ML20212E552
| ML20212E552 | |
| Person / Time | |
|---|---|
| Issue date: | 09/15/1998 |
| From: | Paperiello C NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Hornseth G, Diane Jackson, Martin T NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| Shared Package | |
| ML20212E401 | List: |
| References | |
| NUDOCS 9909270036 | |
| Download: ML20212E552 (3) | |
Text
j l
6 pa a%
g UNITED STATES
- j.
j NUCLEAR REGULATORY COMMISSION e
WASHINGTON, D.c. 20666-0001 September 15, 1998 MEMORANDUM TO: Thomas O. Martin, Chief Generic Safety Issues Branch Division of Regulatory Applicatiens Office of Research Geoffery P. Hornseth, Materials Engineer Material and Chemical Engineering Branch 1
Division of Engineering Office of Nuclear Regulatory Research Deborah A. Jackson, Project Manager Electrical Materials and Mechanical Engineering Branch Division of Engineering Technologies Office of Nuclear Regulatory Research FROM:
Carl J. Paperiello, Director
/ gQLW b
Office of Nuclear Material Safety
/
a-and Safeguards SUBJECY:
DIFFERING PROFESSIONAL VIEW PANEL On 8 September,1998, Ron Parkhill submitted, via E-mail" Differing Professional View"(DPV)
)
regarding a Spent Fuel Project Office position on inspection of cask closure welds in austenitic j
stainless steel casks. Each of you has agreed to serve on a panel to review the DPV. I appreciate your willingness to accept this task. The panel members are:
Thomas O. Martin, Chairman Geoffery P. Hornseth Deborah A. Jackson As listed in Management Directive 10.159, the panel shall -
I Review the DPV and make recommendations to the Office Director or Regional l
Administrator (i)
Determine whether sufficient documentation was provided by the DPV submitter for the panel to undertake a detailed review (ii)
Request technical assistance through the submitter's Office Director or Regional Administrator, if necessary (iii).
The panel should normally review the DPV within 7 calendar days of receipt to determine if enough information has been supplied to undertake a detailed review of the issue. The panel j
should informally contact the employee or the manager who forwarded the DPV to discuss the information provided and request any additional information needed.
Once the panel has received the necessary information to begin a review, the panel normally should take no more than 30 calendar days to make a recommendation to me.
i
Attachment:
DPV 9909270036 990719
(
,6/ 4 PDR
-.. ~ _
e, I
DIFFERING PROFESSIONAL VIEW.
o
-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.
i.
- 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 j
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 rabed 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 uso. ' 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 loaded.
2)
The governing 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 3.11 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 bounda v 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 volumetric examination then the governing code would have excluded ductile ATTACHMENT 1 C.
[
e 4
2-g l
stainless steel from UT examinations, which it does n_21 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.
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 in Revision 12. However, this Code Case has not been adopted by ASME and may l
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 l
_ group it may be prudent to await its' final version, otherwise we are again in L
disagreement with the governing Code.
5)
Reducing the allowable stress to compensate for surface examination (i.e. PT) in lieu of i
a volumetric examination (i.e. UT) is not a method utilized in the governing 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 governing Code requirements should be followed both by the industry, as well as, the NRC. Again, this is a deviation from the governing Code requirements.
l 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 f
m
p-
'd 1
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 governing 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. ! 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.
ATTACHMENT 1
-