ML20196K263
| ML20196K263 | |
| Person / Time | |
|---|---|
| Issue date: | 09/08/1998 |
| From: | Parkhill R NRC |
| To: | Hodges M, Sturz F NRC |
| Shared Package | |
| ML20196K257 | List: |
| References | |
| NUDOCS 9901080142 | |
| Download: ML20196K263 (3) | |
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Frm: Ronald Parkhill To: MWH,FCS Date: 9/8/98 4:55pm subject: Differing Professional view Attached is a fomal differing professional view (DPV) regarding the spent fuel storage canister closure weld examination technique.
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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 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 l l believe that a volumetric examination needs to be performed on the subject weld.
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- 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 loaded.
- 2) The goveming code for the confinement boundary is ASME Section Ill, 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 al! 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 l the canister closure weld since volumetric examination is not done solely to determine flaw sizes have been bounded by a fracture mechanics analysis. Vo!umetric examination is done ;
principally to ensure weld quality during fabrication. It is a verification that the weld meets 1 design requicements 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 governing code would have excluded ductile stainless steel from UT examinations, which it does noldo. 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 govarning Code's requirements.
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- 4) The suggested approach of surface examination for root and finai weld pass as well as, every l . 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 l 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 I 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 examination (i.e. UT) is not a method utilized in the governing code for the confinement boundary (i.e. ASME Section 111, Subsection NB or NC). No amount of allowable 1 stress reduction will compensate for a poorly made weld. This is an example of mixing and l 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 requirements.
- 6) Historically, in the SOC for the VSC-24 rulemaking, Comment #45, the !RC response stated that the ciosure welds meet all ASME requirements except for volumetric e. amination 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 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 weldhg.
- 7) Regulation 10 CFR 72.236(e) requires that the cask be designed to provide redundant sealing of the confinement system. However, the tedundant 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 the goveming code and would verify that there is not a welding process problem (i.e. single l failure). It is very hard for me to appreciate why a regulator would want to abandon the UT l 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 l reoccurring rather than elect to keep the status quo.