ML20043E099

LER 89-018-01:on 890827,cracking Found in Incoloy Alloy 800 Main Steam Ringheaders & Condition Determined to Have Generic Implications.Caused by Creep Failure.Main Steam Ringheaders repaired.W/900531 Ltr

ML20043E099
Person / Time
Site:	Fort Saint Vrain
Issue date:	05/31/1990
From:	Fuller C, Gramling J PUBLIC SERVICE CO. OF COLORADO
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-89-018, LER-89-18, P-90150, NUDOCS 9006120074
Download: ML20043E099 (10)

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@ Public Service' =1 16805 WCR 19 1/2, Platteville, Colorado. 80651

-1 May 31, 1990 i y Fort St. Vrain Unit No, 1-P-90150 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk ,

Washington, D.C. 20555 Docket No. 50-267.

4

SUBJECT:

Licensee Event Report 89-018-01, Final Report

REFERENCE:

Facility Operating License No OPR-34 Gentlemen:

Enclosed, please find a copy of Licensee Event Report S-No. 50-267/89-018-01, Final, submitted. per the requirements- of u.

10 CFR 50.73(a)(2)(v). ,

If _ you have any questions, please contact Mr. M. H. Holmes at (303) 480-6960.  ;

Sincerely, e

>e 4$ l C. H. Fuller Manager, Nuclear Production and Station Manager CHF/1mb i

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Enciosure-cc: Regional Administrator, Region IV Mr. J. B. Baird ATTN:

Tecnnical Assistant Division of Reactor Projects Mr. R. E. Farrell  !

Senior Resident Inspector, FSV l 9006120074 900531 .

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CAU$t tyttlW COMPONENT Ng$0' TO ' 68 $*VII ITITI" CO"'ONINI t ONR B Al B SIG1 1 G IO 1613 Y l i i i l I i l I I I i l i l 1 1 I I I I SveFLlutNT AL hlP041 S KPGCitD Itai ses0Niet Day vtAm t t$ 199 yet, eenwho.e Sk9ttfl0 SV0utSSION DA tti NO l l l t ..cf ,t-,. .e , . e-.e. . e . -. e.e, ~.e. .,,e -e ~,,-,es - n .i l On August 25, 1989, Public Service Company sf Colorado (PSC) discovered a leak in the main steam ringheader of one of the Loop 1 steam generator modules. Upon further investigation, on August 27, 1989, 37 crack indications were found in I the Incoloy Alloy 800 ringheaders in 8 of the 12 modules, and the condition was determined to have generic implications.

l PSC's metallurgical examination determined that the cracking was caused by creep i failure, with the principle failure mechanism being a reduction of creep l ductility of the material. Various factors could have contributed to this i reduction of creep ductility, including cold working without solution annealing, I low titaniym/ carbon ratio, high titanium plus aluminum content, and a coarse-l y 41ned M crostructure. .

Bred on the extent of the ringheader cracking and other operational I considerations, on August 29, 1989, PSC decided to permanently shut down Fort St. Vrain.

Weld repair activities have been performed to the extent necessary to ensure j decay heat removal capability during shutdown and defueling conditions, i

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011 012 0F 01 9 um e . s .mm anc a w on BACKGROUNDt l The two Fort St. Vrain (FSV) steam generators each include six once-through modules arranged in parallel within the cavity of the Prestressed Concrete Reactor Vessel (PCRV) as shown in Figure 1. Each module includes an Economizer-Evaporator-Superheater (EES) section and a reheater section.

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l The EES tube bundle in each module includes 54 tubes that are combined into 18 l subheaders. The 18 subheaders are directed out of the bottom of the PCRV and l are collected in a main steam ringheader for each module, from which steam is l then directed to the main steam piping.

l The twelve main steam ringheaders are manufactured from nickel-iron-chromium l Incoloy Alloy 800, in accordance with the specifications of ASME SB-407 Grade 1.

They are fabricated from 5.563" 0.0. pipe with a nominal wall thickness of l 0.75", and are formed into 51" diameter rings, each with eighteen 1.315" 0.0.

nozzle attachments. Fabrication methods, pro;edures and practices were in accordance with ASME Boiler and Pressure Vessel Code Section Ill, Class A, 1965 Edition, through the Winter 1966 Addenda.

EVENT DESCRIPTION:

On August 25, 1989, with the reactor shut down for control rod (AA)*

replacement, a water leak was discovered in the main steam ringheader ( AB)* for Loop 1 steam generator module B-1-4 At the time, the Loop I EES section was being supplied with condenspte for decay heat removal, at a pressure of less than 300 psig.

The leak was observed to be coming from an approximately 5 inch long crack in the ringheadet, roughly parallel to one of the nozzle attachment welds. The crack was in the base metal.

To determine the extent of the cracking problem, PSC removed the insulation and performed a liquid penetrant examination on the remaining welds of the B-1-4 ringheader. One additional crack indication was observed. Based on the potential generic nature of the crack indications, PSC then performed liquid l penetrant examination on the most accessible portions of all of the Incoloy main I steam ringheaders and found a total of 37 crack indications. Crack indications I were not found on four of the twelve ringneaders.

On August 27, 1989, PSC management rev'ewed the extent of the ringheader cracking. The generic nature of the condition could conceivably have impaired the Safe Shutdown Cooling capability of the plant and at 2255 hours0.0261 days <br />0.626 hours <br />0.00373 weeks <br />8.580275e-4 months <br />, NRC notification was made per 10 CFR 50.72(b)(2)(iii).

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l The root cause of cracking in the Incoloy Alloy 800 main steam ringheaders has l been determined by metallurgical examination to be creep failure, with the  !

I principle mechanism being a reduction of the creep ductility of the material, l PSC's metallurgical examination identified several potential contributing l factors in the FSV ringheader material that reduce creep ductility in Incoloy l Alloy 800 material, These factors were not known at the time the ringheaders l were manufactured, and they include the following:

l 1. Cold working during forming, without subsequent solution annealing, l 2. A low titanium (T1) to carbon (C) ratio, l 3. A Ti plus aluminum (A1) con,ent in excess of 0.6'4, and l 4. Cearse grained microstructure.

SAFETY ANALYSIS:

The main steam ringheader crack that was found on August 25, 1989 and the more extensive crack indications found on August 27, 1989 could conceivably have impaired the Safe Shutdown Cooling capability of Fort St. Vrain. As such, this condition is being reported per the requirements cf 10 CFR 50.73(a)(2)(v).

I Safe Shutdown Cooling can be accomplished with either Loop I or Loop 11 steam l generator EES sections. If the through-wall crack on loop I steara generator module B-1-4 had occurred during power operations, and if it had progressed to the point of complete ringheader failure, PSC could still have safely shut down the reactor with the Loop II EES section. Also, the reheater sections of both l steam generators were available for core heat removal, although the reheaters l have been determined inadequate for Safe Shutcown Cooling heat removal from full power operating conditions, I The cracking observed on the eight ringheaders is considered to have generic I implications, but PSC does not consider it likely that all Safe Shutdown Cooling I capability would have been lost. Complete loss of Safe Shutdown Cooling l capability would have required the simultaneous through-wall cracking and I f ailure of a ringheader in both Loop I and Loop II. A ringheader crack would I most likely leak before a complete break and the ringheaders are accessible l during plant operation, With main steam conditions during operations of 1000 l degree F at 2400 psig, a ringheader steam leak would be detectable by plant i opeettors in time to support a safe plant shutdown.

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01510 lo lo l 21617 Q 118 014 0F Feet st. Vrain. Unit No 1 8 ! 9_ 011 019 van e - . a . . man = = o.- an w nn i In addition, if a ringheader failed / ruptured during power operations, the Steam Line Rupture Detection / Isolation System (SLRDIS) would have detected and isolated the break. Safe Shutdown Cooling could then have been achieved with equipment qualified for the resulting harsh environment. The rupture of a ringheader is bounded by the complete offset rupture of a main steam line as analyzed in FSAR Section 1.4.6. The consequences of this event are acceptable ,

and public health and safety would be protected.

Although highly unlikely, if both Loop I and 'oop II EES sections failed and forced circulation was lost from full power 1 nditions, the PCRV liner cooling system is capable of maintaining containment integrity, so that public health I and sr.fety would be protected. Although fuel damage would occur during such an I event, activity would be contained, as analyzed in FSAR Section 14.10, Permanent loss of Forced Circulation, DBA-1.

METALLURGICAL EVAL.UATIONi l PSC performed a metallurgical examination on samples of the FSV ringheader i cracks, including macroscopic evaluation, replication, microscopic evaluation I (using both light and scanning electron microscopes), chemical analysis, and l hardness testing. (Reference 1) l The configuration of a typical EES ringheader is shown in Figure 2. The I through-wall crack found in module B-1-4 was observed adjacent to the nozzle l weld designated N-118, as illustrated. The linear indications on all l ringheaders were located adjacent to nozzle welds.

l The ringheaders were originally annealed at 1800 degrees F, followed by a water l quench. The material was then cold formed to shape at 1000 degrees F and no l final solution annealing heat treatment was performed. The ringheaders were I designed for typical operating conditions of 1025 degrees F at 2400 psig.

I Average main steam temperatures were actually controlled at 1000 degrees F l during plant operation, with individual module temperatures typically controlled l to within 20 degrees of the average.

l In general, the ringheader material properties were found to be typical for l Incoloy Alloy 800 material. The chemical analysis of the base material met the l Alloy 800 requirements, the grain size was very coarse but in accordance with l ASME SB-407 requirements for Alloy 800 material, the microstructure was typical I of Alloy 800 material, and the hardness was consistent with hardness values l published for Alloy 800 material that had experienced similar service I conditions.

l The metallurgical examination found no contaminants that would be expected if j l the cracking was due to corrosion. Also, the crack surfaces were noted to have I a heavy oxide coating, indicating that they had existed for some time.

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l' l Failure of the ringheaders was determined to have occurred by intergranular l cracking, with negligible material deformation. Examination of failed areas l revealed extensive grain boundary microvoids associated with the cracks, which l indicates that failure occurred due to creep damage. Creep damage is a i

l thermally-dependent deformation process which occurs in a material after l l exposure to constant stress at high temperature. Creep cracking occurs by the f l initiation and coalescenet of microvoids within the material, which eventually l forms microcracks, leading to macrocracking and ultimate failure.

l PSC's metallurgical analysis examined several factors that are now known to l affect the creep failure susceptibility of Alloy 800 material. These factors l are as follows:

l- Ti/C ratio and grain size. The Ti/C ratio was 4.86 which is on the low end l and typically would result in a coarse grain size, as was observed. The l degree of embrittlement is reported in the literature to be more severe for l coarse grained materials than for fine grained materials, l

l- Ti + Al content. Th'e Ti + Al content was 0.694, and it has been reported l l that Alloy 800 material with a Ti + Al content in excess of 0.6'. exhibits l l poor creep ductility.

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l- Degree of cold working and solution annealing heat treatment temperature, i l Cold working up to 10% has been observed to decrease creep ductility in l Alloy 800 material, and to increase notch sensitivity. Solution annealing I at 1500 to 1550 degrees F is recommended to reduce susceptibility to i l premature failure. Since the ringheaders were not solution annealed after l I cold working, the effects of cold working are considered to be significant '

I factors in their premature failure.

l The ringheaders were not solution annealed during manufacture because many

- l factors which influence the creep ductility of Incoloy Alloy 800 material, as l noted above, were not known at the time of design or manufacture.

l PSC's metallurgical examination identified other possible contributing factors, l l the significance of which could not be determined without additional testing and l l a complete stress analysis. These include higher than expected operating l temperature or stress, and unanticipated notch effects due to the weld joint .

l configuration. Due to PSC's decision to permanently shut down Fort St. Vrain, I l additional analysis and testing are not planned.

l l' l REPAIR ACTIVITIES l PSC investigated the feasibility of repairing the cracks to allow continued l

l operation. Several areas were ground, liquid penetrant tested, and found l l acceptable for repair. However, PSC was not confident that all of the cracks l could be repaired and restored to full operating conditions due to the l microcracking which was discussed previously in the Metallurgical Evaluation.

l This microcracking would not be identified during the liquid penetrant l examination but it could contribute to later ringheader failure, j geoa= =.na .u s oro ie*+e:4 5*4ss

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011 Ol 6 0F 0 19 un a . a .ammw ame a answim PSC had previously announced that plant operations would not continue beyond June 30, 1990. In light of this limited amount of potential operation, PSC decided that complete repair and restart efforts were not feasible and Fort St.

Vrain was shut down permanently.

Under reactor shutdown conditions, decay heat removal is normally accomplished l by supplying condensate to a steam generator EES or reheater section in a single I loop, where each loop contains six steam generator modules. PSC re-evaluated the ringheader cracks for the maximum service conditions available with the condensate pumps, which have a shutoff head of 340 psig. Using conservative conditions of 400 psig and 300 degrees F, a minimum calculated wall thickness of 0.0758 inches was determined. A minimum acceptance criteria of 0.250 inches was then conservatively established and weld repairs were made to cracks in modules B-1-1, B-1-3, and B-1-4. No other weld repairs were required. With these repairs, the EES ringheaders of either loop are acceptable for heat removal during the remaining FSV shutdown conditions. Because of the reduced heat removal requirements during shutdown conditions, any future conceivable ringheader failures would have no safety significance.

CORRECTIVE ACTION:

On August 29, 1989, PSC management announced that Fort St. Vrain would not be restarted and that the operational phase of the plant had ended.

l The main steam ringheaders have been repaired to the extent necessary for decay l heat removal during shutdown and defueling conditions. Shutdown heat removal l will use condensate with a maximum pressure of 340 psig.

I l

l Reference 1. PSC Metallurgy Laboratory Report No. 225, Failure Analysis of the l Fort St. Vrain Steam Generator Main Steam Ring Headers, February l 12, 1990.

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