ML13109A454

Response to Request for Additional Information (RAI 71) Regarding Confirmatory Action Letter Response (TAC Me 9727)

ML13109A454
Person / Time
Site:	San Onofre
Issue date:	04/16/2013
From:	St.Onge R Edison International Co, Southern California Edison Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
TAC ME9727
Download: ML13109A454 (8)

Text

SOUTHERN CALIFORNIA Richard 1. St. Onge EDISON Director, Nuclear Regulatory Affairs and

. EEmergency Planning An EDISON INTERNATIONAL Company April 16, 2013 10CFR50.4 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001

Subject:

Docket No. 50-361 Response to Request for Additional Information (RAI 71) Regarding Confirmatory Action Letter Response (TAC No. ME 9727)

San Onofre Nuclear Generating Station, Unit 2

References:

1. Letter from Mr. Elmo E. Collins (USNRC) to Mr. Peter T. Dietrich (SCE), dated March 27, 2012, Confirmatory Action Letter 4-12-001, San Onofre Nuclear Generating Station, Units 2 and 3, Commitments to Address Steam Generator Tube Degradation

2. Letter from Mr. Peter T. Dietrich (SCE) to Mr. Elmo E. Collins (USNRC), dated October 3, 2012, Confirmatory Action Letter - Actions to Address Steam Generator Tube Degradation, San Onofre Nuclear Generating Station, Unit 2

3. Letter from Mr. Richard J. St. Onge (SCE) to Document Control Desk (USNRC), dated February 25, 2013, Response to Request for Additional Information (RAIs 2, 3 and 4) Regarding Confirmatory Action Letter Response, San Onofre Nuclear Generating Station, Unit 2

4. Email from Mr. James R. Hall (USNRC) to Mr. Ryan Treadway (SCE), dated March 15, 2013, Request for Additional Information (RAls 68-72) Regarding Response to Confirmatory Action Letter, San Onofre Nuclear Generating Station, Unit 2

Dear Sir or Madam,

On March 27, 2012, the Nuclear Regulatory Commission (NRC) issued a Confirmatory Action Letter (CAL) (Reference 1) to Southern California Edison (SCE) describing actions that the NRC and SCE agreed would be completed to address issues identified in the steam generator tubes of San Onofre Nuclear Generating Station (SONGS) Units 2 and 3. In a letter to the NRC dated October 3, 2012 (Reference 2), SCE reported completion of the Unit 2 CAL actions and included a Return to Service Report (RTSR) that provided details of their completion.

SCE provided the response to RAls 2, 3 and 4 in a letter dated February 25, 2013 (Reference 3). By e-mail dated March 15, 2013 (Reference 4), the NRC issued Requests for Additional Information (RAIs) regarding the response to RAIs 2, 3 and 4. Enclosure 1 of this letter provides the response to RAI 71.

P.O. Box 128 San Clemente, CA 92672

Document Control. Desk April 16, 2013 There are no new regulatory commitments contained in this letter. Ifyou have any questions or require additional information, please call me at (949) 368-6240.

Sincerely,

Enclosure:

1. Response to RAI 71 cc: A. T. Howell III, Regional Administrator, NRC Region IV J. R. Hall, NRC Project Manager, SONGS Units 2 and 3 G. G. Warnick, NRC Senior Resident Inspector, SONGS Units 2 and 3 R. E. Lantz, Branch Chief, Division of Reactor Projects, NRC Region IV

ENCLOSURE 1 SOUTHERN CALIFORNIA EDISON RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING RESPONSE TO CONFIRMATORY ACTION LETTER DOCKET NO. 50-361 TAC NO. ME 9727 Response to RAI 71

RAI 71

Reference 1, Response to RAI 2 - It is stated on page 4 of 18 that a median value of initiation time was selected for each tube based on 1000 trials. For purposes of evaluating a conservative probability estimate that one or more tubes do not meet the 3 delta P criterion, why is it conservative to consider a median value of initiation time for each tube, rather than sampling from the distribution of initiation times developed for each tube during a given Monte Carlo trial of the tube population? Would sampling the distribution of initiation times for each tube be a more conservative approach, as it would be expected to stretch out the tails of the resulting overall probability distribution for not meeting the 3 delta P criterion? For a probabilistic assessment such as this, what is the justification for not considering a potentially large source of uncertainty associated with a key input parameter?

RESPONSE

Note: RAI Reference 1 is SCE's "Response to Request for Additional Information (RAIs 2, 3, and 4) Regarding Confirmatory Action Letter Response," dated February 25, 2013.

The industry guidelines were followed in performing the tube integrity calculations to meet the structural integrity performance criteria (SIPC) margin requirements of three times normal operating pressure differential (Reference R1). The performance acceptance standard for assessing tube integrity requires the worst-case degraded tube to meet the SIPC margin requirements with a probability of 0.95 at 50% confidence (Section 2.4 in Reference R1). The technical basis for the 95-50 performance standard is documented in Reference R2. Input distributions for probabilistic components of the analysis such as degradation growth rates are allowed to be best estimates.

Why is it conservative to consider a median value of initiation time for each tube, rather than sampling from the distribution of initiation times developed for each tube during a given Monte Carlo trial of the tube population?

A median value of initiation time was selected to provide a conservative (higher) estimate of tube to tube wear (TTW) growth rate in the model used to respond to RAI 2. Sampling the distribution of initiation times for each tube would result in an earlier estimate (maximum likelihood) of TTW initiation and a corresponding lower -T-w growth rate.

In response to this RAI, Intertek completed an analysis of the simulation model output results to demonstrate that the use of median TTW initiation times adequately models the distribution of all TTW initiation times. The analysis simulation results allowed an evaluation of the relative likelihood for the initiation time for each instance of TTW for each tube. The maximum likelihood for a distribution is determined from the mode of the distribution. Figure 1 illustrates the mode of a distribution for the TTW initiation times calculated for a typical tube (R90 C84 in 3E-088). For a simulated population, the sample mode represents the maximum likelihood estimate (greatest chance to occur) and is the peak location of the histogram. The initiation time defined by the mode is the most likely outcome for that tube from the simulation. Also shown in Figure 1 is the median value for the same sample distribution. The sample median is the value where 50% of the occurrences fall to either side of that value when the results are ranked in ascending order. For the majority of the tubes with TTW, the median value of each tube-specific sample provided an initiation time that occurred later in the operation cycle compared to the mode value. This resulted in a higher TTW growth rate which is more Page 2 of 6

conservative when calculating the final depth of TTW and the resulting probability of meeting the 3 delta P criterion. Approximately 85% of the tubes showed this behavior where the median initiation time was typically less than 0.2 years at power. Examples of tubes that had a strong bias towards early initiation times are shown in Figure 2a.

Approximately 10% tubes had initiation times mid-way through the cycle as shown in Figure 2b.

In this case, the median value and the mode value are essentially the same and the resulting TTW growth rates are no different between the median and mode value cases.

Tubes with initiations beyond mid-cycle are shown in Figure 2c. For very few tubes, typically tubes with very low wear indices and/or few affected AVBs, results give very late initiation times and unrealistically high TTW growth rates. These occurrences are not strongly related to the AVB wear index and are more likely caused by impacts from unstable neighbor tubes.

There were only a few instances (less than 5%) where this was observed.

Would sampling the distribution of initiation times for each tube be a more conservative approach, as it would be expected to stretch out the tails of the resulting overall probability distribution for not meeting the 3 delta P criterion?

The corresponding distributions developed from the initiation times (all simulated values vs.

median values) are plotted together in Figure 3. Figure 3 illustrates the global distribution of TTW initiation times for each Unit 3 steam generator. It can be seen that the two distributions are comparable. This is a direct consequence of the diversity in distributional form exhibited by the individual tube initiation time samples shown in Figure 2.

Because of the diversity among the individual tubes, the cumulative distribution for median initiation times covers the full range of this key parameter including the upper extremes. It is not expected that sampling the distribution of individual initiation times for each tube would be significantly more conservative since the tails of the simulated population are represented by the distribution of the median times.

For a probabilistic assessment such as this, what is the justification for not considering a potentially large source of uncertainty associated with a key input parameter?

The median estimates based on 322 calculated values adequately represent the range of uncertainty in initiation times from the full set of calculations (322,000 values). The ability of the distribution of median times to produce a similar distributional behavior for the full simulated data justifies its use in the OA. The potential source of uncertainty associated with the TTW initiation times is considered and included in the assessment.

REFERENCES RI. "Steam Generator Integrity Assessment Guidelines," Revision 3, Electric Power Research Institute, EPRI Report 1019038, (November 2009).

R2. "Technical Basis for Steam Generator Tube Integrity Performance Acceptance Standards,"

EPRI, Palo Alto, CA: 2006. 1012984.

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Histogram of Initiation Times 250 I 1 250 Sapl Mode I ITube R90 C84 Distribution 200 Sampl Media 150 0

E 100 -

z so I-n -.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Initiation Time (Yearn at Power)

Figure 1 - Sample Median and Mode for a Typical Tube Histogram Page 4 of 6

300 U

250 o 200 1Iso 100 U

a) .... ............

Initiation Times (Years at Power) 200 180-140-120-100-so-E Z 4O z Go-40 20-0" b)

IniUaton Times (Years at Power) 200 ISO F.Rl124-180 a

a 140 120-o 100 so-E 40 20 l C) &0 Initiation Times (Years at Power)

Figure 2 - TTW Initiation Times from the Simulation for Selected Tubes in 3E-088 Page 5 of 6

3E-088 IL U.

0 Z

7N U-0.04 I I i 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Initiation Time, (Years at Power) 3E-089 1.0 0.9 0.8 U.

0.7 0

z 0.6 IL aC 0 0.5 0..

oU 0.4 5 0.3 S

0.2 0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Initiation Time, (Years at Power)

Figure 3 - Cumulative Distributions for Initiation Times (Median vs. All Data)

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