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more on davis bessie
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> From: Phoenix_Firebird_One [mailto:firebird.one@alum.mit.edu]
> Sent: June 29, 2002 9:03 AM
> To: DBRVH_LTBL@yahoogroups.com
> Subject: [DBRVH_LTBL] NRC Response to Paul Blanch (posted in our files
> section also)
>
> UNITED STATES
> NUCLEAR REGULATORY COMMISSION
> WASHINGTON, D.C. 20555-0001
>
> June 19, 2002
>
> Mr. Paul M. Blanch
> 135 Hyde Rd.
> West Hartford, CT 06117
> Dear Mr. Blanch:
>
> Thank you for your E-mail correspondence to the Nuclear Regulatory
> Commission (NRC) dated May 9, 2002. Your letter indicated that you
> had reviewed the recently issued NRC augmented inspection team (AIT)
> report and the Davis-Besse root cause analysis (RCA) report. From
> these documents and comments made during the public meeting of May
> 7, 2002, you detailed six questions in your e-mail message and
> requested these be considered as formal questions from the meeting.
> Your questions have been evaluated by the staff and the responses to
> your specific concerns as they relate to Davis-Besse, are discussed
> below:
>
> Question 1 :
> According to the RCA and the AIT, the cracking on Nozzle #3 was only
> axial. If this is true, then why did this nozzle fall over. In order
> to do this, it had to have circumferential cracking? Am I missing
> something? I think one of the enclosed photos clearly shows the
> circumferential crack around the II J" weld .
>
> Response 1 :
> The cracking in Nozzle #3 at Davis-Besse was only axially-oriented
> according to the non- destructive examinations performed by the
> licensee. The nozzle "fell over" during the repair process when the
> nozzle was machined to a location above the J-groove weld and the
> nozzle lost the support of the J-groove weld. None of the
> photographs available to the NRC staff support the presence of a
> circumferential crack in Nozzle #3.
>
> Question 2:
> If the control rod drive mechanism had not fallen over, was Davis-
> Besse planning to clean the head, or as in the past, restart with a
> significant boron remaining on the vessel head?
>
> Response 2:
> Although the licensee for Davis-Besse did not have a regulatory
> commitment to clean the reactor pressure vessel (RPV) head, the
> licensee had stated a plan to clean the head during
> the most recent refueling outage to facilitate visual examination of
> the head at the next refueling outage. The RPV head cleaning was a
> contingency in case the planned new replacement head was not
> available at the next refueling outage.
>
> Question 3:
> The AIT and the RCA are consistent in the discussions about
> circumferential cracking. That is, the circumferential crack
> initiates from the outside diameter (OD) to the inside diameter. If
> this is the case, then how is it that circumferential cracking is
> considered primary water stress- corrosion cracking (PWSSC)? Can I
> assume that the circumferential cracking is the result of axial
> cracking?
>
> Response 3:
> Circumferential cracking initiated from the OD of the nozzle has
> been identified at plants both above and below the J-groove weld.
> For cracks initiated below the J-groove weld, the environment for
> these cracks is strictly primary water, and hence PWSCC is the
> mechanism of interest. For OD cracking above the J-groove weld, the
> water that initially fills the annulus between the nozzle and the
> reactor pressure vessel head comes from through-wall axial cracking
> and it has a composition similar to that of the primary water. As
> additional leakage occurs, the composition of the water in the
> annulus should become more concentrated due to the evaporation of
> the water itself. The description of this mechanism as PWSCC is
> intended to highlight that the source is primary water as opposed to
> secondary or raw water.
>
> Question 4:
> During the meeting there were many discussions about axial crack
> growth rate, but I did not hear any discussions about
> circumferential crack growth rates. It is my assumption that the
> circumferential cracks present the greater risk from possible rod
> ejection accidents than the axial cracks.
>
> Response 4:
> In general, circumferential cracks do present the greater safety
> risk from possible rod ejection events. For the conditions
> identified at Davis-Besse, the safety implications of the RPV head
> wastage outweighed those of the circumferential cracking identified.
> Because the licensee's root cause summary report attributes the RPV
> head degradation to boric acid leakage from
> axial through-wall cracking, the focus of the May 7, 2002, public
> meeting was the growth rate of the axially oriented cracking in
> Nozzle #3.
>
> Question 5:
> Is it possible that a through wall axial crack may occur and remain
> visually undetected due to a tight interference fit at the top of
> the head and then cause undetected circumferential cracking during
> an operating cycle?
>
> Response 5:
> The possibility of initiation of a circumferential crack from an
> undetected axial through-wall crack is a concern that the NRC has
> addressed in Bulletin 2001-01. Specifically, the Bulletin describes
> a "qualified visual examination" which includes a demonstration of a
> leak path from the annulus between the nozzle and the reactor
> pressure vessel head spanning from the J-groove weld to the outside
> surface of the reactor pressure vessel head. If a licensee cannot
> conclude that a leak path exists for any particular nozzle at a high
> susceptibility plant, then Bulletin 2001-01 would indicate the need
> to perform volumetric examination capable of detecting OD-initiated
> circumferential cracking. In addition, a circumferential crack that
> develops during an operating cycle would not be expected to have
> sufficient growth during that cycle to raise a safety concern.
>
> Question 6:
> The following statement is made on page #3 of the NRC AIT
> report: "The cracks in these five nozzles initiated from the outside
> diameter of the nozzle near the J-groove weld." Again, a crack
> initiating from the OD does not appear to be PWSCC unless the crack
> was below the J-groove weld.
>
> Response 6:
> The OD-initiated cracks in four of the five nozzles described on
> page 3 of the AIT report are located on the nozzle below the J-
> groove weld, and hence, in this case, the cracking is due to PWSCC.
> The OD circumferential crack in Nozzle #2 is above the weld, and as
> described in Response 3, the conditions for this crack are not true
> PWSCC, but primary water is the source of the environment.
>
> In conclusion, I would like to thank you for your correspondence and
> sincerely hope this has answered your questions. The NRC is closely
> monitoring the Davis-Besse issue and that the rules and regulations
> that provide for safe operation of nuclear power plants are
> followed, and thus public health and safety is maintained.
>
> Sincerely,
> Anthony J. Mendiola, Chief,
> Section 2 Project Directorate III
> Division of Licensing Project Management
> Office of Nuclear Reactor Regulation
> __
--
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