ML031290367

Request for Additional Information Steam Generator Report

ML031290367
Person / Time
Site:	Fort Calhoun
Issue date:	05/09/2003
From:	Wang A NRC/NRR/DLPM/LPD4
To:	Ridenoure R Omaha Public Power District
Wang A, NRR/DLPM, 415-1445
References
TAC MB6954
Download: ML031290367 (8)

Text

May 9, 2003 Mr. R. T. Ridenoure Division Manager - Nuclear Operations Omaha Public Power District Fort Calhoun Station FC-2-4 Adm.

Post Office Box 550 Fort Calhoun, NE 68023-0550

SUBJECT:

REQUEST FOR ADDITIONAL INFORMATION - FT. CALHOUN STATION STEAM GENERATOR REPORT (TAC NO. MB6954)

Dear Mr. Ridenoure:

By letters dated July 1, 2002 (ML021910122), and December 3, 2002 (ML023400505), Omaha Public Power District (OPPD), provided the results from their 2002 steam generator tube inspections for the Fort Calhoun Station, Unit 1 (FCS). By letter dated September 17, 2002 (ML022490219), the NRC staff summarized a phone call held during OPPD's 2002 steam generator tube inspections.

The staff has reviewed OPPDs submittals and has determined that additional information is needed to complete our review of the steam generator inspection results. A request for additional information is enclosed. This request was discussed with Richard Jaworski of your staff on April 22, 2003, and it was agreed that a response would be provided by July 30, 2003.

If you have any questions, please contact me at (301) 415-1445.

Sincerely,

/RA/

Alan B. Wang, Project Manager, Section 2 Project Directorate IV Division of Licensing Project Management Office of Nuclear Reactor Regulation Docket No. 50-285

Enclosure:

Request for Additional Information cc w/encl: See next page

ML031290367 NRR-088 OFFICE PDIV-2/PM PDIV-2/LA PDIV-2/SC NAME AWang EPeyton SDembek DATE 5/8/03 5/8/03 5/9/03 DOCUMENT NAME: G:\PDIV-2\FortCalhoun\mb6954rai.wpd Ft. Calhoun Station, Unit 1 cc:

Winston & Strawn Mr. Daniel K. McGhee ATTN: James R. Curtiss, Esq. Bureau of Radiological Health 1400 L Street, N.W Iowa Department of Public Health Washington, DC 20005-3502 401 SW 7th Street, Suite D Des Moines, IA 50309 Chairman Washington County Board of Supervisors Mr. Richard P. Clemens P.O. Box 466 Division Manager - Nuclear Assessments Blair, NE 68008 Omaha Public Power District Fort Calhoun Station Mr. John Kramer, Resident Inspector P.O. Box 550 U.S. Nuclear Regulatory Commission Fort Calhoun, NE 68023-0550 P.O. Box 310 Fort Calhoun, NE 68023 Regional Administrator, Region IV U.S. Nuclear Regulatory Commission 611 Ryan Plaza Drive, Suite 400 Arlington, TX 76011-4005 Ms. Sue Semerera, Section Administrator Nebraska Health and Human Services Systems Division of Public Health Assurance Consumer Services Section 301 Cententiall Mall, South P.O. Box 95007 Lincoln, NE 68509-5007 Mr. David J. Bannister, Manager Fort Calhoun Station Omaha Public Power District Fort Calhoun Station FC-1-1 Plant P.O. Box 550 Fort Calhoun, NE 68023-0550 Mr. John B. Herman Manager - Nuclear Licensing Omaha Public Power District Fort Calhoun Station FC-2-4 Adm.

P.O. Box 550 Fort Calhoun, NE 68023-0550

REQUEST FOR ADDITIONAL INFORMATION STEAM GENERATOR TUBE INSPECTIONS OMAHA PUBLIC POWER DISTRICT FORT CALHOUN STATION, UNIT 1 DOCKET NO. 50-285 By letters dated July 1, 2002 (ML021910122), and December 3, 2002 (ML023400505), Omaha Public Power District (OPPD), provided the results from their 2002 steam generator (SG) tube inspections for the Fort Calhoun Station, Unit 1 (FCS). By letter dated September 17, 2002 (ML022490219), the NRC staff summarized a phone call held with OPPD during their 2002 steam generator tube inspections.

The NRC staff has reviewed the licensees submittals and has determined that additional information is needed to complete our review. Also, in a conference call held on April 22, 2003, OPPD stated they will be reevaluating their SG inspection plans for the next outage. Your revised inspection plans would be useful for our review. Please respond to the following questions:

1. It is the NRC's understanding that the reporting criteria at FCS for dents was established at 3 volts and that motorized rotating probe coil (MRPC) inspections were performed at various dented locations in the steam generator. With respect to dented locations:

(a) Please discuss the basis for the selection of 3 volts as the screening criteria for identifying dents rather than some lower threshold such as 2 volts. Probe wobble can mask dents. The issue is at what point do dents and/or probe wobble signals need to be further investigated with rotating probes to ensure an effective inspection. The licensee may want to review the lessons learned from the Comanche Peak Special Inspection (ML030090566) in which probe wobble masked a dent and, in turn, resulted in a flaw not being reported. This response should include a discussion of outer diameter stress corrosion cracking (ODSCC) and primary water stress corrosion cracking (PWSCC).

(b) Please clarify the number, location, and magnitude of the dents, including whether the dent is at a drilled hole tube support, an eggcrate support, or some other support structure. Also, if possible, separate the dents at drilled hole locations from the dents at eggcrate locations.

(c) For each flaw detected during the outage, indicate the magnitude of the dent at that location and indicate whether the flaw (1) was initially found during the bobbin screening, (2) was only identified with the MRPC, (3) was identified during the initial bobbin screening and confirmed by MRPC, or (3) was only identified with the bobbin after the MRPC results were available.

(d) For those dents not at drilled hole tube supports (since all dents at drilled tube supports were examined by MRPC), it appears that 20 percent of these locations

on the hot-leg were originally scheduled to be examined by MRPC (and was subsequently expanded to included 100 percent of the dents at the first two hot-leg tube supports). Please clarify whether the "hot-leg" includes the hot-leg diagonal bar and the vertical supports (V1, V2, and V3). Please discuss the inspection results for these dents. If any flaws were identified, discuss the size of the flaw and the size of the dent at this location.

(e) Regarding the expansion of the MRPC inspection to include all hot-leg dents at the first two hot-leg tube supports, please clarify why the expansion was limited to this region. The staff notes that both stress and temperature affect a tubes susceptibility to stress corrosion cracking. As a result, a larger dent at a lower temperature may be as severe (from a stress corrosion cracking standpoint) as a smaller dent at a higher temperature (material properties being equal). This response should include a discussion of ODSCC and PWSCC.

While the NRC staff understands the licensees position that magnitude of the dent does not necessarily equate to severity, there is still an issue that a more severe dent at a higher elevation with a lower temperature may crack earlier than a lower location. As a result, once cracking is observed, some assessment of not only the temperature, but also the severity of the dent/ding should be performed. The NRC staff notes that although the population of flaws may follow a trend this does not ensure there are not exceptions (for example, the experience at Westinghouse plants where flaws are detected at higher tubes supports in one outage and then at lower tube supports in subsequent outages).

(f) It is the NRC staffs understanding that dent sizes at FCS range up to 100 volts in magnitude. Discuss whether the bobbin probe is qualified to inspect dents with that magnitude. Discuss the extent to which the bobbin probe is qualified to inspect dented regions exceeding a specific voltage threshold (e.g., 5 volts).

2. For locations with dings, please provide information similar to what was provided for the dented locations. For example: (1) clarify the screening criteria (e.g., 3 volts),

(2) provide a summary of the number, location, and severity of all dings, (3) provide a list of all flaws associated with dings along with the amplitude of the ding, (4) provide the basis for any expansion of the inspection, etc.

3. Please clarify what is meant by the term "previous less than zero indications." This term was used in bullet 4 on page 3 of the December 3, 2002, submittal.

4. With respect to the MRPC examinations performed from DBH to H5 in the critical area and the MRPC examinations performed in the 90-degree bends outside the critical area, discuss the results from the examination. If flaws were identified, indicate whether the flaw was initially found during the bobbin screening (or whether the flaw was only identified with the bobbin after the MRPC results were available, or whether the flaw was only identifiable from the MRPC data). If flaws were identified, discuss whether the scope of the inspection was expanded. If not, discuss why not.

5. Discuss the technical basis for the critical area (superposition of partial drilled hole tube support plate locations) discussed in question 4.

6. On page 6 of the December 3, 2002, submittal, it was indicated that historical data reviews were performed for the single axial indications and that of the 74 indications identified, 33 indications showed no change and 3 showed change. Please clarify how the 74 indications were detected during the 2002 outage (by bobbin, by MRPC only, by bobbin only after MRPC identification, etc.). Please discuss whether the "change" referred to is a change in the bobbin coil data and/or the MRPC data.

From above, 33 indications were identified and confirmed as flaws by MRPC in 2002. In evaluating the previous data (presumably bobbin data) for these indications, there was no change in the signals from the prior inspection. Given there was no change in the bobbin data and flaws are known to exist at these locations, discuss why it was appropriate to use historical comparisons of the bobbin data as a basis for not MRPC inspecting other bobbin indications identified in 2002. That is, if in 2002 a bobbin indication is identified and a historical comparison is made to the 2001 bobbin data and there is no change (regardless of the results of previous MRPC examinations), wouldnt the results from the evaluation of the 74 indications discussed above indicate that there is a potential for a flaw to be present. Were any of the flaws that were identified in 2002 and that exhibited "no change" since the 2001 data, inspected by MRPC in 2002?

At locations where the bobbin shows no change and for which a previous MRPC examination did not confirm a flaw (i.e., non-relevant indications), please discuss whether any random MRPC examinations have been performed to confirm the adequacy of the screening criteria for determining when an MRPC examination should be performed? The NRC staff understands that the "change" refers not only to change in the bobbin data but also to the rotating probe data. The staffs concern is whether the "change" criteria (for determining when to "spin" bobbin indications) is sufficient given that the inspection results show that many of the indications show "no change" but are still flaws. That is, given that there may be no change in the bobbin signal (from one outage to the next) and that the initial review of the RPC data during the previous outage did not result in the identification of a flaw, isnt it possible that a flaw exists at this location? It would appear from the results of the inspection that it is possible.

7. Circumferential indications were detected at dented hot-leg drilled supports. The dent size associated with these indications ranged from 2.3 volts to 22.09 volts. Discuss whether circumferential indications could also be present at hot-leg dents at non-drilled hole tube supports. If not, discuss the technical basis. If so, discuss whether all "dented" non-drilled hole hot-leg tube supports were inspected with an MRPC. Also, to what extent have rotating probe examinations been performed at the eggcrate locations? The staff also notes that the smallest dent associated with these circumferential indications was below the dent screening criteria used at FCS (2.3 volts).

8. For the two tubes which were restricted because of a severe geometric condition, discuss how this condition occurred and whether it has been getting more severe with time. Discuss what actions were taken to confirm that these tubes satisfied the performance criteria.

9. It was indicated that "signal confirmation requirements" for reporting flaws at eggcrate supports was eliminated based on recent experience from another CE-designed steam generator. Please discuss what is meant by "signal confirmation requirements.".

Please discuss why this signal confirmation was not eliminated for all locations

(i.e., regardless of whether the flaw was at an eggcrate location). For flaws identified this outage at non-eggcrate locations, discuss whether they could have been reported in the previous outage if the signal confirmation requirements had not been imposed at these locations.

10. During the inspection, a bobbin probe with a diameter of 0.540-inch or 0.560-inch was used to inspect the tubes during 2002. Discuss why a probe of larger diameter (that would improve the fill factor) is not used during the inspections? Include in your response a discussion of the noise levels in the tubes and the severity of the dents and how they compare to the qualification data for these probes for the examination of 0.750-inch diameter tubes with 0.048-inch wall thickness. Discuss whether the fill factor is an essential variable for the bobbin techniques used at FCS. If so, provide the limits for the qualification. Specifically, discuss how many data points are available in the qualification data set for these sized probes for the examination of the size of tubing used at FCS.

It was indicated during the April 22, 2003, call that the Examination Technique Specification Sheet for the bobbin technique now indicates the maximum probe size used during qualification. Does this imply that the use of smaller probes would no longer be considered adequate without a site specific demonstration? If so, what site specific demonstration was performed.

A statement was made that the noise levels in the tubes has not been systematically measured. If this is true, how was it demonstrated that the EPRI-qualified technique is applicable at FCS? If the probability of detection (POD) at Fort Calhoun (based on the rotating probe data, which isnt necessarily a true POD) is less than the POD from the qualification data set (a more realistic POD), what does this imply on the ability of the bobbin probes used at FCS to detect flaws at dented and/or non-dented locations?