ML15127A526

Draft Request for Additional Information (Attachment 2)

ML15127A526
Person / Time
Site:	Peach Bottom
Issue date:	05/05/2015
From:	Plant Licensing Branch 1
To:
Ennis R
References
TAC MF4792
Download: ML15127A526 (6)

Text

OFFICIAL USE ONLY - PROPRIETARY INFORMATION DRAFT REQUEST FOR ADDITIONAL INFORMATION REGARDING REPLACEMENT STEAM DRYER ANALYSIS EXELON GENERATION COMPANY, LLC PEACH BOTTOM ATOMIC POWER STATION - UNIT 2 DOCKET NO. 50-277 Proprietary information pursuant to Title 10 of the Code of Federal Regulations (10 CFR) Section 2.390 has been redacted from this document.

Redacted information is identified by blank space enclosed within double brackets as shown here (( )).

Attachment 2 OFFICIAL USE ONLY - PROPRIETARY INFORMATION

OFFICIAL USE ONLY - PROPRIETARY INFORMATION On August 25, 2014, the Nuclear Regulatory Commission (NRC) issued Amendment Nos. 293 and 296 to Renewed Facility Operating License Nos. DPR-44 and DPR-56 for Peach Bottom Atomic Power Station (PBAPS), Units 2 and 3 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML14133A046). These amendments authorized an increase in the maximum licensed thermal power level for PBAPS, Units 2 and 3, from 3514 megawatts thermal (MWt) to 3951 MWt, which is an increase of approximately 12.4%. This change in power level is considered an extended power uprate (EPU).

By letter dated February 3, 2015, as supplemented by letters dated March 24, 2015, and April 5, 2015 (ADAMS Accession Nos. ML15034A573, ML15083A559, and ML15096A008, respectively), Exelon Generation Company, LLC (Exelon, the licensee) requested NRC approval of a revision to the methodology, previously approved during the EPU review, for establishing the PBAPS Unit 2 replacement steam dryer (RSD) strain limits. Exelons request was submitted to support the continuation of EPU power ascension above 3514 MWt.

By letter dated March 27, 2015 (ADAMS Accession No. ML15033A489), the NRC approved power ascension to 104% of 3514 MWt for PBAPS, Unit 2. By e-mail dated April 10, 2015 (ADAMS Accession No. ML15110A098), the NRC stated it had no objections to continued power ascension to 108% of 3514 MWt. After reaching 108% of 3514 MWt, Exelon collected data regarding steam dryer structural integrity in accordance with EPU license condition 2.C(15). Exelon provided the data and other requested information for NRC staff review on April 22, 2015. At 7:40 pm EDT on April 22, 2015, the NRC staff confirmed receipt of the information. This confirmation started a 96 hour0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br /> review period, for the 108% hold point, in accordance with the EPU license condition. A conference call was held on April 24, 2015, to discuss this information.

The NRC staff has reviewed the information the licensee provided and has determined that additional information, as described below, needs to be provided to help inform the staffs decision on the acceptability of the proposed change in methodology as well as power ascension beyond 108% of 3514 MWt.

Request for Additional Information (RAI) questions 1 through 9, regarding the revision to the methodology, were addressed in Exelons letter dated March 24, 2015. RAI question 10 was addressed in Exelons letter dated April 5, 2015. The information Exelon provided on April 22, 2015, included a response to a supplemental RAI to RAI-10, designated as RAI 10-S1. As such, this RAI is designated as RAI-11.

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RAI-11

As described below and detailed in the proprietary attachments to this RAI (Excel and Power Point files), the non-main steam line acoustic (NMSLA) portion of Exelons evaluation at 108%

of 3514 MWt, based on the proposed methodology change, may be under-estimating the dryer stress because the licensees approach for implementation of steps 2 and 3 of the methodology needs to be corrected. ((The modal participation factors (MPFs) calculated by the licensee are not only lower but also were applied to modes with unreasonably high resonance frequencies, which can lead to under-estimation of dryer stresses. This may be due to using the strain power spectral densities (PSDs) rather than strains thus losing the signs and the phasing effects)).

Since the dryer minimum alternating stress ratio (MASR) at the projected extended power uprate (EPU) conditions is 1.09 with a very small margin, the NRC requests the licensee to re-compute the dryer MASRs and demonstrate the structural integrity of the PBAPS Unit 2 replacement steam dryer (RSD) based on the following information.

The results from the methodology submitted on April 22, 2015, are not reasonable. While the updated methodology is improved in that it considers ((all modes in the finite element (FE) model at all frequencies, the four modes with the highest modal participation factors (MPFs) at the peak measured strain frequencies are not appropriate. Figure 1 shows an example of the top four modes computed by the current methodology at 12.5 Hz. The modes which should be dominating the measured strains at 12.5 Hz are those in the skirt, and are all much lower in frequency than the modes identified by the method. Similar phenomena are observed at all the other peak frequencies, as shown in the attached XLSX file)).

Using ((higher order mode shapes)) could lead to significant stress under-predictions, ((since these mode shapes are often localized, with perhaps high response at strain gage locations, but potentially much lower response (and stress) at remote, but high stress locations. The actual modes which contribute the low frequency peaks below 43 Hz are simple, and sometimes global, with multiple components vibrating and straining.

The strain mode shapes provided previously by the licensee at the strain gage measurement locations may be used to help select a more appropriate modal basis set to use in the methodology. Figure 2 shows all of the modes below 50 Hz plotted at sister pair strain gage locations 3 and 4. Several fundamental outer hood modes are clearly evident in the plot. The displacement mode shapes corresponding to these modes have also been provided by the licensee, and show simple panel vibration on the outer hoods, but with varying relative phase, as shown in Figure 3. Similarly, the strain mode shapes have relative phasing. The phasing of the strain mode shapes, as well as of the measured strains, must be considered when computing MPFs)).

The licensee is therefore requested to provide an updated stress estimate, ((along with associated MPFs, comparisons of simulated vs. measured strains at the dryer strain gage locations before and after application of bias error correction. To ensure that the proper modes are used at the critical peak frequencies, the current method used by the licensee should be amended to use the original modal strains (not the square, which eliminates the relative phasing information) and measured frequency response functions (FRFs) between the on-dryer strain gages. The FRFs may be determined with the cross-spectral densities between the gages using standard signal processing techniques. Once the modes which best match the FRF distributions are found, they may be scaled to the measured PSD levels. Although the licensee OFFICIAL USE ONLY - PROPRIETARY INFORMATION

OFFICIAL USE ONLY - PROPRIETARY INFORMATION uses the top 4 modes for each frequency in the current method, it is possible that more may be required at selected peak frequencies. Also, to ensure that modes well outside the reasonable range of frequencies are not used, only modes within a frequency window that is no more than

+-30% around a given analysis frequency should be considered. It may be necessary to perform separate analyses for the upper and lower dryer if the coherence is low between the upper and lower strain gages. Along with the acceptance criteria cited by the staff in the original RAI 10, an additional criterion is added that the modes used in the peak frequencies below must correspond to the appropriate dryer sections. These criteria are based on examining the on-dryer strain spectra provided by the licensee. An example identifying the inner hood peak frequencies is shown in Figure 4. Further modifications to the methodology may be required if the proper class of strain mode shapes are not used.

- 12.5 and 13.25 Hz - skirt modes

- 16, 18.5, and 21.25 Hz - middle hood modes

- 24.25 Hz - inner hood modes

- 36.25 Hz - inner and outer hood modes

- 37 and 38.75 Hz - outer hood modes))

((

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OFFICIAL USE ONLY - PROPRIETARY INFORMATION Figure 4. Inner hood strains - peaks near 24 and 36 Hz

))

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