NUREG/CR-6850, Volume 1 - NRC

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Overview of the Fire PRA Methodology2.3.2.1 Proper Polarity Three-Phase Hot Shorts on AC Power ConductorsIn developing analysis rules for three-phase proper polarity hot shorts, three cases wereconsidered and an approach for each case developed. The three cases are summarized below.Detailed discussions are provided in the associated analysis task procedures.Case 1: Grounded AC System with Thermoset-Insulated Cable: Three-phase proper polarity hotshorts are evaluated as extremely low probability events for grounded three-phase AC powersystems. It is the opinion of the Technical Development Teams that this failure mode is not risksignificant. It is recommended that this failure mode not be included in the Fire PRA cableselection process for grounded three-phase AC circuits involving thermoset-insulated cable.Case 2: Ungrounded AC System or Thermoplastic-Insulated Cable: For ungrounded systemsand thermoplastic-insulated cable, it can be reasoned that three-phase proper polarity hot shortsare of low probability, but not as low as that for grounded systems with thermoset cable (Case 1).It cannot be conservatively argued that the failure mode likelihood is low enough to screen outfor all cases. It is recommended that for these configurations, three-phase proper polarity hotshorts should be considered only for high consequence equipment. See volume 2, section 2.5.6for the definition of high consequence equipment.Case 3: Armored Cable or Cable in Dedicated Conduit: Three-phase proper polarity faults arenot considered credible for armored power cables or for a single triplex cable in a dedicatedconduit.2.3.2.2 Circuit Selection and Analysis – System-Wide Safety Signals.The systematic treatment of system-wide safety signals (e.g., safety injection, containmentisolation, feedwater actuation, etc.) is difficult. These signals are manifested as auxiliary contactswithin the control circuits of important plant equipment. Integrating logic combinations ofinstrument circuit failures that might trigger a safety signal in the context of scenario specificfailure modes exceeds the capability of analytical tools generally available to a plant (forexample, rolling up the combined instrument circuit failures of a two-out-of-three logic networkfor a containment isolation signal). Consequently, this aspect of circuit analysis is treatedqualitatively, rather than quantitatively, in this report.2.3.2.3 Multiple High-Impedance FaultsThe recommended method does not include quantitative assessment of multiple high-impedancefaults that may cause circuit protection features to activate (e.g., breaker opens or fuse blows)above the first level of electrical protection for the cables assumed to be threatened by the fire.The multiple high impedance fault mode for electrical power circuits is considered to be of avery low likelihood based on:• The narrow range of fault quality (i.e., insulation resistance of the fault) needed to createa sustained fault current below the first level of circuit protection, and2-8
Overview of the Fire PRA Methodology• The need to have multiple sustained faults of the necessary fault quality.2.3.3 Fire Frequency and Fire Modeling AssumptionsThe fire frequency estimates, fire severity estimates, and fire severity approach are consideredan integral set. That is, the fire frequencies have been estimated using an analysis and eventscreening approach that is explicitly tied to the subsequent assumptions relating to fire severitycharacteristics. The fire frequency estimates cited in this report are based on the assumption thatonly a subset of the reported fire events is risk-relevant. In particular, the screening of fire eventsas non-challenging or potentially challenging (i.e., for exclusion from or inclusion in the firefrequency estimates) includes some implicit credit for factors that have in some past studiesbeen credited through a fire severity factor. Thus, the assumed fire severity characteristics reflectthe fire event set retained in the fire frequency calculations. Any alterations to either the firefrequency value or the fire severity characteristics approach should be considered in this jointcontext. The authors recommend that practitioners exercise extreme caution in this regard.2.3.4 Low Likelihood FiresA state-of-the-art Fire PRA should explicitly include the risk contribution from low likelihood–high intensity (and hence potentially high consequence) fires. Past practices where fire modelingconsidered only a point estimate of the most likely fire will not capture this contribution. Therecommended fire severity approach provides an appropriate treatment of these low likelihoodfires.2.3.5 Long Duration FiresA state-of-the-art Fire PRA should explicitly include the risk contribution from low likelihood–long duration fires. Past practices related to fire suppression analysis, such as in the IPEEEanalyses, have often dismissed the potential for fires lasting longer than 15-20 minutes for mostplant areas. In contrast, the event experience clearly shows that fire durations in excess of 30minutes are not uncommon. The recommended fire suppression analysis approach provides anappropriate treatment of low likelihood, long duration fires.2.3.6 General Application of these MethodsThe methods documented in this report are considered to represent the current state-of-the-art.In many ways, the methods should be viewed as a self-consistent set. Modifications made to anyone task can impact the validity of other tasks. However, that does not necessarily imply thatall tasks should be executed in the full level of detail and complexity implied by the cited stateof-the-art.Rather, in many cases, analysts may exercise discretion in determining the scopeand depth of their analysis given the intended application. The authors of this document urgepractitioners to exercise caution, carefully document their decisions, and provide associatedbases.2-9
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NUREG/CR-6850, Volume 1 - NRC

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