a response within 3 months of the date of the GL. In the 3 month response, the licensee is requested to describe the alternative course of action that it proposes to take, including the basis for the acceptability of the proposed alternative course of action.
REQUESTED INFORMATION To more efficiently address the evaluations necessary to confirm that gas accumulations are maintained at a level less than the amount that challenges design margin of Emergency Core Cooling System (ECCS), Decay Heat Removal (DHR),
Residual Heat Removal (RHR), and Containment Spray System (CSS), TVA intends to perform preliminary evaluations for gas voids in safety-related systems by October 11, 2008. These evaluations may identify confirmatory walkdowns that are best performed during prolonged outages. Therefore, TVA will complete walkdowns, as necessary, by the first refueling outage after December 2008 for each respective operating unit. Unacceptable piping voids will be entered into the Corrective Action Program. This approach is reasonable based upon the following:
Operating experience at TVA's plants has shown that due to their relative design and construction, maintenance, operation and testing, gas accumulation is an understood process. Therefore, it is predictable and it can be managed in a manner that ensures safety-related systems are not challenged. TVA has proactively managed gas intrusion issues identified through industry issues including INPO Significant Operating Experience Report (SOER) 97-01 and our Corrective Action Program. Enclosure 2 provides further plant-specific information regarding how TVA has proactively addressed gas accumulation issues.
TVA will follow the development by the Boiling Water Reactor/Pressurized Water Reactor Owners Groups (B/PWROGs) of the information needed to determine an acceptable void size and integrate applicable results into the evaluation processes. It is expected that an acceptable void size or limit will be determined by the end of next fall. TVA expects that information developed from the B/PWROGs will confirm that relatively small voids do not have an adverse effect on safety-system performance.
ENCLOSURE2 PLANT-SPECIFIC DISCUSSION TVA has proactively addressed gas intrusion issues identified by industry issues including INPO Significant Operating Experience Report (SOER) 97-01 and the Corrective Action Program. These are discussed on a plant-specific basis below:
SEQUOYAH UNITS 1 AND 2 The flow path from the Refueling Water Storage Tank (RWST) to the various ECCS pumps stair-steps downhill, with no designed intermediate high spots in the piping.
Most of the horizontal piping from the RWST is expected to be dynamically vented during periodic testing of the CSS. The piping that is not swept by the CSS Pump tests has relatively short horizontal runs, and is smaller diameter piping, making the possible void volume due to grade errors small. In addition, ECCS suction piping locations that have the greatest vulnerability to voiding due to physical layout and proximity to sources of gas are periodically examined by Ultrasonic Testing (UT).
Other than at these known gas traps, there is no history of piping voids in the ECCS and CSS suction piping that has adversely affected pump operation.
There is some history of voids in the discharge of the various ECCS pumps.
Pressure pulsations have been noted in RHR and CSS pump discharge piping in the past. A number of ECCS high point vent valves have been added, and accessible high points are currently vented each month. Various parameters known to cause or influence gas void buildup are routinely trended to validate that the piping remains full between vents. These parameters include the results of the periodic UT exams, Cold Leg Accumulator (CLA) level and pressure changes, RCS leakage, RHR and Safety Injection (SI) header pressures between pump runs, results of high point and pump casing vents, and in the RHR System based on the known total void for header pressurization time after pump start.
The majority of the CSS piping, including the heat exchangers, is dynamically vented during quarterly pump tests. Following refueling outages, CSS is dynamically vented for a period of time sufficient to remove all significant voids.
WATTS BAR UNIT 1 The flow path from the RWST to the various ECCS pumps stair-steps downhill, with no designed intermediate high spots in the piping. Most of the horizontal piping from the RWST is expected to be dynamically vented during periodic testing of the CSS.
The piping that is not swept by Containment Spray Pump tests has relatively short horizontal runs, and is smaller diameter piping, making the possible void volume due to grade errors small.
Furthermore, a continuous gas vent path from three high points in the Centrifugal Charging Pump (CCP) suction piping is routed to the Volume Control Tank (VCT) gas space. This continuous vent path allows gases in the piping to pass to the VCT, thereby eliminating the potential for gas pocket formation.
Based on data collected since June 1998, there have been a few instances of gas accumulations in ECCS pump discharge piping which were detected by periodic venting or ultrasonic testing. The gas accumulations were dispositioned in I
accordance with the Corrective Action Program. Corrective actions have included the addition of numerous high point vents and correction of procedure deficiencies to resolve issues with inadequate filling and venting of systems after refueling outages.
The CSS pumps are at the lowest elevation of the Auxiliary Building and are periodically run, taking suction from the RWST and returning flow to the RWST by a recirculation path that includes the CSS heat exchangers. The CSS does not communicate directly with gas-containing systems such as the RCS (hydrogen) and the SI System (nitrogen in the CLA), so those gases cannot be present in the system.
WATTS BAR UNIT 2 Watts Bar Unit 2 is currently under construction. Therefore, the systems identified as potentially impacted by gas accumulation will not be required until fuel load.
BROWNS FERRY UNITS 1. 2 AND 3 The Unit 1, 2, and 3 Browns Ferry Nuclear (BFN) Plant ECOS network consists of the High Pressure Coolant Injection (HPCI), the Core Spray (CS), the Low Pressure Coolant Injection (LPCI) mode of RHR, and the Automatic Depressurization System.
The GL applicable portions of the ECCS are the HPCI System, CS, and the LPCI and shutdown cooling modes of the RHR System. The suppression pool provides the required source of water for the ECCS. Although no credit is taken in the safety analyses for the condensate storage tank (CST), it is capable of providing a source of water for the HPCI, RHR and CS systems. Gas accumulation in each of these systems is managed as follows:
The flow path piping has the potential to develop voids and pockets of entrained air.
Maintaining the pump discharge lines of the HPCI, CS, and LPCI subsystems full of water ensures that the ECCS will perform properly, injecting its full capacity into the RCS upon demand. This also prevents water hammer following an ECCS initiation signal. One acceptable method of ensuring that the lines are full is to vent at the high points. This technical specifications surveillance is performed every 31 days. The 31-day frequency is based on the gradual nature of void buildup in the ECCS piping, the procedural controls governing system operation, and operating experience.
The HPCI System is periodically tested with the pump suction aligned to the CST.
The system flow rate during these periodic tests is the maximum flow rate expected under accident conditions, so pump performance is periodically shown that the system is not adversely affected by voids present in this suction piping. The HPCI pump can also take suction from the Torus. This piping is at the suppression pool/HPCI system low point, and there is no method for gas accumulation at these locations. Therefore, the HPCI suction piping is expected to remain full after it has been filled and vented. The HPCI discharge piping is periodically vented at the system high point.
The RHR and CS systems are routinely tested with suction aligned to the suppression pool. The pump/system flow rates during these periodic tests are expected to be sufficient for dynamic venting of their suction piping and portions of the discharge piping. There has not been any indication of water hammer during testing of the RHR and CS systems or when the RHR is placed in shutdown cooling.
The RHR and CS discharge piping is filled by a "Keep Fill" system to prevent the possibility of water-hammer. The RHR and CS systems discharge piping high point 2
vents are periodically checked for water flow to ensure that the lines are filled. Based on the above, the current RHR and CS testing and "Keep Fill" methods are adequate to prevent significant water hammer events.
The ECCS venting, testing, and checking during routine and daily maintenance and operations activities described above are augmented by walkdowns by other plant personnel. The plant system engineers procedurally perform walkdowns of the ECCS to maintain a current awareness of system condition and performance and to obtain system performance monitoring data including periodic observation of surveillance activities, infrequently performed tests/evolutions, sensitive activities, maintenance activities, etc. This includes observation that hangers, supports, and snubbers are made up properly, aligned correctly, and have sufficient hydraulic fluid levels.
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ENCLOSURE 3 NEW COMMITMENTS
- 1. TVA intends to perform preliminary evaluations by October 11, 2008. These evaluations may identify confirmatory walkdowns that are best performed during prolonged outages.
- 2. TVA will complete walkdowns, as necessary, by the first refueling outage after December 2008 for each respective operating unit.