ML20087A153
| ML20087A153 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 01/03/1992 |
| From: | Joshua Wilson TENNESSEE VALLEY AUTHORITY |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9201080218 | |
| Download: ML20087A153 (8) | |
Text
{{#Wiki_filter:,
- (*
i-) '4 s .x 4 5: A nwsee w v a r a m. r m : e c m s., n n 4 i JacA L W%un vn nn det lilq-n_6 fire'er W January 3, 1992 U.S; Nuclear Regulatory Commission ATTN: Document Control Desk Washington. 3.0. 20555 u -Gentlemen: r -In the Matter af ) Docket No. 50-328 ' Tennessee' Valley-Authority ) SEQUOYAH NUCLEAR PLANT (5QN) - TDIPORARY REQUEST FOR RELIEF FROM AMERICAN SOCIETY -0F MECHANICAL ENGINEERS ( ASME) CODE FOR PIN-HOLE LEAK ON UNIT 2 LOWER CONTAINMENT VENT COOLER 2-A
Reference:
_ !"A letter to NRC dated December-2, 1991, "Sequoyah Nuclear Plant (SQN) - Teaporary Relief.From American Society of Mechanical Engineers (ASME)' Code for Weld Leakage on Component Cooling System -(CCS) Heat Exchangers (HX's) OB2 and 2Al" This letter provides a temporary request for relief from the ASME code regarding-replacement of a cooling coil on.SQN's Unit 2 lower containment vent cooler 2-A.- At the prese..t time a pin-hole leak exists on the cooler. coil-(1 1/2-inch-diameter copper supply header). This relief request is being submitted under 10 CFB 50.55alg)(5)(lii) and is in accordance with guidance provided by NRC' staff as outlined in the above reference. The? cooling medium for the subject cooler is-essential raw cooling water. '(ERCW). TVA has evaluated the operability of the cooler'with regard to ERCW flow rate requirements and the effects of spray on adjacent tubes and.the structural integrity.of the cooler. TVA's evaluation for operability. ' indicates that the subject cooler will perform its design basis function. Until repair or-replacement is performed, an inspection of the cooler will be conducted on a weekly: basis to assess structural integrity and to ensure that there is.nc further-degradation beyond the available ERCW flow margin. TVA ' will perf orm the 'ASME code repair or replacement before start-up f rom the Unit 2' Cycle 5 refueling outage. l 9201080218 920101 3 t PDR ADOCK 0300032 j (l 1 .p -PDR.
,O .s _m. m. 1 o 4 + U.S. Nuclear Regulatory Commission Page 2.
- January _3, 1992
' l J ' Enclosura 1 pr ovides TVA's request for relief from the ASME Section XI 1 code. Enclosure 2 contains TVA commitments associated with this -submittal. Please direct questions--concerning this __ssue to D. V. Goodin at .(615) 843-7734, i Sincerely,- 1 / [s() 15 et4. 1 l J J . Wilson. . Enclosures; 1 cc: (Enclosures): Mr. D. E. LaBarge, Project Manager i U.S. Nuclear Regulatory Commission -One White F1'.nt, North 4 11555 Rockville ?ike 'Rockville.-Maryland-20852 NRC Resident-Inspector Sequoyah Nuclear Plant 2600 Igou Ferry Road Soddy Daisy, Tennessee 37379 Mr. B. A'. Wilson, ' Project: Chief U.S. Nuclear Regulatory Commission = Region II 101 Marietta Street. NW,' Suite 2900 . Atlanta.-Georgia.30323 t l-i.: I s mer T'
- m.
'A>
m. _.._m _ _. _ _ _ _ - ~ _. ~. _, -... _ _. _ _ j v ENCLOSURE 1 'SEQUOYAH NUCLEAR PLANT (SQN) REQUEST _FOR RELIEF._-- ESSENTIAL RAW COOLITG WATER (ERCW) LOVER.CChrAINMENT VENT COOLER (LCVC) 2-A UNIT: SQN Unit 2 i .COMPONE TS: LCVC 2-A N 3 SYSTEM: ERCW. ASME CODE CLASS: 3 -FUNCTION: - SQN Unit 2 is equipped with four LCVCs (2-A thru 2-D). The' 2B and 2-D coolers serve as the 'B' train coolers and are supplied by a 'B' train EROW header. The 2-A and ;-C coolers serve as the 'A' train coolers and are supplied by the 'A' -train =ERCW supply header. The design basis function'of the LCVCs is to ensure that adequate heat removal capacity is available to provide long-term coolin-following a non-LOCa (losc of coolant. accident) event (refer _to Technical Specification-{TC) 3.6.2.2). During. normal operation (ModeL1), the LCVCs may be utilized te maintain lower containment air temperature between the !S - ) - limit of~1001 degrees'FahrenheitH(F) and 125 degrees F (refer-to T3 3.6.1.5). IMPRACTICAL CODE 1 REQUIREMENTS: When an-American.1 Society of Mechanical' Engineers (ASME) - Section XI. code repair or repiccement is. performed,'it is-required.to be conducted in accordance with ASME -Section XI.- IWA-4000 ' er IWA-7000 respec tively to res tore . structural integrity to the original design requirements. Temporary relief is requested as the-cooler call is not scheduled for replacement until.the Unit 2' Cycle 5 refueling outage. BACKGROUND: On December.ll, 1991, a pin-hole leak was discovered in a 1 1/2-inch-diameter copper 1 supply header on LCVC 2-A. A. work request was initiated -to repair-the leak. Uponl discovery of_the leak, TVA evaluated the' operability of the LCVC:2-A. The estimated leakage from-LCVC 2-A~is 11ess.than 1 gallor. per minute (gpm). : The required flow rate:is 200 gpm. The actual flow rate-is 281 gpm basedLon -'the last performance of the ERCW flow balancing test. LThe
- spray fromLthe leak was not of sufficient velocity to damage adjacent tubes. Spray from.the leaking tube.did not present a danger to surrounding equipment.
Since sufficient 1 flow with adequate margin is being. supplied to the' room cooler,'it'was determined that operability _of LCVC 2-A was not impaired. p i l -e,.,v ~ y
l l l on December 20, 1991, the cooler was removed f ro.n service and the pin-hole was repaired by brazing. Upon returning the cooler to service, a new pir-hole leak was discovered on the same 1 1/2-inch-diameter copper supply header. The estimated leakage from the new pin-hole leak remains within 1 gpm. Because of the small clearance (4-5 inenes) between the supply header and the cooler housing, access to perform a braze repair is extremely limited. Accordingly, the probability of cn adequate brare repair is low. An evaluation was performed te addrems the structural integrity of LCVC 2-A with the leakage present (see the attachner.t to this enclosure). This evaluation indicates that 'a leaking header doec not present a danger to the integrity of the ERCW system or LCVC 2-A. ALTERNATIVE REQUIREMENTS: IVA will continue to operate with the pin-hole leak until the cooler coil can be replaced. An inspection of the cooler will be performed on a weekly basis by the system engineer. The system eng,neer will assess the structural i integrity of LCVC 2-A and verify that there is no further degradation beyond the available margin of ecoling water flow rate. f f s"-M,- 4.2.=
ATTACHMENT TO ENCLOSURE 1. I STRUCTURAL INTEGRITY EVALUATION FOR LOWER CONTAINMENT VENT COOLER 2-A d
.m d I 3 SEQUOYAH NUCLEAR PLANT (SQN) + ENGINEERING EVALUATION OF THE STRUCTURAL INTEGRITY OF THE UNIT 2 LOWER CONTAINMENT VENT COOLER (LCVC) 2-A Problem Definition A pin-hole leak has developed in the Unit 2 LCVC 2-A. which utilize the essential raw cooling water system as the heatThese room coolers, ~ transfer mechanism, a.e composed of a structural housing containing a system of copper tubes' joined to copper headers by a brazed connection. lhe location of_the pin-hole leak is in al1-1/2-inch-diameter copper supply header. Component Structural Description The LCVCs are end-use devices. As an end-use d ' ice, the component performs three functions. The component is a flow path q,ressure boundary), a.suppor: structure,'ani a-processor of the flowing fluid. The process is heat . transfer. The coolers separate the three functions (flow path, structure, and proceas} by_use of sub-components. A copper tube distribution network provides_a_ flow path from the piping system interface to and including the bea* exchanger area. Framework, housings, and bracing within the framework .prov'.de component structure support. For a cooler, the prceess requires primury emphasis on heat transfer characteristics. Copper tubing achieves good heat transfer eneracteristics at the expense of rigid structural . charac te ris tics. Consequently, the eingle cepper supply header does not significantly contribute _to the structural strength of the room cooler < The structural' strength is provided by the framework, housing, internal bracing, and tube bundle. Structural Integrity Assessgenj A structural and metallurgical assessment of the pin-hole leak in the LCVC 12-A cooler was made by TVA. This assessment concluded that the pin-hole leak in the ' copper supply header will " -t degrade the structural integrity of ' the
- co31er. The basis for this conclusion is a~s follows:
1. Stresses 'in the copper header are normally 1ow and wo-J.d not cause the ~ size of the pin-hole to increase. Deadweight and seismic stresses will'be l low because the copper supply header is a short span that is well suppcrted by the piping supports anc cooler framework. Therefore, crack growth frem applied loads is not expected. 2. The most likely cause of the pin-hole leak is fluw-induced erosion of the wall of the copper header. Althcugh this mechaniam is'still pecsent, '~ ~ experience has :shown that increased leakage is minimal following the past formation-of the initial leak. This i:' basically because of the high fracture toughness properties of the copper tubing and the low fluid -operating pressures. L i
. ~. - - -..- -~ . ~.
- s !'
- Leakage'will be monitored and trended to ensure that essential cooling capacity of the_ cooler is maintained. This trending will also provide assurance - that _ the structural integrity is not degraded.
. 3.L The cooler framework, housing, and bracing provide the essential structural strength of the cooler. A single pin-hole leak in a single copper header has no significant effect an the strength of the structure. -Conclusion A pin-hole leak in the copper header of SQN's LCVC 2-A will not degrade the structural integrity of the cooler. I i l l_
ENCLOSURE 2 CO>tSiITMENTS 1. TVA will repair or replace. the pin-hole leak on lower containment vent cooler 2-A before start-up from the Unit 2 Cycle 5 refueling outage. 2. Structural integrity and ficw margin of the 'ower containnent vent cooler 2-A will be ensuted on a weekly basis by monitoring the leakage for further degradation. Weekly monitoring will continue until replace. ment is complete (bef ore start-up f rom the Unit 2 Cycle 5 refueling outage). Y _ _. _ _ _ _ _ _ _ _. _ _ _ _ _ _ _}}