ML20206D341

Suppl Technical Evaluation Rept Based on Review of Addl Responses & Site Audit of Individual Plant Exam of External Events at Susquehanna Steam Electric Station,Units 1 & 2

ML20206D341
Person / Time
Site:	Susquehanna
Issue date:	12/31/1999
From:	Kazarians M, Khatibrahbar, Sewell R ENERGY RESEARCH, INC.
To:	NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
Shared Package
ML17164B026	List: ML20206D333 ML20206D341
References
CON-NRC-04-94-050, CON-NRC-4-94-50 ERI-NRC-98-502, GL-88-20, NUDOCS 9905040058
Download: ML20206D341 (21)

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ERl/NRC 98-502 6

SUPPLEMENTAL TECHNICAL EVALUATION REPORT BASED ON REVIEUJ OF ADDITIONAL RESPONSES AND SITE AUDIT OF THE INDIVIDUAL PLANT EXAMINATION OF EXTERNAL EVENTS AT SUSQUEHANNA STEAM ELECTRK STATION, UNITS 1 AND 2 FINAL REPORT December 1998 i

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l Energy Research, Inc.

P.O. Box 2034 Rockville, Maryland 20847-2034 l Work Performed Under the Auspices of the United States Nuclear Regulatory Commission Office of Nuclear Regulatory Research i j

9905 40058 990427 ]ontract w

No. 04-94-050ashington, D.C. 20555 DR ADOCK 050003 7 l __ _

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ERI/NRC 98-502 SUPPLEMENTAL TECIINICAL EVALUATION REPORT BASED ON REVIEW OF. ADDITIONAL RESPONSES AND SITE AUDIT OF TIIE INDIVIDUAL PLANT EXAMINATION OF EXTERNAL EVENTS AT SUSQUEIIANNA STEAM ELECTRIC STATION, UNITS 1 AND 2 FINAL REPORT December 1998 M. Khatib-Rahbar Principal Investigator Authors:

R. T. Sewell and M. Kazarians 2 Energy Research, Inc.

P.O. Box 2034 Rockville, Maryland 20847 I

Work Performed Under the Auspices of the United States Nuclear Regulatory Commission Office of Nuclear Regulatory Research Washington, D.C. 20555 Contract No. 04-94-050 8

EQE International. Inc.,2942 Evergreen Parkway, Suite 302. Evergieen, CO 80439 2

Kazarians and Associates,425 East Colorado Street. Suite 545, Glendale, CA 91205

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, a TABLE OF CONTENTS EXECUTIVE

SUMMARY

(SUPPLEMENTAL) . , , , . . . . . . . . . . . . . . . . . . . . . . . . ... iii i I NTROD U CTIO N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . I 1.1 Objectives of Supplemental Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I 1.2 . Overview of Supplemental Review Activities . . . . . . . . . . . . . ...........1

.l.2.1 Seismic............................................2 1.2.2 Fire..............................................2 2 SUPPLEMENTAL REVIEW ISSUES AND CONTRACTOR REVIEW FINDINGS . . . . 4 2.1 Seismic . .. . . . . . . . . . . . . . . . . . . ....................... .....4 2.1.1 Requests for Additional Information . . . . . .. . . . . . . . . . . . . ...... 4 2.1.2 . Site Walkdown Issues, Review Approach and Findings . . . . . . . . . . . . . 5 2.1.2.1 Review of the Issues and New Documentation . . . . . . . . . . . 5 2.1.2.2 Wal kdown . . . . . . . . . . .. . . . . . ......... .......5 2.1.2.3 Review Findings . . . . . . . . . .... ............ ..6 2.2 Fire .. . .. . . . . . .. . . .. ... . . . . . . . . . .....................'7 2.2.1 Requests for Additional Information . . . . . . .... .............7 2.2.2 Site Walkdown Issues, Review Approach and Findings . . . . . . . . . . . . . . r 2.2.2.1 Review of the Issues and New Documentation . . . . . . . . . . 9 2.2.2.2 Walkdown . . . . . . . . . . . . . . . . . ................lu 2.2.2.3 Review Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3- OVERALL EVALUATION AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . 13 3.1 S eism i c . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2 Fire...................................................13 4- ~ SUPPLEMENTAL IPEEE INSIGHTS, IMPROVEMENTS, AND COMMITMENTS . . 14 4.1 - S eismic . . . . . . . . . . . . . . . . . . . .. ............. . . . . . . . . . . 14 4.2 Fire................................................... 14 i I

5 R E FEREN CES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ., . . . . . . . . . . 15

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EXECUTIVE SUN 1NIARY (SUPPLEN1 ENTAL)

This supplemental technical evaluation report (STER) documents the approach and findings of additional review consideration or the individual plant examination of external events (IPEEE) conducted for the Susquehanna Steam Electric Station (SSES), Units 1 and 2. This supplemental technical evaluation review was performed by Energy Research, Inc. (ERI) on behalf of the U.S. Nuclear Regulatory Commission (NRC).

The supplemental review was initiated because the Step-l review of the IPEEE submittal and of the licensee's first-round responses to requests for additional information (RAls) concluded that additional information was needed for the NRC to make a determination as to whether or not the intent of Generic Letter (GL) 88-20, Supplement 4, had indeed been met for the plant. The scope of the supplemental review was limited to consideration of seismic and fire events.

The activities undertaken for supplemental review of the SSES IPEEE consisted of the following tasks:

• Development of Step-2 RAls to supplement or clarify the licensee's IPEEE submittal or earlier responses, as necessary;

• Preparation of a site audit plan, after determination of the need for a site visit had been made;

• Completion of the site audit, including personnel interviews and plant walkdowns; evaluation of the adequacy of the licensee's responses and site-audit findings; and determination of remaining issues and commitments, if any;

• Review of preliminary analyses and other written information presented by the licensee during the site audit, and review of the licensee's subsequent responses to review comments received during the site audit: and a Final documentation.

This STER updates ERI's earlier final technical evaluation report (TER) (ERl/NRC 95-512, entitled

" Technical Evaluation Report on the ' Submittal-Only' Review of the Individual Plant Examination of Externa! Events at Susquehanna Steam E'ectric Station, Units I and 2"), and serves as a qualitative supplemental assessment of the SSES, Units 1 and 2, IPEEE submittal, particularly with respect to the objectives described in Generic Letter 88-20, Supplement 4, and the guidance presented in NUREG-1407 pertaining to seismic and fire events.

Seismic The supplerrental seismic review focused on considerations of issues or concerns remaining from the Step-1 review, which were presented to the licensee in the form of a second round of RAls and a list of weaknesses described in a site audit plan. The licensee responded to all remaining seismic issues by means of verbal res;)onses during the plant site audit held on August 4-5, 1998, as well as by means of an interactive plant walkdown.

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e The supplemental review activities have resolved all of the remaining seismic review issues. Following are some related principal findings and observations:

The review has deemed that seismic capacity calculations generaily followed accepted procedures.

In addition to those seismic-related plant improvements already documented in the original IPEEE .

submittal, the licensee demonstrated that additional procedural and hardware improvements, as well as seismic-awareness training, had also been undertaken. Furthermore, the licensee implemented some minor corrective actions in response to seismic review observations made during the site audit walkdown.

The site audit further revealed that considerable effort and resources had been devoted to the seismic IPEEE evaluation, that licensee personnel had an exceptionally high degree of involvement in such effort, and correspondingly, had developed considerable ir. sight as to potential severe accident behavior of the plant for seismic events.

No outstanding concerns or commitments have resulted from the supplemental seismic review.

Fire Several issues were identified during the Step-l review of the fire analysis sections of the IPEEE submittal, which had raised serious doubts that the submittal met the intent of GL 88-20. The following'three fire-related issues were of particular importance: (1) screening of compartments based solely on the. argument of low combustibles; (2) excluding the remote shutdown panel (RSP) from the fire analysis; and (3) reporting of an unrealistically low core damage frequency (CDF). The total CDF was found to be 10* per refueling cycle, which is significantly smaller than the CDF typically reported in other IPEEE submittals.

In addition to these concerns, two other potential weak points were also deemed worthy of further consideration. These related to screening small (<50 hp) pump motor fires and assuming that a cabinet fire would not damage equipment or cables outside the cabinet.

As the principal aspect of the supplemental fire review, the site audit for fire events included a review of the foregoing issues, consideration of brief summaries of existing and new documents and analyses, a plant walkdown, and review of IPEEE findings with the licensee. The following review observations and conclusions regarding the fire IPEEE issues are based on the discussions with the licensee, review of j previously available documentation, plant walkdown, and review of documentation provided by the j licensee during and following the site audit:

• The new screening methodology applied to those fire zones tnat had been screened out based on low combustible loading is reasonable. j i

a The methodology used for quantifying the CDF for the previously screened-out fire zones may not 1 be appropriate. From review of the information supplied by the licensee following the site audit, it {

can be inferred that the licensee has assumed that the severity of a fire and the probability of q suppression failure are independent. This assumption fails to take into account the possibility of  ;

damage before suppression takes place. l l

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The possibility of using the remote shutdown panel was addressed as part of the licensee's Thermo-Lag analysis effort.

The cabinets, and layout of cables and equipment near them, were found to be typical of nuclear power plants with similar characteristies. The possibility of fire propagation outside the electrical cabinet of origin has been included in the analysis.

• Cables and equipment near small motors were examined, and except for one small motor, there are no cases where cables and equipment are located within close proximity of these motors.

Based on the information provided by the licensee, and the observations of the review team during the site audit, the only remaining significant weak point in the tire analysis involves the licensee's assumption that the severity of a fire and the probability of suppression failure are independent, which has led to the calculation of small frequencies for all fire scenarios. liowever, since the same assumption has been applied consistently across all significant fire scenarios, it can be argued that, despite the improper assumption, the licensee has, in fact, identified the significant tire scenarios at the plant.

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1 INTRODUCTION This supplemental technical evaluation report (STER) documents the approach and findings of additional

,. review consideration of the individual plant examination of external events (IPEEE) for the Susquehanna Steam Electric Station (SSES), Units 1 and 2 [1]. The supplemental review, conducted by Energy Research, Inc. (ERI), was initiated because the Step-l review [2] of the IPEEE submittal and of the licensee's first-round responses to requests for additional information (RAls) concluded that additional )

information was needed for the U.S. Nuciv Regulatory Commission (NRC) to make a determination as to whether or not the intent of Generic Lener (GL) 88-20, Supplement 4 [3], had indeed been met for the plant.

This STER updates ERI's earlier final technical evaluation report (TER) (ERI/NRC 95-512, entitled

" Technical Evaluation Report on the ' Submittal-Only' Review of the Individual Plant Examination of External Events at Susquehanna Steam Electric Station, Units 1 ed 2"), and has specifically considered seismic and fire events.

The remainder of this section of the STER provides a description of the objectives of the supplemental review, as well as an overview of the supplemental review activities. Sections 2.1 and 2.2 of this report present ERI's findings related to the seismic and fire supplemental reviews, respectively. Section 3 summarize ERl's overall conclusions and recommendations from the seismic and fire reviews. Section 4 summarizes any additional IPEEE insights, improvements, and licensee commitments, and Section 5 provides a list of supplemental references.

1.1 Obietives of Supplemental Revieg The objective of this supplemental review is to evaluate additional information needed to determine the extent to which the IPEEE process used by the licensee, Pennsylvania Power & Light Company (PP&L),

meets the intent of Generic Letter 88-20, Supplement 4. Insights gained from this supplemental review are intended to provide a reliable perspective that assists in making such a determination. This review has involved a quaiitative evaluation of information supplied by the licensee in the form of verbal responses to second-round RAls, as well as in the form of interactive plant walkdowns. The emphasis of this review has been to resolve, to the extent possible, specific concerns or weaknesses encountered in the Step-l review, particularly with respect to the guidelines established in NUREG-1407 [4].

1.2 Overview of Supplemental Review Activities k The general activities undertaken for supplemental review of the SSES IPEEE consisted of the following tasks:

• Development of Step-2 RAls [5] to supplement or clarify the licensee's IPEEE submittal or earlier responses, as necessary;

• Preparation of a site audit plan [6], after determination of the need for a site visit had been made; Energy Research, Inc. 1 ERI/NRC 98-502

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Completion of the site audit, including personnel interviews and plant walkdowns; evaluation of the adequacy of the licensee's responses and site-audit findings; and determination of remaining issues and commitments,if any; Review of preliminary analyses and other written information [7] presented by the licensee during i the site audit, and review of the licensee's subsequent responses [8] to review comments received during the site audit; and

• Final documentation.

The site audit served as the primary basis for resolving issues. The site audit took place on August 4-5, 1998, on PP&L's premises adjoining the Susquehanna plant site. The site audit was attended by three J NRC representatives. two ERI subcontractors representing the NRC, several licensee personnel, and j licensee contractors.  ;

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1.2.1 Seismic The seismic supplemental review activities involved discussions with structural / mechanical engineers (PP&L staff and contractors) regarding seismic capacity calculations, screening approach, and current ,

disposition ofidentified outliers / anomalies. Discussions were also held with systems engineers regarding j the seismic success paths and their applicability to both accident prevention and accident mitigation (containment performance). The review also included a plant walkdown of some identified outliers and of some representative hardware improvements undertaken by the licensee.

1.2.2 Fire Several issues were identified during the Step-l review of the fire analysis sections of the IPEEE submittal, which raised serious doubts that the submittal met the intent of GL 88-20, Supplement 4; resolution of these issues was deemed to necessitate a site visit to the plant. The following three issues were of particular importance: (1) screening of compartments based solely on the argument of low combustible loading; (2) excluding the remote shutdown panel (RSP) from the fire analysis; and (3) reporting a core damage frequency (CDF) that was unusually smaller than fire CDFs reported in other IPEEE submittals or fire probabilistic risk assessments (PRAs). The total CDF was found in Reference [1] to be 10* per refueling cycle, which is significantly smaller than the CDF typically reported in other IPEEE submittals.

In addition to these concerns, two other potential weak points were addressed: (a) aniassumption that fires originating in a cabinet would not affect cables and equipment outside that cabinet; and (b) an assumption that small motors (<50 hp) do not pose a fire threat or constitute a fire ignition haurce for other materials.

To gain an appreciation of how the licensee had dealt with the issues mentioned above, the fire audit team needed to gain an overall understanding of how the licensee had conducted the IPEEE, and of what types of documentation had been generated. The audit team also needed to gain an understanding of the overall plant layout and internal configuration with respect to the cable spreading rooms, RSP, and the control room. Therefore, the audit team reviewed the documentation supporting the statements made in the submittal, and conducted a walkdown of certain key areas of the plant. Also, the audit team interviewed key licensee personnel who had been involved in the preparation of the fire portion of the IPEEE. These Energy Research, Inc. 2 ERI/NRC 98-502 l

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o personnel assisted in identifying relevant documentation, including various data and computer files, and assisted in conducting the walkdown.

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w 2 SUPPLEMENTAL REVIEW ISSUES AND CONTRACTOR REVIEW FINDINGS The supplemental review focused on resolving issues raised in requests for additional information, by means of direct discussions and plant walkdowns with licensee staff and representatives. The issues, licensee responses, and review conclusions are summarized in the following subsections.

2.1 Seismic

'2.1.1 Requests for Additional Information The principal issues investigated during the seismic IPEEE site audit were docun mted in RAls to the licensee [5], in the TER [2] resulting from the submittal-only review, and in a subsequent site audit report

[6]. These issues pertained to unresolved questions, potential weaknesses identified in the seismic IPEEE analyses, and the potential need for plant improvements. The specific concerns addressed are as follows:

1. The approach described in the Susquehanna IPEEE submittal for assessment of high confidence of low probability of failure (HCLPF) capacities was apparently neither the conservative deterministic failure margin (CDFM) method, fragility analysis method, or other conventional approach. The SSES submittal indicated that HCLPF assessments were based on the original seismic design or on qualification reports, but the details of the methodology were not provided.

In a first-round RAI that discussed this concern, the licensee invited the NRC to review the calculations during a site audit.

During the site audit, the audit team reviewed several HCLPF calculations and supporting data (including seismic design documents, drawings, seismic qualification report [SQRT] binders, etc.).

The purpose of this review was to check whether or not the llCLPF calculations have been based on established practice, to ascertain whether or not the approach and findings were reasonable, and to confirm whether or not the calculations were consistent with IPEEE screening decisions and criteria, as well as with the installed condition and configuration at the plant. To achieve the last of these objectives, a walkdown was performed as discussed in Section 2.1.2.

2. The seismic IPEEE identified certain concerns with respect to low seismic capabilities for (a) adjacent panels and cabinets in close proximity that are not fastened together, (b) unanchored color video CRTs, and (c) HCLPF capacities of four components (two valves, an automatic transfer switch, and a motor control center). Also, anomalous conditions were encountered during the walkdown evaluation of seismic-fire interactions. However, no hardware changes, or other enhancements, have been documented to rectify these concerns.

During the site audit, the audit team reviewed the current status of the licensee's resolution approach for these items. Some representative modifications that have been made were evaluated by consideration of relevant drawings and by physical inspection.

3. For the two identified low-capacity valves (which may potentially be damaged through adverse seismic-spatial interactions) the submittal stated that these valves will be manually operated following a seismic margin earthquake (SME).

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o During the site audit, the audit team evaluated the licensee's basis for this recovery action. The IPEEE systems engineer was interviewed as to the timing and purpose of the required action. In addition, one of the valves was included in the review walkdown, and photographs of the other valve (which was inaccessible) were examined.

Aside from the foregoing items, the TER identified some additional weak points of the IPEEE submittal.

These additional points included the following:

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The submittal's treatment of seismic containment performance was limited. A licensee response to a related RAI relied simply on the argument that core damage would not occur, and did not provide sufficient information on plant seismic capacity related to accids @igation.

During the site audit, the IPEEE systems engineer was interviewed regarding the completeness of the seismic success paths with respect to the containment performance objective of successful accident mitigation.

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The submittal did not apply screening criteria to non-seismic failure.s and human acus.. , as recommended in NUREG-1407 for an Electric Power Research Institute (EPRI) seismic margin assessment (SMA).

During the site audit, the IPEEE systems engineer was interviewed to clarify the licensee's treatment of non-seismic failures and human actions.

2.1.2 Site Walkdown issues, Review Approach and Findings 2.1.2.1 Review of the issues and New Documentation Walkdowns of specific areas of the plant were conducted to support the licensee's presentation of additional information regarding the supplemental review issues. During the seisms review walkdown, plant personnel were interviewed, and following the walkdown, additional documentation (including calculations and drawings) was consulted. Following the site audit, the licensee also submitted information responding to review observations made during the seismic walkdown [8J. This additional information was also considered as part of the present seismic review.

2.1.2.2 Walkdown i The seismic review walkdown was performed during the afternocm of August 4,1998. The walkdown was essentially limited to the items listed in the ERl/NRC site audit plan. Below are identified the scope of plant areas, components, and issues included in the seismic audit walkdown:

Diesel Generator Building "E" Automatic Transfer Switch OATS 556 (interaction with steel channel)

Batteries / rack Transient storage areas Control Structure Energy Research, Inc. 5 ERI/NRC 98-502

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Control room Relay room and cabinets / instrumentation panels (cabinets tied together)

Ifeating, ventilation, and air conditioning (IIVAC) equipment Fans OV-116A,B (anchorage)

Switchgear (breaker hoists removed and not removed [non-Q equipmenti)

Batteries / racks (block wall interaction)

Non-safety metal cabinet (containing combustibles)

Transient storage areas

• Reactor Building Valve HV-251-F024B (interaction with handrail)

Load centers (breaker hoists removed)

Motor control center (interaction with IIVAC duct)

Containment instrument gas bottles and bottle storage (top-strapped only)

Transient storage areas Circulating Water Pumphouse Diesel driven fire pump (attached lines)

Diesel day tank Block wall interaction Batteries / racks (securement concern)

CO2yard tank (weak anchorage concern)

Transient storage areas Emergency Service Water (ESW) Intake Building ESW pumps Spray pond a

Diesel Generator "B" Room Diesel generator room Diesel day tank OT528B (anchorage concern)

In general, the review walkdown consisted of qualitative considerations of anchorage capability, adequacy of installed construction, potential for adverse physical interactions, and state of seismic housekeeping.

(Some relatively minor seismic housekeeping problems were noted during the seisn}ic review walkdown, and Reference [8] indicates that the licensee responded to these identified problems with appropriate corrective actions.)

2.1.2.3 Review Findings Based on the discussions and documentation reviews conducted during the site audit, and based on review of information provided by the licensee in Reference [8], the following review findings have been made:

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Seismic capacity calculations have followed the CDFM approach and have been exceptionally well documented. The approach appears to have been thorough and based on correct implementation.

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2. Some meaningful hardware and procedural improvements have been made. Specifically, the licensee has implemented the following seismic-related plant improvemer.ts or beneficial actions:

Removal of Preaker hoist trolleys above safety electrical cabinets Bolting several cabinets together to prevera adverse interaction and potential relay chatter Installation of new, anchored CRTs in control room cabinets Implementation of housekeeping improvements and a seismic housekeeping procedure Seismic awareness training for plant sthff

3. For some potential interaction problems reported in the IPEEE (e.g., valve interaction and liVAC duct-to-cabinet interaction) no improvements were made because the potential severity of the interaction appeared to be comparatively minor. Also, for the case of potential valve interactions, the licensee indicated that the valves would be needed only after a significant period of time, during which manual operations cotiid be established in the event that the automatic valve operators were in fact disabled.

4. The licensee's consideration of seismic containment performance appears appropriate.

5. The licensee has acquired an understanding of the potential recovery actions needed following an i earthquake, and has presented a justifiable basis for crediting such actions.

6. Seismic walkdown screening for the IPEEE appears to have been executed in a conservative manner.

7. Although systems analysis work performed for the individual plant examination (IPE) served as a basis for developirg seismic success paths, apparently no specific systems and element selection walkdown was performed and no systems engineer participated consistently in the seismic capability walkdown. This represents a deviation with respect to established SMA guidance.

However, there was indeed ongoing communication between the seismic walkdown team and the systems engineer. Although greater involvement of the systems engineer in the seismic walkdown would have been desirable, the implications of this deviation are judged to be comparatively minor.

j Overall, the seismic-related issues raised in the site audit plan have been satisfactorily resolved '

4 2.2 ike 1

The fire supplemental review addressM the pertinent issues through a site audit where plant features were visually inspected, and supporting documentation and reports were reviewed.

2.2.1 Requests for AdditicnalInformation In preparation for the site audit, a set of RAls [5] and a site audit plan [6] were submitted to the licensee.

The principal issues (bases for RAls) that were investigated during the fire IPEEE site audit included the following:

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1. There are at least two cable spreading rooms, 0-25E and 0-27C, that were screened out prematurely. The licensee had screened tnese cable spreading rooms out on the basis of their low combustible loading. Given that these rooms may contain a large number of redundant and diverse circuits, and that they contain cables protected with Thermo-Lag fire barriers, the screening method was not considered valid. Indeed, there is much uncertainty in the likelihood of a fire that could damage cables in such compartments, flowever, given the potential severe impact on safe shutdown equipment, such compartments should not be screened out without gaining an underF*anding of the possible accident sequences associated with cabb failures (i.e., sequences of equipment / system losses or other failures such as vital instrumentation circuits). A potential

.ulnerability had been overlooked.

2. The possibility of having to use the RSP was not modeled in the IPEEE fire analysis. It was assumed that evacuation of the control room would not be needed. This assumption overlooks the possibility of uninhabitable conditions in the main control room, or a decision of the shift supervisor to order abandonment of the room. Lack of analysis of the RSP, which is installed fer mitigating a control room or cable spreading room fire, was considered a significant omission in the fire analysis.

3. The overall core damage frequency attributed to fire events was estimated to be 10* per refueling cycle (12 to 18 months), which is much smaller than frequencies typically reported in other IPEEE submicals. The reason for the small frequency was deemed to lie with the small conditional core damage probability adopted from the Individual Plant Examination (IPE) model, which in turn was deemed to be dominated by human actions and recovery activities. The human actions and recovery analyses of the IPE cannot be used for fire-induced core damage frequency evaluation without an adjustment for the influences of fire on the operators and recovery actions (e.g., the effect on man-power, control room habitability, control panel :tarms and instrumentation reading, use of equipment from the opposite unit, potential shutdown of both units following a single fire, etc.). It may be added that in a part of its response to Reference [5], the licensee states, "The approach taken in the fire PRA is that no additional purpose is served by postulating highly uncertain control room errors, because the steps taken to minimize such errors are currently in place." From this statement, it can be inferred that the licensee did not conduct a prchabilistic analysis of those human actions for which a written procedure and training were established. The core damage frequencies for various fire scenarios, as reported in the IPEEE submittal, were considered to be overly optimistic.

.4 In addition to the three issues listed above, the following two concerns were also noted:

1. The licensee had assumed that fires originating in a cabinet will not affect cables and equipment outside the cabinet. This assumption may be valid only if there are no openings on top of the cabinets, cabinet doors and structural elements (i.e., doors, frame, body, etc.) of the cabinets do not have temperature sensitive weak points, and combustible loading inside the cabinets cannot create an excessively strong fire.

1 The licensee had assumed that small motors (< 50 hp) do not pose a fire threat or fire ignition source to other materials. This assumption can be valid only if there are no cables from other circuits above or near these motors.

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e 2.2.2 Site Walkdown Issues, Review Approach and Findings The site audit was conducted in the following steps:

Review of the issues and brief summaries of existing and new documents and analyses

• Site walkdown

• Review of findings with the licensee staff 2.2.2.1 Review of the issues and New Documentation The first step of the site audit involved a review of the issues identified in Section 2.2.1 with the licensee's analysis team. The main purpose of this review was to provide clarification of the key points of the issues for the licensee's benefit in preparing the proper responses. Also, as part of this step, the licensee's fire analysis team provided an overview of the analyses and documentation related to the issues, and presented a brief summary of studies that have either been completed or been initiated since August 1996 (i.e., since the publication of the response to the initial RAls). Such studies included the following:

Revisions to the IPE quantification of the human error probabilities and other related models.

A re-analysis of those fire zones that were screened out based on low combustible loading [7].

An analysis of safe shutdown possibilities assuming failure of the Thermo-Lag barriers.

The licensee provided ready access to a large set of relevant documents, including the IPE, fire hazard analysis and Appendix R related reports, cable routing related information, computer outputs, and layout drawings.

The following points were made by the licensee relevant to the foregoing issues:

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The possib!'ity of control room abandonment and use of the RSP is being analyzed as part of the Thermo-Lag evaluation effort.

Most human actions considered in the IPE internal events analysis are outside the control room, and they appear in the anticipatcd transient without scram (ATWS) event tree, which is not relevant to fire scenarios. The human actions modeled as part of the fire scenarios take place in i the turbine building, and involve local alignment of the control rod drive (CRD) systems.

The areas with low combustible loading that were screened out were re-analyzed in some level of detail.  !

I A re-analysis of the dominant fire scenarios was conducted using the new screening criteria, and an interim report [7] was presented to the audit team. Subsequent documentation provided by the z

licensee [8] identified the new set of dominant fire scenarios as the following: j Fire Zone 1-2B: northern section at Elevation 670' of the reactor building that contains cables for high pressure coolant injection (HPCI) and Divisions I and II ESW l

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Fire Zone 0-28B-II: a battery charger area in the control structure that contains the electrical panels and associated cables for DC Channels A and B Fire Zone 0-27C: the upper cable spreading room in the control structure that contains cables for DC Channels A and B Fire Zone 0-25E; the lower cable spreading roorn in the control structure that contains cables for HPCI and Division I of residual heat removal (RHR)

The control room A collection of 15 fire zones (listed below) that were originally screened out based on low combustible loading:

- Fire Zone . Fire Area Descrintion 0-24F CS-3 In Control Structure; computer room.' ceiling 0-25A CS-29 In Control Structure 0-26B CS-6 In Control Structure; a cable chase 0-26C CS-7 In Control Structure; a cable chase 0-26D CS-7 In Control Structure; a cable chase 0-26S CS-6 In Control Structure; a cable chase 0-26T CS-7 In Control Structure; a cable chase 0-26V CS-7 In Control Structure; a cable chase 0-27B CS-32 In Control Structure 0-27F CS-6 In Control Structure; a cable chase 0-27G CS-7 In Control Structure; a cable chase 0-27H CS-7 In Control Structure; a cable chase 1-3C-N R-1B A compartment in the Reactor Building 1-4B R-1B A compartment in the Reactor Building 1-61 R-1B In Reactor Building Aside from the control room and Fire Zone 1-28, the rest of the fire zones listed above were screened out in the original IPEEE submittal.

i 2.2.2.2 Walkdown A site fire review walkdown was conducted, and the following areas were visited:

The reactor building, particularly Fire Zone 1-2B The RSP room in the reactor building Switchgear rooms in the reactor building The passage from the control room to the RSP The control room The relay room The upper and lower cable spreading rooms Energy Research, Inc. 10 ERl/NRC 98-502

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The DC power and battery charger area in the control structure

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I In the control room, the internals of a few control panels were inspected. it must be noted that the control I room includes a raised floor. The space between the two floors is used for routing cables. The fire  !

protection for this space uses CO2 gas. The operators must hold down a button to initiate and maintain CO2discharge. The IPEEE submittal did not address this area as a separate fire zone.

f 2.2.2.3 Review Findings

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1 Based on the discussions with the licensee, review of available documentation at the time of site visit, the findings of the site walkdown, and review of Reference 18] that was prepared in response to the comments I made during the site visit, the audit team reached the following conclusions regarding the supplemental I fire review issues:

1. The new screening methodology for those fire zones that had been screened out based on low combustible loading is reasonable. Note, however, that Thermo-Lag is present at Susquehanna l and credit was taken for some fire zones. From the discussions provided in Reference [8] it can be inferred that only five tire zones were screened out because of the credit given to Thermo-Lag.

The CDF was estimated for several other tire zones that could not be screened out despite the presence of Thermo-Lag.

2. The possibility of using the RSP is addressed in the control room analysis conducted as part of the Thermo-Lag resolution effort. Although the analysis is entirely qualitative and does not follow the same probabilistic methodology as that used in the IPEEE, the overall approach employed for control room abandonment analysis is deemed to be reasonable. The effects of loss of offsite power, spurious actuation of equipment, and failure of automatic functions have been included in the analysis. local operator actions (i.e., those performed outside the main control room and the RSP) have been credited to overcome those failures. Procedures are available for using the RSP and for conducting the local actions. From the discussions provided in Reference [8), it can be inferred that the licensee has taken timing of various actions into consideration as well.

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3. The methodology described in Reference [7] for quantifying the CDF for the two cable spreading rooms, and several other fire zones, was found by the audit team to be deficient. It was based on erroneous assumptions about independence among events. For example, in the case of the upper cable spreading room, the formula used for computing the CDF include $ a multiplication for

" Probability of large fire" by " Probability of detection and suppression failure". The two events l represented by these two probabilities are not independent. Large fires can occur in many different ways. For example, if detection and suppression systems fail, a small fire can grow and become a large fire. Also, if a large quantity of combustible materials is introduced into an area, ,

accidental ignition of those materials may lead to a rapidly growing large fire that may fail a l critical set of cables and equipment before the detection and suppression systems function as designed. This concern was mentioned to the licensee staff during the exit meeting of the site visit.

In Reference [8), in the discussions regarding upper cable spreading rooms, the licensee mentions a sensitivity analysis, where "it is assumed that the suppression system is overcome by a large fire Energy Research, Inc. I1 ERl/NRC 98-502

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and rendered ineffective". Using this assumption, the resulting CDF is found to be 2.4 x 10-7, which is three orders of magnitude greater than the CDP presented in the final results. From this discussion it can be inferred that the licensee has not employed the proper methodology for quantifying tire damage frequencies, and therefore, the CDF for the upper and lower cable spreading rooms, and for the 15 fire zones that were originally screened out based on low l combustible loading, may be overly optimistic. The significance and the impact of this deficiency l on the final conclusions of the IPEEE submittal, and especially on the identification of fire vulnerabilities, are discussed in Section 3.2 of this report.

4. Only a few electrical cabinets were observed during the walkdown. Those cabinets and the layout of cables and equipment near them were found to be typical of nuclear power plants with similar characteristics. However, it must be noted that, in the case of relay cz5inets, the licensee has l

installed a common " dome" on top of sets of contiguous cabinets, with openings from every cabinet i

section going into the common dome. The dome contained the header for the Halon fire protection system for the set of cabinets. ]

The possibility of fire propagating outside an electrical cabinet has been addressed in Reference

[8]. The possibility of fire damage in a specific control room panel (control panel IC601) and in electrical panels of Fire Zone 1-2B is described in Reference [8}. For control room panel 1C601, a probabilistic analysis of the fire scenario and operator response was conducted. The scenario has many similarities with those addressed in the qualitative analysis of control room aband mment. For a fire in control panel IC601, it is also assumed that the control room will be abandond Special attention is given to the specific procedures for using the RSP, local actions, and the timing of the actions. It is concluded that the CDF associated with this scenario is 5.1 x 10* per refueling cycle. The CDF is based on a probability of 3.3 x104 (0.033%) for operators failing to conduct the local actions. Although this value is small for a human error probability associated with a complex set of actions, it is deemed to be reasonable given that there would be 60 minutes available for completing the tasks, and considering that there are written procedures available to the operators.

For Fire Zone 1-2B, it is postulated that a large fire affects every item within the zone. The licensee has traced the cables associated with offsite power, and it was concluded that there are no such cables in this fire zone. No credit was given to the presence of Thermo-Lag raceway wraps. Given this assumption, it is concluded that a fire in this zone leads to loss of the ESW pumps and loss of high pressure systems. The CDF for this zone is concludi to be 2.1 x 10* per refueling cycle.

5. A special study had been conducted for the possibility of fires in small (<50 hp) motors leading to damage to adjacent cables and equipment. A damage envelope of 3 feet horizontal a,d 8 feet venical was assumed. The licensee investigated 42 raceways to verify that safe shutdown is not affected from such fires. In one case, it was concluded that safe shutdown may be affected. For this case, the raceway is covered by Thermo-Lag, is 6 feet above the motor, and the motor is %

horsepower. The analysis is deemed to be adequate in resolving this concern.

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0 3 OVERALL EVALUATION AND CONCLUSIONS 3.1 Seismic As noted in Reference [21, the approach chosen by the licensee for conducting the seismic IPEEE of Susquehanna Steam Electric Station (SSES) addresses the major elements specified in NUREG-1407 for a focused-scope plant, and the licensee has implemented a number of appropriate actions for resolution of noted concerns.

The present supplemental review has further t.otad that the seismic IPEEE involved a very significant effort, and was conducted in a competent manner. The licensee had a significant participation in the seismic IPEEE, and had a major contribution to authorship of the seismic analysis; this is consistent with the spirit of the IPEEE program.

No major weaknesses in the seismic IPEEE are apparent, however, it is noted that a greater involvement of the systems engineer in seismic walkdowns would have been desirable.

3.2 Eine As noted in Reference [2], the licensee has expended considerable effort in the preparation of the fire analysis portion of the IPEEE. The IPE plant impact model and probabilistic fire initiation and propagation models have been used to evaluate the tire-related CDFs. However, the licensee has also used a qualitative methodology to evaluate control room abandonment scenarios.

The strengths of the licensee's fire IPEEE effort mentioned in Reference [2] are further validated by the information provided by the licensee during the site audit and in Reference 18]. The new screening methodology is deemed to be reasonable, the possibility of using the remote shutdown panel was addressed, and the possibility of fire propagation outside the electrical cabinet of origin has been included in the analysis.

Based on the information provided by the licensee, and the observations of the review team during the site audit, the only remaining significant weak point in the fire analysis involves the licensee's assumption that the severity of a fire and the probability of suppression failure are independent, which has led to the calculation of small frequencies for all fir;: scenarios. However, the impact of the optimistic CDF values on the final conclusions may be minimal, since despite the erroneous assumpthn, the licensee has identified the significant fire scenarios at Susquehanna. The identification of significant fire scenarios depends on the screening methodology and ranking (using CDF) of the scenarios based on the detailed analysis. The ranking of fire scenarios is based on the relative values of CDF and not the absolute values.

Therefore, since the same treatment has been applied consistently across all significant fire scenarios, it can be argued that, despite the improper assumption, the licensee has, in fact, identified the significant fire scenarios at the plant.

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O 4 SUPPLEMENTAL IPEEE INSIGHTS, IMPROVEMENTS, AND COMMITMENTS 4.1 Seismic Although the plant HCLPF capacity has been assessed in the SSES seismic IPEEE as being 0.21g, this y capacity is governed by potential interaction concerns that may not be sufficiently severe to disable

/ associated success paths.

In addition to the items mentioned in Reference [1], the supplemental seismic review and Reference [8]

revealed that the following plant improvements were implemented at SSES:

• Bolting several cabinets together to prevent adverse interaction and potential relay chatter

• Installation of new, anchored CRTs in control roon cabinets

• Implementation of seismic housekeeping improvements and a seismic housekeeping procedure

• Seismic awareness training for plant staff No licensee commitments resulted from the supplemental seismic review.

4.2 Elte The site audit provided an excellent opportunity for the reviewers to gain an understanding of the fire risk at SSES. The issues raised in the fire TER were addressed by the licensee. In Reference [8], the licensee has provided a list of improvements attributed to the fire IPEEE. The improvements are mainly procedural. They include restrictions on combustible materials storage, han on smoking inside buildings, special covers for barrels, and opening of the floor drain in the Lower Cable Spreading Room to allow sprinkler system water to drain.

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5 REFERENCES

1. "Susquehanna Steam Electric Station Individu.il Plant Examination of External Events (IPEEE),"

Pennsylvania Power & Light Company (PP& L). June 27,1994.

2. " Technical Evaluation Report on the 'Submittal-Only' Review of the Individual Plant Examination f of External Events at Susquehanna Steam Electric Station, Units I and 2," ERI/NRC 95-512, Final Report, February 1998.

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3. " Individual Plant Examinatioc of External Events (IPEEE) for Severe Accident Vulnerabilities -

10CFR50.54(f)," U. S. Nuclear Regulatory Commission Generie Letter 88-20, Supplement 4, June 28, 1991. I

4. J. T. Chen, et al., " Procedure and Submittal Guidance for the Individual Plant Examination of External Events (IPEEE) for Severe Accident Vulnerabilities," U. S. Nuclear Regulatory l Commission, NUREG-1407, May 1991.

5. " Request for Additional Information on Susquehanna IPEEE Submittal," memo from M.

Cunningham to J. Stolz, U.S. Nuclear Regulatory Commission, March 11,1998.

6. " Site Audit Report For the Step-2 Review of the Individual Plant Examination of External Events at Susquehanna Steam Electric Station, Units 1 and 2," ERI/NRC 98-501, Site Audit Plan, July 1998.

7. " Cumulative Core Damage Frequency from Dominant Fire Risk Zones," Pennsylvania Power &

Light Company. (An interim report prepared by the licensee's fire analysis team and presented to the site audit team during the site audit.)

8. "Susquehanna Steam Electric Station, Response to Audit Issues on IPEEE Submittal, Units I and 2", PLA-4983, Letter and attachment from R. G. Byram of PPL to the USNRC, October 15, 1998.

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