25th DOE/NRC NUCLEAR AIR CLEANING AND TREATMENT CONFERENCE

Transcription

1 DEEP BED CHARCOAL FILTER RETENTION SCREEN IN-PLACE REPLACEMENT AND REPAIR Wiiam Burns and Rajendra Paude Commonweath Edison Company LaSae County Station Raymond Rosten and Wiiam Knous Duke Engineering and Services, Inc. Napervie, IL Brian Schume Power Cutting Inc. Lake Buff, IL Abstract During fiter unit testing on the contro room fitration unit it was noted that the deep bed screen was corroded in severa areas. This originated an investigation and evauation by station personne. The root cause for the corroded areas was attributed to water eakage past the instaed deuge system vaves (into the fitration unit) creating a ocaized corrosive environment.on the charcoa retention screen. Options to repair or repace the charcoa adsorber were investigated. It was determined that a screen repair and repacement coud be performed. Because of the time constraints and compexity of the repair, considerabe prepanning was required. A specia weding method was deveoped invoving the use of remote cameras and weding devices that woud accommodate the sma space between the screens. Severa screen sections were repaced and post testing was competed satisfactoriy. This repair method resuted in significant cost savings and imited the fiter unit unavaiabiity time by decreasing the scope of the work and affording a unique repair method. I. Introduction The LaSae County Station is a two unit Boiing Water Reactor (BWR). The main contro room is common for both Units 1 and 2. The contro room ventiation system utiizes two 100% capacity equipment trains with each emergency makeup train fiter unit sized for 4000 cfm. The emergency makeup fiter units were manufactured by CVI and utiize a two-inch deep bed design for the charcoa adsorber section. The CVI charcoa adsorber unit is designed with stainess stee charcoa retention screens. During the eary operation of the fiter units, the fire protection deuge system was inadvertenty operated and wetted the charcoa beds. The isoation vaves were then cosed to prevent recurrence. In addition, eakage past the isoation vaves aso provided chronic wetting of the charcoa. As a resut the water combined with the charcoa impregnant and produced a mixture corrosive to stainess stee. LaSae station personne discovered significant corrosion on the charcoa retention screens during routine visua surveiance activities performed to satisfy NRC Reg. Guide 1.52 () commitments. An Operabiity Evauation performed by the station committed to further evauation for repair or repacement of the corroded components. II. Desim of the Fiter Unit The LaSae contro room ventiation system emergency makeup fiter unit is the safety reated suppy of air to the contro room during accident conditions to meet the contro room habitabiity requirements of Genera Design Criteria 19 (OCFR!50 Appendix A(*?. The fiter unit is designed to remove gaseous radoinucides from the infuent air. The fiter unit is a draw through design with the fan on the outet and consists of an inet damper, moisture separator, heater, prefiter, upstream HEPA fiter, charcoa adsorber, downstream HBPA fiter, fan and outet damper. The CVI design of the adsorber utiizes a series of air boxes to distribute fow throughout the unit. It is an a weded stainess stee design The deep bed adsorber has a top fi hopper with a series of two-inch wide 375

2 vertica beds and a coection system beow the beds for charcoa remova. The air enters an inet air box horizontay, turns 90 horizontay to fow through the bed, turns again 90 horizontay and exits through an outet air box. The air box design utiizes 24 gauge perforated sheet as the charcoa retention screen to aow airfow through the charcoa and maintain the charcoa in pace. See Figure 1. Design of Unit AIR INLET -D AIRcuTLET > TOP VFVy Figure 1 - Design of Charcoa Adsorber FND VIFW III. Probem Statement and Issues The screens in the contro room fitration charcoa adsorber unit had areas with significant corrosion. The integrity of the screens had been evauated and found to be acceptabe but degraded To restore the materia condition of the fiter unit, screen repair or repacement woud be necessary. Corrosion of charcoa retention screens was initiay discovered in the standby gas treatment system charcoa adsorber. This adsorber is an eight inch wide bed which was repaired, with significant dibicuty, by conventiona methods. Later when the corrosion was discovered on the contro room fitration units, it was determined that the conventiona repair methods coud not be performed. The issues to overcome in the impementation of this project are as foows: Working area restrictions - Since the deep beds are ony two inches wide and the inet and outet accesses are approximatey three inches wide, workspace was extremey confined. Externa interferences with conduits/cabes, 376

3 25th DOE/NRC NUCLEAR AIR CLEANING AND TREATMENT CONFERENCE LCO window - The repairs to the fiter unit must be competed within 7 caendar days. This incudes remova of charcoa, screen repair, repacement of charcoa, and post-maintenance testing. The retention screens are weded on the charcoa side. LV. Options Evauated There were a number of options considered during the deveopment of the project. The foowing summary of the major options evauated: is a 1. Cean and Inspect - The Cean and Inspect option was considered if the station determined that the corrosion eves of the charcoa retention screens was imited to a minor surface corrosion. This coud be determined with a simpe ceaning of the corrosion ayer and a measurement of the perforated pate at the damaged section. This data woud then be utiized to determine the structura integrity of the perforated sheet under the most severe conditions (seismic oading). This option was not pursued as a compete soution because of the risk associated with a negative inspection was high with no contingency for actions avaiabe. Therefore, this option was impemented ony in areas that were not significanty affected by the corrosion. 2. Repair - The Repair option was considered in a variety of forms. For exampe, repairs of sma damaged x2 or ess) were considered to be feasibe by attaching a sheet meta patch over the hoe with sheet meta screws and spot weds. Since these repairs required access to the damaged area with hand toos, this type of repair was ony designed to be instaed in the accessibe areas of the screens (i.e. the areas cosest to the inet or outet air boxes). Another repair option evauated was to repace of an individua screen section. The air boxes are designed with support channes on the air side that divide the screens into three distinct sections. The screens are spot weded to the channes at uniform intervas. Therefore, the individua screen sections can be cut out and repaced independent of the other screen sections. This requires the use of specia toos to access the far end of the screens for the cutting, grinding, weding, and inspection to remove the existing screens and insta the repacement screens. The repacement of the screen sections was the main repair option chosen. 3. Repacement - The repacement option consists of two separate options. The first was a ike-for-ike repacement of the deep bed unit by CVI and the second was to repace the deep bed fiter with Days. The first option to insta another deep bed unit was rejected because it woud require a great dea of modification work to remove intetierences, it was expensive, and the probems associated with the deep bed units woud remain (e.g. abor intensive oading/unoading of charcoa). The repacement of the deep bed unit with trays was not considered as a possibiity for the seven day LCO and woud have required a Technica Specification change to aiow a onger fiter unit outage. The repacement of the deep bed units with trays is no onger being considered as an option due to expense and difficuty of performing the repacement. V. Pan of the Unit Renairs After consideration of the aternatives, the foowing pan was deveoped to repair the units. The damaged screen sections were initiay identified by photographsinspection. This was utiized to estabish the extent of expected repairs.. The charcoa woud be removed from the unit to aow for screen access. The damaged screen sections woud be competey repaced. Other sma repairs woud be instaed as required. 377

4 25 h DOE/NRC NUCLEAR AIR CLEANING AND TREATMENT CONFERENCE New charcoa woud be instaed. Post maintenance testing woud be performed. The screen repacement was fuy panned incuding the use of a mock-up charcoa adsorber assemby. The project team deveoped a new screen sectiona design The design was based on instaation of the screen sections on the air side of the air box assembies such that the screen woud fit between the channes separating each section (see Figure 3). To insta the screen sections, the existing screens were cut out This operation required specia tooing due to the restricted access to the fu depth of the air boxes. Various too assembies were deveoped (see Figure 2) that coud be manipuated from either the inet or outet air box. The mock-up unit was instrumenta in the deveopment of the required tooing and other preparations prior to the repair activities on site. Since the whoe repair activity was to be competed in a seven day window from removing the unit from service unti the competion of the post-maintenance testing, each activity was cosey scrutinized and added to the overa equipment outage schedue. With a of the other factors considered the actua repair time aotted was three and a haf days. With this window defined, the teams practiced on the mock-up unti a repairs coud be competed within the three and a haf-day schedue. VI. Deveonment of Tooing Assembies To address a phases of the repair process, four ong hande toos were designed and manufactured. The tooing consisted of. Cutting torch. Grinding / ceaning too Video inspection camera Spot weding and camping too The first design of the cutting too was a sma pasma-cutting torch. During mock up testing it was identified that the pasma cutting process eft a jagged and non-uniform cut requiring grinding and dressing the cut areas, It was apparent that the process needed to be refined to reduce the screen remova time. During testing it was determined that the existing screens coud be cut using a modified manua Tungsten Inert Gas (TIG) torch. Torch amperage settings were adjusted to minim&e therma distortion. This ensured that the screen cut woud be smooth and straight. The grinding toos consisted of an air-operated right ange grinder and straight head rotary grinders. The grinding toos served three purposes, 1) to cean the corrosion off the screens; 2) to cean up any burrs on the screens after the cutting process; and 3) to grind off any unacceptabe spot weds during the instaation process. The video inspection camera consisted of a sma right ange video camera mounted on a poe connected to a VCR/monitor. The camera was used to visuay inspect the condition of the existing screens aong with the charcoa adsorber frame area once the damaged screen was removed. It was aso used to perform a visua inspection of the new spot weds. The spot weding toos aso camped the repacement screen whie weding the repacement screen to the charcoa adsorber frame. Two separate toos were deveoped to aow for vertica and horizonta weds. The wed head consisted of a modified TIG torch and gas cup encased in a machined auminum housing. This auminum housing aso contained an air cyinder that acted as the screen camping mechanism during repacement The wed head functioned as foows; after the damaged screens were removed and the repacement area ceaned and inspected, a new screen was paced in its proper ocation. The wed head woud then be 378

5 positioned into its proper ocation. Once proper repacement screen position was verified, the operator woud energize the camping mechanism, which in turn woud appy pressure to camp the new screen against the existing charcoa adsorber frame. Once the operator verified the proper fit-up, the spot wed woud be instaed using an automatic weding program. The weding program was computer controed and addressed upsope - downsope time, weding amps, arc time, arc ength aong with pre-fow and post fow gas coverage. Figure 2 - Long-Handed Tooing Assembies VII. Execution of the Panned Activities Pi-e-Repair - A of the equipment necessary to perform the repair activities were pre-staged to the mezzanine eve where the charcoa fiter assemby was ocated. This incuded running a the power cords for the weding machines, providing the service air hoses, and assembing a of the necessary tooing in the immediate area. Aso the charcoa remova equipment was staged and checked out to minimize the time required to remove charcoa. In addition, the out of service was prepared in advance to minimize the time required to take the unit out of setvice. After the unit was tagged out of service, the individua screen sections were inspected to verify the areas requiring repair. Next, the charcoa was removed from the unit and the screen areas were ceaned of corrosion. Repair - Two crews for each of two shifts were assembed to make the repairs. One crew woud work on the inet side and the other on the outet side of the charcoa fiter assemby. The screen sections were cut out utiizing the TIG cutter. The repacement screens were then instaed and subsequenty inspected. A tota of 21 of 96 screen sections (48 screen sections per fiter train) were repaced. 379

6 Post-Repair - Foowing the inspections of the repacement screens, the charcoa was refied in the unit Fina ceaning of the interior of the charcoa -fiter assemby was performed. Finay, the postmaintenance fiter testing was performed which incuded fow, fiter differentia pressure, and in-pace fiter testing to verify proper operation of the unit Subsequent air fow distribution testing has aso been satisfactoriy competed. Figure 3 - Screen Repairs VIII. Concusion The repair of corroded charcoa retention screens in a deep bed fiter unit can be accompished successfuy in pace within the aotted Technica Specification LCO with extensive prepanning incuding the use of a mock-up and the seection of a project team. The project team key members incuded engineering, maintenance, operations, the origina equipment manufacturer, and a speciaized weding contractor. IX. References 1. Reguatory Guide 1.52 Rev. 2 Design, Testing and Maintenance Criteria for Atmosphere Ceanup System Air Fitration and Adsorption Units of Light-Water-Cooed Nucear Power Pants. 2. Code of Federa Reguations OCFR50 Appendix A Genem Design Criteria 19 Contro Room. 380