ML15086A333

Request for Additional Information Regarding Replacement Steam Dryer Analysis, Exelon Generation Company, Llc. Peach Bottom Atomic Power Station - Unit 2

ML15086A333
Person / Time
Site:	Peach Bottom
Issue date:	03/27/2015
From:	Richard Ennis Plant Licensing Branch 1
To:	Bryan Hanson Exelon Generation Co, Exelon Nuclear
Ennis R, NRR/DORL/LPLI-2, 415-1420
Shared Package
ML15086A310	List: ML15033A489 ML15086A333
References
TAC MF4792
Download: ML15086A333 (3)

Text

OFFICIAL USE ONLY - PROPRIETARY INFORMATION 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 (( )).

Enclosure 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 (ADAMS Accession Nos. ML15034A573 and ML15083A559, 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.

The NRC staff has reviewed the information the licensee provided in Exelons letters dated February 3, 2015, and March 24, 2015, and has determined that there is reasonable assurance that the PBAPS Unit 2 RSD will maintain its structural integrity for continued power ascension to the next hold point specified in license condition 2.C(15) (i.e., 104% of 3514 MWt). However, the staff has determined that additional information, as described below, needs to be provided to help inform the staffs decision on the acceptability of power ascension beyond 104% 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. As such, this RAI is designated as RAI-10.

RAI-10

The NRC staff has the following observations on the information submitted so far:

Using ((measured strains and finite element (FE) modal strains and stresses)) to infer the alternating stress state in a steam dryer is, in principle, acceptable.

Statistical studies of the peak measured strains on the dryer hood and skirt by the licensee appear to show that ((combining MSL-based and dryer-strain based time signals by the square root of the sum of the squares (SRSS) method )) is consistent with guidelines in NUREG 0484. The licensee indicated that cumulative distribution function (CDF) curves were developed, ((using data from steam dryer strain gages (SG) 4 and 20 to provide a quantified basis for using the SRSS method.)) To further validate the licensees findings from its analysis, the NRC staff requests that the CDF curves for SG4 and SG20 be provided for review.

The ((incomplete subset of modes used, and the approach used for estimating modal amplitudes for those modes,)) does not have a sound technical basis. Therefore, this raises concerns regarding the stress analysis of the PBAPS Unit 2 steam dryer. All ((modes (particularly of the outer hood))) do not seem to be included in the current analysis, raising the concern that the ((peak stress computed (5022 psi))) may not be an appropriate upper bound. Also, the ((modal amplitudes are computed using participation factors based on the OFFICIAL USE ONLY - PROPRIETARY INFORMATION

OFFICIAL USE ONLY - PROPRIETARY INFORMATION unjustified assumption of uniform unit load acting on the dryer.)) These problems with the approach used to date lead to ((strain simulations at identical sister pair locations that are grossly different and inconsistent with measurements,)) adding further uncertainty to the accuracy of the current calculations.

The general methodology as submitted by the licensee is reasonable, but the ((modes used in the simulation)) must be complete and representative of ((low frequency (less than 50 Hz)))

strains and stresses in all major dryer components (outer hoods, inner and middle hoods, and skirt), and the approach used to determine the ((amplitudes of the modes that participate)) in the analysis must be revised as follows:

a) Due to possible inaccuracies in the ((FE modal frequencies compared to those estimated from the available measurements)), a frequency shift of at least +/-10% must be considered to ensure that all ((appropriate modes)) are included in the analyses.

b) ((FE modes (displacement and strains))) used in the analysis should be submitted, and demonstrated to be reasonable for use on the outer hood, middle/inner hood, and skirt, regardless of whether the ((FE resonance frequencies lie within the non-MSL acoustic frequency bands.))

c) The ((modal amplitudes)) should be based on attempting to match as closely as possible

((modal strains with measured strains in the non-MSL acoustic frequency bands.))1 d) The simulated ((peak strains (including any bias))) should bound the measured ones.

e) Simulated ((peak strains for sister pairs of strain gages)) should be reasonably consistent with those measured.

f) The ((predicted strains)) must be used to estimate the top 5 maximum stresses and their locations on the upper and lower dryer.

g) For each of the ((hood modes below 50 Hz)), please provide ((modal displacements (for unit amplitude) and modal strain)) at each of the strain gage locations on the hood.

h) Please confirm whether the outer hood, middle hood and inner hood are dynamically coupled.

1

((Additional Guidance regarding matching FE modes to measured strains A useful means of determining the simulated modal strain shapes that align best with measured strains is to employ a concept similar to the Modal Assurance Criterion (MAC). The MAC is an accepted method in the modal analysis community for finding experimentally measured mode shapes which match best with numerically simulated shapes. The procedure may be used for either displacements or strains. This method may be helpful for developing subsets of FE strain mode shapes which are most appropriate for use on the upper and lower dryer regions. Subsequently, modal amplitudes for these modal subsets may be computed using least squares fits to the measured strains, or other approaches. You can learn more about the MAC (used routinely in the modal analysis community) at http://www.sandv.com/downloads/0308alle.pdf ))

OFFICIAL USE ONLY - PROPRIETARY INFORMATION