ML20196K258
| ML20196K258 | |
| Person / Time | |
|---|---|
| Issue date: | 11/30/1998 |
| From: | Parkhill R NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| Shared Package | |
| ML20196K257 | List: |
| References | |
| NUDOCS 9901080138 | |
| Download: ML20196K258 (5) | |
Text
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l Nnvembsr 30,1998 l l MEMORANDUM TO: Executive Director for Operations FROM: Ron Parkhill l
(/ '
l Spent Fuel Project Office f Office of Nuclear Material Safety y d-@,
and Safeguards '
l
SUBJECT:
Differing Professional Opinion (DPO)
The attached DPO augments my Differing Professional View forwarded to GFPO management on September 8,1998 and is being forwarded to the EDO for review in accordance with NRC Management DirectL J 10.159.
A differing professional view panel documented their findings in a report forwarded to Carl i Paperiello on November 10,1998 and on November 24,1998 Dr. Paperiello documented his '
evaluation of the DPV panel's recommendations. Both are attached Attachments:
- 1) DPO
- 2) DPV,9/8/96
- 3) Memorandum from DPV Panel forwarding Report,11/10/98
- 4) Memorandum from Carl Paperiello,11/24/98 l
cc W.Kane, SFPO W.Hodges, SFPO F. Sturz, SFPO T. Martin, Res J. Thoma, EDO i
I 9901090138 990106 O E
[M .
EDO -- G980712
ATTACHMENT 1 1
This differing professional opinion ( DPO) is in regard to the type of nondestructive examination l (NDE) to be used on the closure weld for dual-purpose spent fuel storaga canisters that are
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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 !
that could go undetected by a surface examination technique, like liquid penetrant (PT), even if i that surface examination was performed after some intermediate welding passes. Additiona!!y,j the governing 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 repositcry 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 til Class 1 or 2 requirements, which has been the standard for all storage canisters licensed to date. The governing 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 i construction and since a specific code did not exist for spent fuel storage canisters. To aid the sta"in identifying areas were the Code cannot be met, the applicants are requested to identify al cosiations 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 fuelloading), 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.
't The ASME Section 111 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 resu;ted 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 weld to be a full penetration weld which would make it easier to volumetric examine; requiring Page 1 of 4
q fabricators, especially those with fabrication problems, to possess a valid ASME Certificate of 1 Authorization; use of Authorized Nuclear Inspectors (ANls) during fabrication; and prohibiting slag producing welding processes such as shielded metal arc welding or flun .>re arc welding.
Examples of fabrication problems that could have been alleviated or avoideo if construction had been iri accordance with Code requirements include: closure weld cracking due to an improper welding process, undocumented welds to base metal that resulted in cracking in area of closure !
weld, excessive weld grinding, and improper reading of radiographs. Therefore, since many I aspects of the Code's proven process to ensure proper construction are not currently required I by the regulator (i.e. the closure weld configuration, fabricators are not Code certified, and no l
, Code required independent third party inspection is utilized), and there have been historical l fabrication problems with the closure weld, 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 impracticel and unrealistic for the near term licensing of dual purpose storage casks. ASME Section ll1 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 6,pecific for dry cask storage casks, but is to apply the requirements of the goveming code (i.e., ASME Code Section lil, Subsections NB or NC) which has already 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 pr, cess 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 panelinappropriately 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 concerned in public confidence, it should have recommended that the requirements of the governing 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.
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d) In the second panel recommendation, the panel correctly recognizes that s@
examinations performed in a high radiation area could lead to inaccurate results and could result in creating flaws if chemicals are not properly removed. However, the paWsoluuon 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 period) to confirm 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 fabrication 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 111.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 ruamination had failed to identify flaws induced by a poor welding process until after 19 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 standard to the maximum extent possible and require volumetric examination.
f) Paragraph ill.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 alternative to a volumetric examination.
I do not feel that curface 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 lll Code for nuclear components does not give any relief from volumetric examination requirements just because the materialis a ductile stainless steel.
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This issue is solely with the adequacy of fabrication of these storage canisters, where the ASME Section til 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 cetect subsurface flaws that could be a result of a welding process problem. The licensed c'onfiguration 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 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 these Code required quality assurance aspects are not being utilized, and there has been a histo _ry of fabrication problems, it would seem appropriate to perform the Code required
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Folbihetric 5i5iWi65 tion to ensure that the closure weld has been properly made in spite of these other shortcomings.
/?>n S H go/fy Ron Parkhill Page 4 of 4 l