ML14178A481

Second Set of Responses to April, 2014, Requests for Additional Information Regarding STP Risk-Informed GSI-191 Licensing Application

ML14178A481
Person / Time
Site:	South Texas
Issue date:	06/25/2014
From:	Gerry Powell South Texas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
GSI-191, NOC-AE-14003101, TAC MF2400, TAC MF2401
Download: ML14178A481 (8)

Text

Nuclear Operating Company SaultP Thea.s Pr~fr'd li/edt/c Ce; e)Stjuaw~ PO. RM _M9 K,2&i?AJhII Taw. 774,13l June 25, 2014 NOC-AE-14003101 10 CFR 50.12 10 CFR 50.90 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555-0001 South Texas Project Units 1 & 2 Docket Nos. STN 50-498, STN 50-499 Second Set of Responses to April, 2014, Requests for Additional Information Regarding STP Risk-Informed GSI-191 Licensing Application (TAC Nos MF2400 and MF2401)

References:

1. Letter, G. T. Powell, STPNOC, to NRC Document Control Desk, "Supplement 1 to Revised STP Pilot Submittal and Requests for Exemptions and License Amendment for a Risk-Informed Approach to Resolving Generic Safety Issue (GSI)-191," November 13, 2013, NOC-AE-13003043 (ML13323A183)

2. Letter, Balwant Singal, NRC, to Dennis Koehl, STPNOC, "South Texas Project, Units 1 and 2-Request for Additional Information Related to Request for Exemptions and License Amendment for Use of a Risk-Informed Approach to Resolve the Issue of Potential Impact of Debris Blockage on Emergency Recirculation During Design-Basis Accidents at Pressurized-Water Reactors", April 15, 2014, AE-NOC-14002526 (ML14087A075)

This submittal responds to a portion of the requests for additional information (RAI) provided in Reference 2 with regard to the STP Nuclear Operating Company (STPNOC) risk-informed application to address GSI-191 (Reference 1). The responses provided are listed in the Attachments. Reference 2 included a June 13, 2014 due date for the RAI responses. STP and NRC subsequently rescheduled the expected completion date for responding to all of the RAls to June 27, 2014. However, STP currently expects the completion date to be July 10, 2014.

There are no regulatory commitments in this letter.

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STI 33840341

NOC-AE-1 4003101 Page 2 of 3 If there are any questions, please contact Mr. Wayne Harrison at 361-972-8774.

I declare under penalty of perjury that the foregoing is true and correct.

Executed on:

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m14 G. T. Powell Site Vice President awh Attachments:

1. Response to APLAB Request for Additional Information

a. CASA Grande - LOCA Frequencies: RAI 2

b. STP PRA Model - General: RAI 2

c. STP PRA Model - Human Reliability Analysis: RAI 5 Enclosures to Attachment 1:

1. STP-RIGSI191-RAI-APLA-III-2, Rev. 1, "RAI APLA-lII-2: Modeling LOCA Frequency and Break Size under DEGB only Breaks", University of Texas

2. "Determination of Adequacy of Plant-Specific PRA to Support Risk-Informed Resolution of GSI-191, Rev. 1"

2. Response to ESGB Request for Additional Information:

a. Chemical Effects: RAI 3, 7,11, 17, 20, 22

3. Response to SCVB Request for Additional Information: RAI 1, 2, 3, 4, 5, 6, 7, 8, 9

4. Response to SNPB Request for Additional Information: RAI 1, 2, 3, 5

5. Response to SSIB Request for Additional Information:

a. Transport: RAI 12

b. Head Loss and Chemical Effects Bump Up: RAI 25, 26

c.

NPSH and Degasification: RAI 30, 31, 32, 34, 35

d.

In-Vessel and Boric Acid Precipitation: RAI 37

e. Debris Bypass: RAI 39

f.

Defense in Depth and Mitigative Measures: RAI 41

6. Definitions and Acronyms

NOC-AE-1 4003101 Page 3 of 3 cc:

(paper copy)

(electronic copy)

Regional Administrator, Region IV U. S. Nuclear Regulatory Commission 1600 East Lamar Boulevard Arlington, TX 76011-4511 Balwant K. Singal Senior Project Manager U.S. Nuclear Regulatory Commission One White Flint North (MS 8 B1) 11555 Rockville Pike Rockville, MD 20852 NRC Resident Inspector U. S. Nuclear Regulatory Commission P. 0. Box 289, Mail Code: MN1 16 Wadsworth, TX 77483 Steven P. Frantz, Esquire A. H. Gutterman, Esquire Morgan, Lewis & Bockius LLP Balwant K. Singal Michael Markley John Stang U. S. Nuclear Regulatory Commission John Ragan Chris O'Hara Jim von Suskil NRG South Texas LP Kevin Polio Cris Eugster L. D. Blaylock CPS Energy Peter Nemeth Crain Caton & James, P.C.

C. Mele John Wester City of Austin Richard A. Ratliff Robert Free Texas Department of State Health Services

NOC-AE-1 4003101 Response to APLAB Request for Additional Information a.

b.

C.

CASA Grande - LOCA Frequencies: RAI 2 STP PRA Model - General: RAI 2 STP PRA Model - Human Reliability Analysis: RAI 5 Enclosures to Attachment 1:

1. STP-RIGSI191-RAI-APLA-II-2, Rev. 1, "RAI APLA-II-2: Modeling LOCA Frequency and Break Size under DEGB only Breaks", University of Texas

2. "Determination of Adequacy of Plant-Specific PRA to Support Risk-Informed Resolution of GSI-191, Rev. 1"

NOC-AE-1 4003101 Page 1 of 4 APLAB, CASA Grande - LOCA Frequencies: RAI 2 RG 1.174, Section 2.3.4, "Plant Representation," states that PRA results should be derived from a model that realistically represents the risk associated with the plant.

NUREG-1829 states that, in general, a complete rupture of a pipe is more likely than a partial rupture. It appears, however, that STP's methodology leads to the opposite result (i.e., a rupture of a given size is more likely to be caused by a partial rupture of a large pipe than a complete rupture of a smaller pipe). Please illustrate the results of your method by comparing the frequency of partial versus complete breaks for a set of representative pipe sizes. Please describe whether the methodology described in the STP pilot is consistent with the assumption of NUREG-1 829 or provide justification for an alternate approach.

STP Response:

The implemented 'continuum break model' for STPNOC yields a conditional likelihood of a double-ended guillotine break (DEGB) of 0.165.

Two alternative models are explored to determine the impact of the continuum model. The first alternative model assumes that all breaks are DEGB breaks. If the frequencies from NUREG-1829 are preserved, this first alternative model results in a lower frequebcy of breaks for larger pipes. A second alternative model was explored in which the 'bottom-up' weighting factors were not used.

In other words, for this second alternative model, all pipes within a size category had the same break frequency. In this second alternative model, the conditional likelihood of a DEGB is 0.0746, indicating the bottom-up portion of the base model used results in an increase in the frequency of a DEGB by approximately a factor of 2.

Details are documented in Enclosure 1, STP-RIGSI191-RAI-APLA-III-2, Rev. 1, "RAI APLA-lII-2: Modeling LOCA Frequency and Break Size under DEGB only Breaks",

University of Texas

NOC-AE-1 4003101 Page 2 of 4 APLAB, STP PRA Model - General: RAI 2 RG 1.174, Section 2.3.3 states that the PRA model should be technically adequate for the application. Please explain the basis of the statement on page 11 of LAR Enclosure 3 "since the STPNOC's PRA is compliant with RG 1.200, Revision I for internal events, it is compliant with RG 1.200, Revision 2 for assessing the risk associated with GSI-191."

Please explain how this conclusion reached.

STP Response:

The risks associated with GSI-191 phenomena are only considered to be potentially significant for internal events. Therefore, while the STP Probabilistic Risk Assessment (PRA) has not been demonstrated to be fully compliant with RG 1.200, Revision 2, the differences between Revision 1 and Revision 2 are concerned with external events.

Since external events are not required input for the assessment of GSI-191 phenomena, this implies that the STP PRA is considered to be sufficient for addressing the risks associated with GSI-191 phenomena.

"Determination of Adequacy of Plant-Specific PRA to Support Risk-Informed Resolution of GSI-191, Rev. 1" included as Enclosure 2 supports this position.

NOC-AE-1 4003101 Page 3 of 4 APLAB, STP PRA Model - Human Reliability Analysis: RAI 5 RG 1.174, Section 2.3.2 states that the level of detail of the PRA model must be sufficient to model the impact of the proposed change. On page 37 of Volume 3, other important longer-term actions are listed, which include: (1) Securing one containment spray pump if all three containment spray pumps are successfully initiated; (2) Securing all containment spray pumps later in the event; (3) Switchover to ECCS sump recirculation after the RWST [refueling water storage tank] has been drained; and (4) Switchover to hot leg injection. Please explain how the CASA Grande results were developed to address the various combinations of success and failure of these operator actions. Please also explain how the consistency between the actual PRA scenario and the GSI-191 basic event failure probabilities developed in CASA Grande was assured.

STP Response:

The operator actions on Page 37 of Volume 3 are listed as:

1. Securing one Containment Spray System (CSS) pump if all three CSS pumps are successfully initiated

2. Securing all CSS pumps later in the event

3. Switchover to Emergency Core Cooling System (ECCS) sump recirculation after the Refueling Water Storage Tank (RWST) has been drained

4. Switchover to hot leg injection The first two actions are included in the STP Probabilistic Risk Assessment (PRA) logic model as switches but their success or failure status does not affect the sequence of events modeled. They are included in the model in case later it is desired to perform sensitivity studies. Their status does not affect the probability of success or failure of actions 3 and 4. Action 1 is directed by procedure "Reactor Trip or Safety Injection",

OPOP05-EO-EOOO, Conditional Information Page. Action 2 is directed by procedure "Loss of Reactor or Secondary Coolant", OPOP05-EO-EO10, step 16C, this also requires containment pressure to be less than 6.5 psig and provided the Technical Support Center (TSC) concurs.

Action 3, switchover to sump recirculation, is assumed to result in core damage if failed. This is consistent with the base model and has not been changed in the model updated to consider GSI-191 phenomena. Short of sufficient debris to cause loss of sump recirculation, the presence of partial debris in the sump is not anticipated to impact the operator's performance of this action. Please also see the response to RAI APLAB, STP PRA Model - Human Reliability Analysis: RAI 6 in STP letter to NRC dated May 22, 2014, NOC-AE-1 4003103, (ML14149A434).

Action 4, switchover to hot leg injection, follows action 3 in the sequence, to align for ECCS sump recirculation to the cold legs. Preceding action 3 is always successful when action 4 is queried since action 4 is only of interest if core damage has not already occurred. Therefore, no dependence is assumed between actions 3 and 4.

The status of action 3 is always assumed successful in CASA Grande, before considering the GSI-191 phenomena since its failure already guarantees core damage for medium and large Loss of Coolant Accidents (LOCAs). Since actions 1 and 2 do not impact the STP PRA logic model, there is no need to interface the exact timing of

NOC-AE-1 4003101 Page 4 of 4 the actions in CASA Grande with the response in the STP PRA. Action 4, switchover to hot leg injection, occurs after the times of greatest interest in CASA Grande, in particular those involving failures of sump recirculation. Fuel flow blockage failure is bounded by the failure criteria for boron precipitation.

Boron precipitation related to GSI-191 phenomena ends at the time of switchover to hot leg injection. Once hot leg injection is successful this eliminates the potential for excessive boron precipitation after that time. If switchover to hot leg injection fails, the status of boron precipitation after that time is not of interest because boron precipitation is assumed for the fraction of breaks in the cold leg. Consequently, information about switchover to hot leg injection assumed in CASA Grande also need not be transferred to the STP PRA logic model for evaluation.

Reference:

1.

Letter, G.T. Powell, STPNOC, to NRC Document Control Desk, "First Set of Responses to April, 2014, Requests for Additional Information Regarding STP Risk-Informed GSI-191 Licensing Application - Revised,"

May 22, 2014, NOC-AE-14003103 (ML14149A434)