ML20094P231

Forwards Response to 920323 Confirmation of Action Ltr Re Ice Condenser Lower Plenum Floor Movement & Degradation. Doors Restored to Operable Status & Current Configuration Determined Acceptable from Sys Performance Standpoint

ML20094P231
Person / Time
Site:	Sequoyah
Issue date:	03/27/1992
From:	Joshua Wilson TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
CAL, NUDOCS 9204070292
Download: ML20094P231 (18)

Text

{{#Wiki_filter:_ _ - - ~ - . ~ - d se < ten ,1, . s m,1 s a J t Wo vo rw.p t Qv,Au. .r March 27, 1992 U.S. Nuclear Regulatory Commission ATTN Document Control Desk Washington, D.C. 20555 Gentlement In the Matter of ) Docket Nos. 50-327 Tennessee Valley Authority ) 50-328 SEQUOYAll NUCLEAR PLANT (HQN) - ICE CONDENSER LOWER PLENUM FLOOR MOVEMENT AND DEGRADATION

Reference:

Letter from Stewart D. Ebneter to TVA, Attn Mr. J. R. Bynum, dated March 23, 1992, " Conf 2rmation of Action Letter" l This letter provides the information requested under Items 1, 2, and 3 in the referenced letter regarding the subject issue. The informatica provided by this submittal will be discussed in the NRC meeting scheduled for 11 a.m., on April 3, 1992, in the PRC Region II offices. This tueeting will complete Item 4 of the referenced letter. TVA has identifJed upward movement and cracking of the Units 1 and 2 ice condenser floor wear slabs that form the top layer of the ice condenser floor assembly. This upward movement also resulted in mechanical interference with the bottom of a number of lower ice condenser inlet doors, thereby increasing the required opening force beyond the technical specification (TS) limit of 675 inch-pounds. The condition was discovered during inspections of the lower inlet doors during the Unit 2 Cycle 5 refueling outage on March 16, 1992, and verified on March 17, 1992, following furcher engineering inspection and review. Movement of 27 of 48 doors was found to be inhibited, along with localized wear slab cracking and upward displacements of up to several inches. The immediate apparent cause of the wear slab degradation vas water intrusion, freezing, and expansion within the floot assembly. Confirmatory inspection of Unit 1 on March 13, 1992, which was operating near 100 percent power three months into Cycle 6 operation, identified similar indications, although to a lesser extent, affecting 11 of 2 3 doors. TVA conducted an orderly shutdown of Unit 1 to effect resolutiun to the observed problems, and hot standly was reached at 0247 Eastern standard time on March 19, 1992. G 9EO O>0292 920327 f,f Ug/ A PDR ADOCK 05000327 ~ /[/.- P PDR

U.S. Nuclear Regulatory Commission Page 2 March 27, 1992 1VA has taken short-term actions to establish or verify operability of the Unit 1 ice condenser and to bound the effects of the condition for that unit under power operation. The doors have be~ ~estored operable, the current configuration has been determined acceptable from both systems and structural performance standpoints, and an at-power monitoring plan has been estabilshed to verify continued operability. Evaluations supporting these efforts have been reviewed by the onsite Flant Operations Review Committee, an independent structural consultant, ice condenser uesigner, and a system specialist from Westinghouse Electric Corporation. Additional actione have bean taken to prevent further degradstion of Unit 2 during the ongoing outage maintenance activities and are being taken to establish or verify the operab!11ty of ice condenser systems, structures, and components before Unit 2 start-up from the outage. Additional parallel investigation and corrective efforts are also ongoing. -It is expected that the resultant modifications and/or repairs implemented for Unit 2 during this outage will constitute permanent long-term corrective actions. Ilowever, at-power monitoring will be conducted to verify the effectiveness n' ' hose actions and to determine the optimum long-term actions for Unit $ 1VA considers that the actions taken and planned provide assurance that the SQN ice condensers will remain capable of performing the intended accident mitigation function and that the causes of the condition are being addressed to prevent future recurrence. Details concerning 1VA's evaluation and corrective actions are provided 3 in Enclosure 1. Supporting documentation and evaluations for Unit i have .been provided to the onsite resident inspectors and are referenced in Commitments made in this submittal are listed in Please direct questions concerning this issue to M. A. Cooper at (615) 843-8422. Sincerely. W% J ;L. Wilson Enclosures cc: See page 3

_...._____.m j a U.S. Nuclear Regulatory Connission Page 3 March 27, 1992 ) i cc (Enclosures): Mr. D. E. ImBarge, Project Manager j U.S. Nuclear Regulatory Coninission One White Flint, North 11555 Rochv111e Pike Rr kv111c, Maryland 20852 NRC Realde4t inspector r Sequoyah !ucicar Plant 2600 Igou Ferry Road Soddy Daisy, Tennessee 37379 e Mr. B. A. Wilson, Project Chief ( U.S. Nuclear Regulatory Conunission Region 11 101 Marietta Street, NW, Suite 2900 ...lanta, Georgia 30323 3 5 l l l I o .m. ,,, - - -,. _. ~. -. -, - -.. ~..... _..,., -.. ~ ~ _ - -. _ .,.. - -... _., _. - _. ~... - _. _

1 ENCf.0SUllE 1 ICE CONDENSER LOWER PLENUM FLOOR MOVEMENT AND DEGRADATION

~_ - t ENCLOSURE 1 1. PACKGROUND A. Desigo_Descr.iptlun The Sequoyah Nuclear plant (SQN) containments are an ice condenser i pressure suppression design. The internal design pressure for the containment is 12 pounds per square inch gauge (psig), and the design temperature is 250 degrees Fahrenheit (F) The primary containment structure is a freestanding welded steel s6ructure wfth a certical cylinder, hemispherical dome, and a flat circular base. The internal concret< structural design allows the containment to be divided into four main areas for containment pressure design evaluation the lower compartment, the dead-ended compartment, the upper compartment, and the ice condenser (see Attactuuents 1 and 2 for containment and ice condenser sections). The ice condenser is a 4 passive device containing bo*ated ice that is utilized in the event of a loss-of-coolant accident (LOCA) or hig t-energy link break (!!ELB) to absorb thermal energy. Thl6 ensure. that steam is condensed and the pressure energy is reduced to ensure containment integrity in the early stages of an accident. During a postulated LOCA er llELD, which can only occur in the lower compartment, steam emanating from the break location, along with the untmal atmos ee of the lower compartment, is directed almost exclusively to the auw i condenser section via pressure-driven flow through the lower ice doors. These lower ice doors, which are held closed during normal operation by the cold air head pressure occurring inside the ice condenser, are designed to open at a differential pressure of 1 pound per square foot (psf). After entering the ice condenser, the steam and/or air mixture is directed upward toward the ice bed where the ice removes energy from the mixture, resulting in the conoensing of the steam, cooling of the air, and melting of a portion of the ice. Discharge from the ice condenser is made to the upper compartment here continued cooling is ensered by operation of the containment < pray system. This flow pattern is maintained by the pressure-driven blowdown forces until the end of the blowdown period. After this initial blowdown phase, the continued movement of the steam and/or air mixture is ensured by the air return fan system that forcibly returns the changing atmosphere in the upper compartment to the lower compartment region by the way of the dead-ended compartment. The ice condenser is divided into 24 bays, with each bay having a pair of inlet doors in the lower plenum (see Attachments 3 and 4 for the lower elevation door and plan views). These door panels are provided with tension spring mechanisms that prodace a small closing j torque on the door panels as they open. The magnitude of the closing torque is equivalent to providing approximately 1-psf pressure drop through the inlet ports with the doors open to a position equivalent to the full port flow area. The zero lond poFltion of the Spring mechanisms IS such that, With zero dif f erentia? ;

• essure across the door panels, the gasket holds the door slightly open. The setting provides assurance that all doore

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2_ will be open slightly, upon removal of cold air head, therefore eliminating significant inlet maldistilbution for very small incidents. r'or larger incidents, the doors open fully and flow distribution is controlled by the flow area and pressure drops of inlet ports. The doors are provided with shock absorber assemblies to dissipate the large door kinetic caergies generated during large break lucidents. The ice condenser floor assembly consluts of an 18-inch-thick 6,tructural slab, spanning between the crane *:c.11

• concrete columns at the steel containment vec e1 (SCV) (see Attachment 5 for the floor assembly details). A vapor barrier (copper armored sisolkraft) is installed on top of the structural slab, with a 15-inch-thick foam concrete layer poured as an insulator. The foam concrete is a very light-weight concrete consisting of portland cement, water, and a foaming agent.

On top of the foam concrete, a 1-inch-thick layer of grout was placed to provide a level surface for the placement of a 1/4-luch steel plate that has the glycol piping attached to it. Within each bay, the steel plate consists of two pieces that wete " butted" together and spilced with a plate on the top. The aplice plate was tack welded to the two bottom plates. The 4-inch concrete wear slab is poured on the top of the steel plate. Itcinforcing steel at 6 inches on center in both the radial and circtunferential direction was placed with 1/2 inch cover irom the top of the slab. Each bay of the wear slab is independent of the next bay n,I separated with expension joints. The wear slab in also independet, of the crane wall with an expansion joint occursint at the juncture of the crane wall and wear slab. The design required that the expanolon joints be sealed with a sealer 1/2 inch deep. B. IdentiLied.. Condition Upward movement and cracking of the wear slabs were identifled in both SQN Units 1 and 2. The cracking is located principally near points of rigid restraint.. This upward movement also resulted in mechanical interference with the bottom framen of a number of lowec inlet doors, thereby increasing the required opening force beyond the technical specification (TS) limit of 675 luch-pounds. The condition was discovered of the lower inlet doors during the Unit 2 Cycle 5 ref ueling; outage inspections on March 16, 1992, and verified on March ' 7,1992, f ollowing f ur ther engineering inspection and review. The movement of 27 of 48 doors was found to be inhibited, along with localized wear slab cracking and upward displacements of up to several inches. The inunediate apparent cause of the wear slab degradation was water intrusion. freecing, and expansion within the floor assembly. A confinmatory inspection of Unit 1 oa March 18, 1992, which was opeiatii.g near 100 percent power thece I months into Cycle 6 operation, identified similot indications, although to a lesser extent, affecting 11 of 48 doors. TVA conducted an orderly shutdown of Unit 1 to effeet resolution to the obsers -d problems, and hot standby was reached at 0247 Eastern standard time on March 19, 1992. The condition resulted in potential operability impacts of the ice condenser from both performance and structural standpoints.

1 3 TVA has concluded that the wear slab movement and degradation resulted f ro'n water intrusion, f rreting, and expansion within t he floor assembly. Several paths for floor assembly water intrusion were created by initial construction and design deficiencies. A design change during construction and approved by Westinghouse allowed the removal of the requirement for a sealed metal flashing at the crane wall expansion joint and for elimination of the expansion joint and sealant over two short sides of the middle column supr.3rt. Additionally, walkdowns have detemined that the sealer in the expansion joints, which is required by the designer, is " missing" in several areas. It was also found that the steel plates were warped in some cases at the time of initial installation and were installed on cured hardened grout, lesdang to gnpa within the floor assembly. Subsequent maintenance practAcce resulted in significant ext osure to water during outage rnalatena' ice activities (e.g., defrosting and c1 caning) and did not establish appropriate controls to prevent water from accumulating or standing on the floor and in the drains during those maintenance activities. Additionally, humid, lower-containment air present in the floor i drains during operation would provide a mechanism for condensation and freezing wittin available gnps and passages. L The combination of installed conditions and maintenance practices could create cycles, of water intrusion, f reezing, and expansion over time that vould create progressively larger passages for future intrusion leading to the observed condition. Through inspection and evaluation, it is believed that the ice largely exists in passages beneath the steel plate. The foam concrete shows no evidence of movement. In summary, TVA concludes that the cause of the condition is the combination of the installed ccnfiguration deficiencies and maintenance practices described above. Further de*. ails concerning this assessment will be documented in the ongoing incident investigation (Reference 5). TVA has established and is implementing a comprehensive corrective action plan to address identified ice-condenser, lower-plenum floor movement and degradation for both Units 1 and 2. This plan includes immediate actions necessary to return Unit 1 to operation (Section II), short-term actions to address the ongoing Unit 2 outage activities aad its subsequent return to power operation (Section III), and longer-term monitoring to assess effectiveness of the actions taken and to confirm the optimum long-term resolution for Unit 1 (Section IV). Details concerning each of these areas are addressed below. II. IMEPJATE_ ACTIONS _l!NILLRESTARLACCEETARILLTLEASIS Unit-1 was operating near 100 percent power, approximately three months into the Cycle 6 operation, at the time of discovery of applicability on that unit. TVA concluded that neither the SQN ice condenner door TS (LCO 3.6.5.3) nor the ice bed TS (LCO 3.6.5.1) appropriately addressed the existing condition and that the as-found condition werranted unit shutdown. Accordingly, an orderly shutdown was initiated. Continued operability of the ice condenser must be verified and/or established before restart. I-

4 To restore operability to the Unit 1 lower inlet doors, a temporary modification (Reference 1) wan performed to remove the lower "L-shaped" sheetmetal flashing and gasket mounted on the wear slab and connected to the flashing piece t.at forms the jam for the lower inlet. doors. This removed the interference with the bottom of the doors and provided physical margin for wear slab growth without interference with the doors. A second temporary modification was implemented to replace the insulation bags installed under the flashing with a double layer of Armaflex rubber Jnsulation fixed in place with adhesive. This provided improved sealing of air-leakage paths and ensured retention under accident conditions. Refer to Attachment 6 for the premodification and pcstmodification configurations. Following removal of the flashing and gasket described above, a pull-force surveillance test was successfully perf ormed for all of the Unit 1 lower inlet doors, verifying TS i operability. The removal of the flashing and gasket provides acceptable configuration during power operation, but would require further alteration to support future outage maintenance activities. As discussed in Sections III and IV, the long-term overall resolution and configuration for Unit I will be determined from the ongoing resolution for Unit 2 and subecquent m. itoring of the effectiveness of those i actions. In parallel with the door activities, detailed walkdowns and inspections were performed to identify and assess impacts on interfacing components and to establish the baseline configuration for future monitoring. Specifically, a generalized walkdown and review of both the lower plenum area and the lower elevation below the ice condenser floor were i performed to look for cracks, spalling, or other signs of distress. Floor assembly components and structural members were reviewed for corrosion and obvious deformation. Various interfacing features (e.g., conduit) were evaluated for signs of distress or damage because of the a slab movements. The turning vanes were inspected to assess if the floor had displaced to the point-of contact with the vanes, if the bolting was i deformed, and if the wear slab in contact with the vanes was cracked. Visual observations, review of configuration, and operating history were performed for the glycol floor piping to identify any evidence of damage and operational itupacts. The 12-inch floor drains were inspected for deformation, and sealing joints were inspected for damage and consistency with the as-designed configuration. In conjunction with the direct visual examinations, a boroscope was utilized to verify the absence of indications of excessive corrosion on the SCV in the vicinity of affected ice condenser components. A boroscope was similarly utilized to inspect the exposed, interior floor assembly passages of the floor drain for assessment of icr formation extent and locationt results of that ;nspection are available on tape. A detailed elevation survey and crack mapplug of the wear slab were-performed to document the present configuration. Fron the_above inspections, a 50.59 safety evaluation utilleing bounding evaluations (Reference 2) was performed that verified the structural integrity of components necessary to ensure functional capability of the ice condenser system and acceptability of the existing configuration relative to ice condenser operability.- The structural slab evaluation l consisted of the inspection described above, which did not identify any ~

. apparent cracking or areas of distress as well as potential downward loading on the structural slab pilor to wear slab cracking and loading transmitted f rom wear slah and turning vane contact. The potential impacts were determined to be acceptable relative to design loading and capacities. The dead weight impact of water and/or ice within the floor assembly was determined to be minirial. Wear slab evaluation concluded that the slab would maintain position during a seismic condition and that the existing deformation did not prevent it from protecting the glycol piping. No evidence of glycol piping damage was identified through coriucted inspections and a review of ice condenser or glycol temperatures. Upward loads transmitted by the wear slab on column anchor bolts were also evaluatedt expansion joints prevent any loading a on the columns themselves, rotential impact on bolt capacity was found to be insignificant. Conservative bounding evaluation of potential loading on the turning vanes and associated bolting concluded that functionality would be maintained. To ensure that any potential further degradation does not impact operability, a periodic monitoring plan has been established (Reference 3) that will consist of at-power monitoring of floor movement every other day for the first week, once a week for the next month, and once a month thereafter if no observed changes are identified. Evaluation of monitoring results will determine appropriate changes in inspection scope or frequency. Several options are provided to minimize the as low as reasonably achievable impacts, including remote or upper containment monitoring of wear slab displacement transducers for each bay or utilization of a camera projected through the ice passages from the upper plenum for visual observation. Lower plenum entry for inspection is provided as a backup method. The monitoring instruction provides criteria for assessing inlet door operability impacts and conducting further engineering assessment to ensure the continued validity of the above-described evaluation. Formal operational guidance (Reference 4) was established based on the ice condenser door TSs from Duke Power and the ongoing Methodically Engineered Restructured and Improved Technical Specifications effort. This guidance will remain in placo to address any future identified impairments to inlet door performance until a formal TS amendment is submitted and approved. The evaluations supporting the above-described actions were reviewed by the onsite safety review committee, the SQN Plant Operations Review Committee. The overall evaluation was reviewed by the ice condenser designer and system performance specialist from Westinghouse. The supporting structural evaluation was reviewed by an independent structural specialist. A detailed final inspection to verify appropriate work closecut, material configuration, and area housekeeping will be conducted before entering Mode 4. 1VA considers that the above-described actions have bounded the effects of the subject condition and established appropriate baais for verifying continued ice condenser operability of Unit 1 over the remaining Cycle 6 operation. I SHORT-TEIOLA('j l0NS2_ UNIT _2_RESTARI ALOhfTAn1LITUASIS I11. Additional shc et-term actions have been or are being taken to address resolution for Unit 2 and direct, long-term resolution and recurrence controls for80oth units. These actionc can be broken into three categories: (1) actions to address ongoing Unit 2 outage maintenance activities, (2) actions to establish configuration acceptability of Unit 2 for restart from the outage, and (3) ongoing investigation efforts to establish long-term hardware and operational and/or maintenance improvements. It is expected that the long-term hardware modifications and/or repairs .1 ba implemented on Unit 2 before restart irom the ongoing outage. Upon identification of the condition, schedu C ice condenser outage activities for Unit 2 were placed on hold to allow for detailed luspections of the as-found condition similar to those conducted for Unit 1, and to establish additional et,ntrols to prevent further possible damage during the upcoming maintenance activilles. Inspections and mapping have been completed as performed for Unit 1, and a detailed wear sinb elevation survey and SCV inspection will be completed before restart. No observed distress or damage was identifled on the structural slab. Limited as-found pull data for the inlet doors was obtained, l'ntted by potantial for damage to the more severely affected doors. A full structural and performance evaluation will be performed for Unit i simflar to that performed for Unit 1 and will receive the same level of reviews for acceptability. A detailed change analysis is being performed to better establish the relative contributions of the currently identified causes and contributing factors and to allow optimization of long-term actions relative to the material condition and operation and/or maintenance practices. This analysis includes the historical conditions and practices experienced at the SQN units, obtained from interviews and reviews of procedures and surveillance and inspectica results. Differences between the SQN units and the other Westinghouse ice condenser units are also being evaluated in detail from design, construction, operation, and maintenance standpoints; initial review indicates similar design features but dissimilar maintenance practices. The planned outage activities were evaluated for potential impact, spe ifically directed at activities that would expose the floor assembly to additional water intrusion, such as the floor and wall panel defrosting activities. The floor drains have been inspected and cleaned to provide f ree flow of any water acetunulation. The defrosting maintennnce practices have been revised to include additional controls and provisions to minimize watet accumulatica en the floor. Operational procedures are also being examined to ~[tect imptovement, e.g., glycol outage control and restoration. To restore operability of the inlet doors, a modification will be implemented to install a sheetmetal flashing configuration that will allow for vertica movement. This will provide a long-term configuration that is acceptable from both operation and maintenance perspectives. An additional modification will be implemented before start-up to seal exposed wear slab interfaces and joints ut water intrusion paths, and to seal wear slab cracks as appropriate Possible methods to drain water and/or ice accumulation from the floor assembly are being evaluated. Voids and separation in the iloor drains will also be repaired as appropriate. On-line monitoring of Unit 2 will be performed as previously described for Unit 1 to verify continued ice condenser operability and assess the effectiveness of the dercribed actions. In sununary, these short-term activities are intended to ensure continued capability of the Unit 2 ice condenser to perform the required accident mitigation function and provide additional basis from which to establish long-term resolution for both units. IV. LONG-TEl21 ACTIONS __ UNITS _1 AND 2 Continued at-power monitoring during mycle 6 operation and inspections conducted during the Cycle 6 refueling outages will be used to assess the effectiveness of actions previously taken and to establish the optimum long-term resolution for Unit 1. TVA also pinns to submit a permanent Tf; license amendment to provide appropriate action statement (s) for impeirnents to the opening capability of the ice condenser doors. Current in-process analyses (Ref erence 6) indicate that some level of impattinent may be found acceptablet the results of these analyses will be addressed in the associated licensee event report to be issued by April 15, 1992. Until the final TS amendment is developed, submitted, and approved, prudent operational guidance will be provided through TVA's technical specifientloo interpretation O SI) process. V. REFERENCES 1 4 1. Temporary Alteration Chnnge Form (TACF) 1-92-014-061, Revision 0 (RO), and Safety Evaluation, li38 920323 800 2. TACF 1-92-016-061, RO 3 periodic Inntruction (PI) 0-PI-SXX-061-001.0, " Ice Condenser Lower Menum Floor Monitoring," and *1ACF 1-92-0017-061 4. TSI 92-01 5. Incident Investigation .1-S-92-026 (unissued) 6. Operational Evaluation of the Binding of Lower Ice Condenser Lacrs in SQN Unit 1

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r-x m,c 2-I f-t - n ,4,e g ,'l l x Mo O== r*- b[ o :c e mO O e-g xmm DO O U -.4 % MW M w O n m e k ~ P"" 3= mZ ta 74 rt \\/ ? g 2 fr' structural and perf ormance evaluation will be perf onned for Unit 2 to that performed for Unit 1 and will receive the s..ne level of > w

• acceptability. This will be completed before t u a t 2 restart h

om the Cycle 5 tefuellas outage. Q 1, ce operability of i be Unit 2 inlet doors, a modification will be imw-

d before Mode 4 to install a sheettetal flashing configuration 1.411ow f or vertical movement. This will provide a long-term tion that is acceptable from both operation and maintenance per

,( es. 4. ka toditsanal mcdification will be implemented before Unit 2 start-up <'iode 4) to seal exposed wear slab interfaces and joints at water intrusion paths, and u seal wear slab cracks as nppropriate. 5. Volds and separation in the Unit 2 floor drains will also be repaired as appropriate before Mode 4. 6. Cn-line nanitoring of Unit 2 will be pertormed as previously described f or Unit 1 to verLfy contirimd 3ce condencer operability and assess the effectiveness of -he de cribed actio A. The mor.it.oring plan will be establi-hed beitto 4:4m 4. 7. Contit.ead at-power e.onitoring during (fcle 6 operation and inspections conducted during *he Cycle 6 refueling outages will be used to assess the effectiveness of actions previously taken and to establish the optimum long-term resolution for Unit 1, 8. TVA also plans to submit a permanent TS license amendment by June 1, 1992, to _covidr appropriate actic.. statement (s) for impairments to ice condenser door opening c v bility. a .-}}