NOC-AE-04001691, Clarification of Sensitivity Study Insight Review by Working Group and Expert Panel

Clarification of Sensitivity Study Insight Review by Working Group and Expert Panel

ML040790248
Person / Time
Site:	South Texas
Issue date:	03/16/2004
From:	Head S South Texas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NOC-AE-04001691, STI:31708030
Download: ML040790248 (4)

Text

Nuclear Operating Company South TcsPo/tEkdc Ga11hXngSttan PO. 5282 Mdsrth r=7748J3 March 16, 2004 NOC-AE-04001691 File No. G25 STI: 31708030 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555-0001 South Texas Project Units 1 and 2 Docket Nos. STN 50-498, STN 50-499 Clarification of Sensitivity Study Insight Review by Working Group and Expert Panel

Reference:

1) SECY-97-229, "Graded Quality Assurance/Probabilistic Risk Assessment Implementation Plan for the South Texas Project Electric Generating Station," dated October 6, 1997

2) Letter, J. A. Zwolinski to W. T. Cottle, "South Texas Project Units 1 and 2 - Safety Evaluation on Exemption Requests from Special Treatment Requirements of IOCFR Parts 21, 50, and 100 (TAC Nos. MA6057 and MA6058)," dated August 3, 2001 South Texas Project (STP), as the industry's proto-type pilot for the 10CFR 50.69 effort, is implementing a methodology change on how sensitivity study insights are considered.

This methodology change does not alter the language in the Operations Quality Assurance Program (OQAP) or in UFSAR Section 13.7, nor do we believe the change impacts the intent of the basis for approval of the exemption from certain special treatment requirements. However, STP wishes to keep the NRC staff informed of lessons learned which add efficiency and robustness into the categorization process. Therefore, STP is not seeking NRC approval of this change, but is providing the change to the NRC for information.

The NRC approved the basis for revisions to the STP Operations Quality Assurance Program utilizing a risk-informed approach as stated in the Reference 1 safety evaluation report. In addition, STP was granted an exemption from certain special treatment requirements of IOCFR Parts 21, 50, and 100 as stated in Reference 2. The STP UFSAR contains commitments with respect to the exemption as well as allowances for making changes to UFSAR Section 13.7 without prior NRC approval.

NOC-AE-04001691 Page 2 of 4 During data reviews of the recently approved Revision 4 to the STP Probabilistic Risk Assessment (PRA) Model, STP identified that the approach utilized for considering sensitivity study insights is more conservative than specified in the Graded Quality Assurance (GQA) Safety Evaluation Report (SER) (Reference 1). Section 3.2.2 of the GQA SER specifically states:

The licensee performed a variety of sensitivity studies to provide additional assurance that important SSCs are not inappropriately categorized because of PRA modeling limitations and uncertainties.... All components categorized in the base case as being less significant to plant safety, but categorized as HSS in any of the above sensitivity studies, will be identified and described, and relevant comments prepared for special consideration by the WG and the EP.

Previously, the STP implementing procedures have conservatively treated all PRA sensitivity study insights. This methodology required that if three or more of the Scheduled Maintenance Case studies, or if any of the other sensitivity studies (e.g.,

Operator Recovery, Common Cause Failures, Induced Steam Generator Tube Rupture, and Increased Failure Rate of LSS Components), reflected a higher risk ranking than the PRA base case ranking, the associated component PRA risk ranking was automatically increased to the higher risk ranking. This higher risk ranking was presented to the Working Group as the PRA ranking, and the Working Group was not procedurally allowed to alter the PRA ranking.

Therefore, the previous methodology was more restrictive than the approach stated in the GQA SER. Per the GQA SER, if a component were PRA risk ranked lower than HSS, but ranked as HSS in any of the sensitivity studies, this information would be presented to the Working Group and to the Expert Panel for special consideration. The Working Group would then deliberate the technical merits of the component's sensitivity study insight, and determine if the SSC final categorization should be a higher value than the PRA base case. The Working Group consideration would also factor in other available deterministic information to aid in its final conclusions. Once the final conclusion is reached, the documented information would be forwarded to the Expert Panel for their independent assessment of the Working Group's results.

The STP UFSAR, Section 13.7.2.3, makes reference to aggregate PRA sensitivity studies (not component sensitivity studies), and does not specify the detail or methodology on how the aggregate sensitivity study insights are to be considered. The UFSAR currently states:

To determine the impact of a potential change in reliability of the LSS components on the overall plant risk, a sensitivity study is performed as part of the periodic updates to the PRA to determine the cumulative impact on CDF and LERF from postulating a factor of 10 increase in the failure rates for all modeled LSS components and non-categorized low ranking PRA components.

NOC-AE-04001691 Page 3 of 4 The increases in CDF and LERF are determined to be acceptable using the guidelines for changes as outlined in Regulatory Guide 1.174.

To address defense-in-depth issues related to Late Containment Failures, a similar sensitivity analysis is performed as part of the periodic updates to the PRA. This study postulates an increase in component failure rates by a factor of 10 for all modeled LSS components and non-categorized low ranking PRA components.

STP compares the resulting late containment failure frequency with its nominal frequency to assure that the delta increase in the late containment failure frequency is small, in support of adhering to the defense-in-depth philosophy stated in Regulatory Guide 1.174.

Based on the STP Working Group and Expert Panel experience to date, it was concluded that the approach to sensitivity study consideration should be adjusted to closer align with the approach stated in the GQA SER. This approach, as changed, includes separating the PRA base case ranking from the PRA sensitivity study insights. The Working Group is presented with both the PRA base case risk ranking (which the Working Group cannot alter) and any sensitivity study results which indicate a higher risk rank than the PRA base case (applies to all higher risk ranks; not for HSS only as specified in the GQA SER). The Working Group deliberates the technical merits of the sensitivity study insights along with other available deterministic information. The results of the sensitivity study evaluation is then documented and presented to the Expert Panel for independent assessment and approval. STP believes that this methodology adds realism and robustness to SSC categorizations when considering sensitivity studies, and thus improves the effectiveness of the categorization process.

This methodology change does not impact the basis for NRC approval of the risk-informed Operations Quality Assurance Program or the exemption from certain special treatment requirements. In addition, the language in UFSAR Section 13.7 is not altered.

STP, as the industry's proto-type pilot for 10CER 50.69 activities, is communicating this change to keep the NRC staff informed of lessons learned which add efficiency and robustness into the categorization process If there are any questions regarding this change, please contact me at (361) 972-7136.

Scott M. Head Manager, Licensing

NOC-AE-04001691 Page 4 of 4 cc:

(paper copy) (electronic copy)

Bruce S. Mallett A. H. Gutterman, Esquire Regional Administrator, Region IV Morgan, Lewis & Bockius LLP U. S. Nuclear Regulatory Commission 611 Ryan Plaza Drive, Suite 400 L. D. Blaylock Arlington, Texas 76011-8064 City Public Service U. S. Nuclear Regulatory Commission David H. Jaffe Attention: Document Control Desk U. S. Nuclear Regulatory Commission One White Flint North 11555 Rockville Pike R. L. Balcom Rockville, MD 20852 Texas Genco, LP Richard A. Ratliff A. Ramirez Bureau of Radiation Control City of Austin Texas Department of Health 1100 West 49th Street C. A. Johnson Austin, TX 78756-3189 AEP Texas Central Company Jeffrey Cruz Jon C. Wood U. S. Nuclear Regulatory Commission Matthews & Branscomb P. O. Box 289, Mail Code: MN116 Wadsworth, TX 77483 C. M. Canady City of Austin Electric Utility Department 721 Barton Springs Road Austin, TX 78704