ML17059B471
| ML17059B471 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 03/19/1997 |
| From: | Thadani A NRC (Affiliation Not Assigned) |
| To: | Terry C NIAGARA MOHAWK POWER CORP. |
| References | |
| NUDOCS 9703210187 | |
| Download: ML17059B471 (14) | |
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11 II Carl Terry, BWRVIP Vi
-Chairman Niagara Mohawk Power Company
\\ I lost ~iffice Box 63
- Lycoming, NY 13093
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'UBJECT:
QUESTIONS FOR TELEPHONE CONFERENCE, CALL WITH BWR VESSEL AND INTERNALS PROJECT (BWRVIP) REGARDING RECENT OCCURRENCES OF UNEXPECTED DEGRADATION OF BWR'INTERNALS
Dear Hr. Terry:
'f Several instances of degradation of BWR internals have occurred in recent
- months, in particular the cracking of the vertical shroud welds and the degradation of the shroud tie rod spring at Nine Mile Point, Unit 1
(NHP1).
The NRC staff has determined that a discussion with the BWRVIP Executive Oversight Committee (EOC), regarding the BWR Owners Group's generic activities addressing these
- issues, should be held at the earliest opportunity.
Specifically, we would like the BWRVIP to address the safety significance of the observations at NHPl, and the actions the BWRVIP intends to take to address this issue generically.
Therefore, as previously discussed with you by my staff, there will be a
telephone conference call with you and other members of the BWRVIP EOC on
- Thursday, March 20, 1997, at 1:00 p.m.
EST.
Please call (301) 415-7605,
- and, at the tone, enter 4321789.
If you have any questions, please contact C.
E.
(Gene)
Carpenter, Jr., of my staff, at (301) 415-2169.
To ensure that the discussion will be productive, attached are an initial list of questions regarding the NRC staff's generic concerns on the recent core shroud cracking and other generic internals degradation concerns.
The NRC staff does not expect that you will be able to answer all of the questions during the conference call, but that it will set the direction for the discussion.
NM HHCE~ Co~t W
Enclosure:
As Stated Sincerely, fJ",)
P I'I Ashock C. Thadani, Associate Director for Technical Review Office of Nuclear Reactor Regulation cc:
See next page DISTRIBUTION:
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I Carl Terry, BWRVIP Vi
-Chairman Niagara Mohawk Power Company Post Office Box 63
- Lycoming, NY 13093
SUBJECT:
QUESTIONS FOR TELEPHONE CONFERENCE CALL WITH BWR VESSEL AND INTERNALS PROJECT (BWRVIP) REGARDING RECENT OCCURRENCES OF UNEXPECTED DEGRADATION OF BWR INTERNALS
Dear Hr. Terry:
Several instances of degradation of BWR internals have occurred in recent
- months, in particular the cracking of the vertical shroud welds and the degradation of the shroud tie rod spring at Nine Mile Point, Unit 1
(NHP1).
The NRC'staff has determined that a discussion with the BWRVIP Executive Oversight Committee (EOC), regarding, the BWR Owners Group's generic activities addressing these
- issues, should be held at the earliest opportunity.
Specifically, we would like the BWRVIP to address the safety significance of the observations at NHP1 with regard to the acceptability of continued operation of BWRs, or whether immediate actions by BWR licensees are necessary, and what actions the BWRVIP intends to take to address this issue in the longer term.
Therefore, as previously discussed with you by my staff, there will be a
telephone conference call with you and, other. members of the BWRVIP EOC on
- Thursday, March 20, 1997, at 1:00 p.m.,EST<.'--
Please call (301) 415-7605,
- and, at the tone, enter 4321789.
To ensure that the discussion will, be productive, attached are an initial list of questions regarding the NRC staff,',s gener'i'c concerns on the recent core shroud cracking and other generic 'internals. degradation, concerns.
The NRC staff does not expect that, you.will be abl'e, to answer-all~,of the questions during the conference call', but',that;it,will set'he dir'ection for the discussion.
Please respond 'by letter to these questions'ithin 60 days.
If you have any questions; please,'contact-'..q E.
(Gene)
Carpenter, Jr., of my staff, at (301) 415-2169; ,,
Sincerely,,',
, Ashock C.'Thadani, 'Associate D'irector for Technical Review--..
Office of Nuclear Reactor Regulation 1
Enclosure:
As Stated cc:
See next page DISTRIBUTION:
Central File GCLainas RHWessman HEHayfield EHHackett
- PUBLIC,
" SJCol 1 ins/FJHiragl i a RCJones-RPZimmerman KAKavanagh NRR/EHCB Reading DOCUMENT NAME:
G: iBWRVIPiVIPEOC-(.DOC To receive a copy of this document, indicate in the box:
attachment "E" = Co y with attachment "N" = No co y
- See'previous concurrence "C"
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CECar enter*
EMCB:SLS E
RAHermann*
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- Lycoming, NY 13093 1
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SUBJECT:
QUESTIONS FOR TELEPHONE CONFERENCE CALL WITH'WR VESSEL AND INTERNALS PROJECT (BWRVIP) REGARDING'ECENT OCCURRENCES OF UNEXPECTED DEGRADATION OF BWR INTERNALS
Dear Nr. Terry:
Several instances of degradation of BWR internals have occurred in recent
- months, including jet pump riser assembly elbow cracking at LaSalle Unit 2, and cracking of the vertical shroud welds at Nine Mile Poin 1=.
The NRC staff has determined that a discussion with the BWRVIP Execut'versight Committee (EOC), regarding
- the, BWR Owners Group's generic activ'es addressing these
- issues, should be hei'd at the earliest opportunity J
Therefore, as previously iscussed with you my staff, there will be a
telephone conference call w' you and oth r members of the BWRVIP EOC on
- Thursday, March 20, 1997, at
- 00 p.m.
Please call (301) 415-7605,
- and, at the tone, enter 4321789.
To ensure that the discussion wil e productive, attached are an initial list of questions regarding the NRC s 6's generic concerns on the recent core shroud cracking and other gener'c in mals degradation concerns.
The NRC staff does not expect that yo will be ble to answer all of the questions during the conference call, t that it ll set the direction for the discussion.
Please respond y letter to th e questions within 60 days.
If you have any question
, please contact C.
(Gene)
Carpenter, Jr., of my staff, at (301) 415-216 Sincerely, lb k
j Brian W. Sheron,'irector
/
Division of Engineering Offic'e of Nuclear Reactor Regulation
Enclosure:
As Stated cc:
See next page DISTRIBUTION: I't Central File GCLaina's,',",
" ',~', NEMayfiel d PUBLIC
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!l EMHa'ckett -<
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'"C" ='opy without attachment "E" - Co with attachment "N"'
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Carl T'erry, BWRVIP Vice-Chairman Niagara Mohawk Power Company Post Office Box 63
- Lycoming, NY 13093
SUBJECT:
'UESTIONS FOR TELEPHONE.CONFERENCE CALL WITH, BWR VESSEL AND INTERNALS PROJECT (BWRVIP) REGARDING RECENT OCCURRENCES OF UNEXPECTED DEGRADATION OF BWR INTERNALS Dear Mr. Te Several instanc of unexpected degradation, of'BWR inte als have occurred in recent
- months, in uding jet pump riser assembly elbo cracking at LaSalle Unit 2, and crackin of the vertical shroud welds at ine Mile Point 1.
The NRC staff has determi d that a discussion with th BWRVIP Executive Oversight Committee (EOC), regard g the BWR Owners Group'eneric activities addressing these
- issues, ould be held at the arliest opportunity.
Therefore, as previously, disc sed with you y my staff, there will be a
telephone conference call with ou and oth members of the BWRVIP EOC on
- Thursday, March 20, 1997, at 1:0
.m.
E Please call (301) 415-7605,
- and, at the tone, enter 4321789.
To ensure that the discussion will b roductive, attached are an initial list of questions regarding the NRC staf 's'neric concerns on the recent core shroud cracking and other generic ntern ls degradation concerns.
The NRC staff does not expect that you w' be ab to answer all of the questions during the conference call, but that it wil set the direction for the discussion.
Please respond b letter to the questions within 60 days.
If you have any questions, ease contact C.
E.
(Gene)
Carpenter, Jr., of my staff, at (301) 415-2169.
Sincerely, Brian W. Sheron, Director Division of Engineering Office of Nuclear Reactor Re ulation
Enclosure:
As S ated cc:
See nex page DISTRIBUTION'entral F'
GCLainas MEMayfield PUBLIC RHWessman q ',<
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','~', KAKavanagh ACThadani RPZimmerman'
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W IP G ne c Concerns During the January 16, 1997, meeting between the BWRVIP EOC and the NRC staff, the NRC staff questioned the BWRVIP as to whether individual licensees will commit to follow BWRVIP guidelines once the NRC staff has evaluated and approved the various submittals.
The BWRVIP executives committed to inform the NRC staff by April 1997 if the BWRVIP will coordinate individual licensees commitments to follow approved BWRVIP submittals.
Please confirm that the BWRVIP will respond on the agreed upon schedule.
Have all vertical welds at all BWRs had a baseline inspection performed?
- a. If so, what methods were used?
If inspected previously, can estimates be made for crack growth rates?
- b. If not, what is the technical basis for not inspecting?
During a March 1996 meeting, the BWRVIP agreed to collect and provide to the NRC staff additional information related to the scope and results of inspections.
It was agreed that this information would be provided immediately following the end of the outage season (approximately June and December).
To date, this information has not been provided to the NRC staff.
The NRC staff requests that the BWRVIP expedite providing this information.
Please ensure that the provided information reports whether any other plants have performed any axial weld examinations, and if so, the results of the inspections?
As a result of recent degradations, the NRC staff requests the BWRVIP re-evaluate the scope and schedule of internals inspections it has proposed.
Provide a technical justification to the NRC staff why the proposed schedule does not need to be accelerated and the scope of inspections currently being conducted does not need to be expanded.
The NRC staff's review and approval of the preemptive core shroud modifications was based on the assumption that all vertical welds would maintain structural integrity.
Provide justification to demonstrate that the design of these modifications would not be affected by the presence of cracked vertical welds, both in terms of leakage and structural integrity.
Also address multiple cracking of the vertical welds.
What is considered to be the primary driving force for cracking the vertical welds?
The stabilizer assemblies will be thermally preloaded during normal operating conditions.
The tensile load in the tie rod results from the thermal expansion coefficient for the new. stabilizer hardware being less than the thermal expansion coefficient of the shroud.
What is the effect of this thermal preload on the potential for crack growth in the vertical welds?
What is the minimum length of the vertical welds needed to maintain the structural integrity of the shroud and repair assembly assuming that the
, ATTACHNENT
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horizontal welds H-2, H-3, H-6A, H-6B, H-6, and H-8 have cracks of the maximum size allowed by the BWRVIP guidelines7 9.
During a combined steamline break and design-basis earthquake event, the tie rod load has been determined to be in excess of 300,000 lbs.
Provide calculations to demonstrate that the existing cracks in the vertical welds V-9 and V-10 will not fishmouth during such an event, and that any potential leakage would remain small.
10.
"BWR Core Shroud Inspection and Evaluation Guidelines," dated September 2,
- 1994, and Revision 1 dated March 1995, states the following:
"There are other welds and welded components attached to the shroud, such as vertical welds, ring segment
- welds, and attachment welds for core spray piping, LPCI piping, lugs and other cases where welded attachments may have been
- removed, such as alignment supports for installation and vibration test equipment fixtures.
Core spray piping is already covered by inspection requirements intended to detect SCC.
For other welded shroud attachments, the welded region is small enough that even if through wall SCC were to develop, the safety consequences due to leakage from the core region into the shroud annulus region would be considerably less significant than the scenarios evaluated for circumferential welds.
There would be no significant safety concerns resulting from cracking of these welds for any operating condition, given that the structural margins of the circumferential welds are maintained.
Therefore, inspection of these welds is not currently required."
Based on current vertical weld cracking experience, are the safety-consequences still significantly less than circumferential weld cracking?
11.
By letter dated September 21, 1994, the BWROG issued specific operator guidance for the detection of shroud cracking of sufficient magnitude to cause separation of the shroud along the crack during normal operations.
The intent of the letter was identification of horizontal weld cracking of the.shroud.
By letter dated December 28, 1994, the NRC staff transmitted its safety evaluation of the August 5,
- 1994, BWR shroud cracking generic safety evaluation, Revision l.
The safety evaluation stated, in Section 4.6, that "[s]ince the limiting crack size is in excess of the weld length it is not possible for a critical flaw to develop from a vertical weld unless such a crack propagates into an adjoining plate.
This situation is highly unlikely because there "is no driving force to support flaw penetration into the adjacent unstressed material."
Confirm that the conclusion provided by the August 1994 document remains valid. If not,
'would the vertical weld cracking be sufficient to'separate, and would it be detectable during normal operationsT 12.
The BWRVIP should provide a date regarding when the inspection of other shroud support assemblies and attachment welds will be addressed.
Also include any on-going development program or program to be implemented in facilitating such an inspection.
ATTACHMENT
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- 13. During the installation of core shrouds, how much ovality was allowed by
- design, and how much was actually measured in the field measurements?
Was this accounted for in the stress calculations?
14.
The NRC staff has expressed a concern during several meetings that the BWRVIP has not completed a review into the effects of possible multiple failures caused by any single failure.
What assurance does implementation of the BWRVIP guidelines, and actual inspections performed to date, give that cracking of internals will not lead to coincidental. failures of internal components with unacceptable consequences?
- 15. The core shroud lower wedge support is held in place by a spring latch device which attaches it to the tie rod lower spring assembly.
This spring latch device was found to have failed at one location and degraded at other locations at Nine Mile Point Unit 1 during a recent inspection after just one fuel cycle of operation.
In light of this failure, what assurance is there that other plants that have installed tie rod repairs also do not have failed spring assemblies?
What design modifications and/or other measures are being considered to prevent similar failures at other installations?
In light of this failure, what assurance is there that similiar degradation does not exist in other plants, and, if so, what is the safety significance? ATTACHMENT
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