ML20209H332

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Rev 2 to 0PGP01-ZA-0304, Probabilistic Safety Assessment Risk Ranking
ML20209H332
Person / Time
Site: South Texas  STP Nuclear Operating Company icon.png
Issue date: 07/31/1997
From: Moldenhauer A, Stillwell D
HOUSTON LIGHTING & POWER CO.
To:
Shared Package
ML20209H296 List:
References
0PGP01-ZA-0304, PGP1-ZA-304, NUDOCS 9907200188
Download: ML20209H332 (7)
v • d • e


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SOUTil TEXAS PROJECT ELECTRIC GENERATING STATION D0527

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L";r a '! L ., OPGP01-ZA-0304 Rev.2 Page 1 of 7 Probabilistic Safety Assessinent Risk Ranking Quality Non Safety-Related Usage: Available Effective Date: 07/31/97 m

A. C. Moldenhauer D. W. stillwell *A Risk Mgmt & Industry Relations PREPARER TECIINICAL USER COGNIZANT ORGANIZATION Table of Contents Pace

1.0 Purpose and Scope

.. ....... ...... ..................... .... ..................................................2 2.0 Definitions.....................................................................................................................2 3.0 References............................................................................................. ......................3 4.0 Re sp o n s ib il it i e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 5.0 Procedure............................................................................................................3 Addendum 1 - Maintenance Rule... .... ............... . . . . . . . . . . . . . . . . . . . . . . . . . ..........................4 Addendum 2 - Graded Quality Assurance ... .............. . . .... ....... ... ...... ..... ... . .... ... . .... ..... .. .... . 5 1

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OPGP01-ZA-0304 Rev.2 Page 2 of 7 -

Probabilistic Safety Assessment Risk Ranking

1.0 Purpose and Scope

This procedure describes the methods and criteria used to rank according to risk significance the systems, structures, and components (SSCs), and operator actiops within the scope of the Probabilistic Safety Assessment (PSA). This procedure is applicable to those items contained in the STP risk models. It applies to Maintenance Rule, Graded QA and other administrative programs that use risk ranking as part of the process as specified in references 3.1 and 3.2.

l 2.0 Definitions 2.1 Risk Ranking: the process by which SSCs and operator actions within the scope of the PSA analysis are grouped based on their significance to the risk of nuclear accidents.

2.2 Importance Measures: standard calculations which quantify the risk significance of elements of the PSA relative to a figure of merit (FOM) such as core damage frequency (CDF) or large early release frequency (LERF).

1 2.3 Basic Event: a component of a system fault tree which represents a failure mode of a single SSC, although to reduce the number of cut sets, SSCs are sometimes combined.

2.4 PSA System: a related group of components that together perform a particular function within the scope of the PSA. PSA systems are similar to but not the same as plant _

engineered systems. PSA systems are based more on logical rather than physical 4 component relationships, and can contain more or fewer components than the corresponding plant systems.

2.5 Fussell-Vesely (FV): an importance measure defined a ~ ratio of the difference between the FOM with the component failed and the FOM with the component successful, to the value of the average FOM multiplied by the component's importance. ,

2.6 Risk Achievement Worth (RAW): an importance measure which is defined as the ratio of the FOM given the component is guaranteed failed to the average FOM.

2.7 Risk Reduction Worth (RRW .f.m importance measure which is defined as the ratio of the FOM given the component is 100% reliable to the average FOM.

2.8 Common Cause Failure: a single, shared event that adversely affects two or more components at the same time.

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0PGP01-ZA-0304 Rev.2 Page 3 of 7 Probabilistic Safety Assessment Risk Ranking I

3.0 References  !

3.1 OPGP02-ZA-0003," Comprehensive Risk Management" ;

3.2 OPGP04-ZA-0604,"Probabilistic Safety Assessment Program" 3.3 OPGP04-ZE-0313," Maintenance Rule Program" 3.4 STP Maintenance Rule Basis Document 3.5 EPRI TR-105396,"PSA Applications Guide", August 1995 3.6 ST-HS-HS-034969," Basis and Validation for using the STP PSA for Ranking of the Safety Significance for SSCs", August 1996.

3.7 OPGP05-ZE-0001,"PSA Analysis / Assessments." l 4.0 Responsibilities 4.1 The Administrator, Risk and Reliability Analysis ensures that the requirements of this procedure are implemented.  !

1 5.0 Procedure 5.1 The basis and validation for ranking the safety significance of SSCs for each application of the PSA is described in Reference 3.6.

5.2 This procedure will be performed after every major PSA model, update.

5.3 The at power PSA model will be quantified for the nominal average operation case for both Level 1 and Level 2.

5.4 Maintenance Rule: The impact on the Maintenance Rule Basis Documem shall be performed as described in Addendum 1.

5.5 Graded QA The sensitivity studies and risk ranking for the Graded QA program shall be performed as outlined in Addendum 2..

5.6 The results for the risk ranking study shall be documented in accordance with the PSA Analysis / Assessments procedure, Reference 3.7.

5.7 The results of the risk ranking process and supporting documentation shall be sent to the appropriate Expert Panel or Working Group according to the relevant program procedure.

OPGP01-ZA-0304 Rev.2 Page 4 of 7 Probabilistic Safety Assessment Risk Ranking Addendum 1 Maintenance Rule Page1of I 1.0 Requirements The RRA requirement for providing input to the Maintenance Rule Program is contained in section 3.6.3 of OPGP04-ZE-0313 (Ref 3.3): " Provide a Maintenance Rule Basis Docum impact review for any changes to PSA processes or procedures that could affect Maintenance Rule functions, performance criteria, or goals."

2.0 Maintenance Rule Basis Document Ranking systems is required for the Maintenance Rule,10CFR50.65, and is described in Section 5 of the Maintenance Rule Basis Document,(Ref 3.4). PSA modelinformation and ranking results are presented in the tabs entitled:

Tab 7, "PSA Modeled Functions and Systems List" Tab 9, "RRW and RAW Ranking."

3.0 Procedure

3.1 Review Section 5, and Tab 7 of the Maintenance Rule Basis Document, and note any changes resulting from the PSA model update.

3.2 From the quantified RISKMAN model acquire a system importance report for both CDF and LERF.

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3.3 Send the documented review to the Maintenance Rule Working Group Chairman.

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OPGP01-ZA-0304 Rev.2 Page 5 of 7 Probabilistic Safety Assessment Risk Ranking m_

Addendum 2 Graded Quality Assurance Page 1 of 3 m

1.0 Requirements 3 The requirements for PSA inputs to the Graded QA Program are referred to in OPGP02-ZA-0003 (Ref 3.1). Figure 1 of Addendum I and Figure 1 of Addendum 3 describe processes that start with a PSA risk ranking of High, Medium, and Low.

2.0 Risk Ranking From the base case at-power model for nominal average operation, generate the RAW and FV basic event importances relative to CDF and LERF. Categorize the basic events according to the criteria in Table 2-1 below. The rank given to a basic event will be the highest between that determined on the basis of CDF and that determined on the basis of LERF. Cross reference basic events with the components they represent to determine component risk ranking.

Use Table 2-1 to determine component risk ranking. Components in the " Medium (Further Evaluation Required)" risk rank category will require additional evaluation and final classification based on deterministic considerations by the Program (e.g., Graded QA) Working Group and approval by the Comprehensive Risk Management Expert Panel to provide reasonable assurance that the controls established for SSCs ensure they will perform their intended critical function (s).

3.0 Sensitivity Studies For each of the sensitivity studies determine the risk ranking described in the previous paragraph.

List all components that were ranked Low in the base case, but that fall in the High category in a sensitivity study. Such components whose classification changes under various sensitivity studies are to be identified, and relevant comments are to be prepared for consideration by the Graded QA Working Group or Comprehensive Risk Management Expert Panel.

3.1 Scheduled Maintenance

Description:

Quantify the at-power nominal average operation PSA separately for each one of the scheduled-maintenance state types modeled under top event GENST in event tree PMET.

Purpose:

Considers the effect of unavailability due to the 12 week rolling maintenance schedule on nisk ranking.

7 OPGP01-ZA-0304 Rev. 2 Page 6 of 7 Probabilistic Safety Assessment Risk Ranking Addendum 2 Graded Quality Assurance , Page 2 or 3 3.2 Recovery Descrip jon: Quantify available and appropriate risk models based on the removal of all operator recovery actions.

Purpose:

Provides risk ranking with primary emphasis on equipment availability, reliability, and removes credit for human intervention.

3.3 Common Cause Failures

Description:

Quantify available and appropriate risk models based on the removal of all common cause failure contributions.

Purpose:

Provides focus of risk ranking based equipment combinations outside the scope of common cause failures.

3.4 Multi-System Effects of a Component Type

Description:

For selected component types common to more than one system that are low risk and candidates for changes in QA requirements, vary the failure rates and requantify the models. Selection should based on active components that appear in a majority of system level analyses such as relays, check valves, motor operated valves, etc.

Purpose:

To determine the impact of changes that affect more.than one system.

3.5 LERF De.:cription: Set split fraction ISS to one halfit's normal value and reevaluate LERF.

Purpose:

LERF is dominated by induced steam FC"erator tube rupture, which is a highly uncertain phenomenon. This change considers the effect of reducing the assumed failure rate on risk ranking.

4.0 Define Equipment Groups - classify equipment based on the above results and document for the Comprehensive Risk Management Expen Panel.

OPGP01-ZA-0304 Rev.2 Page 7 of 7 Probabilistic Safety Assessment Risk Ranking Addendum 2 Graded Quality Assurance Page 3 of 3 Table 2-1 Rank Ranking Criteria High RAW 2100.0, or FV 2 0.005 and RAW 2 2.0 or FV 2 0.1 l i Medium (Further Evaluation FV < 0.005 and 100.0 > RAW 2 10.0 l Required)  !

Medium FV 2 0.005 and RAW < 2.0, or FV < 0.005 and 10.0 2 RAW 2 2.0 Low FV < 0.005 and RAW < 2.0 I

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