ML20062H719

Responds to 800630 Memo Re Peer Review of Differing Prof Opinion on Pressurizer Relief Line Weld Repair.Repair Did Not Penetrate Pipe Wall;Hydrostatic Testing Unnecessary. Discussion of Repair Issues Encl

ML20062H719
Person / Time
Site:	Sequoyah
Issue date:	07/24/1980
From:	Shao L NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
To:	Vollmer R Office of Nuclear Reactor Regulation
Shared Package
ML20062H675	List: IR 05000327/1980012 ML20062H670 ML20062H676 ML20062H682 ML20062H690 ML20062H693 ML20062H695 ML20062H700 ML20062H704 ML20062H707 ML20062H710 ML20062H713 ML20062H719
References
NUDOCS 8008210284
Download: ML20062H719 (9)

Text

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so; UNITED STATES NUCLEAR REGULATOAY coMM:ssiCN

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.....< JUL 2 41980 MEMORANDUM FOR: Richard H. Vollmer, Director .

Division of Engineering, NRR FROM: L. C. Shao, Assistant Director for General Reactor Safety Research, RES

SUBJECT:

PEER REVIEW 0F A DIFFERING PROFESSIONAL OPINION, SEQUOYAH UNIT ONE PRESSURIZER RELIEF LINE WELD REPAIR

Reference:

Memo to Shao from Vclimer, " Peer Review of a Differing Professional Opinion," dated June 30, 1980 In May 1979 Sequoyah Unit One pressurizer relief line was deformed as a result of a support malfunction during hot functional testing of the reactor coolant system. In order to restore the pipe to its original configuration, TVA used a weld draw-bead technique, employing two deeply grooved welds that i.ere ground out and rewelded, in the 6-inch stainless steel pipe. The NRC staff accepted this repair, but Mr. J. Halspatz expressed concerns about the repair. Subsequently, a 304 stainiess steel mockup was evaluated and in situ metallography on the field repair was performed.

Based on tneir review of the mockup and field repair, the NRC staff iccepted the TVA repair procecure. Nevertheless, Mr. J. Halapatz of tne NRC staff suomitted a differing professional opinion regarding tne acceptance of the weld recair. On June 30, 1980, you (by referenced memo) requested that a peer review of Mr. Halapatz's differing professional opinion be conducted.

In completing tnis review, I have drawn on information and evaluations developeo by my staff, principally J. Muscara, C. Z. Serpan, Jr., and J. E. Richardson. We have reviewed the information included in your memo, have conoucted interviews with NRC personnel from NRR, IE Headquarters and SD, and have held two telephone interviews with personnel from TVA. Both J. Muscara and C. Z. Serpan have met with TVA at their headquarters offices and at a nuclear plant construction site to review and evaluate previously available and aeditional inforr.ation. In completing the conclusions and recommendations, I have considered all of the metallurgical information provided by my staff and have also considered the overall safety implications of this repair weld against the history and experience of other welds and repairs in nuclear service.

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R. H. Vollmer 2 M 2 '- 2 Two main points of the differing professional opinion of Mr. J. Halacat:

are that (1) the draw-bead welds might have penetrated to the pipe I.D.

thereby requiring a hydrotest, and (2) the draw-bead welding might have caused sufficient sensitization of the piping to render it susceptible to intergranular stress corrosion cracking (IGSCC). Some of Mr. Halapatz's contentions regarding item (2) have merit.

The following paragraphs describe the results of this peer review with respect to weld penetration and sensitization, with conclusions and recomendations following; a discussion of technical issues and comparison of the weld repair to BWR nonconforming service-sensitive lines is enclosed.

First, with respect to weld penetration, adequate information has been presented and reviewed by NRC IE Region II to indicate that the weld did not penetrate tne I.D. Further, a hydrotest in this case would not provide any additional useful information on weld integrity. We believe that the NDE performed. i.e., radiography and UT, have shown the soundness of the repair.

With respect to sensiti:ation, it is concluded that the pressuri:er relief pipe has been sensitized under the repair welds cown to the inside surface and most likely has been made susceptible to IGSCC by the repair process.

This opinion derives from a review of the available information attached to your referenced memo, a review of research results of the past 5 years, and from results of the second mockuo prepared by TVA, as disclosed to C. Z. Serpan, Jr. ano J. Muscara during a visit to TVA on July 17 and 18,1980.

Those results show a region of sensitization below the bottom of the weld groove that is sufficiently large as to encroach on the pipe I.D. on the field recair. This sensiti:ation, apparent to one degree or another all the way frem the weld root to the wall I.D., when coupled with the relatively hign 02 content in tne steam phase in the line (reported by TVA to be a maximum of 0.2 ppm), plus high residual tensile stresses, could result in initiation of IGSCC.

Conclusions and Recommendations We believe that the Sequoyah Unit One pressurizer relief line draw-beac repair (1) did not oenetrate the pipe wall so the hydrostatic testing is not necessary, and (2) did become sensiti:ed and, thus, could be susceptible to IGSCC in service. As stated above, in addition to metallurgical reasons, the conclusions and recommendations are based on an evaluation of the overall situation which includes field expvience, operating stresses, successful experiences in detecting small leaks, and behavior under accident loads. Therefore, although this stainless steel line most likely is sensiti:ed to some degree and will operate in an environment of 02 concentration

R. H. Vollmer 3 JUL 2 41980 somewhat similar to that of BWRs under full power operating conditions wnich have experienced cracking in service, this specific repair weld appears to be neither worse nor more serious than many other welds that are currently in service, especially those welds in BWR nonconfoming service-sensitive lines (see page 6 of the enclosure). We, therefore, reconrnend resolution of this matter to be the same as for service-sensitive nonconforming lines as stated in NUREG-0313, Revision 1. -

Altnough we believe the above reconsnendations will satisfactorily resolve this issue, if one wishes to better understand the status of the repair weld, or to minimize the potential crack growth of the repair weld, other tests could be performed on the second mockup or on the pressurizer relief line itself. For the mockup: (1) Electrochemical Potentiokinetic Reactivation (EPR) tests could be perfomed to evaluate the degree of sensitization on the irside surface of the pipe, and (2) Slow Extension Rate Testing (SERT) of inside surface specimens could be perfomed in a simulated service environment (saturated steam at 650'F and 2485 PSI containing 0.2 ppm 07 )

to determine the susceptibility to IGSCC. For the repaired pipe, technTques exist for conducting in situ EPR tests on the inside surface or near-inside surface to determine the degree of sensitization. In the absence of conducting any further tests, acoustic emissior, can be used for limited area monitoring of the repaired section of the pipe to detect, on-line, the possible growth cf IGSCC and thereby avoid any leakage in service.

It is pointed out that the whole issue of weld repair is not well addressed oy the ASME Boiler and Pressure Vessel Code, nor by the NRC in Regulatory Guices or other technical positions. It would be beneficial for this section of the Code to be strengthened or other NRC positions to be developed to better address the sensitization problem in weld repairs.

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L. C. Shao, Assistant Director for General Reactor Safety Research Division of Reactor Safety Research

Enclosure:

Discussion of Tecnnical Issues cc w/ encl:

R. J. Budnitz, RES H. R. Denton, NRR E. G. Case, NRR T. E. Murley, RES C. Z. Serpan, RES J. E. Richardson, RES l J. Muscara, RES )

J. Halapatz, NRR '

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DISCUSSION OF ISSUES RELATED TO DRAW-BEAD WELD REPAIR OF SE000YAH UNIT ONE PRESSURIZER RELIEF PIPE TVA prepared a mockup to qualify the procedure for draw-bead field repair.

Documentation and quality control of the draw-bead. repair appear to be w tain normal requirements, but are not adequate for a careful review of what was actually done or to evaluate the metallurgical consequences of the repair.

Altncugh the draw-bead repair is stated to be Code acceptable, and is practiced in fisid repairs, the Code does not address the relevant topics of residual welding stresses, sensitization and susceptibility to stress corrosion cracking of austenitic stainless steels all of which can result from repair welding and can be responsible for field failures. Further, the draw-bead weld repair technicue is not specifically discussed and accepted by the Code, but upon a cuery to ASME by TVA, ASME found this draw-bead method as being implied in the rules for weld repair.

First Mockuo - 30a SS - The original mockup prepared and studied by TVA and j reviewed by NRC staff and consultant was not adequate to address the issues i of sensiti:ation and cracking with respect to the field repair. The mockup was of 304 stainless steel while the field repair was of 316 stainless steel.

Althougn the welding heat inout was claimed to te much higner in the mockup ,

wnen TVA was interviewed, they agreed that the travel sceed both in the mockup and in the 'ield were not measured. TVA stated that the mockup constitutes a conservative representation of field repairs because of the higner heat input ano hign carbon 304 stainless steel used in the mockuo. Further, TVA claims tnat the field repair was acceptable because the specimers from nis mock-up

:assed the A262E intergranular corrosion test. It is agreed that mcckup

! soecimens passed the A262E test, even :nougn metallograony did snow a J

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2 considerable level of sensitization; however, the A262E tests over the last 5 years nave been reported to be insensitive to levels of sensitization in welded stainless steel that are nevertheless high enough to crack stainless steels in operating plants.

Therefore, the mockuc was unrepresentative and also it cannot unquestionably be considered conservative. Further, the corrosion tests performed do not accurately predict the susceptibility to IGSCC, and tnus, such tests cannot be used to establish the acceptability of the repair.

The review shows that the caroon level in tne 304 stainless steel mockup was not cocumentec out was saic to be abcut 0.065 w/o. Questions still exist witn resoect to accurate information on weld neat input in neckup versus field altnough the mockup heat inout was reported to be about 179 Kj/in. versus about 2u Kj/in, fcr-the field.

In situ Meta 11ocrachy - At a meeting between TVA and NRC staff on March 13, 1980, NRC requested, and TVA subsequently conducted, in situ metallography on the outside surface of the draw-bend weld repaired pressuri:er relief pipe. The in situ metallography was witnessed by IE Region II personnel. TVA and IE Region II concluded from an evaluation of this in situ metallography that the pice HAZs were not sensitized, tnat sensiti:ation cid not encroach on the pipe insioe surface, and that, therefore, the weld repair was acceptable and the pipe would not be susceptible to IGSCC. NRR also accepted these conclusions.

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3 It is pointed out nat outside surface metallography can be totally misleading with respect to the state of sensitization on the inside surface of the pice.

Recent research results have snown that the outside surface of a pipe may not sensitize appreciably due to welding even though the inside of the pipe does. Very often the inside surface of the pipe is sensitized enough to render it susceptible to cracking hile the outside has very little or no sensitiration.

If the outside surface of piping is found to be sensit.ized then :ne inside surface will be even more sensitizec; tnis situation can arise wnen the heat of material is especially sensitive or is already sensiti:ed in :ne as-received condition before welcing.

There are several important reasons wny the outside surface does not sensitize nearly as mucn as tne inside. Firstly, the oatside surface coes not experience as much excesure to heat (total time at sensitization temperatures) as the inside surface. Secondly, two parameters have been found most important in accelerating weld sensiti:ation; these are the thermal cycles and the strain cycles experienced by the underlying (or adjoining) base material. The themal cycles and strain cycles increase the kinetics of sensiti:ation. Since the inside surface of a welded pipe experiences more cumulative thermal and strain cycles tnan the outside surface, more sensiti:ation is found on the inside '

surface. With respect to the draw-bead weld repair of the pressuri:er relief pipe, the 316 stainless steel material used has slower sensiti:ation kinetics than 3Ca stainless steel, so that under nomal comparable welding conditions, 31e would sensiti:e less. The weld recair was acconolished by filling tne groove once then grinding out tne weld metal and refilling :ne groove. This

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procedure, then, has imposed twice the thermal / strain cycles on the base material underlying the root bead leading to the insioe surface than on a nor nal penetration welc. The outside surface did not experience very many ther nal/ strain cycles and can be expected to be nonsensitized or only lign:ly sensitized. The inside surface, however, because of tne accelerating effect of thermal / strain cycle: and the large number of cycles received car, be exoected to be sensiti:ed. Therefore, one cannot infer from the outside surface metallography results, that the inside surface of the draw-bead repaired pipe would not be sensitized.

Secone Mockue - 316 SS - On July 17 and 18, 1980, C. Z. Serpan and J. Muscara reviewed results of a second mockup prepared by TVA in February 1980. This mockup more closely represented the actual pressurizer relief pipe material, anc furtnemore, the welder was instructed to use tne same welcing procecure as for the' field repair. The mockup material used was 316 stainless steel of .051 percent C (pressuri:er pipe is 316 SS of .052 percent C). The mockup contained one penetration weld and two draw-bead repair welds. The two draw-bead welds penetrated approximately 2/3 and 1/2 the oipe wall thickness, respectively. It was evident from the metallography tnat (a) the full penetration weld (wnicn was not reground and rewelded) showed no sensitization on either the outsice or inside surface; (b) the draw-bead welds (wnicn were reground anc reweloed) showed no sensitization on the outsice surface but showed serlsitization along the fusion lines extending up from the root pass to within about 1/4 T of the wali 0.D., as well as a considerable :ene of sensitization 1

in the base material uncerneath the root pass leading towards and even reaching l l

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the inside surface; (c) the sensitized base metal extended about 0.2 inches '

below the root pass towards the inside surface, although the degree of sensiti:ation decreased from the root bead towards the inside surface.

By relating the results of this mockup to the field draw-bead repair, the absence of sensitiration on the outside surface of the field pipe, as shown by in situ metallograohy, is confirmed. Also confirmed is our evaluation that sensiti:ation to some degree extends to the inside surface of the repaired pressuri:er relief pipe since there is less than 0.2 inches of material left on the pipe while the mockup shows a sensitized :ene extending about 0.2 inches from the root bead tcward the inside surface.

Corrosion tests were also performed on this second mockup using ASTM A262 practice E tests. These tests snowed the absence of cracking. As described earlier, A262 practice E tests are not very sensitive for detecting tne susceptibility to IGSCC in LWR environments for materials with sensitization levels wnich are nevertheless high enough'to be susceptible. As a good example of this test's poor discrimination to cracking susceptibility, we highlight the results of the TVA first mockup whien used a high carbon (said by TVA to be .065 percent) 304 stainless steel and a very high heat inout resulting in a condition which would most certainly have failed in LWR environments containing 0.2 pom oxygen: test samples from this 304 stainless steel mockup passed the AS*M A262 practice E test.

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Com:arison of tne Weld Repair to BWR Ncnconformine Service-Sensitive Lines -

The weld recair to the Secuoyan pressuri:er relief line is at least as good as many welds in SWR nonconfoming service-sensitive lines investicated by the Dipe Crack Study Group (NUREG-0313). This conclusion is derived from the following:

1. The average car 0cn content in the 304 stainless steel BWR pipe material found to be cracking in service, aoout 0.06 cercent, is about the same as the pipe material in the Sequoyah line (316 stainless steel - 0.052 percent).

2. Under full operating conditions, oxygen content of the BUR coolant is slightly higher than that of a DWR pressuri:er relief line (0.2-0.4 con vs 0.005-0.2 pom). Tne im::ortance of this is tnat oxygen is a major centributor to intergranular stress corrosion of sensiti:ed stainless steels.

3. Over the cast several years, there have been numerous cases of cracks in BWR stainless steel lines while there have been no reports of cracks in the pressurizer relief lines of PWRs.

J. It is likely that some BWR lines, because of their geometry or location. l may be stressec nigner than tne PWR oressuri:er relief line esoecially wnen large eartncuakes are considered.

5. Tne safety significance of one 6-incn PNR relief line weld is much less than the safety significance of a larger amount of welds in SWR nonconforming service-sensitive lines.