2CAN119804, Forwards Supplemental Info to Support risk-informed ISI Pilot Application.Addl Info Re Sample Selection & Expansion Criteria for Svc Water Sys Will Be Included in Separate Submittal

Forwards Supplemental Info to Support risk-informed ISI Pilot Application.Addl Info Re Sample Selection & Expansion Criteria for Svc Water Sys Will Be Included in Separate Submittal

ML20196F479
Person / Time
Site:	Arkansas Nuclear
Issue date:	11/25/1998
From:	Vandergrift W ENTERGY OPERATIONS, INC.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
2CAN119804, NUDOCS 9812070049
Download: ML20196F479 (9)

Text

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• j C Entergy operztiona,Inc.

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1448 S R. 333 RussoMe. AR 72801 Tel 501858-5000 November 25,1998 2CANI19804 J l

U. S. Nuclear Regulatory Commission ,

Document Control Desk

l Mail Station OPI-17 l Washington, DC 20555  ;

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Subject:

Arkansas Nuclear One - Unit 2 l Docket No. 50-368  ;

! I *maa No. NPF-6 i Information to Support Risk-Informed Inservice Inspection Pilot Application  !

! l Gentlemen-1 L Entergy Operations submitted the results of the risk-informed inservice inspection (ISI) pilot

! plant application study for Arkansas Nuclear One, Unit 2 (ANO-2), to the NRC by letters  ;

L dated Sap *W 30,1997 (2CAN099706), and March 31,1998 (0CAN039809). The pilot l plant application is to be used as an alternative, per 10CFR50.55(a)(3)(i), to certain ASME

Code requirements for the remainder of ANO-2's second inspection interval (ending March 25, 2000) and for the subsequent thirdm' spection interval A meeting was held with the NRC Staff to discuss the ongoing review of the apphcation on September 9 and 10,1998. /j During the maating. draft responses to NRC questions on the application were discussed. The /

NRC Staff transmitted these questions in the meeting summary dated September 25,1998 (2CNA099801). Entergy Operations' response to the questions was submitted October 8,1998 (2CAN109801). Further discussions were subsequently held with 7 yf the Staff on October 20,1998, and October 27,1998. Based on these discussions, Entergy L Operations agreed to provide the attached supplemental information. Additional information concerning sample selection and expansion criteria for the service water system will included in a separate submittal.

Ifyou have any questions concerning this letter, please contact me.

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U. S. NRC November 25,1998 2CANI19804 Page 2 V truly yours, e

?

Ji D. Vander '

Director, Nuclear Safety

.JDV/jd attachment cc: Mr. Ellis W. Merschoff Regional Administrator  ;

U. S. Nuclear Regulatory Commission Region IV 611 Ryan Plaza Drive, Suite 400 Arlington, TX 76011-8064 NRC Senior Resident Inspector Arkansas Nuclear One P.O. Box 310 London, AR72847 Mr. Chris Nolan NRR Project Manager Region IV/ANO-2 U. S. Nuclear Regulatory Conunission NRR Mail Stop 13-H-3 ,

One White Flint North 11555 Rockville Pike Rockville, MD 20852

Attachment to 2CANI19804 Page I cf 7 Entergy Operations Response to NRC Questions on Risk-Informed Inservice Inspection Pilot Plant Application l

Supplement to NRC Question No. 26 Please provide a brief description of how ANO-2 has ensured that the higher safety significance oflower frequency large early release fraction (LERF) scenarios are reflected in the consequence categorization of the piping segments.

Entergy Operations Response 1

From the ANO-2 probabilistic risk assessment, the dominant contributors to the large I release containment events (less than 7% of all severe accidents), include plant damage states (PDSs) with:

e loss of containment spray (CS) and containment cooling, caused by a total loss of service water or a station blackout, e containment isolation failure coincident with a station blackout event or a loss of a DC bus event, and e containment bypass accidents.

As can be seen from above, the sequences relevant to the ANO-2 risk-informed inservice inspection (RI-ISI) application, not explicitly described in the October 8,1998, submittal (2CAN109801), but potentially important from a large (& carly) release perspective, I consists of those involving either CS pipe breaks or SWS pipe breaks. These issues are ,

discussed below. I CS Pipe Breaks In the ANO-2 RI-ISI analysis, many of the CS consequence segments are ranked

" Medium", based on the unexpected frequency of challenge, "between test" exposure time, and one backup train. If not isolated, the break would eventually result in a ! ass of all emergency core cooling, either due to the loss of refueling water tank outside containment, or due to the equipment flooding. Based on a review of the containment pressure response, either containment spray or containment cooling successful operation would prevent containment failure during a severe accident. Loss of CS, or a single train of CS, is not expected to change the conditional LERF.

Attachment to 2CANI19804 Page 2 of 7 SWS Pipe Breaks In the ANd-2 RI-ISI analysis, all events leading to a total loss of SWS (and containment cooling) are already evaluated as "High" consequence based on the conditional core damage probability (CCDP) contribution. Events leading to a loss of one SWS train are evaluated as " Medium" consequence, and enough margin in the CCDP exists (less than IE-5) such th:.1 cont snment performance would not change the consequence rank.

Atwhment to 2CANI19804 Page 3 of 7 Supplement to NRC Question No. 27 I i Please provide estimates for change in core damage frequency (ACDF) and ALERF as a result ofimplementing the proposed risk-informed inservice inspection changes.

Entergy Operations Response While the qualitative assessment previously provided showed that there will be a net decrease in CDF and LERF due to the significant increase in inspection locations in High and Medium risk regions, a quantitative estimate of the net change in CDF and LERF has i been developed. Any attempt to estimate numerical changes in risk due to changes in the  ;

l inspection program is difficult due to several factors including the inherent uncertainty in  ;

passive component reliability prediction, and the need to predict the changes in piping reliability due to changes in the inspection program.

Two approaches have been used to estimate changes in CDF and LERF to further demonstrate compliance with Reg. Guide 1.174. These approaches make use of existing service experience to estimate rupture frequencies for different systems and degradation

! mechanisms. The application of these approaches in presented below:

Bounding Estimate of Removed Locations  ;

1 One way to demonstrate that - ptable risk impacts will not occur from implementation of the proposed RI-1SI program is to estimate the risk impact due to removing locations from the inspection program. This was done by using the pipe rupture frequencies (calculated based on service experience) for locations proposed for removal from the inspection program, and taking the product of these frequencies with the corresponding estimates of CCDP. The results of this assessment for CDF are 4

summarized in Table 27-4, which shows that the change in CDF is on the order of 10 per year. Using a similar approach the increase in LERF due to removing inspection locations that could potentially contribute to a containment bypass is on the order of 10* per year.

These estimates are indicative of negligible contributions to risk from eliminating inspections and are consistent with the criteria of RG 1.174. Since a number oflocations are either added or have enhanced inspections in the high and medium risk categories, it is l clear that the net change in risk will indicate a net reduction in CDF and LERF.

Estimate of the Net Change la Risk due to Inspection Enhancements l

l Another approach to providing an estimate of the net change in risk due to both positive i and negative influences of inspection program changes was developed for the pilot l application for Vermont Yankee. This same approach has been applied to the ANO-2

! pilot application with results as provided below.

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Attachment to 2CANI19804 Page 4 of 7

.A bounding estimate for the CCDP and the pressure boundary failure (PBF) likelihood are I given in Table 27-5. As shown in Table 27-5, the highest CCDP in the ANO-2 RI-ISI analysis is 1.4E-2, which corresponds to large loss of coolant accident (LOCA) events.

The likelihood of PBF is a function of the presence of different degradation mechanisms and/or the susceptibility of these locations to degradation mechanisms. The failure potential rank is based on the relative failure likelihood instead of an absolute number.

The likelihood of PBF for a piping location with no degradatica mechanism present is given as xo. and is expected to have a value lower than IE-8/ location, per year.

Assuming that performing an inspection on a specific location will lower the risk at this location, with the likelihood equal to the probability of detection (POD), the difference in risk can be estimated as given in Table 27-6. In general, this is a conservative assumption, )

in view of the fact that there are several factors that are impossible to inspect for, and I-- inspections are generally performed only once within a ten-year period consistent with the current ASME Section XI Code.

In Table 27-6, POD. is for the current Section XI inspection method, while POD is for the RI-ISI inspection method A change in POD is only considered if a thermal fatigue degradation mechanism is present. Otherwise, POD. = POD., such as for Risk Category 4 i or 6. At ANO-2, all welds in Risk Categories 2 and 5 are subject to thermal fatigue.

• If no credit is given for the improvement in the inspection method POD = POD = 0.3 (xo ~ lE-8/yr)

The reduction in risk resulting from implementation of the RI-ISI program is:

ARar = -2.07

• POD.

• xe ~ -1E-8/yr

. With credit given for the improvement in the inspection method (i.e., thermal fatigue):

POD -0.3 POD =0.9 (xo ~ lE-8/yr)

The reduction in risk resulting from implementation of the RI-ISI program is:

ARcor = -5.34

• xo ~ -5E-8/yr In general, containment performance for PWRs has shown a conditional LERF of 0.1.

Conditional LERF (CLERF) is not expected to be affected by the reduction in inspections. i In two segments, where a LOCA outside containment is isolated by a single motor operated valve, the number ofinspections has increased In other segments considered in the containment bypass analysis, there is more than one isolation valve, and CLERF is estimated to be lower than the bounding value of 0.1. In SW, which is a significant contributor to LERF, four existing Section XI inspection locations have been removed.

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Anschmant to 2CAN119804 Page 5 of 7 The existing Section XI inspection techniques for these locations would not capture the dominant degradation mechanism , i.e., localized corrosion. However, even if those inspections are conservatively credited, their removal would not have a significant impact on the ACDF/LERF assessments (i.e., ACDF <1E-10/yr). Therefore, the bounding value of 0.1 is used for CLERF. As such, the Arum would be expected to be an order of magnitude lower than the ARcor provided above.

ARuw ~ -5E-9/yr The above evaluation clearly shows that implementation of the proposed ANO-2 RI-ISI Program willimprove plant safety. -

In summary, the qualitative and quantitative evaluations performed in response to this RAI provide a convincing case that there will be net reductions in CDF and in LERF due to implementation of the proposed RI-ISI program.

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Attachment to 2CANI19804 Page 6 of 7 Table 27-4 Bounding Estimate of Risk Impact Due to Removed Inspection Locations 1 RI-ISI L Ceasequesa t arado.s Inspecsed 1 ..

[ Risk Desmage 1

CDF lacrease Due to

. . . .Systeams'Category L Rank ( ' Mechanissas Current 1  : RI-ISI - Inspections Reasoved Inspections CS 4 High None 0 4 4 Improvement l 6 Medium None 21 0 21 3.2E-Il l 7 Low None 0 0 0 No Change  !

CVC 2 High TF 2 4 2 Improvement 4 liigh None 0 9 9 Improvement 1 6 Medium None 0 0 0 No Change )

7 Low None 0 0 0 No Change i EFW 5 Medium 17 0 3 3 Improvement 6 Medium None 0 0 0 No Change 7 Imw None 0 0 0 No Change HPSI 2 High TF 6 9 3 Improvement 4 High None 0 2 2 Improvement 5 Medium TF,IGSCC 6 4 -2 3.0E-10 17 2 0 -2 1.35E-11 6 Medium None 77 0 -77 2.7E-10 7 None None 0 0 0 No Change LPSI 2 liigh "IT 3 3 0 No Change 4 Iligh None 14 19 5 Improvement 6 Medium None 14 0 -14 4.9611 MIW 5 (3) Medium TF, CC (FAC) 1 2 1 Improvement 17 (FAC) 3 4 1 knpa,vement 6 (3) Medium None (FAC) 8 0 -8 7.2E-12 MS 6 (3) Medium None (FAC) 10 0 -10 1.0E-11 6 Medium None 3 0 -3 3.0E-11 7 Low None 0 0 0 No Change RC 2 High TF 10 12 2 Improvement 4 Iligh None 34 23 -11 1. lE-08 5 Medium TF 2 2 0 No Change 6 Medmm None 0 0 0 No Change SW 2 High FA (COR) 0 0 0 No Change 4 (2) High None (COR) 0 0 0 No Change 6 (5) Medium None (COR) 4 0 -4 4.0E-l1 7 (6) Low None (COR) 0 0 0 No Change All High Risk Region Various 21 28 7 Improvement RI-ISI Medium Risk Region Various 62 "n 10 Improvement Systems low Risk Region Various 137 ,._

0 -137 1.2E 08

! CVC = chemical volume control EFW = emergency feedwater HPSI = high pressure safety injedon

! LPSI = low pressure safety injection MFW = main feedwater MS = main steam RC = reacto",aolant

! 17 = thermal fatigue IGSCC = intergranular stress corrosion cracking CC = crevice corrosion FAC = flow accelerated corrosion E.C = crosion cavitation COR = general corrosion l

Attachment to 2CAN119804 Page 7 of 7 TABLE 27-5

, , Bounding Estimates for Risk Matrix

CONSEQUENCE RANKl;

• LIKELIHOOD'OF PDFi iRanki. tCCDP.! LRank n Relative PBF 1 L(Upper Bound) : T Likelihood.;

High 1.4E-2 High 200xo Medium 1.0E-4 Medium 20xo Low 1.0E-6 Low x.

TABLE 27-6 Estimate of Change in Risk il Risk) Risk (CDF); .;No. of f E No. ofj '

A Risk (CDF)1

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' Category =-

LCurrenti  :: Pro' posed =

Upper Bo'und K RI-ISI: -I l ~

L[1NR))  ! Inspections" Inspectional 1 2.8xo N/A N/A N/A 2 2.8E-lxo 21 28 (21 POD. - 28 POD.)

• 2.8E-lxo

3 2.0E-2xo FAC Program Applies 0 4 1.4E-2xo 48 57 -9 POD,

• 1.4E-2xo 5 2.0E-3xo 14 15 (14 POD.- 15 POD.)

• 2.0E-3xo 6 1.0E-4xo 137 0 137 POD

• 1.0E-4xo 7 1.0E-6xo 0 0 0 TOTAL [5.80 POD - 7.87 POD.]xo l

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