Fire PRA Cable Selection (Task 3)

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Task Overview

Background

This task provides instructions and technical considerations associated with identifying cables supporting those components selected in Task 2. In previous Fire PRA methods (such as EPRI FIVE and EPRI's Fire PRA Implementation Guide [TR-105928, no longer available on epri.com]) this task was relegated to the SSD analysis and its associated databases. This document offers a more structured set of rules for selection of cables.

Purpose

Conducting a Fire PRA in accordance with this procedure necessitates an analysis of fire-induced circuit failures beyond that typically conducted during original Fire PRAs. The circuit analysis elements of the project are conducted in three distinct phases:

  • Fire PRA cable selection (Task 3),
  • Detailed circuit failure analysis (Task 9), and
  • Circuit failure mode likelihood analysis (Task 10).

This task provides methods and instructions for conducting the first phase of circuit analysis–selecting Fire PRA cables (Task 3). The purpose of Task 3 is to identify for all Fire PRA components the circuits/cables associated with the components and the routing/plant location of the identified circuits/cables. These relationships can then be used to determine the Fire PRA components potentially affected by postulated fires at different plant locations.

In most cases, it is advantageous to perform some or all of Task 9 (detailed circuit failure analysis) coincident with Task 3. The degree to which Task 3 and Task 9 are combined is highly dependent on numerous plant-specific factors. Considerations for combining the two tasks are incorporated in relevant sections of Chapter 3.

Scope

Task 3 provides methods and technical considerations for identifying cables to be included in the Fire PRA Cable List. This task contains the following key elements:

  • Identify cables associated with Fire PRA equipment,
  • Determine plant routing and location for the Fire PRA cables,
  • Identify Fire PRA power supplies, and
  • Correlate Fire PRA cables to Fire PRA equipment and plant locations (fire compartments and/or fire areas).

Implementation of plant-specific quality assurance and configuration control requirements that might apply to a Fire PRA is not within the scope of this task. Nor does this task address validating the accuracy of plant-specific data extracted from plant drawings, documents, or databases. Each plant should follow appropriate quality assurance, administrative, and configuration control procedures applicable to the work conducted. The need to validate input source documents should be addressed as part of assembling the prerequisite information.

Related Element of ASME/ANS PRA Standard

Cable Selection and Location (CS)

Related EPRI 1011989 NUREG/CR-6850 Appendices

Appendix B, Appendix for Chapter 3, Example

Appendix I, Appendix for Chapter 9, Examples of Components Circuits Analysis

Supplemental Guidance

NEI 00‑01

NEI 00‑01 documents the cable selection and circuit analysis processes as well as the criteria for deterministic analyses and Fire PRAs.

Fire-Induced Spurious Operations

Additionally, since the publication of NUREG/CR-6850, significant research on fire-induced spurious operations has occurred. This has resulted in the publication of three EPRI/NRC reports on this subject.

Joint Assessment of Cable Damage and Quantification of Effects from Fire (JACQUE-FIRE): Phenomena Identification and Ranking Table (PIRT) Exercise for Nuclear Power Plant Fire-Induced Electrical Circuit Failure (EPRI 1026424 / NUREG/CR‑7150 Volume 1)

This report documents a Phenomena Identification and Ranking Table (PIRT) exercise on fire-induced electrical circuit failures that may occur in nuclear power plants as a result of fire damage to cables. The results and conclusions of the PIRT panel are a primary input to the follow on PRA Expert Panel documented in Volume 2.

Impact on EPRI 1011989 NUREG/CR-6850 Guidance
  • The spurious operation of a three-phase AC motor due to proper polarity hot shorts on three-phase power cabling is incredible.
  • The spurious operation of DC compound-wound motor due to proper polarity hot shorts in the motive/power cabling is incredible.
  • The ignition of a secondary fire from an open circuited CT secondary circuit with a turns ratio of 1200:5 or less is incredible (Note: This is later modified and clarified in EPRI 3002009214 / NUREG/CR-7150 Volume 3).
  • The guidance given in Nuclear Energy Institute, NEI 00-01, Rev. 2, Appendix B.1, can be applied safely to fire safe-shutdown methodologies throughout the plant in resolving concerns associated with Multiple High-Impedance Faults (MHIFs). (Note: Appendix B.1 of NEI 00-01, Rev. 2 offers a basis for concluding that MHIFs need not be considered provided there exists breaker coordination for any circuits damaged by the fire that should previously have been assessed for the effects of MHIFs and appropriate testing and maintenance is performed)

Joint Assessment of Cable Damage and Quantification of Effects from Fire (JACQUE-FIRE): Expert Elicitation Exercise for Nuclear Power Plant Fire-Induced Electrical Circuit Failure (EPRI 3002001989 / NUREG/CR‑7150 Volume 2)

This report documents the results of the PRA panel's expert elicitation that is used to develop the conditional probabilities of hot-short induced spurious operation & duration of various control circuit configurations.

Impact on EPRI 1011989 NUREG/CR-6850 Guidance
  • A new failure mode, the Ground Fault Equivalent Hot Short (GFEHS) has been identified. A GFEHS occurs when fire damage results in conductors of the same or different cable developing a relatively low impedance fault with a ground plane. For the GFEHS to cause a hot short induced spurious operation, both a source and target conductor must short to the same ground plane and have a compatible power supply. Source conductors are those which can supply energy (voltage, current). Target conductors are associated with end devices such as a conductor connected to a solenoid or a conductor connected to a contactor of a motor starter.
    • Spurious operation caused by shorting conductors through a surrogate ground path in ungrounded circuits as a result of fire induced damage, is a failure mode which occurred during the DESIREE-Fire testing and is referred to as “Ground Fault Equivalent Hot Short.” The probability of this failure mechanism with respect to cable-to-cable hot shorts is such that it warrants consideration for including in future testing programs and, subsequently, in analyzing post-fire safe-shutdown conditions.
  • Conditional probabilities and spurious operation duration are covered in Task 10.
NRC Correspondence
  • NRC Memo dated June 14, 2013, Interim Technical Guidance on Fire-Induced Circuit Failure Mode Likelihood Analysis and Enclosure (ADAMS Accession Nos. ML13165A209, ML13165A214)
  • NRC letter to NEI, dated December 16, 2013, Interim Technical Guidance on Fire-Induced Circuit Failure Mode Likelihood Analysis and Enclosure (ADAMS Accession Nos. ML13238A280, ML13165A214)
  • NRC Memo dated February 12, 2014, Supplemental Interim Technical Guidance on Fireinduced Circuit Failure Mode Likelihood Analysis (ADAMS Accession No. ML14017A091)
  • NRC letter to NEI, dated April 23, 2014, Supplemental Interim Technical Guidance on Fire-Induced Circuit Failure Mode Likelihood Analysis (ADAMS Accession No. ML14086A165)
(The memos and letters dated June 14, 2013, December 16, 2013, February 12, 2014, and April 23, 2014 referenced interim guidance until the issuance of NUREG/CR-7150, Volume 2, in May 2014. NUREG/CR-7150, Vol. 2 should be referred to as the official guidance. The purpose of the memos and letters is to trace NRC acceptance of the guidance.)

Joint Assessment of Cable Damage and Quantification of Effects from Fire (JACQUE-FIRE): Technical Resolution to Open Issues on Nuclear Power Plant Fire-Induced Circuit Failure (EPRI 3002009214 / NUREG/CR‑7150 Volume 3)

This report provides technical recommendations to resolve long-standing issues related to evaluation of multiple spurious operations and deterministic post-fire safe shutdown analyses. Note that the approaches and content from EPRI 3002009214 / NUREG/CR‑7150 Volume 3 was incorporated into NEI 00‑01 Revision 4 and endorsed by the NRC in RG 1.189 Revision 4.

Impact on EPRI 1011989 NUREG/CR-6850 Guidance
  • Clarified circuit failure modes and terminology including: proper polarity, latching versus non-latching, high impact components, failure mode classification and GFEHS
  • Revised several PIRT panel positions following the conclusions of the PRA Expert Panel
    • Consideration of insulation type for the aggressor cable conductor is eliminated. This was eliminated due to the impracticality of tracking the conductor insulation for aggressor cables. As a result, the reported classifications are a function of the conductor insulation of the target conductor.
    • Classification of inter-cable hot shorts for thermoset insulated conductors from "implausible" to "plausible" based on the PRA expert panel probabilities that did not support a classification of "implausible"
    • Grouped single break ungrounded AC (from common CPT or distributed) with DC due to similarities in circuit failure type classification.
    • Classified inter-cable hot shorts for double break design with TP insulated conductors as "implausible" for latching and "incredible" for non-latching circuits. This classification was deferred until the PRA Expert Panel completed their estimation.
  • Developed technical design considerations for shorting switch applications
  • Disposition of secondary fires due to a fire-induced open circuited current transformer

Methodology for Modeling Plant Trip Probabilities in Nuclear Power Plant Fire Probabilistic Risk Assessment (EPRI 3002016053)

Typically, a Fire PRA assumes a reactor trip for fire scenarios unless it can be demonstrated that no Fire PRA components are impacted and the reactor will not trip. As a result, very few Fire PRA scenarios are screened from quantification. The operating experience in the United States was reviewed and showed that only one in six events results in a plant trip, and the events are strongly correlated with a few ignition source bins mainly related to electrical distribution and the turbine generator. This report documents a methodology and screening process that can be applied to fire scenarios for ignition source bins where a plant trip is unlikely to occur. For qualifying scenarios, a 0.1 conditional trip probability can be applied.

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