ML20212E456

Forwards Formal Differing Professional View Re Spent Fuel Storage Canister Closure Weld Examination Technique

ML20212E456
Person / Time
Issue date:	09/08/1998
From:	Parkhill R NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
To:	NRC
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J From: Ronald Parkhill, pfnf.)

To MWH,FCS Date 9/8/98 4:55pm

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Differing Professional View Attached is.a' formal differing professional view (DPV)'regarding the spent

-fuel storage' canister closure weld examination technique.

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4 f This Differing Professional View (DPV) is being initiated because SFPO staff concerns 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 surface examination for the subject weld whereas I feel that a volumetric examination is justified

' for the following reasons.

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 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 technical review section sent e-mails supporting volumetric examination of these welds and raised numerous concerns with the proposed position. Last Friday I was informed that management had decided that surface examination wasagood 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 examinatio n needs to be performed on the subject weld.

1) Subsurface flaws will go undetee' ted 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 ibeen 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 Ill, Subsection NB or NC

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. as stated in the current SRP and historical practice for storage canisters. As such, volumetric examination is required to be performed on all 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 would 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 governing 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 volumatric examination then the goveming code would have excluded ductile stainless steel from UT examinations, which it

~ does .n_qt do. UT examinations of stainless steel components are performed routinely as part of Part 50 ISI programs.- The exclusion of ductile stainless steel from volumetric examination is a deviation from the governing Code's requirements.

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4) The suggested approach of surface examination for root and final weld ' pass as well as, every b

1/4 inch of weld is what has been proposed in a draft ASME Code Case, currently in Revision L

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 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.'

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5) 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 governing code for the

. confinement boundary (i.e. ASME Section lli, 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 governing Code I requirements. i i

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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 fuel in the cask. Today l We know that this documented basis for not doing the volumetric examination is not correct. We o 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.

a 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 volu~ metric

- 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 l closure welds since all other confinement welds are volumetrically inspected, it is required by l L 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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• NUCLEAR REGULATORY COMMISSION WASHINGTON, o.C. 2066H001 November 10, 1998 MEMORANDUM TO: Carl J. Paperiello, Director Office of Nuclear Material Safety and Safeguards FROM: Thomas O. Martin, Chief Generic Safety issues Branch Division of Regulatory Applications k'Oh f Office of Nuclear Regulatory Research

SUBJECT:

DIFFERING PROFESSIONAL VIEW PANEL On September 15,1998, I received a memorandum from you providing a differing professional 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 Homseth, NRR, and Deborah Jackson, RES.

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

' members with any questions or comments on this report.

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Attachment; As stated cc:

R. Parkhill, SFPO W. Hodges, SFPO G. Hornseth, NRR D. Jackson, RES J. Craig, RES j

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

. 1. Background This report discusses the review of the Differing Professional View (DPV) dated September 8, i 1998, submitted by Ron Parkhill(Attachment 1) to Carl Paperiello, 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 Professional Views or Opinions." The panel niembers 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 (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. 4 lli. 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

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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

' the pertinent code and regulatory requirements the difficulty of conducting UT

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radiological and ALARA aspects l- A.; Flaw Types and Causes Encountered for Welding Austenitic Stainless Steel

' Based upon staff and industry experience, as supported by a literature survey, the Panel concluded:

1) j 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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B. The Pertinent Code and Regulatory Requirements Mr. Parkhill refers to the ASME Section lil as the gov'eming code for the confinement boundary.

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

L ' 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 lif eThe 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 i chosen this code for convenience of providing a high integrity boundary. The SRP also states i that the staff has relied upon Section 111 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 i

' 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. Radiologicaland 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 !!d 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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'm f IV. Panet 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 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 spee!.'ic to the construction and use of these casks be endorsed and applied. The NRC has not consistently applied minimum acceptable requirements for the level ofinspection 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 Panel, not conducive to promoting public confidence in the use of these structures. This is exacerbated by the inconsistency or 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:

.Yr Thomas O. Martin, Dl5V Panel Chairman t wY d Deborah A. Jac Aw%d GeoffreW. Horpeth s

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

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 Director ins that a complete wntten 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 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 ur. detected by surface examination techniques unless the flaw penetrates the surfa6e 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 loaded.

2) The goveming code for the confinement boundary is ASME Section lit, 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 a!! 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 would 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 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 ductile ATTACIMENT 1

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\ l stainless steel from UT examinations, which it does D2.t do. UT examinations of stainless steel components are performed routinely as part of Part 50 ISI programs. The exclusion'of goveming Code'sductile stainless requirements. steel from volumetric examination is a deviation from

4) 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 j i

in Revision 12. However, this Code Case has nt'. 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 1 l

not included consideration of additive spacings between pts being more than the calculated critical crack size (e.g. If he 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 adepting a preliminary position from the consensus Code i

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 examination (i.e. UT) is not a method utilized in the goveming code for the confinement boundary (i.e. ASME Section Ill, 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 govemitig 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 dosei 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 I 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

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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 1

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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ATTACHMEliT 1

'y 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. Parkhil!(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 ;f this discussion.

BACKGROUND Four instances of 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 necessary 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 ex amination method, ultrasonic test (UT), was consequently employed by the industry.

POTENTIAL FLAW TYPES IN AUSTENITIC STAINLESS STEEL WELDS <

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 different weld defects compared to those encountered when welding carbon steel. One major difference in possible mechanisms involves hydrogen (or delayed) cracking.

Carbon steel is potentially susceptible to hydrogen cracking. The metallurgical reasons are well understood and the problem rnay be eliminated by employing one or more of several measures. 1 Austenitic stainless steel. due to its different atomic structure, is well recognized as being nearly immune to hydrogen cracking Thus, the need for a volumetric method to inspect for this type of flaw is eliminated.

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

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Attachnent 2 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.

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

T 1 he most commonly employed stainless filler metal (type 308) has a ferrite number.sufficiently

'high to meet this requirement. Hot cracking may also be alleviated by employing certain welding techniques: maintaining a convei bead shape, avoiding high heat input, avoiding wide weld passes or weave. ,

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

Lack of penetmtion 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 bead.. An excessively large weld puddle or wide weld pass would suggest possible LOF. LOF i 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 discernable visually. Also, due to the less fluid characteristics of the stainless steel weld puddle (compared to carbon steel), a LOF defect would have a 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 are 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 I

controlled welding program is low. The likelihood for generating such flaws is considered to be

- higher in 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 regarded as the most potentially serious due to their linear, crack-like morphology. Less potentially serious defects of a non-linear morphology may also i

. 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 c amater 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,  !

or insufficient shielding gas. The conditions resu!!ing in porosity may be readily avoided by 1 2

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• 5 Attachment 2 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 shielded) 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-gar 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 werk 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 ofloss of shielding 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 and fusion will also be poor and frequently manifest itself in a more rounded bead with notches 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 fiaws that would have escaped detection. It would l

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

l Verification of the welding process and controls could also be performed. This would i

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 f

' 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

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• Attachment 2 connected weld cracks occurred when the initial tack welds were placed. 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 then determines the maximum depth of weld deposit that can be made before a PT is performed.

REFERENCES Mio Weldino Handbook. Union Carbide Corp., New York,1981 Weldina Enoineerino, Continuing Engineering Education Course Notes, R.L. Edwards, P.E., j 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. 1 i

Introductory Weldino Metallurov, 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 Spent Fuel Storace System: Performance Testino, EPRI NP-6941, Electric Power Research Institute, Palo Alto,1990.

Weld inteority and Perforrrance, ASM Intemational, Metals Park, Ohio,1997.

4

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ly ( t UNITED STATES'

' .- g NUCLEAR REGULATORY COMMIS810N WAsmNeTON, D.C. 30000 4501 November 24, 1998 L; ,

o ,...,,

MEMORANDUM 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. g-and Safeguards

~

SUBJECT:

' RECOMMENDATIONS FROM DIFFERING PROFESSIONAL VIEW PANEL. ,

The Differing Professional View Panel which was convened to review your Differing Professional l

_ View, has issued its findings (Attachment). The panel supported employing PT of the weld I 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 i 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,

's Code case is 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 j Code case such that can be issued. Consistent with agency policy, we would expect to either i

= 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 i evaluation of the cask to determine the appropriate NDE technique, whether volumetric or~ ,

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 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. Jackson, RES

- W. Hodges, SFPO MP Y ffd

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