ML17258A686

FAQ 16-0010 Alt Method Ign Freq Weighting-Final Rev B 08302017

ML17258A686
Person / Time
Issue date:	06/09/2017
From:	Casto G NRC/NRR/DRA
To:
Robinson J
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ML17258A687	List: ML17258A638 ML17258A686
References
NUREG/CR-6850
Download: ML17258A686 (2)

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FAQ Number 16-0010 FAQ Revision B FAQ TitleAlternative Methodology to NUREG/CR-6850 for Maintaining FPRA Ignition Frequencies Weighting Factors Page 1 of 2 Plant: Date:June 9, 2016 Contact: Joseph R. Renner Phone:(925) 943-7077 Email:jrenner@jensenhughes.com Distribution: (NEI Internal Use)

FPRA TF BWROG PWROG Purpose of FAQ:

This FAQ proposes an alternative method to NUREG/CR-6850 for maintaining Fire PRA ignition source fire frequencies and establish a framework for maintenance of cable loading weighting factors.

In the event of a change in the number of ignition sources in a plan t, current guidance provided by NUREG/CR-6850 requires the recalculation of ignition source frequencies. The formula yielding the ignition frequency for a given ignition source includes a weighting factor, WIS,J,L. This weighting factor is the ratio of the count of the ignition source in question, divided by th e total count of ignition sources of the same type in the same generic pl ant location. See NUREG

/CR-6850, section 6.3.1 for more details.

The guidance in this FAQ would eliminate the need to recalculate the NUREG/CR-6850 Task 6 results and the individual fire scenario frequencies for Task 11 if the count of ignition sources changes. This change provides a more pragmatic approach for evaluatin g the addition or removal of ignition sources.

Application of the proposed method would address the following adverse effects that ma y be encountered when updating a Fire PRA

1. The reduction of calculated risk in relativ ely high-risk fire scenarios resulting from the addition of ignition sources in low-risk locations.

2. The increase of calculated risk in rela tively high-risk fire scenarios resulting from the elimination of ignition sources.

As risk models mature and are utilized in evaluating risk informed plant change evaluations (e.g. the use of NFPA-805 self-appr oval models), consideration for the risk implications of adding or removing of equipment will increase. Therefore, it is expected that the location chosen for the addition of new igniti on sources be separated from risk sensitive targets. In these cases, were the gui dance in NUREG/CR-6850 to be used, the increase in the number of countable ignition sources (i.e. all ignition sour ce types other than transients and large system ignition sources) would decrease the frequency applied to other, po tentially higher risk scenarios, with the same ignition source types. Therefore, the inclusion of a new ignition source would produce the un desired effect of artificiall y reducing calculated plant risk elsewhere in the model. Conversely, the re moval of an ignition source from the analysis would redistribute ignition frequency to the remaining scenarios resulting in an increase of the plant fire risk in those scenarios.

Furthermore, the current method from NUREG/CR-6850 imposes a burdensome process for maintaining cable loading weighting factors when implementing modifications that add or remove cables. With regard to changes in combustible cabl e loading, risk insights from the Fire PRA models are generally only impacted by large-scale modifica tions that include a significant change in combustible cable loading. Therefore, this FAQ establishes a framework to treat a change in cable lo ading as resulting in a negligible change in ignition frequency, thereby eliminating the burden of maintaining cable loading information.

In summary, the process proposed by this FAQ would maintain constant ignition source and cable loading weighting factors.

However, it is recognized that during specific PRA model updates, a rebase line of the weighting factors might be desirable.

Therefore, the intent of this FAQ is to provide an alternative approach to NUREG/CR-6850 to pragmatic ally enhance the applicati on of the Fire PRA models and re duce unnecessary model updates.

Relevant NRC document(s):

NUREG/CR-6850, Section 6.5.7 Details: NRC document needing interpretation (include document num ber and title, section, pa ragraph, and line numbers as applicable):

NUREG/CR-6850, Section 6.5.7 Circumstances requiring interpretation or new guidance:

Adding and removing ignition sources produces an undesired effect when redistributing the ignition frequencies for evaluating the effect on plant risk increases and decreases in risk informed change evaluations.

Adding or removing cables within a plant location is generally immaterial to the existing transient fire ignition frequencies.

This FAQ would establish a basis for not updating the cable loading factors when evaluating modifications that add or remove cables within a compartment.

Detail contentious points if licensee and NRC have not reached consensus on the facts and circumstances:

N/A FAQ Number 16-0010 FAQ Revision B FAQ TitleAlternative Methodology to NUREG/CR-6850 for Maintaining FPRA Ignition Frequencies Weighting Factors Page 2 of 2 Potentially relevant existing FAQ numbers:

N/A Response Section:

Proposed resolution of FAQ and the basis for the proposal:

The proposed approach would be a deviation from NUREG/CR-6850 Section 6.5.7: Ignition Source Weighting Factors.

The ignition source weighing factor, WIS,J,L, for countable items is calculated as the number of ignition sources within a given compartment divided by the tota l number of ignition sources in the generic plant location.

After initially establishing a weighting factor in accordance with NUREG/CR-6850 Section 6.5.7.1, the proposed method would permit subsequent calculations to use a constant denominator (i.e. the total number of ignition sources in the generic plant lo cation would remain unchanged). This method would effectively yield a static weighting factor for use in evaluating the addition or elimination of plant ignition sources. Upon the addition or elimination of an ignition source, the static weighting factor woul d be used and applied to the ignition source in question. This would eliminate the requirement of the analyst to reca lculate an upda ted weighting factor based on a revised tota l ignition source count and apply it to all impacted scenarios in the analysis.

The proposed method is predicated on the assumption that the initial weighting factor calculation sufficiently establishes a pl ant-specific ignition frequency on a per-component basis. Therefore, the subsequent addition or elimination of ignition sources wou ld have no effect on the frequencies currently applied to the existing igniti on source scenarios and the ensuing change in calcula ted plant risk would produce ill ogical risk insights.

If appropriate, provide proposed rewording of guidance for inclusion in the next Revision:

The proposed method would provide the fo llowing as an acceptable optional appr oach to ignition frequency changes.

-Establish a total ignitio n source count in accordance with guidance in NUREG/CR-6850 for developing the Fire PRA model (i.e. no change to the current method). An accepted revision of the Fire PRA (i.e. the current model of record), may be used as the b asis for the "initial" ignition source count.

-Establish a per count ignition frequency for each countable item in a given plant generic location. For example, 100 pumps (bi n 21 of NUREG/CR-6850) located within the ge neric location 'L' would result in a calcu lated weighting factor of 1/100 = 0.01 for each pump in location L. Multiply this weighting factor by the appropriate bin total frequency from the latest approved data, yielding a frequency for a si ngle pump ignition source.

-Any subsequently added ignition sources should use the per-source calculated frequency for scenario development. Any subsequently eliminated ignition sources should simply be removed from the Fire PRA with no consequential impact on any of the remaining fire scenarios. Therefore, neither the addition nor eliminati on of ignition sources should impact the ignition freque ncies for other fire scenarios.

-The same approach is applicable to th e cable fire frequenc y weighting factor, W Cable,i , for cable fires caused by welding and cutting (Bins 5, 11, and 31) and for cable run fires (Bin 12). That weighting factor should remain static and should continue t o be used in the model to evaluate modifications when cables are adde d or removed from the analysis. In the event a plant modificati on substantially changes the numerator of the weighting factor for a PAU (using the same metric originally used to establish the weighting factor) then an update of the fire frequency for that tr ay should occur. Otherwise, no change is necessary. In this context, "significant" is defined as a 5% change.

When new generic ignition frequency data becomes available, then an update with this data should be performed consistent with the licensee's maintenance and upgrade process. Currently, prior to the development of industry-wide, component-based fire ignition frequencies, new generi c ignition frequency data will be applied during the update on a plant wi de basis and the ignit ion source frequency distributed equally among all components corresponding to the bin. Fi re frequencies throughout the plant shou ld be calculated with current counts of components and current cable loading when a periodic update is performed.