ML20105A900

Rev 2 to PRA North Anna Power Station Svc Water Preservation Project,Part One, Final Rept

ML20105A900
Person / Time
Site:	North Anna
Issue date:	08/31/1992
From:	Afzali A, Donovan M, Zikria F HALLIBURTON NUS ENVIRONMENTAL CORP.
To:
Shared Package
ML20105A897	List: ML20105A894 ML20105A900 ML20105A904
References
PRA-920831, NUDOCS 9209180039
Download: ML20105A900 (122)

Text

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t PROBABILISTIC RISK ASSESSMENT NORTII ANNA POWER STATION SERVICE WATER PRESERVATION PROJECT PART ONE Augu'st: 1992 Final Report Revision 2

-l Prepared for a VIRG. IA ELECTRIC AND POWER COMPANY By.

A. 'Afzali F. Zikria.

M. Che'ok J. Tokar Approved by: N e--

.- M. D Donovan -

Assistant General Manager - .

-j Risk 'and Reliability Division -

-HALLIBURTON NUS ENVIRONMENTAL CORP' ORATION 910 Clopper Roat Gaithersburg, Maryland 20878 -

9209180039'920911" PDR-P~

ADOCK.05000338 - -

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j Revision one is produced to reflect two additional cables in the Appendix A Cable List.

l Additional cables and their functions are shown in Appendix A and are identified by correction

! ' mark on the right hand column of the page.

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l-I Revision 2 is produced to reflect the correction of two typing errors on pages 35 and 39. The corrected items are underlined.

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Table of Contents

1.0 INTRODUCTION

1 1.1 Background 1 J.2 Analysis Objectives 2 2.' TECHNICAL APPROACII 4 2.1 General Approach 4 2.2 Description of Excavation and Backfill Activities 7 2.2.1 Possible Consequences of Damage to Ducts 12 2.2.2 Possible Consequences of Damage to Unit 1 Service Water Lines 13 2.3 Construction Hazard Assessment 14 2.4 Asse sment of Hazards from External Events 14 2.4.1 Frequency of Seismic Events 18 -

2.4.2 Frequency of Tornadoes /High Wind Hazard 21 2.5 Assessment of Risk from SW Unavailability 23 3.0 RESULTS 25 3.1 Risk Contribution of Excavation During Unit i Operation 25 3.1.1 Construction Mishaps 25 3.1.2 External Events 34 3.2 Service Water Unavailability 37

4.0 CONCLUSION

S AND RECOMMENDATIONS 40 4.1 Conclusions 40 4.2 Recommendations 41

5.0 REFERENCES

42 APPENDIX A: FUNCTIONAL IDENTIFICATION OF CABLES APPENDIX B: GENERAL METHODOLOGY FOR FINDING THE CHANGE IN CDF DUE TO CONSTRUCTION MISHAPS APPENDIX C: SERVICE WATER MODEL

SECTION 1 INTRODUCTION The probabilistic risk assessment (PRA) of the fir;t part of the Service Water Preservation Project at Virginia Power Comnany's North Anna Power Station (NAPS)is documented in this report. The background and objectives of the study are described in this section. The methodology for performing the risk analysis is provided in Section 2. The results of the risk analysis calculations are presented in Section 3. Section 4 contai .s a summary of the i conclusions and recommendationr of the analysis.

1.1 BACKGROUND

The Service Water System piping at the North Anna Power Station is experiencing wall degradation because of general corrosion and relatively rapid wall loss in localized areas because of microbiologically induced corrosion. Several sections of small diameter :albon steel pipes have been replaced with stainless steel material. A pipe rehabilitation project has been undertaken to clean, weld repair and coat large diameter buried service water piping.

The restoration work on the service water system will be conducted in five different parts. The work in Part I will be performed duri' g the 1993 Unit 1 Steam Generator repair / refueling outage and will involve repair and partial replacement of the service water lines to Unit 1. The work in Part 2 will be conducted with both units operating and involves installation of manways on 36" main service water headers. The work in Part 3 involves repair and partial replacement of service water lines to Unit 2 ad is planned for the Unit 2 octage in 1993. Part 4 will consist of repair and pa tial rg?acement of the auxiliary service water lines while both units are operating. Part 5 will consist of repair of the service water piping connected to the component cooling heat exchangers and will be conducted during a planned outage in 1994 or 1995.

During the restoration pro,imt, it will be necessary to excavate and backfill in the Unit I and Unit 2 alleyways a.d in the yard north of the Turbine Building. New manway accesses will be 4

installed on '6" main headers west of Unit 2 (outside the auxiliary boiler roem), on 24" auxiliary service water lines in the north yard and below the basement of the Turbine Building.

The service water system is shared between the two units. It provides the ultimate heat sink capability for normal plant operation and for mitigating the consequences of a design basis event.

Performing these restoration projects will require isolation of one train of service water for brief 1

periods to install and subsequently remove blocking devices to the affected sections. Unit operation at power with a single train of service water is allowed for periods of up to seven days by plan: technical specifications (Reference 1). However, reducing the redundancy of the service water system through repeated use of the seven day action statement may contribute to l increasing the risk of an accident resulting in core damage.

Excavation of the buried service water lines will expose the lines and some electrical conduit ducts to possible hazards which may interrupt power to critical equipment. The normal design basis protection against natural phenomena afforded by the earth and concrete will be temporarily removed during the excavation. This will require a temporary exemption, to be approved by the Nuclear Regulatory Commission, from 10 CFR Part 50, Appendix A, " General Design Criteria (GDC) for Nuclear Power Plants". Specifically, criteria 2 (GDC-2) requires design protection against the effects of natural phenomena (e.g., earthquake, tornadoes) for components, systems, and structures important to safety.

To evaluate these possible risks, a PRA of applicable parts of the service water preservation

project will be performed. This report documents the PRA of Part 1 of the project, affecting the Unit I service water lines. T!e PRA analyses of other parts of the project will be 5 documented separately.

1.2 ANALYSIS OBJECTIVES i

l The principal objective of this PRA analysis of Part 1 of the service water preservation project

is to determine the effects, if any, of the project activities an the core damage frequency (CDF) 2

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of the operating units. An additional objective is to identify specific measures to further reduce the risks associated with the project. The third objective of the analysis is to provide a documented basis for a temporary exemption GDC 2 of 10 CFR Part 50, Appendix A " Design Basis for Protection Against Natural Phenomena."

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SECTION 2 TECliNICAL APPROACII This report covers the risk assessment of Part 1 of the service water preservation project; i.e.,

repair and partial replacement of the Service Water (SW) lines to Unit I components. The risk analysis for the remaining parts of the project will be provided in separate reports. The approach for performing this risk analysis is described in the following sec ions.

2.1 GENERAL APPROACH The overall approach for performing the probabilistic risk assessment (PRA) of the Unit 1 SW preservation project is illustrated in Figure 2-1.

The first step is to itemize the equipment that will be iselated or unavailable during the construction period as well as potential hazards which could damage other components or structures during construction operations. A review of the preliminary design change package and other project plans was made to determine the details of the scope of work to be performed.

Systems and components to be isolated during the project were itemized.

A plant walkdown of the affected areas was performed at the beginning of the PRA task to help in the hazard identification process.

Plant drawings were reviewed and a survey of the areas of construction was carried out to identify structures or components which will be exposed to damage by construction activities or external hazards like earthquakes or tornadoes. Using data in the Individual Plant Examination (IPE) system notebooks, plant P&Ips and information provided by Virginia Power, components in safety and support systems which would be affected by these hazards were identified. The result of this step was a listing of possible hazards, the systems or structures which could be damaged, the components which would be unavailable, and the effect of the component unavailability on plant operations.

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'5 Figure 2-1 Approach for Probabilistic Risk Assessment of Service Water Restoration Project

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1. Itemize Potential llazards and Component Unavailabilities

- Planned equipment unavailability

- Equipment unavailable due to accidental damage during constmetion

2. Revise IPE Model to Quantify CDF During Construction Period.

- Potential Initiating Events caused by equipment unavailability Unavailability of Safety or Support Systems

- Modify maintenance unavailabilities

3. Evaluate Countermeasures

- Identify critical items

- Determine countermeasures for specific items

- Calculate reduction on CDF for candidate countermeasures a

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The second step in the approach was to revise the North Anna IPE model to quantify the Units 1 and 2 CDF during the construction period. In revising the model, the planned unavailability of portions of the SW system wts accounted for by failing that portion of the service water '

header. Potential initiating events and system unavailabilities which could result from the I hazards identified in step one were also included. An additional modification to the model was

} to remove the following maintenance unavailability terms for planned maintenance actions which j would not be conducted during the one header operation of the SW system:

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o Three out of four SW pumps and both Auxiliary SW pumps 3

j o 4160 VAC bus 1H, IJ,2H, and/or 2J

} o Diesel Generators which would power operable SW Pumps i

This modification was based on the assumption that during the periods that one SW header is 1

unavailable, no maintenance will be performed on components affecting the reliability of the 1

other SW header. 4 i

f The revised IPE model was used to determine the change in Units 1 and 2 CDF during the f construction period. The result of this step is the quantification of the increase in the probability l of a core damage event during part one of the SW preservation task as compared to the baseline

! value determined from the IPE.

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! The third step was to evaluate the effectiveness of any countermeasures to reduce CDF during l- the construction period. Candidate counterrneasures include methods to accurately. locate

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s underground ducts prior to excavation, barriers to prevent damage, or changes to maintenance j and test schedules for systems important to accident mitigation.-

j . r Based upon review of the planned activities for the Unit 1 SW system preservation' work and a.-

{ survey of the work area, it was determined that the planned approach involved two sets of j activities.which could potentially increase the risk of the operating units.-- Thew activities are

! summarized as follows: -

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1. Exca' cation of the Unit 1 SW lines will be performed prior to the outas e while the unit is operating. Backfill of the area may be performed after restart, depending on the duration of the preservation task and other vage activities. During the I excavation and backfill, there is a possibility of damage to the SW lines or the j overlaying cable ducts resulting from construction mishaps. Additionally, the SW lines and cable ducts may be more vulnerable to damage from external events (e.g., earthquake).

2. During the restoration work, each SW header must be isolated and partially drained to install and remove blocking devices on the Unit 1 SW lines. This will result in a loss of redundancy in the SW system for the operating Unit 2 for six periods of up to seven days.

Tne approach for assessing the possible risk impact and effective countermeasures for each of these items is discussed in the following sections.

2.2 DESCRINION OF EXCAVATION AND PACKFILL ACTIVITIES The excavation site for the Unit 1 SW lines is located between the Quench Spray Pump House and the Main Steam Valve House to the south, and the Service Building to the north (see Figure 2.2). This area is called Unit I alleyway.

The SW lines are composed of four 24" pipes, two supply and two return headers, to and from the Unit 1 Recirculation Spray heat exchangers. In addition, four 4" SW pipes for the Unit I control room air conditioning condensers, each connected to a 24" SW line at an elevation of approximately 258'-0", are present (see Figure 2.3).

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Cable duct lines P and N are buried directly above the SW lines. Figure 2.3 shows a cross-sectional view of the planned excavation site.

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The project consists of replacing the 24" SW pipes in the Unit I alleyway and will be performed in three steps. In Step 1, the excavation will start and will proceed to the level of the SW pipes.

, Step 1 is planned to take about 30 days and will be performed prior to the Unit I shutdown.

Following Unit I shutdown, Step,2 will begin. This step is anticipated to last about 120 days or the entire outage period. During this step, the SW pipes will be replaced and repairs to some parts of SW pipes will be performed. Step 3 is the backfill of the excavation site which may be performed after Unit I restart.

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The following specific steps will be performed during the excavation and backfilling operations.

Excavation Steos

1. Breaking of the existing pavement, between the Service Building t and MS Valve House, within the alleyway will be performed using pneumatic equipment (i.e., air compressor and jackhammers).

Removal of debris will utilize a backhoe-loader (wheel type) to 6mp the material into dump trucks.

l 2. The subgrade directly below the pavement will be machine excavated with a backhoe-loader. Machine excavation will be .

performed up to a point approximately 2 feet above the N and P electrical duct '- :. All excavated material will be transported away from the excavation area with dump trucks as in item 1.

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3. All remaining excavation will be performed with hand powered tools and conventional pick and shovel tools. Excavated material will be hauled out of the pit using a conveyor that will transport the material directly into dump trucks for removal. Excavation will proceed until four SW pipes are exposed and will continue approximately 2 feet below the bottom of the pipes for a total depth of 28'-6" below the alleyway pavement.

Shoring of the excavation will be required, although the method of shoring has not yet, been determined. In any case, pile driving equipment will not be used. It is anticipated that soldier piles, installed in short segments, with timber lagging spanning horizontally between the piles will be utilized. Installation of the piles and lagging will be with a truck mounted crane. Itis 10

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anticipated that the excavation will remain open during the scheduled Unit 1 outage.

Backfilline Steos

1. Backfilling operations will begin upon completion of the SW pipe rehabilitation and may occur subsequent to returning Unit I on-line.

Backfilling will be accomplished by unloading backfill material from dump trucks into a concrete bucket (a bucket normally used for placement of concrete) supported by a truck mounted crane. ,

The bestiu material will be placed as needed with the crane and compacted with engine powered, hand operated compactors.

Alternatively, the material may be conveyed into the excavated area in a manner similar to its removal.

2. Vertical compaction of the soil will be performed as close as possib.e to beneath the existing duct lines. Backfilling will continue, closing one side of each duct line then horizontal compaction by hand will be performed within the remaining areas below the duct iines. If proper soil compaction can not be achieved beneath the duct lines, then lean concrete fill may be used within these areas.

3. Upon completion o,f the backfilling operations, the pavement within the excavated area will be restored with new concrete paving.

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During these excavation and backfill steps, there may be a potential for damage to the cable <

l ducts or SW lines due to construction mishaps. The possible risk effects of damage to these systems were reviewed as described in the following sections.

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i 2.2.1 POSSIBLE CONSEOUENCES OF DAMAGE TO DUCTS

! A review of electrical loads supplied by cables in the exposed duct lines was performed. A. list of all the cables in the N and P duct lines was prepared to determine the potential consequence of the damage to the duct lines (Reference 2). The review was conducted in several steps, including:

1. Review of the cables to screen out, based on the system designators, non risk contributing cables.

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2. Identification of the origin (FROM) and tne destination (TO) of all the remaining i cables.

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3. Identification of the Loads (Affected Components). Using the data banks, and pertinent engineering drawings the affected components were identified.

The results of the cable review are presented in Appendix A. No component (or group of j components) failures which would result in an initiating event (plant trip, either manual or

automatic) were identified. Accident mitigating components which could be disabled as a result of the cable damage include:

o two (2) Outside Rec,irculation Spray (ORS) pumps, o two (2) motor driven Auxiliary Feedwater (AFW) pumps, o two (2) Low Head Safety injection (LHSI) pumps.

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l Plant technical specifications require the plant to commence shutdown within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> if e.ny of

these pumps are inoperable. Therefore, the worst case consequence of damage to the P or N cable ducts would be a reduced capability for Unit I safety systems during the brief period prior to unit shutdown.

The IPE model was modined to include damage to the cable duct as a common mode failure resulting in unavailability of the affected pumps. This revised model was used to determine the effect of the probability of damage to the cable ducts on Unit 1 CDF.

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I 2.2.2 PQSSIBLE CONSEOUENCES OF DAMAGE TQ UNIT 1 SERVICE WATER LINES - l i

The SW lines to be excavated are the supply and return headers for the recirculation spray heat exchangers in Unit 1. There are no valves to isolate these lines from the main SW headers, i Technical specincation require both units to be shutdown if one SW header becomes inoperable and cannot be restored within seventy-two hours. This allowable outage time is extended to 168 hours0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> for service water upgrades.

Damage to one SW line which resulted in a small leak would interrupt the excavation activities f and ultimately require shutdown of the SW header to accomplish repairs. A large break would

! require immediate shutdown of the affected SW header and probably result in shutdown of both units within several days. Neither a small or large break in a single SW line would, by itself, initiate a core damage sequence, The IPE model was revised to represent Unit i operation with one train of SW unavailable.

This model was used to determine effect of the probability of SW line damage on units i and 2 CDF. .

Damage to two SW lines connected to train A and B SW headers which resulted in a large break j

or rupture could cause a total loss of SW. This initiating event has been analyzed in the NAPS Individual Plant Examination (IPE). The loss of SW accident sequence, designated T6, can i

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result in core damage due to one of the following reasons. Loss of SW will result in loss of cooling to emergency switchgear which induces a station blackout. Loss of SW will also result in a loss of RCP seal cooling, a loss of component cooling to the RCPs, and a loss of instrument air compressors. Each one of these initiators can result in a plant trip.

The IPE model was revised to include damage to two SW lines as a contributc r to the T6 event frequency. This model was used to determine the effect of the probability of this event on CDF.

1 2.3 CONSTRUCTION HAZARD ASSESSMENT The probability of a construction mishap resulting in damage to the cable ducts or SW lines is dependent on a wide range of situational factors and is very difficult, if not impossible to quantify. For this reason, a qualitative hazard assessment was performed, to identify the possible hazards to the electrical ducts and service water lines during excavation and backfill.

The hssessment was conducted by reviewing the existing project work plans, surveying the site of the excavation and surrounding areas and interviewing plant personnel with expeiknee in previous projects of a similar nature. The hazard assessment was performed by a civil enginen with extensive experience in excavation projects accompanied by a risk analyst.

Each step of the excavation was carefully analyzed to identify possible hazards, their causes, and

! possible preventive measures. The assessment results were reviewed with project engineers and i

construction personnel to ensure the completeness of the hazards evaluation and the feasibility

of the preventive measures. The results of the hazard assessment are presented in Section 4. ,

I 2.4 ASSESSMENT OF HAZARDS FROM EXTERNAL EVENTS During the period of excavation and backfill, the SW lines and cable ducts will not have the '

level of protection from external events normally provided by the soil and concrete pavement.

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External events (e.g., earthquakes, Goods, tornadoes, aircraft impacts, etc.) were analyzed to evaluate the degree of hazard posed by externally initiated events to the safety of the exposed cables ducts and SW lines. The screening approach described in NUREG/CR 4340 (Reference

3) was used to determine the events of possible concern. For external events which could not be qualitatively screened, quantitative analyses to determine the frequency of occurrence were performed.

The screening of external events to select the sigaificant risk contributors consists of several steps. First, all possible external events specific to ti'e site and plant are identified (a Table 2.1). The screening process is performed at two levels. At the first level, all external events which do not directly impact the site of question are eliminated from further consideration. At the second level, events are screened out based on the following screening criteria:

o The event is impossible or has an extremely remote probability o Barriers or protective measures eliminate the effect of the event o The effect of the event is not changed by the excavation t

Using the established criteria, each external event is independently reviewed to find out if more detailed analysis of the event is required. All those external events that are discarded as being  ;

insignificant are documented and the reasons for not performing a more detailed analysis are reported. The reason for exclusion of these events are also provided in Table 2.1.

It should be noted that external flooding was initially considered a credible event. The SW lines in the excavation and the SW manways in the auxiliary building will be open while work on the piping is performed. This introduces a possible flooding path through the open pipe to the auxiliary building. There are two, possible sources of flood of significant magnitude. First, flooding due to the failure of the SW reservoir earth dam. Second, flooding due to the rupture of the RWST. Flood from high lake water is impossible tiecause of the plant elevation which is above the maximum expected lake turface elevation, including coincident wind and wave g activity (NAPS UFSAR Section 2.4). Heavy rainfall cannot cause flooding of high magnitude 15

over a short period of time because the plant site is graded such that surface runoff will flow away from the excavation area.

The possibility of SW reservoir earth dam failure is remote. Even if a catastrophic failure of the dam occurs, the site is gt:ded in such a way that any water spillage will flow away from any safety-related facilities through a ditch into the lake. Similarly, flooding caused by the failure of the RWST is not possible. The base of the tank is below the elevation of the alleyway and the slope will carry water away from the excavation. Therefore, there is no possible external flood source.

It was also determined that the risk of sabotage or deliberate damage to the structures will not be affected. The excavation is completed within the protected area and the SW pipes and cables will have the same level of security as is normally provided elsewhere in the plant.

Events surviving the elimination process (e.g., Earthquake, and Tornadoes /High Winds) were analyzed m more detail, in general, the risk from these external events are assessed using a 1 two-step method:

1. Evaluation of the occurrence frequency of a particular external event as a function of severity, such as peak ground acceleration for seismic analysis, wind speed for high winds, etc.

2. Evaluation of the impact of the event on the duct lines and SW pipes as a function of severity of the event.

The degree to which it is necessary to perform a detailed analysis depends on the frequency of the event occurrence. The following sections discuss the quantification of the frequency from each of the external event of interest.

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TABLE 2.1 Qualitative Screening of External Events Event Cause for Exclusion Aircraft impact Remote, no airports in plant vicinity Avalanche impossible Euthquake Cannot screen Fire in Plant No effect on excavated area Fire Outside Plant but on Site No effect on excavated area Fire Offsite No effect Flammable Fluid Release Significant quantities of flammable fluid prohibited Flooding External No flood source .

Flooding. Internal No effect on excavated area Tornadw Cannot screen High Winds Consider under tornsto Industrial or Military Accident No industrial or nulitary facilities in area Offsite Landslide Impossible LigL. g No effect Metconte impact Extremely remote Pipeline Accident No pipelines in area Ship Impact No effect on excavated area Sabotage Excavation is inside protected area Toxic Gas Release No effect Transportation Accident No effect Turbine Missile No effect Volcanic Activity Extremely remote 17

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2.4.1 FREOUENCY OF SEISMIC EVENTS Excavation activities will remove the surrounding and supporting soil for the safety-related cable ducts, N and P, during the Unit I alleyway excavation. As a result, these structures will not remain qualified for seismic loads during the excavation period. Therefore, an evaluation of the i change in the plant operational risk, as measured by the change in the Core Damage Frequency 4

(UDF), will be necessary, if appropriate seismically qualified supports are not provided.

Cor.servative analyses were performed to evaluate the feasibility of dismissing the increase in risk from seismic cvents, at a low level of detail. Th analysis included an evaluation of the i

probability of an earthquake event during the period that Uait 1 is operating vJth the excavation i

or backfill in progress. To determine earthquake frequency, hazard curves were taken from two j sources (Reference 4 and 5). The first set of hazard curves was from the NRC-sponsored i

Eastern US Seismic Hazard Characterization Program, performed by Lawrence Livermore

! National Laboratories (LLNL). A second set of hazard curves was obtained from the industry-sponsored Electric Power Research Institute Seismic Hazard Methodology Development program. These curves are shown in figures 2.4 and 2.5 respectively.

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'J The NAPS seismic design basis for components in soil is an event with a peak ground acceleration of 0.18g (175 cm/s2 ). For this acceleration value, the 85 percentile earthquake frequencies from Figure 2,4 and 2.5 are approximately 2E-3 and 4E-4 per year, respectively.

These translate to a probability of 3.4E-4 and 6.5E 5 during the two 30-day periods. These values are considerably aDove the IE-6 value typically used as a threshold for non-credible -

events. Therefore, the carthquake probability cannot be used as a screening criteria and the consequences of occurrence of earthquake have to be considered.

Damage to the SW lines and cable ducts will not necessarily result in a core damage event.

However, to determine a change in the seismic-ir.Juced Core Damage Frequency (CDF) from the SW re;toration project, one would need to know the seismic CDF of the plant under normal operating conJitions. This determincion of seismic CDF is beyond the scope of this document.

It is therefore conservatively assumed that the probability of core damage from exposed unsupponed SW lines and cable ducts is at worst equivalent to the mean probability of accident design basis seismic event.

Therefore, it is recommended that measures be taken to ensure that the exposed SW lines and cable ducts will be supported by supports that will meet all appropriate acceptance criteria stated in the UFSAR for a plant design basis seismic event.

2.4.2 FREOUENCY OE_ TORNADOES /HIGH WIND HAZARQ Two data sources were used to quantify the frequency of tornadoes and high wind events. ,

ANSI /ANS 2.3-1983, "Standad for Estimating Tornado and Extreme Wind Characteristics at Nuclear Power Sites", (Reference 6) provides data on the probability of occurrence of various wind velocities. For the Virginia area, these data are summarized on Table 2.2 The probability of high winds during the period of Unit 1 operation with the cable ducts exposed is obtained by multiplying the annual probability by the fraction of the year spent in that configuration. -

21

L Table 2.2 Wind Speed Probability Probability (MPED (per year) (per 60 days) 150 lE-5 1.6 E-6 200 lE-6 1.6 E-7 250 1E-7 1.7 E-8 The NAPS UFSAR (Reference 7), also provides data on the frequency of tornado events. The UFSAR estimates the annual probability of a tornado striking any point in the plant as 3.3 E-5 per year, based on historical data for the area. For the 60 day exposure per .be probability is 5.4E-6.

Both data sources indicate that the probability _of a tornado or high wind event occurring during _

the period that Unit 1 is operating is slightly above IE-6. However, missile damage to the cable ducts during such an event is not certain. The ducts are several feet below grade and would only be damaged by a large missile (e.g., vehicle, large beam) dropping into the excavation.

Determining the probability of missile strikes on the structures would require a simulation analysis. Previous studies of similar situations (Reference 13) have calculated the probability of missile strike on a structure in the range of 6 E-2 to 1.8 E-2. However, a conservative <

assumption in this case is that the probability of missile strike, given a tornado event, is .25.

This accounts for the fact that the excavation is surrounded by buildings on three sides. The probability of large missile strikes could be further reduced by adopting a policy of moving all e

vehicles and other objects at !rast 200 feet from the excavation upon notification of a tornado-watch in the area. ,

22 s

2.5 ASSESShfENT OF RISK FROM SW UNAVAILABILITY The overall objective of this assessment was to perform a probabilistic analysis of the SW system f with particular emphasis on the change in the Unit 2 Core Damage Frequency induced by isolcing each train of SW for periods of seven (7) days to install blocking devices. The analysis includes:

o A reliability study of the North Anna Station SW System in normal operation (i.e.

two main headers operable);

o A reliability study of the SW System in an LCO condition with 168 hours0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> (7 days) as the mission time; o Determination of the change in CDF as a result of SW System being in a LCO condition of seven (7) days duration.

l The general procedure used to perform SW system fault tree analysis is outlined in HNUS General Task Procedure Task FT (Reference 8) with exceptions noted in the North Anna IPE task plan for system modelling (Reference 9). The starting point for the development of the fault tree models is obtained from the North Anna Power Station IPE (Reference 10). The NUPRA computer code (Reference 11) was used to generate the fault trees and to quantify these trees.

The major inputs and sources used for performance of the analysis are those used for the North Anna Power Station (NAPS) IPE. The IPE has used North Anna P&lDs (FM series drawings) to develop system flow paths, and the FE and ESK series drawings to determine power requirements. System descriptions were obtained from the various training manuals, vendor manuals, and the UFSAR. Additional information has been obtained during plant walkdowns, through the video disc information management system (VIMS), or directly from the system engineers at the plant. The North Anna operating procedures, abnormal procedures, emergency 23

I procedures, func'ional restoration procedures, technical specifications, and testing and maintenance procedures were used to determine operational parameters.

The inputs from the other IPE tasks include the system success criteria from Task AS; the basic event unavailabilities and common cause hilure probabilities from Task DB; and the human error probabilities from Task HI.

A detailed description of the SW system analysis is presented in Appendix C.

e 24 i I

SECTION 3 RESULTS The results of the PRA analysis of each of the areas of possible risk contribution are discussed in the following sections.

1 3.1 RISK CONTRIBlTrlON OF EXCAVATION DURING UNIT 1 OPERATION During the excavation and back611, the SW lines and overlaying cable ducts will be exposed to damage from construction mishaps or external event hazards. The IPE model was modified and used to determine the change in Unit 1 CDF in relation to the probability of damage to these structures. The details of these analyses are provided in Appendix B. The results are presented .

in Ogure 3.1 for the electrical ducts and Ogure 3.2 for SW lines.

The damage probability values shown in the Ogures represent the probability of damage to the electrical ducts or SW lines during the period that Unit 1 is in opemtion and the excavation is open. These curves were used to evaluate the CDF contribution of possible construction mishaps and external events. The evaluation results are discussed in the following sections.

3.1.1 Construction Mishaos The results of the qualitative hazard assessment of the exavation and backn11 activities are presented in Table 3.1. This table represents the possible hazards, causes, and preventive measures for each step in excavation and 611ing. The recommended preventive measures are summarized in Table 3.2. Many of these measures may be addressed by existing phnt procedures. Other items would be included in project work plans. All items should be inuuded in training and shift briefs of the work crews performing the excavation.

The only possibility of a mishap would result from failure of personnel to follow the administrative policies used to implement the measures described above. The " Handbook of 25

. - . . _ . . . . . . _.. . - - . - - - - . = - - . -

i i

r J 1 4

i i

1E 05 -

i 'x Single Duct I

x. -- o -

• ' N l

1 E-06 .

\ _

Both Ducts l

s s . _

s s  ;

2. N A s ._ _ J O \ \ l o X \

3 1 E-07

\ \_ -

i .

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< z x x i < N N i

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i 1 E-08 - -

t x~ -

! x i- x, f~

j I

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! 1.00E-01 1.00E-02 1.00E-03 1.00E-04

! Dl'CT(S) DAMAGE PROBABluTY' 3

i i.

e i

1 Figure 3.1: Increase in CDF in relation to Probability of Damage to Cable Ducts L

26'

- . , . . ~ , . ,.,,.,..,,,.c.,,.,_,._

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t 4

(

0.01 - -= -===- - --

= - - - ,

, x x_

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! > 0.0001 m_ _ _ . -

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x

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l

O =

o ,

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,' w =

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=

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i, .

x i 1 E-09 .

\, . . . . .

0.1 0.01 0.001 0.0001 1 E-05 1 E-06 1 E-07 PROBABILITY OF DAMAGE TO THE SW LINES

1

! --*- BOTH SW LINES ONE SW LINE I

i i

i l

1 i

i Figure 3.2: Increase in CDF in Relation to Probability of

)l Damsge to SW Lines i

i 27

e Table 3.1 liarard Awewment of the Unit i SW Excavation and Backflill Comtruction Stem Pmsible liarards Gms:s Preventative Measures

1. Removal of surface Darnage to unidentified buried lluman Error

• A. Research of peninent pavement with utihty. engineering drawmgs (FEs, jackhammers and twihoe- Inadequate data on buried FMs. fps).

loader. Top !8" to 24* of utihty lines excavation. B. Visual inspecti(e of the pertinent sections of the Main Steam Valve llouse and QSPil, for penetrations to ensure that there are no other utihties which can be at nsk.

2. Machine excavation ep to a Daauge to unidentified buried utihty llunun Error

• C. Prior to renm al of soil

'g point approximately 2 feet and/or elecincal duct imes w/baskhoe use steel bar above the N&P cldtrwal with backhoe andler jackhammers. Inadequate data on buried probe or protes or scanning duct hnem. utihty and duct hne location methods to sound out the unexcavated area.

3. Iland excavation to a pomt Damage to elecincal duct lines from iluman Enor* D. Provide protective twrriers.

approximately 2 feet below hand tools. of sufficient strength to i' the bottom of the service Equipment Failure prevent the largest vehicle water pipes with Dump truck or other large anticipated for this work conventional hand powered equipment falling into open from falling into the and n anual pick and shovel excavation damaging or collapsing excavarite.

tools, the duct lines and/or service water pipes.

Damage resulting from equipment, barriers or construction materials being knocked into the open excavation.

m-- -

m_._ _ _ _ _ _ = . - _m_

Table 3.1 Continued l llaard Assewnent of the Unit 1 SW Escavation and IlacLIill Gristruction Steos Possible llaards Opm Preventative Mt 3vres E. All non<ssential

( equipment and construction materials shall be hicated away from the excavation to prevent them from falhng into the excavation area.

F. All equipment and

- barriers locate 1 adjacent to the excavation shall be securely anchored or tied down to present l r ., accidental movement into l'# the excavation.

G. Current vehicle and

• equipment safety inspection.

l Shering failure, damage to duct inadequate design of II. Independent review of the

4. Shoring of excavation design, analysis and lines or service water pipes. shoring and supports for l installation of piles and construction documents for duct lines and service water lagging with truck mounted the excavation shoring and Damage to duct line and service pipes crane. Installation of supports for the duct lines supports for duct lines and water pipes due to dropping of Improper installation of and service water pipes.

service water pipes. materials used for shoring.

! shoring and supports I. QA and/or ngineering He concept of installation lluman Error

• inspection of all nuterials of shonag or support of used for and installation shoring was not develoM Equipment Failure inspection (construction f at the time this analysis was oversight) of all shoring

( perfornul. His is an work and supports i unicsolved concern.

• identified aleve.

l r

Table 3.1 Continued flazard Assessment of the Unit i SW Esca,ation and Backfill Construction Shps Possible llazards Caum . mentative Measures

4. J. Design of supports for the duct lines, service water pipe, and installation of shoring should include  !

provisions to prevent a progressive collapse of the supports and should also allow for muhiple support damage with no af fcct to the supported elements.

The shoring activities g should be scheduled such o that, installation of the shoring would take place prior to excavation of SW lines.

K. Ensure only experienced trained personnel are assigned.

L. Provide direct verbal communication, via dedicated radio if required, as well as visual communication between the equipment operators and the workmen responsible for placement and removal of

.i ' materials.

e

.e  % _ _.2_____

Table 3.1 Continued fluard Assessment of the Utdt I SW Excavation and Backfill Comtruction Steps Possible fluards Causes Preventative Measures M. All slings, hayks, chains, 4.

cables and lifting devices used for transporting of materials into, out of and over the excavation shall be properly designed and rated for the maximum anticipated load they will be used for. These items a

should be inspected prior to each use.

B' lluman Error

• N. Provide independant

5. Duct lines will be supported Collapse of duct line and danuge to supports so that the exposed between the full width of service water pipes due to improper Inadequate as-built - lines are supported by the excayation. engineering evaluation or original information regarding the supports that meet all construction of the duct lines.

design and construction of appropriate seismic the duct lines and service acceptance enteria in the water pipes. UFSAR.

flunun Error

• O. Pmvide attemate method of Backfilling operations. Damage or collapse of duct line

6. backfilling such as a fixed unloadmg of backfillb;g from misguided r;oncrete bucket. conveyor for transporting of Equipment Failure material with a concrete backfill material.

bucket and truck mounted Damage to duct lines from dropping crane. of construction materials on the ducts.

h

~

Table 3.1 Ilazard Awewment of the Unit I SW Eacavation and Hackfill f'unstruction Steps Powihle llanirds Gm Preventative Measures j

6. ' Damage or collapse of duct lines P. Provide geotechnical and/or service water pipes from consultation fm the proper improper uni compaction below materials, method of these items. placement, compaction and testing of the backfill material. Provide QA' and/or engineering iryation of all backfilling and soil compaction work.

\

Fire or explosion Improper use or handlir:g of gas Q. Gasoline for gas powered nowered hand compactors and fuel. hand compactors should be stored a safe distance from fj ' the excavation. Provide restrictions on amount of gas or number of gas containers that can be brought within the excavated area.

R. The affectal work area should be identified and maintained as a no-smoking zone.

See G. K, L, and At above.

• Iluman Error includes the following:

o Improperly trained personnel o inexperienced personnel (equipment operators) o Inadequate communication between key personnel o Sudden illness

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Human Reliability Analysis.. " (Reference 12, table 20-6) provides a human error probability (HEP) value of 0.01 for errors in compliance with administrative controls. Clearly, only a small fraction of the violations would be expected to result in damage to the ducts or SW lines.

However, the .01 value can be considered as a very conservative upper bound for the probability of damage to these structures from construction mishaps.

Using the .01 probability value with the curves in 6gures 3.1 and 3.2, the change in CDF is less than lE-6 for all cases except the case of simultaneous damage to both SW supply lines. The probability of occurrence of this event must be less than 8E-6 in order to limit the change in CDF to less than IE-6. The probability of a mishap resulting in simultaneous large ruptures to two SW lines can be minimized by the following measures:

o Delay excavation of the SW lines until the last several days prior to the outage o Controls and frequent inspections to ensure that no heavy materials with dimensions in excess of 6 feet (the distance between SW lines) are in vicinity of the excavation after SW lines are excavated.

With these measures in place, the likelihood of a mishap which damages both SW supply lines is extremely remote and the increase in CDF would be less than IE-6.

3.1.2 EXTERNAL EVENTS The probability of occurrence during the excavation or fill period of external events which could damage the cable duct lines are shown in Table 3.3.

The probability of a seismic event is signi6 cant and cannot be screened out. The effect of a seismic event which damages the duct lines cannot be quantitatively assessed since a base case seismic PRA has not been completed for NAPS. This result indicates that the cable ducts should be supported to withstand seismic loads equivalent to the plant design basis.

34 1

i

i j i

I TABLE 3.3

}

i PROBABILITY OF EXTERNAL EVENTS DURING EXCAVATION / BACKFILL PERIOD -

l PROBABILITY j' EVENT ANNUAL FREOUENCY - PER 60 DAYS Earthquake 0A

} - 0.4-2E-3 0.65-3.4E-4 4

Tornado 1-3.3 E-5 1.6 to 5.4 E-6 i

f-i '

! For high winds, the results indicate that the probability of occurrence is less than 6 E-6. The l probability of damage to the structures from win'd missiles is conservatively estimated to be 25 i percent of this value (i.e., < 1.2 E-6). With this probability of damage for the cable ducts and i SW lines, the increase in CDF is.less than IE-6 in all cases.

• i i

The results of the risk calculations for possible hazards to the SW lines and electrical ducts are .

l summarized in Table 3.4. The results show that, given adequate seismic supports for the cable ducts, the increase in CDF and the probability of a core damage event during the excavation and j- backfill periods will be less than IE-6.

i 1

i I

i-

~

d-i' L 35 i

Table 3.4: Increase in Probability of a Unit 1 CD Event During Excavation / Backfill Increase Probability of in Unit 1 Occurrence During Increase In - Core Damage IIAZARD 60-days Unit 1 CDF Probability Construction Mishap .01 3 E-7 3 E-9 Damages Cable Ducts Construction Mishap .01 1 E-8 1 E-10 Damages One SW Line Construction Mishap i E-6 < 3.E-7 3 E-7 Damages Two SW Lines Seismic Event (.18g) 3.4 E-4 Unknown Damages Both Cable Ducts ,

Tornado Missile Damages 1.25 E-6 < IE-9 <lE-9 Cable Ducts Tornado Missile Damage 1.25 E-6 < IE-9 <1E-9 One SW Line Tornado Missile <lE-6 < 3E-7 3 E-7 Damages Both SW Lines Total < lE 6 < 1 E-6 r

36 5- i .-i.a.ms.imi ..m.a....mi ......ic. . ..mi.. .....w.i..i

..iam... i.m-.. .

3.2 SERVICE WATER UNAVAILAlllLITY The results of fault tree analysis for the unavailability of the service water system for different cases are provided in Table 3.5. The change in the probability of core damage due to isolation of SW headers, Delta CDsw,,, is given by:

Delta CD3wr, = (change in CDF per year)

• f

, where f= the additional fraction of the year that the SW system would be in one header operation, f= (6*7)/365 = .12 Delta CDswy, =

(4.4E-5)* .12 = 5.1E-6 37 l

l

1

. -)

4 i

4

. Table 3.5 1

i e

Contribution of SW Unavailability to b

l Core Damage Probability i

[ Increase in

Increase in ~ Core Damage i

Unit 2 CDF - Probability i

SW Configuration (Events / Year) Durim6IMxk i

l- One Train Isolated 4.4 E-5 5.1 E-6 i

l l

l One Train Isolated, 1.2 E-5 1.4 E-6 i .

j Capability for Emergency i Pipe Repair i

One Train Isolated, 2.6 E-5 3.0 E-6--

{ Capability for Bearing i

l Cooling to HVAC Chiller 1

I

! One Train Isolated, 6.5 E-6 - .7.4 E Capability for Pipe Repair and Bearing Cooling i

i~

i j_

E I

i i

i l 38 l l1 e

i l

- - . . . . ._;--,,,,n.,:.-.-,;-----,-. . - ,1 -; , ,- . ,, - - ,w ; ; . . . _ , . _ _ . - , , - , . . . . . . . . - . . .-.--....._..a ...-..,n..---,-.-.u..-- - - - - ~

These results show that isolation of the service water headers for the six times necessary to install and remove the blocking devices will increase the probability of a core damage event on Unit 2 by 5.1 E-6.

There are certain measures which would limit the increase in plant operational risk. These recommendations are listed below:

1) Provide coohng for the Unit 2 Emergency Switchgear Room (ESGR), in an event of total loss of SW system. One feasible option is to provide a tempoiary supply and return path to/from the Bearing Cooling System to the Unit 1 air conditioning chillers. (e.g., the Design Change Number 91-009-1, indicates that a temporary supply and return path to/from the Bearing Cooling system will be provided to maintain at least one Unit 1- ,

chiller available). Implementation of this recommendation will allow credit to be taken for providing adequate cooling to the ESGR in the event of loss of SW.

2) Provide emergency pipe repair materials, trained personnel, and a procedure to repair the SW piping in the auxiliary building in an event of pipe rupture. This will allow the quick recovery from the loss of SW system due w pipe rupture. The frequency of T6 initiating e$ent, when in one header operation, will be reduced.

The effectiveness of these measures in reducing the risk of a core damage event is illustrated in Table 3.5. The results show that if both measures are implemented, the increase in Unit 2 core damage probability because of SW unavailability is less than IE-6.

39

l 1

SECTION 4 CONCLUSIONS AND RECO.%DIENDATIONS A summary of the conclusions and recommendations resulting from the PRA are presented in

this section.

4.1 CONCLUSION

S

The conclusion of the PRA analysis of the Unit 1 SW line restoration project is that project will not have a significant effect on the risk of a core damage event for Unit 1 or 2.

i Performing the excavation and backfill activities while Unit 1 is operating wi!* result in a negligible (< IE-6) contribution to CDF and the probability of Core Damage (CD) event

, occurring during these periods. This conclusion is based on the assumption that the cable ducts are seismically supported and appropriate measures are in place to Icevent construction mishaps, l

The probability of a construction mishap which would damage the cable ducts would have to be greater than 0.1 to have a significant effect on core damage probability. With the recommended preventive measures, the likelihood of a mishap of this type is conservatively estimated to be less than 0.01. Therefore, the probability of mishaps during excavation and filling will have no significant effect on risk.

J External events other than earthquakes and tornadoes /high winds were qualitatively screened and it was determined that they have no or an extremely unlikely effect relevant to the project. The l probability of a seismic event during the periods when Unit 1 is operating with the duct lines

[ exposed was found to be significant. Therefore, the 2 ducts will require supports designed to meet seismic qualification standards. The CD contribution of tornado / wind carried missile damage to the duct lines was found to be negligible.

4 40

The repeated isolation of service water headers for the purpose of installing and removing blocking devices will have a small effect (5.1 E-6) on the probability of a CD event for Unit 2.

This effect is primarily from the sequence initiated by a total loss of SW and is dominated by the probability of a pipe rupture in the sperating SW header. The analysis considered recovery measures to restore the SW header or provide cooling to critical components in Unit 2. These measures would further reduce the probability of a CD event during periods of operation with one SW header isolated to less than IE-6.

4.2 RECOMMENDATIONS It is recommended that the design package, work procedures, and worker training modules for the SW excavation and backfill tasks be carefully rcviewed to ensure that the measures to prevent construction mishaps are fully addressed.

It is also recommended that the project include seismically qualified supports for the cable ducts.

These supports should be installed as soon as possible after the duct is excavated.

It is also recommended that provisions for emergency repair of SW piping be established prior to isolation of the SW Header. The modification to provide Bearing Cooling System water as a backup cooling source for HVAC chillers should also be considered.

41 .

1

SECTION 5 REFERENCES

1. North Anna Power Station Technical Specifications.

2. Memorandum from E.L. Cooper, to A.V. Blankley, "lR-7006, Service Water Preservation Project - Phase 1, NAPS,7/2/92.

3. M. P. Bohn, J. A.12mbright, Procedures for the External Event Core Damage Frequency Analyses for NUREG-1150, NUREG/CR-4840, U.S. Nuclear Regulatory Commission, Washington, D.C., November 1990.

4. D.L. Berneruter, et al., Siesmic Hazard Characterization of 69 Nuclear Plant Sites F2st of the Rocky Mountains, NUREG!CR-5250, October 1988.

5. Electric Power Research Institute, Probabilistic Seismic Hazard Evaluation for North Anna Power Station. North Anna Project RP-101-53, April 1989.

6 American Nuclear Society, " Standard for Estimating Tornado and Extreme Wind Characteristics at Nuclear Power Sites", ANSI /ANS 2.3-1983.

7. North Anna Power Station UFSAR

8. "Probabilistic Risk Assessment, General Task Procedure, Task FT, Systems Analysis,"

NUS Corporation, Rev. O, March 21, 1989.

9. HNUS Project 0581 - Task 320M.N, Virginia Power Project NP-1498, Task Plan -

System Modelling Task (SM), North Anna Units 1 and 2, Rev. O, May 1991.

10. North Anna Probabilistic Risk Assessment for the IPE, to be Published.

42

T i 11. VP-NE-760, "NUPRA Version 2.0- PC-Based Software for Prob ne Risk Analysis,"

NAF Virginia Power Technical Report, Rev. O.

12. A. D. Swain, H. E. Guttmann, " Handbook of Human Reliability Analysis with Emphasize on Nuclear Plant Applications" NUREG/CR-1278, Sandia National i Laboratory, Albuquerque, NM,1983.

i i

13. " Tornado Hazard to Class I Electrical Conduits at Pilgrim Nuclear Generating i

Station", BECO SUDDS No. 87-1023, December,1987.

)

1 1

I i

t 43

APPENDIX A f

___ __ _______m_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ . _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _

Appendix A FunctionalIdentification of Cables The affected systems are identified by the functional identification of cables inside duct lines P and N. A list of all the cables in the N and P duct lines were prepared to analyze the consequence of the damage to the duct lines. The analysis is conducted in several steps, I including:

i) Review of the cables to screen out, based on the system designators, non risk contributing cables.

ii) Identification of the origin (FROM) and the destination (TO) of all the remaining cables.

iii) Identincation of the Loads (Affected Components). Using the data banks, and pertinent engineering drawings (FEs, E5Ks, as well as loop diagrams), the affected components were identified.

iv) The list of the affected components were analyzed No component (or group of components) failures which wou'd result in an initiating event (plant trip, either manual or automatic) is identified. Accident mitigating components which could be disabled (made functionally unavailable) as a result ci the cable damage include:

Two outside recirculation spray pumps, two mo;or-driven Auxiliary Feedwater (AFW) pumps, and two Low Head Safety Injection (LHSI) pumps.

The complete list is provided in Table A-1. The key for system designators codes, and equipment mark numbers are provided in the general nuclear standard (STD-GN-008). Items in parentheses are the pertinent references. A complete description of the reference is provided in Table A-2, and a full de cription of the type designators is provided in Table A-3.

T A-1 l

TABLE A-1: UNIT I DUCTS ASSOCIATED'VITH SERVICE WATER REPAlks (SERVICE BUILDING-QUENCil SPRAY EXCAVATION)

Pil ASE I AFFECTED CABLES DUCT LINE N IDKO*OAl CABLE NUMBER FROM 10 AF FF17F D COM W)NENT COMMENTS IMSSANK001 SPARE DUCT (NCABLE) TYPEI IMSfCNK001 SPARE DUCT (NCABLE) TYPEI IQSSNhK001 1 EP<B48A LS-QS103 UNIT I RWST TANK TYPE 3- Tile ABILITY TO MONITOR RWST MAY BE LOST-ISWEAOK100 1-EP CB-19A JB-730 SOV-SWl0l A-1 TYPE 3- LOSS QF THIS CABLE WILL RENDER SOV-SW101 A-1 INOPERABLE.

(ATTACll.1) (l l715-SW-03T) AS A RESULT TV-SW101 A CANNOT BE OPENED IF NEEDED.

- AN D TV-SW101 A ISWEBOK100 1-EP-CB-80A J B-740 SOV-SW1018-1 TYPE 3- LOSS OF Tills CABLE WILL RENDER SOV-SW101 A I INOPERAi!LE.

(ATTACil.1) (ll715 SW438) AS A RESULT TV-SWl01 A CANNOT BE OPENED IF NEEDFD.

AND TV-SW101 B

> IDl10070 A1 a w CABLE NUMBFR @M TO AFFECTED COMPONENT COMMENTS 1RSOAOH001 1 EE-SW-01 H10 1-RS-P42 A 1-RS P42A MOTOR FOR l-RS-P42A WILL BE MADE UNAVAILABLE.

t' ATTACH.1) (FEr8EM) 1D110070A2 CABLE NUMBER FROM T_O AFFECTFD COMPONFNT COMMENTS

!FWEAOH001 1-LE-SW48 ll3 1-FW-P43 A 1 FW-P43 MOTOR FOR l-FW-P-03 WILL BE MADE UNAVAILABLE, (ATTACH.1) (FE-8BE)

< I DH0070 A3 L

181E NUMBER FROM T,J AFFECTFD COMPONENT COMMENTS ISILAOH001 I-EE-SW OI-H9 l-SI P41 A l-Si P41 A MOTOR FOR l-SI-P41 A WILL BE MADE UNAVAILAELE

( ATTACH.1) (FE-88L)

~

1 DOW)70 A2 CABLE NUMBFR FROM TJ gITCTFD COMPONENT COM M ENTS lill ANNC458 ( ATTACII.1) TYPE 4 1 DCf W)70A2 CABLE NUMBER F1(OM 11! AFFFCTED COMPONENT COMM ENTS ISILNFC010 1 -El CB-48A 1B-1172 Mt. ' 1862A TYPE 3- A!SUME MOV IR62A WILL BE DISAHLED.THE MOV IS NORMALLY OPEN AND IS ON Tile RWST SUPPLY LINE TO l-SI-P-IA (FM 96A 1 OF 3)

(NCABLE) (FE-46L)

JB 439 SOV-Sil859 TYPE 3- 50V lR59 WILL REM AIN CLO5ED, NOT ALLDWING TV-1859 TO ISISNOC210 1-EI CB 45-AER (ATTACil.1) ($1035) OPEN.

(TV 1859)

I DO(170A2 CARLE NUMBER FROM TJ AFF FCTFD COMfoNENT COMMENTS SOV-MSil 3 A-1 TYPE 3- SOV MSil3A I WILL BE LEFT UNENERG12ED, CAUSING TV-MSil3A IMSB AOCw a t-El CB 45- AEN 1B-1130 (ATTACil.1) (MS-I I 7) TO REM AIN CLOSED.

(TV-MSil3 A)

JB5 A SOV MSiO9A TYPE 3- LOSS OF Tills CABLE WILL RESULT IN LOSS OF SOV 109A IMSBAOCll5 l-El CB 45- AEQ

" (A sTAcil.1) (MS-Il3) Wil!Cil IN TURN WILL NOT ALLOW TV-SWlO9 TO OPERATE IF NEEDED.

TV-MS109 JB.602A TYPE l- SPARE (FE-46K)

. IMSBAOC315 .l-EICB 05-AER I (ATTACll.1)

SO V Il3B 1 SOV-M31 RAT WILL REMAIN CLOSED (UNENERGlZED), NOT ALLOWING IMSBBoi251 1-El-CB45 - A EN J8-1831 (ATI'ACil. I) (MS-118) TEMSI A3P 10 OPEN.

SOV MSil3C-1 SOV-MSII8-1 WILL RD4AIN (.LOSEO (UNERGlZED, NOT ALl OWING IMSBCOC051 1 El<B05-AER JB.ll32

( ATTACil,1) (MS-119) TV-MS ll3C TO OPEN.

TV-MS101 A-1 SOV-101 A-1 WILL BE DE-ENERGIZ.ED, NOT ALLOWING TV415113 A IMSSAOCJ15 1 ElCB-05 AEP JB415A (ATTACH.1) (MS-Ilo) TO OPEN.

JB417A . TV-MSl01 B-l SOV4tS101B WILL BE LOST. TiiE ABILIT( TO CLOSE TV-MS101B IMSSBOC315 l-EICB45-AEP (MS-Ill) WILL BE DEGRADED.

(ATTACll. I)

JB419A TV-MS101C-1 SOV-MS101C-1 WILL BE LOST. Tile ABillTY 10 CLOSE TV-MS101B IMSSOCJ13 1-El-CB45-AEP (FE-!!2) WILL BE DEGRADED.

(ATTAcil.1) t

. ..m... _ , _ . . . _ _ _ . . .g.., m _.. .. .,._m _. .. .. -- - . - - ~ . _ . . . . . . . . . ~ . . . . . - . . . _ . - . . . . . - . _ . . . . - . . ..

I DCf070A2 '

CABLE NUMBFR FROM T J AF FEGF9 COMrONEW COMM ENT S llIVRNNCOS2 CO! LED (NCABLE) TYPE 2 ISVSNOCll5 l-El CB OS- AEP J B409 TV-SV102A 1 SOV-SV102 2 CANNOT BE ENOGlZED, NOT ALLOWING TV-SVIO2-2 (ATTACil,1) (FE-460) TO OPEN.

l ISVSNOC131 1-EICBM- AEN JB409 TV-Sol 02A l DISABLES LS FOR TV-SVIC24 sNO TESTS ARE A?, LOWED)

(ATTACll.1) (FE-460)

ISWEAOCl00 JB-730 1.EICB 07 SOV.SW-101 A 1 POSITION INDICATION 1.Rilfi'. ON Tile TEST PANEL WILL BE LOST.

(FE-3CF) OPERABILTTY OF Ti1E S(IV W1 LL NOT BE AFFECTED.

- ISWEBdOIOO JB 740 1. El CB-07 SOV-SW-10lB l PO3frlON !NDICATOIN Lt&lWi ON Tile TEST PANEL WILL BE IDST.

(FE-3CF) OPERABILTTY OF Tile SO% A.L NOT BE AFFECTED. .

IDXOO fN Bl tillANNX810 TYPE 4

1. IRMSNNX001 4

! IRMSNNX002 IRAISN N.XOO3 IRMSNNX007 IRMSNNXO21 IRMSNNX022 IRMSNNX023 1RMSNNX027 IMSSFNX001 1-EI CB-44 iT-M5105 NEAR l-FW-P-2 ASSUME FIAW TRANSMITTER IS DISABLED (NCABLE) (FE-46R)

IFWSNNX820 FF-74 T. W JB-1014 PCV-FW 159A ABILITY TO MEASURE AFW DLSCif ARGE PRESSURE IS DEGRADED.

(NCAELE) (FW-053) l T

1 e _ _ . _ _ _ _ _ _ _ . _ _ _ . _ _ _ _

& 4 s DUCT LINE P IDilm7PRI CABLE NUMBFR fROM Il} AFI FCTED COMPONENTtS) COMMENTS 1-EE-SW-02.J 10 1 RS P42B l- RS-P 42 B MOTOR FOR l-RS-P 02B WILL Im t/J*T, 1RSOBPif001 (ATTACli. 4) (FE8BY)

I DiltO7PBl CAEl.E NUMBER FROM 10 AFFFCTED COMPONENT (S) COMMENTS I MOTOR FOR I tW-P43B WILL BE LOST.

l IFWEBPil001 1-EE-SW42-13 i-FW-P438 I-FW-P43 B (ATTACll.1) (FE-8BR) l IDil007PH3 CABLE NUMBER FROM . TJ AFFECTED COMPONENT CAMMENTS 1- EE-SW-0219 l P-CI B l-SI-P Ol B MOTOR FOR l-SI PolB WILL BE LOST.

ISILBPil001

( ATTACH.1) (FE-8BX)

IDX(OlWAl TJ AFFECTED COMPONENT COMMENTS I,

u CABLE NUMBER FROM 1-ElCB 238 LT-RS103 A l-RS-TK 1 ABILfrY TO MONITOR Tile LEVEL OF CASE COOLING TANK 1-RSTK-I IRSOBWX001

( ATTACil.1) (RS-29) %1LL BE DEGRADED.

1-El CB-23B TE-RSl00A (RS 31) ABILITY TO MONTTOR Tile TE*1PERATURE OF Tile CASE CWLING IRSOBWXOO2 (ATTACll.1) TANK 1 RS-1K-1 WILL BE DEGRADED.

IDXODIY Al T AFFECTED COMf'ONENT COMMENTS CARLE NUMBER FROM J LT-R$103B l-RS-TK-1 ABILTTY TO MONITOR Tile LEVEL OF CASE COOLING TANK t-RS-TK-1 B RSODYX001 1-El-CB-23 D (ATTACH.1) (RS-30) WILL BE DEGRADED.

TE-RSIOOB l-RS-TK-1 ABILITY TO MONrrOR TIIE TEMPERATURE OF Tile CASE COOLING TANK IRSODYX002 1-EJCB 23D (RS-32) WILL BE DEGRADED.

(ATTACll.1)

._ .. , , m __. .- . . . . ...m - - _. - . . . - . . -- . . . . .._. ._ m . _ ._- . ., _ . . . . . m-I DCtK17PB2 CARLE NUMBER FROM JJO AFFECTED COMIUNENT COMMENTS IIHANNC459 TYPE 4 IIHANNC493 TYPE 4 IRilS ANCOO2 1-ELSW-01-H14 I-EPCB 84A1 1 RH-P-01 TYPE 3- SWIFCil FOR l-RH-P41 A MOTOR HEATER (NCABLE) (FE-911K)

ISILANC004 - 1.EE-SW41-H9 l EPCB 84Al l-51P41A TYPE 3- SWIFCH FOR l-$1 P41 A MOTOR IIEATER (NCABLE) (FE-9ffK)

IDC007PB2 CABLE NUMBER FROM TJ AFFECTED COMPONENT COMMENTS IRSOANC007 1-EE-SW41 Ill0 l EP<B-84 A l-RS-P41 A SWFICH FOR l-RS-P42A MOTOR llEATER, (FE-9H F)

IQSS7NCol8 . JB-520 l EPCB 28 l-RS. P-2A,1-QS-P-2B ONE OF THE CONTACTS FOR RWS TANK CHILLER PUMPS WILL BE U4$T.

(QS-14) h IHVRANC003 1-EE-SS41-il7 l-EPCB-84 A I-HV F41C SWIFCH FOR l HV-F4tC MOTOR llEATER.

(NCABLE) (FE-9HF)

IllVRANC005 1-EE-SS4247 l-EPCB-84A . . I-HV-F41C -JCH FOR l-HV-F41C MOTOR HEATER.

(NCABLE) (FE-9HF) 1HVRNNC062 ColLED (NCABLE) TYPE 2 IFWEANC008 l-EE SW-01 H3 1 EPCB-84A l-FW-P43 A SWTFCH FOR l-FW-P43A MOTOR 11 EATER.

(NCABLE) (FE9HF)

IFWEBNC008 ~ 1-EE $W4243 1.EP-CB-84B SPARE (NCABLE) (FF 9HF)

Table 'A Complete Description of the References Reference Code Full Description 1- Attach.1 Attachment I to the Memorandum from E.L.

Cooper to R.W. Riley, February 24, 1992.  ;-

2- NCABLE North Anna Power Station Circuit Schedule, Cable Routing System.

3- FE-XXXX 11715-FE-XXXXX, Engineering Drawing Number.

c Table A-3

-Complete Description of Type Designators Cable Tvm Full Descriotion Type 1 Spare Cables (Do not provide. service, presently)

Type 2 Coiled Cables (Do not provide Service presently)

Type 3 Provide service and cannot be screened out based on system designator Type 4 Provide service but is screened out based on system designator A-7  !

l 1

g_.._ . _,,. .

APPENDIZ B O

hu

____ _ m

Appendix B General Methodobgy For Finding The Change In CDF Due To Construction Mishaps B-1 General Methodolon The change in core damage frequency (CDF) was calculated using the NAPS IPE. The NAPS IPE was performed using the NUPRA software system, developed by the Halliburton NUS Environmental Corporation. NUPRA is a PC-based user friendly safety / Reliability workstation for probabilistic risk / safety assessments (PRA/PSA).

The methodelogy used to perform the IPE for North Anna Power Station Units 1 and 2 is based en the performance of a Level 1 PRA. The approach used for the plant models is based on the use of event trees to develop the sequence of events following a plant transient or loss-of-coolant accident (LOCA), and fault trees to model the system failures and successes at each phase of the sequence. Each individual sequence is quanti 6ed by linking together the fault trees for the system and support system failures that lead to a given sequence of events. Each sequence defines a set of conditions leading to inadequate cooling of the core. The core damage frequency is then calculated by adding all the sequences which result in core damage.

The change in CDF was calculated by first establishing the base case CDF. The base case CDF was calculated by quantifying the NAPS IPE models that existed as of June 3,1992. Then, the IPE models were modified to account for the mishaps during the SW Preservation Project. The modified IPE models were quantified to obtain the new CDF. The difference between the two CDF values represents the increase in risk due to construction mishaps. There are two distinct contributions from the construction mishaps to the increase in CDF. The first contribution is as a result of damage to the cable duct line(s) and the second is as a result of damage to the SW line(s). The evaluation of the change in CDF due to damage to duct line(s) and SW pipe line(s) failure is discussed in section B-2 and B-3 respectively.

B-t

B-2 Change in CDF due to Duct LineM Failure The specific steps performed in fmding the change in CDF during the construction period of

{ interest (60 days) are as follows:

l. The IPE model, existed as of June 3,1992, was quantified to establish the base case CDF.

2. The IPE model was modified to account for mishaps during construction.

Specifically, the Auxiliary Feed Water (AFW), Low Head Safety Injection (LHSI), and Outside Recirculation Spray (ORS) fault trees were modified by adding the failure of Ducts N and P independently or by common cause to the failure of relevant pump branches. For example, the LHSI fault tree was modified by adding the FWLHRS-CC-lPUMPS and FWLHRS-SF-I APUMP basic events to the train A failure branch. This means that, in addition to other pump failure modes, the LHSI train A pump can also fait due to failure of the Duct line N or due to the common cause failure of both Ducts (N and P) during l the construction period (see Figure B-1). The train B of the LHSI pump is

, modified in the same manner.

3. All newly added basic events (FWLHRS-CC-IPUMPS, FWLHRS-SF-1 APUMP, and FWLHRS-SF-1BPUMP) were set to 0.1 in the Basic Event Data (BED) file. This was

, performed to ensure that most cutsets with newly added basic events are preserved by being cave the tiuncation value.

4. All fault trees were updated from the BED file.

5. Re-solved the entire IPE model. The following steps were followed:

a. Fault trees were linked together using . Table 7A of the NAPS Quantification Analysis File.

B-2

i R

4

b. Updated the linked trees based on relevant house event BED and then solved for the appropriate gates. The truncation values used for solving each top event unavailability were based on Table 7A.

i r

c. The merge control files or OCLs, which quantifies each individual accident sequences, were solved.

, d. The Scouence probability or SEQ file, which contains records for I

the no..-OK sequences in the event trees, was updated from the newly solved cut set equations assigned to the functional events challenged in the sequence.

e. The concatenation and truncation of all sequence cutset, which is the ANDing together of all the core melt sequence equations into a single plant damage equation, was performed.

f. A sensitivity analysis on the plant damage ~ equation was performed I

to assess the impact of variations of single and both Duct failure prnbabilitie to the core damage frequency.

i

g. The change m CDF for various duct failure probabilities during

[ the construction period was calculated. The change in CDF equals the CDF calculated in step 1 minus the CDF calculateo in step Se, the ruult of which was plotted on a logxlog scale (see Figure 3-1).

B3 1

= - --n w - --e- -- - - ---. -- -

• l l

[- D-3 Channe in CDF due to SW Line(s) Failure l Tis m vation of the SW lines during the construction process, sill expose the lines to

! construction hazards. The SWPP PRA results indicate that the change in core damage frequency

! due to loss of SW (when in one header oneration) during an accident is negligible. On the other

• hand, loss of both SW header during an accident, caused by corstruction mishaps, is considered f

to be insignificant, based on the assumption that if an accident would occur, the construction -

l l activities will be stopped. Thus only change in CDF due to total loss of SW during normal i

operation of the plant (ie no accident) is evaluated here. To quantify the change in CDF due to consequence of the construction hazard (construction mishaps) on the availability of the SW

!' system, the following steps were taken:

. 1) The fault tree for the loss of SW initiating event frequency (T6) was modified to f

I account for the probability (Pa) of the SW header (s) rupture due to construction .

i mishaps. The T6 fault tree was then requantified to obtain the new Initiating i

Event (IE) frequency (f**16).

i i

l 2) Based on the new T6 IE frequency, the contribution of the T6 to the CDP was

[ evaluated by requantification of the T6 event tree (CC F4 l

[ 3) the change in CDF was then obtained by subtraction of the contribution of the IPE T6 initiating event to the base case CDF, CCD'use, from the C"1..

l" The above evaluations were perfctmed for- different = values of Pa. The results of. the quantification is presented in Figure B-2, and the modified fault trees for T6 initiating event I frequency given damage to one or)oth headers are shown in Figures B-3 and B-4 respectively.

It should be noted that during two header operation (if hazard frcm construction activities are 4

not considered) the probability of header rupture is ~ considered to be insignificant.

1 t

B-4 4

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APPENDIZ C e

Appendix C C.1 General Description of the Service Water System This section diausses the Service Water (SW) System and provides a general description of the system operation, flow paths, and interrelationships with other plant systems. The contents of this section are copied from the Service Water System Training Manual, Module NCRODP-13, June 1992. A simplified SW system disgram is presented in Figure C-1.

The Service Water System is common to both reactor units and is designed for the simultaneous operation of various subsystems and components of both units. The purpose of the SW System is to provide long term cooling after a loss of coolant accident (LOCA) and to supply cooling water to the following safety-related components during normal plant operations:

1. component cooling (CC) heat exchangers;

2. recirculatien spray (RS) heat exchangers (available, but not normally tiowing);

3. control room air conditioning condensers;

4. charging pump seal coolers, gear reducers, lube oil coolers; and

5. instrument air compressors.

The SW System also serves as a backup source of water to the:

1. Auxiliary Feedwater system.

2. spent fuel pit coolers, i

C-1

__mm____-__._-m.__.-.m-_ __m.m_________m._m_.--.m_.m____m__-m_m____m._.___m.m.m __.__;

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3. containment air recirculation coolers, and The sources of cooling water for the SW system are the SW reservoir and Lake Anna. These 3 two independent sources of water form the ultimate heat sink for the Nonh Arna Power Station.

4 Flow Paths The SW System has two modes of operation: reservoir-to-reservoir and lake-to-lake. Itis normally operated in the reservoir-to-reservoir mode which uses the SW reservoir as the ultimate i heat sink. The SW reservoir is a large pond with sufficient supply of treated water to provide cooling for four operating units with one of the four units suffering from loss of coolant accident (LOCA). There are two spray headers in the reservoir that spray returning SW into the air to assist in dissipating the heat acquired while cooling the various plant components. Each spray header consists of two pairs (four total) of individual contiollable spray arrays. The spray arrays can be bypassed by two spray bypass lines (one per header) leading directly to the reservoir.

. There are four SW pumps, of which one pump per unit is normally in operation. The pumps l draw SW from the reservoir through a set of traveling screens that filter out debris. The SW

pumps provide the motive force for the flow of the SW through the various compor nts cooled i by the SW System.

1 The SW System supplies cooling water through the plant with two supply headers. Two return headers collect the SW from the cooled components and return the water to the reservoir. At the reservoir, the return headers divide the returning SW among the two spray headers or spray

+

bypass lines. Radiation monitors are used to ensure that no radioactis.e contamination has leaked into the returning SW. ,

, in the lake-to-lake mode of operation, two auxiliary SW pumps draw water directly from Lake Anna through the Circulating Water (CW) System traveling screens. The lake to-lake mode is used as a backup and during SW System maintenance. -The auxiliary SW pumps discharge the C-2

_ _. ~ __ _ _. . . . _ , _ . _ _ .

SW to the same supply headers as the SW pumps used in the reservoir to reservoir mode. The return headers have an auxiliary return header that directs the return SW to the lake. The auxiliary return header is also monitored for radioactivity by a radiation monitor. Two of the CW screen wash pumps serve as makeup pumps for the SW System and can add lake water to the SW reservoir. The auxiliary SW pumps can also be used to provide makeup water to the SW reservoir.

Subsystems Screen wash subsystem. The SW Jystem has its own screen wash subsystem that uses two screen wash pumps and four traveling screens. The screen wash subs' y stem is ir the SW Pump House. The traveling screens 61ter the incoming reservoir water prior to its entering the SW pumps. When suf6cient debris is collected on the screens (or once daily), the screens rotate and are cleaned by spray water from the two screen wash pumps. The screen wash pumps use SW from the reservoir. The debris is collected in a trash basket, and the water is returned t'o the reservoir.

Chemical Addition Subsystem. The Chemical Addition Subsystem purpose is to treat SW System water so as to minimize SW System component and piping corrosion. The subsystem consists of a now loop which originates at one SW Supply Header and returns to both SW Supply Headers. Chemical treatment equipment in the Service Water Chemical Treatment House stores, controls and injects chemicals into the Chemical Addition Subsystem Dow loop.

Chemicals injected into the Service Water System Supply Headers via this now loop are mixed by main Service Water System Header Flow, and Distributed to all parts of the Service Water System.

Air subsystem. The SW air subsystem consists of two air compressors. The air subsystem is in the SW Pump House. The air compressor provides the necessary air to control and monitor the operation of the screen wash subsystem. Air from the SW air subsystem is also used to measure the reservoir level for indication and alarm in the Main Control Room (MCR).

C-3

Sumo subsystems. The SW sump subsystems remove any leakage and runoff water from the SW Pump House, SW Valve House and SW Tie-In Vault. Drainage from various components collects in the sumps in the lower levels of the structures. The sump pumps periodically return the collected water from the SW Pump House and the SW Valve House to the reservoir. Drainage from the SW Tie-In Vault is discharged to grade outside the Vault.  ;

System Interfaces Circulatine Water (CW) System. The CW System provides filtered lake water to the suction of the auxiliary SW pumps and the makeup pumps. The auxiliary SW return header directs returning water into the Unit 2 discharge tunnel for ultimate return to the lake.

l Instrument Air System. Service Water flows through an intermediate cooling loop (part of the IA system) for both instrument air compressors in the Auxiliary Building. IA Compressor I and aftercooler temperature control is a function of the IA system.

1 i

Feedwater System. The SW System provides a backup source of water to the auxiliary feedwater pumps. The auxiliary feedwater pumps provide feedwater to the steam generators on i

a loss of normal feedwater capability. SW is the last source of water to be used to supply the auxiliary feedwater pumps because of the chemical contamination that would result in the steam

. generators.

1 Chemical and Volume Control System (CVCS). The SW System provides cooling water to the charging pumps in the CVCS. Cooling water is provided to the seal coolers, gear reducers, and lube oil coolers. .

Radiation Monitorine System. The Radiation Monitoring System provides a number of radiation monitors that are used to detect possible radioactive leakage into the SW System.

C-4 2

i

)

Primary Ventilation Syskm. The SW System provides cooling water to the MCR air l conditioner condensers. The SW System also provides a backup source of water to the containment air recirculation coolers that are normally supplied with water from the Chilled Water System.

Spent Fuel Pit System. The SW System provides a backup source of water to cool the spent fuel pit cooiers. The coolers are normally cooled by CC, but they can be cooled by SW if CC is not available.

Comoonent Cooline System. SW is provided to cool the CC heat exchangers. Most

the components for which SW is the backup source of water are normally cooled by the CC i System.

Recirculation Soray System. The RS heat exchanFers are cooled by SW on receipt of a containment depressurization actuation (CDA) signal. The RS System is used to depressurize containment during a LOCA or main steam line break in Containment and to provide long term s containment cooling during a LOCA.

J

Reactor Protection System (RP5'. The RPS generates the CDA and safety injection (SI) signals that alter the SW lineup. The CDA signal initiates SW flow through the RE heat

, exchangers and isolates SW flow through the CC heat exchangers. SW, which is not normally supplied to the containment air recirculation coolers, is also isolated on receipt of a CDA signal.

The SI signals starts the SW pumps and ensures that all of the SW reservoir spray header isolation valves are open and the spray bypass valves are closed for maximum SW cooling

, capability.

C.2 Identification of Computer Code Used and Computer Employed The NUPRA computer code was used for fault tree input and solution. ' NUPRA was written by HALLIBURTON NUS Environmental Corporation (formerly NUS Corporation) and is a C-5

personal computer (PC) based code. The solution of fault trees by NUPRA has been verified by hand calculations and also by comparison of results with the mainframe based SETS computer code (an industry and NRC accepted code for fault tree solution).

The version of NUPRA used is Version 2.0.

b C.3 Fault Tree Development The method of fault tree development creates a systematic format for gathering and evaluating system data. The fault tree itself is the logical combination of this information to generate the system unavailability. The SW system fault tree model is shown in Figure C-2 and major topics of the fault tree development are briefly discussed below. Detailed information on these topics are provided in the following subsections.

Determination of System Success Criteria. The success criteria for a system form the basis for the top event for each fault tree.

Development of Simplified Schematic, The simplified schematic models the system as represented in the analysis, and is a critical element of the systems analysis. It identifies those components included in the model, defining their boundaries. It identifies interfaces with other systems and establishes the system limits for analysis as well as identifying areas for interaction between the model and other fault trees.

Determination of Support System Interfaces. Through the generation of a Deper.dency Matrix, the interfaces with support systems such as electric power, control power, instrument air, cooling water, etc. are identified.

Generation of Test and Maintenance Matrices. Surveillance testing and preventative maintenance performed at power can be a noteworthy source of C-6

compo ient unavailability. The matrices identify such prncedures, and provide information needed te evaluate their impact upon component unavailability.

Operating Experience Review. Often, plant operation will identify unforeseen component failure modes or unexpected operating evolutions. Generally these items are not addressed within design documentation, so that a review of plant operatin;; experience is needed to identify and incorporate these failures and evolutions where they impact unavailability.

Identification of Differences between Units 1 and 2. For Nonh Anna, there are few intentional differences between the units, nevertheless, a concerted effort to evaluate dependencies, procedures, etc., is needed to verify similarity or to identify differences so that a unique Unit 2 model can be generated.

C 3.1 System Success Criteria The system success criteria form the basis for the top event of each fault trec. The fellowing Service Water system success criterion has been established:

Provide sufficient water '.hrough SW system header "A" to provide adequate cooling for the RS heat exchangerc control room air conditioning condensers, charging pump seal coolers, y educers, ano lube oil coolers for a 24-hour mission time. In order to meet u. -: functional requirements, one pump and one supply header are required for one unit. Operation of the second unit requires only one additional pump.

C.3.2 Deselopment of Simplit d Schematic The SW System simplified drawing for the SW fault tree is provided in Figure C-1. This simplified schematic was used as the basis for fault tree. development for both Unit I and Unit C-7 1

1 HALUBURTON NUS

. .. ~. . - - -. - -. - .

s j 2 since the SW System is shared between the two units. This schematic was developed by taking Figure 1 -2 from training manual #13 and detailing it per the following drawings:

i j fddirig Revision ll715-Fht-078A Sh.1 35  ;

11715-Fht 078A Sh. 3 26 i1715-Fht-078A Sh. 4 41

11715 FM-078B Sh. I 18 11715 Fht-078C c 2 31

, i1715 Fht-078G Sh. I i1

4 12050-Pht-077A Sh. I 16 i 12050-Fht-077A Sh. 2 13 i

]

C.3.3 Support System Interfaces The SW fault tree support system interfaces are identified with Dependency Matrices. The i

j dependency matrices for the SW System are obtained from the IPE system notebook. All SW j suppoi; systems are modelled based on these dependency matrices.

I 4

, Test and Maintenance Matrices 1

i The Test and Maintenance matrices serve two functions. First, they summarize a review of North Anna Periodic Test and Preventive Maintenance procedures to identify scheduled PT's or PM's at power. These events can lead to component unavailability and can be modeled in the j fault trees with the TM (Scheduled Test and Maintenance) fault. Second, the Test and Maintenance matrices provide input to the Data Base task for quantifying TM faults identified 4

by tne System Analysts.

]- The Test and Maintenance matrices for the Service Water System components is provided in the i

IPE system notebook. From these matrices, it can be seen that TM faults are not required for 4

any_of the components because none of the Periodic Testing procedures require any components C-8

of the SW trains to be made inoperable durin ro c erations. No Prevcntative Maintenance procedures were identified for the SW componentt Uascheduled maintenance will be included in we fault tree based on historical experience at North Anna. Procedures performed only during refueling or cold shutdown were autematically excluded from consideration.

Operating Experience Review Nonh Anna Unit 1 and Unit 2 Licensee Events Peports (LERs) were reviewed to identify operating experience that may be of value to systei aodeling. The review was aimed towards identifying Common Cause Faults (CCFs) or unusual operating modes that could affect system reliability. The LER n-view covered the last five years of operation. None of the LERs reviewed provided additional faults that neede. n explicitly included in the fault tree.

Fault Tree Modelling Assumpt ns This section describes the assumptions used to model the SW fault trees.

1. For valves that are passive (normally open) components in standby, the plugged (PG or PL) failure mode applies. For components that are normally closed (N.C.) plugging is not explicitly modeled, but is assumed to be included in the fait closed (FC) parameter.

The origin of this assumption is NUREG/CR-1363. " Data Summaries of LERs of Valves .

at U S. Commercial Nuclear Power Plants," which cescribes valve plugging as an event that would stop or limit flow through a normally open valve. Since standby check valves are norma 3y closed, no Type 2 (standby) plugging of check valves (CKV) is assumed.

Type 4 plugging failures of, normally open manual valves is not modeled. These faults are considered to be of low probability compared to plugging in standby.

1 Comman cause failures are generally applied only to standby components. Exceptions to this mle are noted where applicable. Using this assumption, common cause failures C-9 i

(CCFs) are not applied to a 2 train system where 1 train is running and I train is in standby.

/ . A 24-hour mission time is assumed for all running components (NUPRA Failure Mode 4). For evaluation of the reliability of the SW system during any 7 day period, the mission time for all pertinent time dependent failures were increased to 7 d.ys.

5. I A Service Water Pumps (1-SW-P-1 A and 2-SW-P-1 A) are assumed to be the normally I running pumps for the SW System. Pumps 1-SW-P-1B and 2-SW-P-1B r.re assumed to be in standby. It is also assumed that the SW pumps are alternated every month. Pump 1-SW-P-1 A is assumed to be aligned to service water supply header 1(A). Pump 2-SW-P-1 A is assumed to be aligned to service water su;,;ly header 2(B). Similarly, the auxiliary service water pumps are assumed to be in standby with one pump aligned to each alternate service water header.

6. Per prior assumption, manual valves in running trains do not experience plugging faults.

7. Per prior assumption, manual valves in standby trains are assumed to be subject to plugging faults if they are normally open.

8. Failure Mode 2 events for the standby manual valves on the outlet of the SW standby pumps and the CW pumps assume a 720 hour0.00833 days <br />0.2 hours <br />0.00119 weeks <br />2.7396e-4 months <br /> interval between valve verification. It is assumed that the SW pumps are alternate every month, thus allowing the valves to be checked once a month. The CW pumps are run at irregular time intervals so it is hard to set an exact time interval but a month's time frame should cover at least one traveling screen wash.

9. The general guidelines for the fault tree analysis is that if an acti <e failure mode is postulated for an MOV, there is no reason to include a plugging failure mode also.

4 C-10

10. SW pumps will start automatically on receipt of an SI signal from either unit or a loss of reserve station power provided that all the following conditions are saet:

1

1) . SW pump handswitch is in Auto after Start position

2) no motor overload

3) no undervoltage or degraded voltage It is assumed for the fault tree that these conditions are met.-

4

11. According to the UFSAR, the water stored in the service water reservoir can provide service water for extended periods for four units should the normal makeup pumps be .

inoperative. Enough water is available to guarantee 30 days'of operation for four. units without makeup. It was originally planned for North Anna to contain four operating units but only two were built. The reservoir was still designed to provide for four units providing more than sufficient water for the two units and, therefore, loss of servin water will not be modeled as a fault for this system.

12. It has been determined, based on operating history, that the traveling . screens to the service water pumps could become plugged within a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> period. Therefore, the traveling screes o the SW pumps need to be modeled. The CW traveling screens, used by the Auxiliary SW System, can become plugged with fish. These screens will be modeled as undeveloped events; one for each screen type. A common cause event will also be defined for the SW screenwells and the CW screenwells.

13. The service water air subsystem supplies air for the reservoir level indicator and for the -

traveling screen differential pressure indicators. Since is has been determined that the level of the reservoir is guaranteed adequate for 30 days, the level indicator will not be' considered ne casary to be modeled for the Service Water System.

14 The self-cleaning strainers (2-CW-S-3A/B) are apparently cleaned by divetting some of C-11 l

J the screen wash flow past the surface of the strainer through 2-CW-MOV-204A/B. It is assutned that the closure of these valves lead to plugging of the strainers.

15. During the periodic testing for the SW and Auxiliary SW pumps, certain valves are realigned. A restoration fault for these valves was not included in the fault tree because the procedure includes a check-off list that should verify that these valves are rettored to their previous position and these valves are also visually inspected monthly to veify that their position is correct, i

16. Opening the cro s-tie between the two Auxiliary SW pumps was not modeled as a j recovery action for the loss of a pump because as part of the success criteria it is I necessary to have SW available to both units. Therefore, it was decided that each

Auxiliary SW pump would be available as an alternate supply of SW for anly one unit.

17. The training manual for the CW system states that; "Normally the screen wash pumps are secured. The operator monitors screen differential level on a periodic basis and, at an indicated level of 2 to 3 inches of water, manually initiates washing of the individual screen."

i Based on this information, the screen wash pumps and traveling screens were modeled to start running when manually started and not through the use of the automatic timer.

18. SW pumps 2-SW-P-1 A and 2-SW-P-4 are assumed to be dedicated to Unit 2 and no credit for their availability is taken for Unit 1.

19. Per 1-MOP-49.07, for one supply header operation, 3 SW pumps are aligned to the operable supply header, with alignment of the fourth pump optional. In this analysis it is assumed that all four pumps are aligned to the operable h:ader.

C-12

~ - - . , -

c -!

4

! 1

! l When one h:ader operation, the following additional ~ assumptions have been made: -

J

! l i

_ (1) 3 out of 4 SW pumps and both auxiliary SW pumps are available (i.e., no maintenance is scheduled for 3 out of 4 SW pumps and .

both auxiliary SW pumps,'during the one header operation).

i l

(2) No maintenance activity is schedule for Electrical Emergency Bus l

[ lH. 2H, IJ, and 2J.

l (3) No maintenance activities on the.DGs associated with the pumps i

!- taken credit for to satisfy assumption 1.

l~ SW Induced Change In Core Damage Frequency i

The loss of SW System contributes to the plant operational risk under two failure scenarios. The l

j first scenario is the loss of service water system during an accident. That is, after occurrence l cf an accident inducing plant trip and during the cooldown process, the'SW System may fail to provide its intended function. The second scenarios is the loss of SW System during normal-operation of the plant, resulting in a plant trip. This analysis will address contribution from both

{-

[ these risk inducing scenarios.

[ Loss of SW During an Accident (CASE 1) i i

l: The North Anna Power Station (NAPS) Incividual Plant Examination (IPE) project models several accidents which would r.ecessitate tripping of the plant (Called Initiating Events-(IE)).-

j Upon tripping of the plant (i.e., occurrence of an IE), several systems and measures (accident j mitigating systems / measures) are taken credit for as being functional to achieve successful safe

~

f- shutdown of the affected umt The IPE models also evaluate the probability of failure of these

, accident mitigating systems / measures. Service water system acts as a support system for several.

Accident Mitigating Systems (AMS) and its failure will render the AMS ineffective. Thus, any

! C-13 I

,,w. < 4,w-. , .-.- +.c -.,...e , , 4* w - = c , -, , . - ~ - . ~ a

degradation in the configuration of the SW System that will decrease the reliability of the system, will in turn affect the availability of the AMS. The accident mitigating system / measures which are modeled as being supported by the SW system are shown in Table C-1.

The change in risk, due to the increase in the unreliability of the SW System induced by the one loop operation of the system is given by:

Delta CDFsu(for unit 2 only)=(CDF2 n - CDFi n)

• f where

=

CDFsu Change in CDF induced by the increase in unreliability of the SW System

=

CDFn 2 Core Damage Frequency when in Two Header Operation .

CDFn i = Core Damage Frequency when in One Header Operation f =

Fraction of a Year in One Header Operation The CDF 2w, and CDF i n, are evaluated by quantification of the latest IPE model for NAPS core damage frequency.

' Loss of SW During Normal Operation (CASE 2)

The NAPS IPE project also evaluates the CDF as a result of loss of service water when no other IE has occurred. SW system supports several normally operating (eg component cooling water system) and standby systems (eg recirculation spray system). Loss of SW is considered to lead to a reactor trip similar to the loss of emergency switchgear room cooling and is designated as T6 initiating event.

The one header operation induced change in CDF can be evaluated by:

I C-14 l.

- - - . _ ____-_______.___m______m__-._-_._..____ _ _______.._ _____..___. _.-_ __-_ - _____..___m_-.._ _..______

i) Evaluating the frequency of the T6 initiating event, based on the historical data

{

(i.e. frequency of T6 as used in the IPE project) . )

I ii)- Evaluating the contribution of the T6 Initiating Event (IE) to CDF with the frequency of T6 as evaluated in step (i), Case *F. This step is carried out by quantification of the T6 IE event tree.

i iii) Evaluating the frequency of the T6 initiating event, assuming that the system would be in one header operation for an entire year.

iv) Evaluating the contribution of the T6 initiating event to CDF with the frequency of T6 as evaluated in step (iii), Ci a*' and requantification of the T6 IE event i .

tree.

iv) Evaluating the change in the CDF by subtracting C inCDP[79g { useGP, Mathematically, these steps are presented by

Delta CDFr. = (C n*' - Ci n9 2

1 l where

= Change in CDF induced by the increase in the frequency of T6 CDFr.

initiating event 4

Service Water Reliability (CASE 3)

J In addition v the above failure scenarios, the SW reliability in two (CASE 3A) and one header (CASE 3B) operation modes, for any 7 day mission time, was also evaluated. The purpose of-this evaluation veas:

1) To identify the most significant contributors to the increase in the unreliability of the SW system during one header operation and recommend mitigating measures 4

C-15

to reduce the unreliability, i

4

-1 o

h

.e e

C . . .

4 J

J C-4 Evaluation of the Impact of the SWPP PRA Reconunendstions on the Change in CDF P

The SWPP PRA identified several modifications which if implemented would reduce the nsk

associated with the pipe preservation project even further. This section describes the proposed

modifications and the methodology used to evaluate their impact.

f

, C-4.1 RECOM.1- Providing Backup Cooling to the Unit 2 Emergency Switch Gear i Room when in One Header Operation.

One of the most important functions of the SW system is to support the ESGR HVAC system.

NAPS IPE project has identified that, under the present plant configuration, loss of SW system

) will render the ESGR HVAC system unavailable, which will have severe adverse effect on the plant accident mitigating capabilities. During normal operation of the plant (2 header operation),

due to high reliability of the SW system, the loss of the ESGR HVAC caused by the loss of SW system is not considered as a likely event. In one header operation mode, on the other hand, j the probability of the loss of SW system, and consequently the loss of ESGR HVAC system, will increase significantly. However, operation of the SW system in one header configuration is limited and the SWPP PRA study indicates that the impact on the plant operational risk is not

! significant. Additionally, during one header operation, the Fire Protection System (FPS) is

aligned to be used as a back up for the SW system to the HHSI pumps component coolers (AP

} 12).

! However, if ESGR is lost, the HHSI pumps will trip rendering the AP 12 measures ineffective.

Thus any operational, procedural, or physical modification of the plant that will reduce the probability of the loss of ESGR (due to excessive heat) in an event of SW failure will reduce the risk even further. Using the Bearing Cooling System (BCS) as a backup for the SW system to the ESGR HVAC chillers is an option which is recommended and its effect has been evaluated in this study. .

C-17

Using the BCS training module and pertinent drawings a fault tree model for the BCS was developed and the unavailability of the BCS was evaluated. Then, the loss of SW initiating event event tree (as modelled in the NAPS IPE) was modified to account for the recovery of the ESGR :ooling and the charging pump coolers (using FPS which its probability of failure was also modelled and quantified). The fault trees for the BCS and FPS are shown in Figure C-3 and C-4 respectively, and the modified event tree is shown in Figure C-5. The contribution of the loss of ESGR cooling to the CDP was then evaluated using the modified event tree. The contribution decrease by factor of 1.7.

t C.4.2 RECOM. 2 Providing The Pipe Repair Capability in Tne Auxiliary Building.

3 The NAPS IPE identified the pipe rupture in the auxiliary building as the most significant contributor to the loss of SW system when in one huder operation. The SWPP PRA further identified that providing the FPS as a backup system for the SW system to the charging (HHSI) pumps' coolers will be less effective, if the SW rupture occurs in the Auxiliary Building, since the ability to discharge the water could be lost in some occasions.

The NAPS IPE has also identified all the SW component which their postulated failure could lead to the loss of SW system. A list these components are shown in Table C-2. As evident from the table the most significant contributors are located in the auxiliary building. Any measure which reduces the probability of these component failing will reduce the loss of SW system initiating event contribution to the CDF. A possible measure is to provide dedicat:d and trained tean. of repair personnel with proper instructions, tools and components to repair ruptured components. Since the postulated ruptured components are of 10 and 4 inch diameter this measure is deemed to be feasible. However, credit is taken only for those rupture which are not double ended ruptures. The result of such a measure would be to reduce the initiating event frequency. Table C-3 shows the modified contribution of the SW piping rupture to the T6 initiating event. Using the SW initiating event fault tree the T6 initiating event frequency was re-evaluated and using the IPE T6 initiating cvent event tree the contribution to CDF was calculated. The contribution of the T6 initiating event, when in one header operation, after the C-18 l

I implementation of the above modification is 1.42E-5 compared to 4.65E-5 which is the contribution of the T6 initiating event frequency, in one header operation, before the modification.

C.4.3 RECOM. I and RECOM. 2 The most beneficial measure is to implement both of the modifications described above. The impact of the both modifications was evaluated by taking the T6 initiating event frequency as calculated for RECOM. 2 and re< valuate the contribution to CDF using tne modified T6 event tree developed for RECOM.1. The resulting contribution to the CDF was found to be 8.53E-6.

d e

f 4

t l

i i

i 4

4 e

C-19

TABLE C-1 Accident Mitigating Systems / Measures Supported by the SW System SYSTDI IP.E FAULT TREE l-Component Cooling Water (CCW) System CC100 2-Chemical Addition System CH100 3-Feed and Bleed Measure FB400 4-High Head Safety Injection System HH100 5-Emergency Switch Gear Room Cooling HV100 6-Recirculation Spray System RS100 c

C-20

\

Table C-2 '

SW Piping Sections Contributing to T6 IE Frequency ~

Components Best -

4 1- 2 36" dia. EJ on the SW Header in the SWPH 6.5E-07 2- 36" dia. Piping up to the Point the Piping 1.6E-08 Enters the ground 3- 4" dia. Piping Supply to the Unit 2 Air Cond. 4.2E-06 Unit up to the Isolation Valve 4 2" dia. Manual Isolation Valve for the Unit 2 6.3E-06 Air Cond.- Unit 5- One 24" dia. MOV on the Aux. SW Supply to the 1.3E-05 ,

Normal Supply Header 6- Piping Section Between the Aux. SW Isolation ' l .7E-06 Valve and the 24" Normal Supply Header 7- 4" dia. Man. Isolation Valve to the Charging 6.3E-06 8- 4" dia. Piping Downstream of the Manual l 4.9E-06 Isolation Valve for the SW Supply to the Charging-Lube Oil Coolers 9- 4" dia. Man. Isolation Valve on the Return from 2.5E-06 the Charging Pumps Lube Oil Coolers 10- 4" dia. Piping Upstream of the Manual Isolation 2.0E-6 Valve on the Return Header from SW Supply to the Charging Pumps Lube Oil Coolers Il- 10" dia. Isolation MOV for the Return Header 8.3E-05 from the SW Supply to the CCW Fuel Pit Coolers 12- 10" dia. Piping Upstream of the Isol. MOV for 1. lE-05 the Return Header from the SW Supply to the CCW Fuel Pit Coolers 13- 24" dia. Isolation MOV on the Return Header to 3.5E-06 CW Discharge Tunnel -

1 C-21

Table C-2 Continued SW Piping Sections Contributing to T6 IE Frequency Components Best 14 - 24" dia. Piping from the Normal Peturn Header up 3.4E-07 to the Isolation MOV on the Return Header to CW Discharge Tunnel 15- 4' dia. Isolation Valve for Return Header from 6.3 E-07 "

the Unit 1 Air Ccnd. Unit 16- 4" dia. Piping Between the Retura Isolation 1. l E-06 Valve for the Unit 1 Air Cond. Unit and the

, Normal Return Header 1

C-22

l' i

4

(

l '

Table C-3 *

SW Piping Sections Contributing to T6 IE if Recom,2 is Implemeated-

[

l l-l Frequency

[

i Components Best -

j 1- 2 36" dia. EI on the SW IIeader in the SWPH 6.5E-07 l .

2- 36" dia. Piping up to the Point the Piping Enters 1.6E-08 l the Ground l

l 3- 4" dia. Piping Supply to the Unit 2 Air Cond. Unit 4.2E-06 l up to the Isolation Valve-l

! 4- 4" dia. Manual Isolation Valve for th: Unit 2 Air Cond. Unit 6.3E-06 -

i.

1 l 5- one 24" dia. MOV on the Aux. SW Supply to the Normal Supply Header 1.3E-05 L

l 6- Piping Section Between the Aux. SW Isolation Valv~e and the 24"

. 1.7E-06

: Normal Supply Header i

7- 4" dia. Man, Isolation Valve to the Charging Pumps Lube Oil Coolers 6.3E-07 ,

!. 8- 4" dia. Piping Downstream of the Manual Isolation Valve for the 4.9E-07 l . SW Supply to the Charging Lube Oil Coolers

~

9- 4" dia. Man. Isolation Valve on the Return from the Charging 2.5E-07

i. Pumps Lube Oil Coolers

[ 10- 4" dia, Piping Upstream of the Manual Isolation Valve on the - 2.0E-07

j. Return Header from SW Supply to the Charging Pumps Lube Oil Coolers -

t

! 11- 10" dia.. Isolation MOV for the Return Header from the SW Supply' 8.3E-06

. to the CCW Fuel Pit Coolers

! 12- 10" dia. Piping Upstream _ of the Isol. MOV for the Return Header 1. lE-6

[

from the SW Supply to the CCW Fuel Pit Coolers 1 l 13- 24" dia. Isolation MOV on the Return Header to CW Discharge Tunnel 3.5E-06 i- CW Discharge Tunnel i

i '

[ 14- 24" dia. Piping from the Normal Return Header up to the Isolation 3.4 E MOV on the Return Header to CW Discharge Tunnel l

4 i

f C-23

_ _ _ . . . __ . . .. _, __ . w _ . _ ,.- .-_..- _ _ . . -- _ _ ..

4-

I i Table C-3 Continued l

! SW Piping Sections Contributing to T6 IE  !

l if Recom 2 is Implemented l

! Frequency i_ Components Best '

4 j; 15- 4 dia. Isolation Valve for Return Header from the Unit 1 Air-. 6.3E-07 i-Cond. Unit I 16- _4" dia Piping Between the Return Isolation Valve for the Unit i 1.1E-06 Air Cond. Unit and the Normal Return Header i

l Total Rupture Frequency per year 4.2E-05

{

3 h '

l I

l' 1

i i '

I i

i-i p.

4 I'

r

?,

)

i; h

! C-24 e

r*v-- s , - - , ,y-** ~..u- er y -v - vw w -v w ,-- -e+

Figure C-1 Simpli'ied Service _ Water System Diagram e

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C-25

0 l 1'_' l 2 I 1 l 4 j $

SEAVICE WATER SYSTEN 0

NAPS UNIT 1 320MAF,N.13 bWI $

asat s s t sa l Mat:m ca E- 35-27-a? aEvMIN  %-N-72 1

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I=ET AatE Se FLC# INCf AATE Se Fi%m FECu 4&CER a g anta e 4

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