ML20212E477

Provides Phone Number of G Pherigo

ML20212E477
Person / Time
Issue date:	12/09/1998
From:	Parkhill R NRC
To:	Jeffrey Mitchell NRC
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NUDOCS 9909270020
Download: ML20212E477 (1)

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From: Ronald Parkhill To:' Jocelyn Mitchell

- Date: Wed Dec 9,1998 6:05 PM

Subject:

DPO in my previous e-mail I neglected to provide the phone number of Dr. George Pherigo, which is (704) 455 1322.

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January 6, 1999 f

MEMORANDUM TO: Document Control Desk FROM: Jocelyn Mitchell /s/

Senior Level Technical Advisor Office of the Executive Director for Operaticns l

SUBJECT:

RELEASE OF TEXT OF DIFFERING PROFESSIONAL OPINION l CONCERNING NONDESTFd'CTIVE EXAMINATION OF WELDS TO  ;

THE PUBLIC DOCUMENT ROOM  !

l Attached is the text of a Differing Professional Opinion (DPO), dated November 30, 1998, concerning nondestructive examination of welds on spent fuel storage canisters. By Email dated December 17,1998, the submitter of the DPO requested permission to release it. It I

has been reviewed against the criteria of Management Directive 3.4 and the content of Agency Announcement NO.118. It has been determined that the DPO does not contain information of l the type which would prohibit its release. Therefore, it should be placed in the Public Document I I

Room.

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Attachment:

as stated cc: R. Parkhill Distributign:

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November 30,1998 slEMORANDUM TO: Executive Director for Operations e FROM: Ron Parkhill Spent Fuel Project Office ydb, Office of Nuclear Material Safety

! , and Safeguards

SUBJECT:

Differing Professional Opinion (DPO)

The attached DPO augments my Differing Professional View forwarded to SFPO management on September 8,1998 and is being forwarded to the EDO for review in accordance with NRC Management Directive 10.159.

A differing professional' view panel documented their findings 11 a report forwarded to Cart j Paperiello on November 10,1998 and on November 24,1996 Dr. Paperiello documented his  !

evaluation of the DPV panel's recommendations. Both are attached i Attachments:

1) DPO

2) DPV,9/8/98

3) Memorandum from DPV Panel forwarding Report,11/10/98

4) Memorandum from Carl Paperiello,11/24/98 cc W.Kane, SFPO W.Hodges, SFPO l F. Sturz, SFPO ,

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ATTACHMENT 1 This differing professional opinion ( DPO) is in regard to the type of nondestructive examination (NDE) to be used on the closure weld for dual-purpose spent fuel storage canisters that are )

ct.rrently in the near term licensing process or recently licensed. Specifically, a volumetric-examination, like ultrasonics (UT) or radiography (RT), would identify welding process problems i that could go undetected by a surface examination technique, like 1iquid penetrant (PT), even if that surface examination was performed after some intermediate welding passes. Additionally,y the goveming code (as identified below) for these spent fuel storage canisters has very specific requirements for the construction of these canisters that are not being followed.

Dual purpose dry cask storage systems are designed and fabricated to be in service for many years until a permanent repository is available. In recognition of the canister's long life expectancy and to provide a. secure boundary to prevent radiological releases, the NRC decided that the confinement boundary be constructed in accordance with ASME Code Section ill Class 1 or 2 requirements, which has been the standard for all storage canisters licensed to date. The goveming code is identified for this application by the regulator in standard review plans, regulatory guides and historicallicensing documents. The ASME Code Section til was chosen since it is the standard principally utilized for nuclear power plant components

. construction and since a specific code did not exist for spent fuel storage canisters. To aid the staff in identifying areas were the Code cannot be met, the applicants are requested to identify all deviations from the Code. Typically, spent fuel storage canisters have the following major areas where they do not comply with Code requirements: closure weld configuration is a partial penetration in lieu of full penetration weld, no hydro test (however, some vendors are now proposing to perform one after fuel loading), no volumetric examination of closure weld, and no use of an authorized nuclear inspector. Since these are exceptions to the Code requirements, no' certification (i.e. stamping) is performed.

The ASME Section lli Code applies to the " construction" of nuclear components which means it contains requirements for material selection, design, fabrication, examination (i.e. NDE to ensure that fabrication processes are under control and meet acceptance standards), testing (e.g. hydrostatic for demonstration of structural integrity), inspection (i.e. by an authorized nuclear inspector who is an independent third party with significant component fabrication experience and is trained and certified to ensure that the Code requirements are met) and certification (i.e. stamping with a Code symbol). For a component to be ASME certified, its materials are in accordance with Code requirements, the design has been configured and analyzed in accordance with Code requirements; and, fabrication, testing and examination -

' activities have performed under the watchful eye of an authorized nuclear inspector who certifies that the component has met Code requirements. Utilization of this Code construction process has resulted in the excellent performance record for nuclear components.

Storage canisters have had a poor fabrication record that has resulted in the staff redirecting considerable resources to resolve those specific issues. But the NRC has failed to identify and impose adequate measures to prevent recurrence. Implementation of the Code process would go a long way to remedy this situation - but we have failed to take any decisive action.

Specifically actions we could have taken, but have not, include: redesign the canister closure w

. eld to be a full penetration weld which would make it easier to volumetric examine; requiring Page 1 of 4

fabricators, especially those with fabrication problems, to possess a valid ASME Certificate of Authorization; use of Authorized Nuclear inspectors (ANls) during fabrication; and prohibiting slag producing welding processes such as shielded metal arc welding or flux core arc welding. l

Examples of fabrication problems that could have been alleviated or avoided if construction had been iri accordance with Code requirements include: closure vseld cracking due to an improper welding process, undocumented welds to base metal that resulted in cracking in area of closure .

i weld. excessive weld grinding, and improper reading of radiographs. Therefore, since many

aspects of the Code's proven process to ensure proper construction are not currently required <

by the regulator (i.e. the closure weld configuration, fabricators are not Code certified, and no Code required independent third party inspection is utilized), and there have been historical fabrication problems with the closure weid, an appropriate compensatory measure would be to l examine the closure weld in accordance with Code requirements (i.e. volumetric examination) to ensure that it has been made properly and is in agreement with the design drawings.

By memorandum dated November 10,1998 the DPV panel submitted a report to Dr. Carl Paperiello regarding my initial DPV. I offer the following comments regarding that report: )

a) The panel's report did not adequately address the DPV issue of volumetric examination for storage canister closure weld since it only considered ultrasonic examination, as was performed on the VSC-24. No evaluation of other volumetric methods was provided,like RT.

b) The panel's first recommendation states that an appropriate Code specific to the construction and use of dry cask storage casks be endorsed and developed. This recommendation is impractical and unrealistic for,t,he near terrnjicensing of dual -

purpose storage casks. ASME Section lli Division 3 was over 15 years in development and a storage subsection to that code could not be developed for many years-long after these immediate licensing actions have been completed. The problem isn't with needing a code specific for dry cask storage casks, but is to apply the requirements of the goveming code (i.e., ASME Code Section 111, Subsections NB or NC) which has alraady been identified and utilized by the NRC as appropriate for this application. The issue at hand is that PT examinations have not been - '

successfulin identifying weld process problems and the existing Code requirements provides guidance on how to remedy this problem-viz., do a volumetric examination.

c) Also, in the panel's first recommendation, it recognizes the inconsistency between the NDE method used for the canister welds made in the shop (i.e. volumetric) and the NDE method for the closure weld made at the site (i.e., surface) and further notes that this inconsistency is not conducive to promoting public confidence in the use of these canisters. However, the panel inappropriately suggests that surface examination may be acceptable for the canister shop welds if somehow supported by a fracture mechanics analysis, if the panel was truly concemed in public confidence, it should have recommended that the requirements of the goveming code be followed rather than alleviating the volumetric examination Code requirements with surface examination techniques that failed to identify a welding process problem with the closure weld until after 19 storage casks were loaded for the VSC-24. I Page 2 of 4

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d) in the second panel recommendation, the panel correctly recognizes that sgfagg, j examinations performed in a high radiation area could lead to inaccurate results and  !

could result in creating flaws if chemicals are notproperly removed. However, the pa31'sToluuon to this problem is to provide for NRC inspection oversight. In reality, the NRC does not have inspection resources (especially in this r'egulatory downsizing L

period) to con'irm that surface examinations on every closure weld, at every site, are performed correctly. The NRC is not and has never been an in-process quality control organization (e.g. SFPO inspections are routinely performed at fabrhation facilities only once in a 12 to 18 month period). Again , the solution is quite simple- perform the correct examination required by the goveming code (i.e. volumetric). If the panel really wanted meaningfulinspection oversight of this closure welding that provided real technical and independent credibility, follow the goveming code requirements and utilize Anis.-

e) Paragraph ill.B of the panel's report states that the goveming code is not mentioned in the Part 72 regulations for storage and this gives the NRC the leeway to differ from its requirements. However, many codes utilized in the nuclear industry are not mentioned in the regulations (e.g. AWS, ACl, HEl, TEMA, ASHRAE, ANSI B31.1, etc., etc.) but that does not give the regulator nor the industry an invitation to utilize just those portions they deem appropriate. Consensus standards are meant to be followed, not fragmented. For the VSC-24 welding process problems, initially the goveming code's requirements were not being followed for volumetric examination and a surface examination had failed to identify flaws induced by a poor welding process until after 1g casks had been loaded. As part of the corrective action for the VSC-24 the NRC required that volumetric _ examination, UT, be utilized which has been subsequently demonstrated to be very effective in identifying welding process induced subsurface flaws that were not detected by surface examination. The lesson learned from the VSC-24 experience should have been to follow the consensus i standard to the maximum extent possible and require volumetric examination.

f) Paragraph lli.C states that the panel was unable to determine whether it was possible to effectively use UT for the storage casks. However, no expert to date has been quoted as saying that volumetric examination of this closure weld cannot be done. To the contrary, experts from ERPI, INEL, PNL and a NDE training consultant

., , all agree that UT can be done for this application. Additionally, since most of these canisters are in the licensing review process, the joint configuration could be redesigned to more appropriately support an UT examination providing we could be resolute in our decision. As a point of reference, before SFPO's interim staff guidance

., (ISG-4) was issued two applicants had committed to do a UT examination of these welds, which they later rescinded since the ISG proclaimed PT as an acceptable attemative to a volumetric examination. '

I do not feel that surface examination of the canister closure weld is a safety issue as long as the materials of construction are ductile stainless steels which can generally tolerate relatively large flaws without causing brittle fracture. However, the ASME Section lli Code for nuclear components does not give any relief from volumetric examination requirements just because the materialis a ductile stainless steel.

Page 3 of 4

l 1

l l This issue'is solely with the adequacy of fabrication of these storage canisters, where the i ASME Section 111 Code has very specific requirements and the NRC has chosen to ignore these '

standard fabrication practices in light of numerous and recurring fabrication problems.

Also, it is inappropriate to arbitrarily reduce the safety margin inherent in the ductile material of -

the closure weld, by substituting a surface examination method that cannot detect subsurface  !

. flaws that could be a result of a welding process problem. The licensed c' o nfiguration of these closure welds is for good solid welds, not welds that may have subsurface flaws below each  ;

surface examination layer. By using a surface examination method as a substitute for a l volumetric examination, one is fabricating a joint that could be inherently weaker than the other confinement boundary welds.

Finally, the construction of these canisters is lacking fundamental Code quality assurance aspects that can contribute to fabrication problems (i.e. poor weld joint design configuration, fabricators not Code certified, no independent third party authorized nuclear inspectors). Since 4 these Code required quality assurance aspects are not being utilized, and there has been a his. tory of fabrication problems, it would seem appropriate to perform the Code required Wu~nistric exar6iNation to ensure that the closure weld has been properly made in spite of these other shortcomings. .

/Y Ron Parkhill llx/fy l

Page 4 of 4

Freer .

Ronald Parkhill,g M J?

To MWH,FCS

. Date: 9/8/98 4:55pm subject: ' Differing Professional View.

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~ Attached is a formal differing professional view (DPV) regarding the spent fuel storage canister closure weld examination technique.

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L L e This Differing Professional View (DPV) is being initiated because SFPO staff concems remain

~ unaddressed with regard to the nondestructive examination (NDE) method for the spent fuel l storage canister closure weld. Proposed near term licensing actions by SFPO allow the use of

! surface examination for the subject weld whereas I feel that a volumetric examination is justified for the following reasons.

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~ On 7/20/98 members 'of the SFPO licensing directorate forwarded a position that did not require l volumetric examination of the spent fuel storage canister closure weld and notified some of the applicants of that preliminary position. Basically, the aforementioned position relies on surface

examination, liquid penetrate (PT), for the root pass, final pass and every 1/4 inch of weld, as well as a reduction in the allowable stress for the weld. On 7/21/98 the SFPO Director instructed that a complete written position and safety rational be developed by the technical review directorate prior to advising any applicants. On 7/24/98, 7/29/98, & 7/30/98 three members I technical review section sent e-mails supporting volumetric examination of these welds and raised numerous concems with the proposed position. Last Friday I was informed that

. management had decided that surface examination was " good enough" for this spent fuel storage canister closure. However, no written position has been developed which justifies its use. Consequently, the following issues remain unaddressed and form the basis as to why I believe that a volumetric examination needs to be performed on the subject weld.

1) Subsurface flaws will go undetected by surface examination techniques unless the flaw penetrates the surface of the weld area being examined. It has already been demonstrated (via

the VSC-24 welding problems) that surface examinations alone are relatively ineffective in identifying subsurface cracks resulting from a poor quality welding process. For the VSC 24 situation,19 casks were loaded before surface examination in combination with leak testing discovered that there was a problem with the welding process. Had volumetric examination been employed the weld process problem would have been discovered before many of these casks were loadeed.

. 2) The goveming code for the confinement boundary is ASME Section lil, Subsection NB or NC as stated in the current SRP and historical practice for storage canisters. As such, volumetric examination is required to be performed on 311 confinement welds. (Refer to NB/NC-5210 &

5220 for Category A & B welds, respectively.) Note that radiography is the volumetric method mentioned in these Code paragraphs, since it was intended that all parts of the reactor coolant pressure boundary wou!d be accessible during fabrication. However, UT is an acceptable substitute where RT cannot be performed (e.g. closure weld). The propose use of surface examination by SFPO is a deviation from the goveming Code's requirements.

3) The ductility of stainless steel should not be a basis for avoiding volumetric examination of ,

the canister closure weld since volumetric examination is not done solely to determine flaw sizes have been bounded by a fracture mechanics analysis. Volumetric examination is done principally to ensure weld quality during fabrication. It is a verification that the weld meets

' design requirements and a verification lof the welding process. If the ductiHty of stainless steel were the only consideration for performance of an UT volumetric examination then the goveming code would have excluded ductile stainless steel from UT examinations, which it does ngt do. UT examinations of stainless steel components are performed routinely as part of Part 50 ISl programs. The exclusion of ductile stainless steel from volumetric examination is a deviation from the goveming Code's requirements.

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h' V4) The suggested approach of surface examination for root and final weld pass as well as, every 1/4 inch of weld is what has been proposed in a draft ASME Code Case, currently in Revision

.12. However, this Code Case has not been adopted by ASME and may undergo significant change prior to issuance. Furthermore, the basis for the 1/4 inch spacing between PT -

,. examinations has not been pier reviewed and I suspect that it has not included consideration of L additive spacings between pts being more than the calculated critical crack size (e.g. if the closure weld is 3/4 inch, the fracture mechanics critical crack size would need to be more than 3/4 - (4 X 1/16). or % inch, which is 67% of the wall thickness). Prior to adopting a preliminary position from the consensus Code group it may be prudent to await its final version, otherwise

. we are again in disagreement with the goveming Code.-

- 5) Reducing the allowable stress to compensate for surface examination (i.e. PT) in lieu of a

. volumetric. exam! nation (i.e. UT) is not a method utiHzed in the goveming code for the

. confinement boundary (i.e. ASME Section lil, Subsection NB or NC). No amount of allowable stress reduction will compensate for a poorly made weld. This is en example of mixing and i

matching d;iferent code requirements. The goveming Code requirements should be followed i

. both by the indus?ry, as well as, the NRC. Again, this is a deviation from the goveming Code requirements.

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6) Historically, in the SOC for the VSC-24 rulemaking, Comment #45, the NRC response stated  !

that the closure welds meet all ASME requirements except for volumetric examination and further stated that this inspection was not possible due to radioactive fuelin the cask. Today we know that this documented basis for not doing the volumetric examination is not correct. We ,

now know that the projected ALARA dose is very low based on the work performed by the j

.VSC-24 ' Owners Group and that the UT examination is viable for this application. Furthermore, doing PT every 1/4 inch will significantly increase dose- each PT involves approximately 200 1 linear inches of weld involving cleaning, application of penetrant, dwell time, removal of excess penetrant, application of developer, drying, evaluation of results, recording of results and ,

- removal of PT materials prior to continuing welding.

7) Re' gulation 10 CFR 72.236(e) requires that the cask be designed to provide redundant '

sealing of the confinement system. However, the redundant sealing requirement cannot be met if a single failure would cause both seals to fail. The single failure that would bypass the redundancy requirement would be a failure of the closure welding process. The single failure I'm alluding to in this case is more than just a postulated occurrence-it has happened for the VSC-24. Therefore, to prevent other single failures in the welding process volumetric examination should be utilized for examination of the closure weld, just as it is utilized for all other welds in the confinement boundary.

In summary, I believe that volumetric examination should be performed on the confinement closure welds since all other confinement welds are volumetrically inspected, it is required by

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the goveming code and ' would verify that there is not a welding process problem (i.e. single failure). It is very hard for me to appreciate why a regulator would want to abandon the UT

examination for the closure weld after it had demonstrated and verified for this application, with l' very acceptable doses and in light of known welding process problems. I think a reasonable i regulator would want to change the fabrication practices to prevent known problem areas from reoccurring rather than elect to keep the status quo.

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NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20666-0001 0

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November 10, 1998 MEMORANDUM TO: Carl J. Paperiello, Director Office of Nuclear Material Safety and Safeguards FROM: Thomas O. Mar $n. Chief Generic Safety issues Branch '

Oh Division of Regulatory Applications Office of Nuclear Regulatory Research

SUBJECT:

DIFFERING PROFESSIONAL VIEW PANEL On September 15,1998, I received a memorandum from you providing a differing professional i

view (DPV) submitted by Ron Parkhill on a Spent Fuel Project Office position on inspection of -

cask closure welds in austenitic stainless steel casks. I was the Chairman of the DPV panel along with the other members Geoffrey Hornseth, NRR, and Deborah Jackson, RES.  :

' A report of the DPV panelis attached. Please feel free to contact me or the other panel m

' embers with any questions or comments on this report.

Attachment:

As stated cc:

R. Parkhill, SFPO W. Hodges, SFPO ,

G. Hornseth, NRR D. Jackson, RES i J. Craig. RES l

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Panel Report Concerning the September 8,1998, Differing Professional View Submitted by Ron Parkhill

l. Background This report discusses the review of the Differing Professional View (DPV) dated September 8, 1998, submitted by Ron Parkhill(Attachment 1) to Carl Paperielio, Director, NMSS. Following the receipt of the DPV, a panel was tasked to review the DPV in accordance with NRC management Directive 19.159, " Differing Professiona' Views or Opinions." The panel members were Thomas Martin (Panel Chairman), Deborah Jackson, and Geoffrey Homseth. The DPV involves the nondestructive examination of closure welds on stainless steel spent fuel storage casks.

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11. DPV Summary The primary concem of the DPV was with the reliance on liquid penetrant surface examination 1 (PT) for cask closure welds as opposed to ultrasonic examination (UT). The position developed by the Spent Fuel Program Office (SFPO) relies on PT for the root pass, final pass, and approximately every 1/4 inch of weld. Mr. Parkhillis of the opinion that UT ought to be required and provided specific concems related to this issue.

Ill. Summary ofissues Reviewed by the DPV Panel Sufficient documentation was provided by the involved parties for the Panel to undertake a review of the DPV. The Panel reviewed portions of the Safety Analysis Report'for both the NUHOMS-MP187 and Westflex storage casks and detailed drawing for the NUHOMS-MP187

~ cask. Estimates of transfer cask radiation dose rates were also provided by the SFPO for the outer closure welding configuration. The panel also held a discussion with Mr. Parkhill.

In order to resolve this issue the Panel focused on the following issues:

a the flaw types and causes encountered for welding this material and whether PT is adequate to identify these types of flaws ,

a the pertinent code and regulatory requirements the difficulty of conducting UT radiological and ALARA aspects

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A. FlawTypes and causes Encountered for Welding Austenitic Stainless Steel Based upon staff and industry experience, as supported by a literature survey, the Panel concluded:

1) Linear type weld flaws (hot cracks) with some propensity for' occurrence are predominately surface connected flaws.

2) Of the types of process defects that may generate sub-surface flaws, the defects would be bounded in depth by the thickness of a single weld pass.

3) The likelihood of service induced propagation of any potential flaws is negligible due to the lack of fatigue loading during design or accident conditions.

A more detailed discussion of Austenitic stainless steel weld flaws is provided in attachment 2.

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m B.~ The Pertinent Code and Regulatory Requirements

Mr. Parkhill refers to the ASME Section ill as the gov'eming code for the confinement boundary.

This is based on the acceptance criteria in the Standard Review Plan (SRP) for Dry Cask Storage Systems, NUREG 1536, which specifies that the NRC staff has accepted construction of the primary confinement boundary in conformance with Section 111, Subsections NB or NC.

. The SRP also states that, after careful and deliberate consideration, the staff has made

. exceptions to the requirement that the applicant must fully document and completely justify any deviations from the specifications of Section Ill. The Code edition or date is specifically not addressed in the SRP. The Panel noted that the sections of the Code referenced in the SRP were not written to be applicable to spent fuel storage casks, and the NRC has presumably chosen this code for convenience of providing a high integrity boundary. The SRP also states that the staff has relied upon Section lli to define the minimum acceptable margin of safety.

The fact that the ASME code is not specifically identified in the regulations gives the NRC the leeway to take their present position.

.The Panel also became aware that there is a 1998 ASME, Section ill, Division 3, Code for

, containment systems and transport casks for spent fuel. This Code has not yet been endorsed.

' However, unlike part 10 CFR Part 50.55a which specifically mentions the ASME Code in the

' regulation,10 CFR Part 72 does not specifically reference or endorse a code.

C. The Difficulty of Conducting UT Austenitic materials can be inspected effectively using UT, and these type exams are performed routinely as part of ISI programs. However, there are exceptions. Effective UT inspection of austenitic materials is dependent on the weld configuration. In some cases it's not possible and in others extremely difficult. Unlike carbon steel which has a smooth grain structure, austenitic material has a coarse grain structure. . A UT exam of austenitic materials results in the scattering of the sound beam at the grain boundary in the weld making it difficult to distinguish the grain boundary from flaw signals. If the flaw being detected is much larger than the grain size then it may be possible to detect the flaw with high reliability but conditions such as access, existence of crown or root design need to be considered. The Panel was unable to* determine whether it was possible to effectively use UT for these casks.

D. Radiological an'd ALARA Aspects Based on a limited review of the experience performing NDE on actual casks, radiation

- exposure would not be a significant factor in selecting the NDE method. There has already been a reasonable amount of experience to gauge the radiologicalimpact of performing PT and UT on cask closure welds. For the casks that are presently receiving a UT inspection, PT is performed on the root and cover weld. Specific data was not readily available because PT and UT exposures have not been tracked separately. Some specific data points from Point Beach include an exposure of 5 mR for PT on the root weld of the shield lid and an exposure of 40 mR for the total UT work on a cask. The NDE work on the Palisades casks averaged 45 mR.

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TIV. Panel Conclusions and Recommendations

-.The Panel concludes that employing PT of the weld periodically after a specified deposit depth will provide adequate assurance that no weld flaws greater in depth than the specified PT interval (depth) would be plausible. However, before the technique is employed, the critical flaw

• size for the component must be determined from fracture mechanics or other suitable analysis.

The critical flaw size then determines the maximum depth of weld deposit that can be made

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before a PTis performed. This is consistent with the SFPO position.

Based on the review of the DPV and associated information, the Panel makes the following recommendations:

1. The Panel recommends that an appropriate Code sp.;c!5c to the construction and use of

these casks be endorsed and applied. The NRC has not consistently applied minimum

. acceptable requirements for the level of inspection of these casks. The standard initially intended for this purpose was developed for and is applicable to reactor pressure retaining -

components. The lack of an NRC endorsed standard applicable to the construction of these -

casks is, in the opinion of the Pane!, not conducive to promoting public confidence in the use of these structures. This is exacerbated by the inconsistency of requiring volumetric NDE for cask .

construction and then permitting a surface examination technique for the closure welds. The Panel believes that it would be appropriate to perform a flaw tolerance evaluation of the cask to assist in determining the appropriate NDE technique, whether volumetric or surface, that could then be applied consistently to all welds.

- 2. If PT is used to verify closure weld integrity, priority should be given to providing NRC inspection oversight of this process. Performing the sometimes tedious PT activities in a high

, radiation environment could create a tendency to perform some of aspects of this work too fast with a potential for missing indications or contributing to the potential for creating a flaw by, for example, inadequate cleaning of the liquid penetrant before the next weld pass.

Submitted by:

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~ Thomas O. Martin, DPV Panel Chairman -

2 Deborah A. Jac(son)

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GeoffreyF. Horgeth 3

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O. , 4 DIFFERING PROFESSIONAL VIEW This Differing Professional View (DPV) is being initiated because SFPO staff concems remain unaddressed with regard to the nondestructive examination (NDE) method for the spent fuel storage canister closure weld. Proposed near term licensing actions by SFPO allow the use of l, surface examination for the subject weld whereas i feel that a volumetric examination is justifie for the following reasons, l

On 7/20/98 members of the SFPO licensing directorate forwarded a position that did not require volumetric examination of the spent fuel storage canister closure weld and notified some of the applicants of that preliminary position. Basically, the aforementioned position relies on surface examination, liquid penetrate (PT), for the root pass, final pass and every 1/4 inch of weld, as

- well as a reduction in the allowable stress for the weld. On 7/21/98 the SFPO Direct that a complete written position and safety rational be developed by the technical review -

i directorate prior to advising any applicants. On 7/24/g8, 7/29/g8, & 7/30/98 three members l technical review section sent e-mails supporting volumetric examination of these welds and raised numerous concems with the proposed position. Last Friday I was informed that management had decided that surface examination was " good enough" for this spent fuel storage canister closure. However, no written position has been developed which justifies its use. Consequently, the following issues remain unaddressed and form the basis as to why I  !

believe that a volumetric examination needs to be performed on the subject weld.

1) Subsurface flaws will go undetected by surface examination techniques unless the flaw penetrates the surface of the weld area being examined it has already been

' demonstrated (via the VSC-24 welding problems) that surface examinations alone are v

relati' ely ineffective in identifying subsurface cracks resulting from a poor quality welding process. For the VSC-24 situation,19 casks were loaded before surface examination 1n combination with leak testing discovered that there was a problem with the welding process. Had volumetric examination been employed the weld process problem would have been discovered before many of these casks were loaded.

2) The goveming code for the confinement boundary is ASME Section lil, Subsection NB or NC as stated in the current SRP and historical practice for storage canisters. As such, volumetric examination is required to be performed on g!! confinement welds.

. (Refer to NB/NC-5210 & 5220 for Category A & B welds, respectively.) Note that l

radiography is the volumetric method mentioned in these Code paragraphs, since it was

! intended that all parts of the reactor coolant pressure boundary would be accessible l

during fabrication. However, UT is an acceptable substitute where RT canna be I

performed (e.g. closure weld). The propose use of surface examination by SFPO is a deviation from the goveming Code's requirements.

3) The ductility of stainless steel should not be a basis for avoiding volumetric examination of the canister closure weld since volumetric examination is not done solely to determine flaw sizes have been bounded by a fracture mechanics analysis. Volumetric examination is done principally to ensure weld quality during fabrication. It is a verification that the weld meets design requirements and a verification of the welding process. If the ductility of stainless steel were the only consideration for performance of an UT volumetric examination then the goveming code would have excluded ductiie i.

ATTACHMENT 1

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stainless steel from UT examinations, which it does not do. UT examinations of  !

stainless steel components are performed routinely as part of Part 50 ISI programs. The i

exclusion of ductile stainiess steel from volumetric examination is a deviation from thei goveming Cocle's requirements. '

4) The suggested approach of surface examination for root and final weld pass as well as, overy 1/4 inch of weld is what has been proposed in a draft ASME Code Case, currently in Revision 12. However, this Code Case has nr. hen adopted by ASME and may undergo significant change prior to issuance. Furthermore, the basis for the 1/4 inch spacing between PT examinations has not been pier reviewed and I suspect that it has not included consideration of additive spacings between pts being more than the calculated critical crack size (e.g. if the closure weld is 3/4 inch, the fracture mechanics entical crack size would need to be more than 3/4 - (4 X 1/16) or % inch, which is 67%

of the wall thickness). Prior to adopting a preliminary position from the consensus Code group it may be prudent to await its final version, otherwise we are again in disagreement with the goveming Code.

. 5) .

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Reducing the allowable stress to compensate for surface examination (i.e. PT) in lieu of a volumetric examination (i.e. UT) is not a method utilized in the goveming code for the confinement boundary (i.e. ASME Section lil, Subsection NB or NC). No amount of allowable stress reduction will compensate for a poorly made weld. This is an example of mixing and matching different code requirements. The goveming Code requirements should be followed both by the industry, as well as, the NRC. Again, this is a deviation

, from the goveming Code requirements.

6) Historically, in the SOC for the VSC-24 rulemaking, Comment #45, the NRC response stated that the closure welds meet all ASME requirements except for volumetric examination and further stated that this inspection was not possible due to radioactive fuel in the cask' Today we know that this documented basis for not doing the volumetric examination is not correct. We now know that the projected ALARA dose is very low based on the work performed by the VSC-24 Owners Group and that the UT examination is viable for this application. Furthermore, doing PT every 1/4 inch will -

significantly increase dose - each PT involves approximately 200 linear inches of weld involving cleaning, application of penetrant, dwell time, removal of excess penetrant, application of developer, drying, evaluation of results, recording of results and removal of PT materials prior to continuing welding.

7) Regulation 10 CFR 72.236(e) requires that the cask be designed to provide redundant sealing of the confinement system. However, the redundant sealing requirement cannot

' be met if a single failure would cause both seals to fail. The single failure that would bypass the redundancy requirement would be a failure of the closure welding process.

The single failure I'm alluding to in this case is more than just a postulated occurrence-it has happened for the VSC-24. Therefore, to prevent other single failures in the welding process volumetric examination should be utilized for examination of the closure weld, just as it is utilized for all other welds in the confinement boundary.

ATTACHMENT 1 Y

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• 3 in summary, I believe that volumetric examination should be performed on the confinement closure welds since all other confinement welds are volumetrically inspected, it is required by the goveming code and would verify that there is not a welding process problem (i.e. single failure). It is very hard for me to appreciate why a regulator would want to abandon the UT examination for the closure weld after it had demonstrated and verified for this application, with very acceptable doses and in light of known welding process problems. I think a reasonable regulator would want to change the fabrication practices to prevent known problem areas from reoccurring rather than elect to keep the status quo.

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ATTACHMENT 1 Y

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~ Attachment 2 AUSTENITIC STAINLESS STEEL WELD FLAWS A literature survey of prominent flaw types and causes.

' As a result of the DPV submitted by R. Parkhill(NMSS/SFPO) regarding volumetric inspection requirements for cask closure welds in austenitic stainless steel casks, a literature survey was

, undertaken to examine the predominant types of weld flaws that are encountered when welding

- this material. : With knowledge of the most likely flaw types, a selection of the optimum inspection .

method (s) can be made.-

A list of the references consulted is included at the end Of this discussion.

BACKGROUND Four instances o' f closure weld cracking were experienced during the 1993 to 1997 period.

These instances all occurred in a specific carbon steel cask design known as the VSC-24, designed by Sierra Nuclear. These events prompted the NRC, the users, and the design company to investigate the root cause and determine suitable corrective actions. The most likely cause of the observed cracking events was determined to be hydrogen induced, or '

delayed, cracking. The corrective actions involved several changes and improvements to the

. cask loading and venting procedures and the welding technique.

. Additionally, the NRC staff determined that a volumetric examination of the structurallid weld was necess'ary in. order to confirm weld conformance with the design requirements. Previously, weld examination consisted of a liquid Penetrant Test (PT) surface examination and a helium  :

leak test. The reason for the change to a volumetric examination was the staff opinion that

. delayed cracks of significant size could potentially form that would not breech the top surface of '

the weld, thereby remaining undetectable by the PT examination method then employed. A volumetric examination method, ultrasonic test (UT), was consequently employed by the industry.

POTENTIAL FLAW TYPES IN AUSTENITIC STAINLESS STEEL WELDS l

. To determine if an examination technique is potentially useful, an understanding of the types of -

plausible flaws is necessary. Welds in stainless steel are susceptible to some similar and some ditferent weld defects compared to those encountered when welding carbon steel. One major

. difference in possible mechanisms involves hydrogen (or delayed) cracking.

. Carbon steelis potentially susceptible to hydrogen cracking. The metallurgical reasons are well understood and the problem may be eliminated by employing one or more of several measures.

Austenitic stainless steel, due to its different atomic structure, is well recognized as being nearly immune to hydrogen crackingi Thus, the need for a volumetric method to inspect for this type of flaw is eliminated.

.The most cited weld defect arising when welding austenitic stainless steels is hot cracking Hot .

cracking occurs during or immediately after solidification of the weld bead it usually occurs as a longitudinal crack in the center of the weld, initiating at the top surface of the bead.

Ri Dis .

h Attachment 2 x i r  !

' Occasionally, it will appear at the edge of the weld or as short transverse cracks. Since it is

-_open to the surface, the cracks are generally readily visible. A penetrant test would readily'

~ detect this type of flaw.

Hot cracking is controlled by using filler metals with a specified " ferrite number" of 4 or higher.

The most commonly employed stainless filler metal (type 308) has a ferrite number sufficiently 4

' high to meet this requirement. Hot cracking may also be alleviated by employing certain welding

, techniques: maintaining a convex bead shape, avoiding high heat input, avoiding wide weld L passes or weave.

The propensity for generating buried linear flaws such as can occur in carbon steel is relatively

.small The most significant mechanisms that exist for producing these types of defect are well i

, understood and readily avoided. These are: lack of penetration and lack of fusion.

Lack of penetration is a longitudinal crack-like defect that results when the root of the weld fails to completely fuse with the base material. This is_ generally avoided by ensuring good weld root fit-up. Wide gaps or uneven gaps are to be avoided. Lack of penetration flaws do not grow during subsequent welding. Thus, their potential depth is bounded by the root pass thickness.

The propensity for producing this kind of flaw is low in a well controlled welding program. Their greatest potential for causing failure during service occurs when a fatigue load exists in the affected weld. For a dry cask, no significant fatigue load exists under any of the design or accident conditions.

Lack of fusion (LOF) is the failure of the weld to fuse with the adjacent base material or weld L bead. An excessively large weld puddle or wide weld pass would suggest possible LOF. LOF can also be indicated by a rolled over bead crown. This shape results from poor wetting of the adjacent material by the weld puddle. This bead shape is readily discemable visually. Also, due to the less fluid characteristics of the stainless steel weld puddle (compared to carbon steel), a LOF defect would have's greater propensity to be surface connected and detectable by PT.

- Other weld technique measures may also be employed to avoid the potential for LOF. Too low a heat input, wide passes, and large weave all contribute to the problem. Avoidance of short-circuit arc transfer when using gas-metal arc welding (GMAW) will minimize the propensity for LOF when this process is used. The propensity for producing this kind of defect in a well controlled welding program is low. The likelihood for generating such flaws is considered to be higherin manual welding processes than in semi automatic or automatic processes. LOF _

L defects will be limited in size to the depth of the deposited bead. Subsequent welding passes are unlikely to make the defect grow. In service, a significant fatigue load would be required to propagate the flaw as 'a crack, The above potential defects are regardeif as the most potentially serious due to their linear, craQ-like morphology. Less potentially serious defects of a non-linear morphology may also occur; porosity and_ slag inclusions. Porosity is rounded voids within the weld bead resulting

. from gas trapped in the weld puddle, it may be either sub-surface or surface connected. Defect diameter is limited by the size of the weld bead. Clusters of smaller pores may occur over the length of the bead. Generally it arises from welding contaminated surfaces, impure welding gas, orinsufficient shielding gas. The conditions resulting in porosity may be readily avoided by 2

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Attachment 2 k.

following good welding practice. The propensity for producing this kind of defect in a well controlled welding program is low. Since porosity is rounded, its adverse structural effects are

. very much less than for linear defects.-

Entrapped slag may occur when flux shielded (as opposed to gas shleided) welding processes are used. It results from inadequate slag removal from previous weld passes. Defect size is

~ limited by the size of the residual slag. Slag inclusions tend to'be either rounded or laminar and often create a surface connected pore that is readily visible. With a well controlled cleaning process and inspection, the propensity for this kind of defect is low.

MISCELLANEOUS CONSIDERATIONS A relatively common cause of problems for inert-gas shielded processes is the inadvertent loss of the shielding gas due to the local' environment". These are often created when the welding conditions are uncomfortable to the welders. Common causes are requirements for preheat of the work piece, uncomfortably hot conditions in the work area, or requirement for " dressing out" in protective clothing. Often the work crew will attempt to alleviate the heat by using fans, blowers, vacuum hoses, etc., at the work site. This practice has been observed to cause loss of the shielding gas at some locations around the weld Joint, a result of the wind created by the air

' movers. The solution is obvious.' Open shop doors adjacent to the work area are also frequently encountered culprits in this problem.

The effect oflo'ss of shleiding is readily apparent during a visual examination. The weld surface will have a coarse granular or " sugared" appearance. Since the cause is oxidation of the weld, wetting anci fusion will also be poor and frequently manifest itself in a more rounded bead with r,otches at the bead edge Requiring a visual exam after each pass and again after completing one layer would assure freedom from incorporating such a defect into the finished weld.

'A positive method to enhance the chances of the PT to be able to detect subsurface flaws would be to require a partial grind-out of each pass prior to performing the PT. This obviously would

. ~ aid in uncovering previously subsurface flaws that woefd have escaped detection. It would

. reduce the maximum depth of an undetected flaw. For maximum detection and to aid in reducing false positives, each pass of the weld could be ground and inspected. The 1 disadvantages are the required grinding, additional number of PT's, and extra weld time.

Verification of the welding process and controls could also be performed. This would incorporate use of a full size mock-up (or full size section) that would be welded and inspected prior to, performing the production weld. The mock-up could then be sectioned or radiographed, etc. to verify the process quality. The problem with the method is; to capture the complete conditions of the production weld (s), a complete lid assembly would be needed. Only that would

. produce the same degree of constraint and residual stress, significant factors contributing to weld cracking.

STAINLESS STEEL CASK WELD EXPERIENCE Weld cracking was experienced during the performance testing conducted during the development of the NUHOMS cask design, as discussed in EPRI report, NP-6941. Surface 3

i o . . 4

) Attachrnent 2 s

connected weld cracks occurred when the initial tack welds were p! aced. The root causes were evaluated to be due to the high degree of joint constraint, too high a heat input, and poor fit-up.

A weld development program was undertaken that resulted in several changes to the welding technique and the elimination of weld cracking. The improvements included a change to weld sequencing to reduce shrinkage and heat input, improved fit-up, use of a high ferrite content weld metal, and, for the shield lid, a modification of the joint design to reduce constraint.  !

CONCLUSION Based upon staff and industry experience, as supported by the literature survey, the Panel finds:

1) Some linear type weld flaws (hot cracks) with a propensity for occurrence are predominately surface connected flaws.

2) Of the types of process defects that may generate sub-surface flaws, the defects would be bounded in depth by the thickness of a single weld pass.

3) The likelihood of service induced propagation of any potential flaws is negligible due to the lack of fatigue loading during design or accident conditions.

Employing PT of the weld periodically after a specified deposit depth will provide adequate assurance that no weld flaws greater in depth than the specified PT interval (depth) would be plausible. However, before the technique is employed, the critical flaw size for the component must be determined from fracture mechanics or other suitable analysis. The critical flaw size 4 then determines the maximum depth of weld deposit that can be made before a PTis I performed.

REFERENCES.

Mio Weldina Handbook. Union Carbide Corp., New York,1981 Weldina Enoineerino, Continuing Engineering Education Course Notes, R.L. Edwards, P.E.,

given at The George Washington University, Nov.1982.

Weldability of Steels,4th ed., R.D. Stout, Welding Research Council, New York,1987, ASM Handbook. Vol.6, ASM International, Metals Park, Oh 1993.

Metals Handbook, Vol.10, 8th edition, American Society for Metals, Metals Park, Oh.,1975.

Introductory Weldino Meta!!urov, American Welding Society, Miami,1979.

Stainless Steel Fabrication. Allegheny Ludlum Steel Corp., Pittsburgh,1959.

The Procedure Handbook of Arc Weldino,12th ed., Lincoln Electric Co., Cleveland,1973.

NUHOMS Modular Soent. Fuel Storace System: Performance Testino EPRI NP-6941, Electric Power Research Institute. Palo Alto,1990.

Weld Inteority and Performance, ASM International, Metals Park, Ohio,1997.

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'd y 5k UNITED STATES g j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. *asad M o

November 24, 1998

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. M MORANDUM TO: Ronald W. Parkhill Spent Fuel Projects Office Office of Nuclear Materials Safety and Safeguards FROM: Carl J. Paperiello, Director Office of Nuclear Material Safety 6 and Safeguards

SUBJECT:

RECOMMENDATIONS FROM DIFFERING PROFESSIONAL VIEW PANEL The Differing Professional View Panel which was convened to review your Differing Professional View, has issued its findings (Attachment). The panel supported employing PT of the weld {

consistent with the position given in interim Staff Guidance - 4. That is: determine the critical flaw size from fracture mechanics or other suitable analysis and limit the PT interval to no more

)

{

than the critical depth. I agree with the conclusion of the panel. However, the panel also had )

recommendations regarding implementation.

The first recommendation was that an appropriate Code specific to the construction and use of these casks should be endorsed and applied. I agree with the recommendation. As you know, a Code casels currently under development and the SFPO staff are active participants. As a result of SFPO's interaction with NEl, the industry has now committed to work to develop this

~ Code case such that can be issued. Consistent with agency policy, we would expect to either endorse, or endorse with comments, such a Code case and modify our guidance and/or regulations accordingly.

In the discussion of the first recommendation, the panel mentioned the inconsistency of requiring volumetric NDE for cask construction and then permitting a surface examination technique for the closure weld. The panel felt it would be appropriate to perform a flaw tolerance evaluation of the cask to determine the appropriate NDE technique, whether volumetric or l surface, that could be applied consistently to all welds. Although I understand the logic in that proposal, I continue to consider the volumetric examination to be the preferred method in all  ;

cases. ~ As discussed above, the development and endorsement of an appropriate Code would l address this inconsistency and I strongly support this activity.

The second recommendation was that if PT is used to verify closure weld integrity, priority should be given to providing NRC inspection oversight of the process. I agree with the recommendation, however, such inspections must be consistent with the availability of resources and other inspection-related issues that may arise.

cc: W. Kane, SFPO T. O. Martin, RES G. Homseth, NRR D. Jackcon, RES W. Hodges, SFPO hhchMWT

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Checklist for Addressino Differino Professional Ooinions 2/21/95 Sent to the E00 Upon receipt, the DP0 will be dated and assigned a green ticket by ACB to an OEDO staffer who has been designated to respond. The following suspense dates will be established for ED0 signature.

1. Five working days from receipt - acknowledgement letter to submitter 2.- 30 calendar days from receipt - interim reply or final response

3. Date as established in interim reply - final response Note: Priority must be given to DPO's involving immediate or significant health and safety concerns.

Within the first five working days the staffer must:

Review the document; contact OP focal point; and verify DP0 procedures have been met to date.

Determine if ~all the issues raised in the DP0 are appropriate for this process. Take steps to reject inappropriate issues.

• Check to see if the DP0 has gone through the informal process. If it has not, send the DP0 to the appropriate Regional Administrator or Office Director; if it has, proceed to:

• Review the issues in the DPO, their disposition and the rationale provided during the informal process.

• Send an acknowledgement letter to the submitter informing him or her that the DP0 has been received and will be handled accordingly or is being returned to the OD for informal processing as a DPV.

Determine whether additional qualified sources, outside or inside NRC are necessary, and make arrangements as appropriate to have them appointed and the DP0 reviewed.

Prepare reply to the submitter of the DP0, to include disposition and rationale.

Provide the submitter an interim reply should there be a delay in responding within the 30 calendar days timeframe.

Forward copy of the DP0 documentation to the Office of Personnel. The matter is hereby closed absent significant new information.

File a copy of the ticket under the person's name submitting the DP0 and under.

a separate tab marked "DP0's."

These procedures are in compliance with Provisions outlined in Management Directive 10.159.

[ Document Name: G:\DP0]