ML20195K065

SER Concurring W/Util Proposed Corrective Actions in Engineering Rept Entitled, Rept of Helium Circulator S/N 2101 Damage & Inlet Piping S/N 2001 Repair & Mod Activities

ML20195K065
Person / Time
Site:	Fort Saint Vrain
Issue date:	06/15/1988
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20195K063	List: ML20195K059 ML20195K065
References
TAC-65992, NUDOCS 8806290335
Download: ML20195K065 (4)

Text

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' ~g UNITED STATES 8 n NUCLEAR REGULATORY COMMISSION

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SAFETY EVALUATION REPORT BY THE OFFICE OF NUCLEAR REACTOR REGULATION ON THE HELIUM CIRCUL/.0R S/N C-2101 FAILURE AND REC 0VERY FORT ST. VRAIN NUCLEAR GENERATING STATION PUBLIC SERVICE COMPANY OF COLORADO LICENSE NO. DPR-34: DOCKET N0. 50-267 l INTRODUCTION A meeting was held with the Public Service Company of Colorado (the licensee) l on March 4, 1988, at the Fort St. Vrain Nuclear Generating Station to review the engineering report, "Report of Helium Circulator S/N C-2101 Damage and Inlet Piping S/N 2001 Repair and Modification Activities." The purpose of the meeting was to discuss the design changes and controls to be implemented at Fort St. Vrain as a result of the analyses performed on ccmpenents from the failed circulator and associated inlet piping assembly.

EVALUATIO_N:

Metellurgical analyses on 24 different components from the helium circulator j showed that component failure and/or damage occurred as a result of caustic stress corrosion cracking. The primary source fcr caustic cortamination was the steam used for the turbine drive. A review of Fort St. Vrain cperation record indicated that major steam contamination occurred during the period from December 1983, until April 1985. Significant amounts of sodium hydroxide and sodium phosphate were injected into the auxiliary bciler steam in order to reduce corrosion in the system. The controlling procedure for the chemistry of the condensate /feedwater during plant shutdown specifies a limit of 1000 ppb sodium where the demireralizers are bypassed and 20 ppb where they are in service.

The procedure is being revised at present so that the permissible sodium limit for the condensate /feedwater during power operation is 3 ppb at the outlet to the dcmineralizers. The revised procedure will require auxiliary boiler system clean-up in the event the sodium content of the auxiliary boiler steam exceeds 3 ppb.

The metallurgical evaluation showed that failure occurred in different materiais in several components. Pitting and stress corrosion cracking (SSC) was observed in Type 410 stainless steel labyrinth seal mounting bolts, steam turbine stator cap screws and socket head cap screws and in the plain carbon steel spring plunger. Stress corrosion cracking was observed in the Type A-286 stainless steel steam duct bolts and pressure tap bolt. SCC was observed in Monel 400 lockwire, Inconel 600 expansion jcint, 5% chrome nozzie bolt assembly, and SAE 4140 nozzle support socket cap screws and the bellows support ring cap screws.

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Pechanical darage was observed in Inconel 718 steam duct and Type 430 stainless steel labyrinth spacer and seal and insulation ccver. Slight pitting and cor-rosion was observed on Type 422 stainless steel steam turbine rotor disk and 5%

chrone special studs. Cracking due to inadequate weld was observed in plain carbon steel steam deflector plate. No damage was observed in Inconel 718 Pelton-wheel bolt and nut, Inconel 600 Pelton wheel lock washer, Inconel 600 lockwire for cap screws and Type 304 stainless steel lockwire for bolts.

The corrrron environment to all the failed materials was the steam used to drive the helium circulator. High levels of caustic carried ever into the circulater by the auxiliary boiler stear was considered to be the major cause of the corrosion and the 500. Althcugh nolybdenum disulfide was used for bolt lubrica-tion, SCC was observed en nonlubricated components, such as the expansion bellows and the fractured surface of the pressure tap bolt, the major cause of the ob-served SCC was net attributed to the degradation prcoucts of melybdenum disulfide.

A survey was corducted to determine the bolting materials for use in the helium circulator rest rcsistant to SCC in a steam or water environtent containing caustic, chlcride and/or molybderum disulfide degradatier piccuct. Thc survey indicated that no bolting mattrial was entirely irrune to caustic SCC under conditions believed to have occurred in the darage helium circulator auxiliary boiler reheat steam. Inconel X-750 in the solution heat treated, overaged conoitior, was generally favored to be the most cerrosion resistart, high strength, high temperature material for use in circulator refurbishment. The heat treat-rent reccrrended is solution anneal at 2025 25 F for I to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and overage at 1300 25'F for 20 to 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br />, corresponding to EPP.! draft specification 2227 - 16322 - HC2.

The prelcad reouf rement for each fastener application in the circulater was evaluated. The objective was to mininize preload stress in order te avoia SCC but to exceed a tightening value where fastener loosening might occur. One of the most critical application was the steam ducting to bearing assembly bolts.

Inconel X-750 was not suitable for this application because of the stress re-quired to maintain secondary interspace pressurization. Type A-286 was origi-nally specified and performed satisfactorily for an estinated 60,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />. As

! an enhanced circulator inspection program is being implemented, including moni-toring of the secondary inrerspace pressure, the specification for these bolts was not changed.

All lockwashers and lockwires exposed to the turbine drive steam are being changed to Inconel 600. Inconel 600 appeared to be superior to plain carbon and icw alloy steel and Type 300 and Type 400 series stainless steel. The steam l inlet piping mounting stud material is being changed from SA-193-B5 to SA-193-27 due te material availability. There are sixteen studs holding the piping to the steam scroll and preload stresses are less than 60% of yield strength. The turbine /Pelton wheel tie bolt and nut (Incorel 718) are not undergoing specifi-cation change. The preload is less than 60% yield strength and SCC was rot cbserved during the examination of the original material.

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s l The method for preloading each fastener was evaluated. The steam ducting to 1 bearing assembly bolts required preload extension of 0.008-in. to maintain the I pressure boundary seal. In addition to specifying parallel end-faces for these I

Type A286 bolts, an ultrasonic methed is being investigated to measure the elongation during component assembly. The turbinc/Pelton wheel tie belts will be  ;

preloaded using a caliper to measure elongation during component assembly. The remainder of the fasteners will be preloaded using a precision torque wrench (Refer to Table 3 of subject report).

Fechanical means is being used to remove residual molybdenum disulfide lubri-cant frcm the fastener holes in the refurbished helium circulator. A revievi of potential lubricant indicated that Dow Corning Mclykote 505 was the preferred substitutt. Although a chemical analysis is planned to ensure environmental ccmpatibility, should constituents be found which may cause SCC, Holykote 505 will not be used. Never-Seez Pure Nickel Special and Felpro Grade N 1000 are being investigated concurrently fcr potential usage.

Technical Specification SR 5.2.18 is to be upgraded to include an enhanced inservice inspection surveillance requirement for fastener replacement and component inspection within a time frame of 40,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> helium circulator operation. The purpose of the prcgram is to insure replacement of failed faster.ers and the spring plunger in the steam turbine /Pelten wheel area and to perfern a visual inspection of the components removed for fastener replacement.

The fastener replacement and component inspecticr. will be repeated within a time frame of 40,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> operation. In addition, a second circulatcr will be disassembled within a ten-year period in conformance to the present Technical Specifications.

A comprehensive nenitcring program has been implemented to enhance circulatcr performance and to assist in the early detection of circulator failurt.

Circulator shaft wobble is at present being monitored on an interim basis. In an effort to increase monitoring capabilities, the comprehensive program should provide continuous wobble monitoring, alam, data acquisition and storage, trending and diagnostic capabilities.

C0hCLUSION The metallurgical analysis performed on the components frcm the failed helium circulator C-2101 indicated that caustic stress corrosion crackina was the pri-mary cause for compcnent failure. Significant levels of caustic (sodium hyroxide and trisodium phosphate) were injected into the feedwater to the auxiliary boiler steam to cure a corrosion problem during the period from December 1983, to April 1985. Component failure occurred in several different materials, some of which are not normally susceptible to SCC.

SCC was identified in the following circulator components: Type 410 stainless steel bolts, Type A286 stainless steel bolts, carbon steel spring 5. lunger, Monel 400 lockwires, and Inconel 600 bellows.

s 4 The staff cor. curs with the actions being taken by the licensee in the refurbish-ment of helium circulator C-2101 and to minimize the potential for the recurrence of SCC at the Fort St. Vrain Nuclear Generating Station. The fastener and lock-ing rechanism are being char.ged to Inconel X-750 and Inconel 600, respectively.

The Inconel X-750 fasteners are specified in the solution annealed and overaged condition. In certain hichly stressed applications, where significant preload in required to provide adecuate pressure capability and Inconel X-750 is not a suitable replacement, the original Type A-286 material is specified. An enhanced inservice inspecticn and monitoring crogram based on 40,000 hour0 days <br />0 hours <br />0 weeks <br />0 months <br /> circulator operation is being implemented. In eddition, water chemistry procedures have been instituted to reduce the sodium content of reheat steam used to drive the circulators. The staff concurs with these actions and believes that the new procedures will ensure the integrity of circulator ccaponents and reduce the poter.tial for the occurrence of stress corrosion cracking.

Dated: June 15,1988 Principal Contributor: F. Litton 1

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