ML20205L243

SFP Soluble Boron Credit Dilution Analysis (Summary of Applicable Portions)

ML20205L243
Person / Time
Site:	Mcguire, McGuire
Issue date:	04/05/1999
From:	DUKE POWER CO.
To:
Shared Package
ML20205L214	List: ML20205L209 ML20205L225 ML20205L234 ML20205L243 ML20205L253
References
NUDOCS 9904140196
Download: ML20205L243 (36)

Text

ATTACHMENT 7 MCGUIRE NUCLEAR STATION 1

SPENT FUEL POOL SOLUBLE BORON CREDIT BORON DILUTION ANALYSIS

(

SUMMARY

OF APPLICABLE PORTIONS)

D AO 9 P ,

PDR ,,

l l l Attachment 7 L Page 1 of 35 Evaluation Of Potential Boron Dilution Accidents For The l

McGuire Spent Fuel Pools Table of Contents 1

i Section Page l

1.0 INTRODUCTION

/ BACKGROUND 2 l 2.0 ASSUMPTIONS 5 2.1 Unit 1 and Unit 2 Spent Fuel Pool Similarity 5 2.2 Boron Concentration 5 2.3 Spent Fuel Pool Water Level 6 2.4 Mixing Factors 7 2.4 Piping Break Sizes 7 3.0 IDENTIFICATION AND SCREENING OF DILUTION INITIATING EVENTS j 8

4.0 EVALUATION OF BOROF OILUTION TIMES AND VOLUMES 8 5.0 EVALUATION OF SFP DILUTION EVENTS 13 5.1 Pipe Breaks 13 5.2 Misalignment of Systems Interfacing with KF System 14 5.2.1 Dilution from Reactor Makeup Water Storage Tank 15 5.2.2 Dilution from the Recycle Holdup Tanks 15 5.2.3 Dilution from Demineralized Water (YM)

System 16 5.2.4 Dilution from the Recycle Monitor Tank 16 5.2.5 Dilution from Nuclear Service Water i System 17 I 5.2.6 KC/KF Heat Exchanger Leak 18 I 5.2.7 Dilution from Drinking Water System 19 I 5.2.8 Boron Removal by Spent Fuel Pool Demineralizer 19 5.2.9 Dilution from Fire Protection System 19 5.3 Loss of Off-Site Power 20 5.4 Evaluation of Infrequent Spent Fuel Pool Configurations 21 q I

6.0 RESULTS 23

7.0 CONCLUSION

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8.0 REFERENCES

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r Attachment 7 Page 2 of 35 I

l Evaluation Of Potential Boron Dilution Accidents For The McGuire Spent Fuel Pools l ~

1.O INTRODUCTION / BACKGROUND l The current criticality analysis for the McGuire Spent Fuel Pool (SFP) takes credit for a solid boron material in the fuel racks known as Boraflex. This material has unexpectedly degraded over time and has lead to a loss of boron in the material. As this degradation has continued, it has become necessary to reduce or eliminate credit for the solid boron in the racks in the criticality analysis. In order to continue l

meeting criticality design criteria, it is necessary to take

credit for soluble boron contained in the SFP water. This calculation will evaluate potential accidents that could add significant amounts of unborated water to the Spent Fuel Pool causing dilution of the pool boron concentration. This l calculation will evaluate the minimum possible boron concentration which could result from a credible boron dilution i l

t accident event. The results will also provide timing estimates of boron concentrations resulting from these accidents.

This analysis is related to reactivity management for the McGuire Spent Fuel Pools. Although no plant operational l parameters or design features are affected by this calculation, it is an input to another calculation of the reactivity impact of various postulated accidents in the McGuire Spent Fuel Pool l

(Attachment 6). The criticality analysis which takes credit for soluble boron is also required to address a bounding case with complete loss of all soluble boron to show that the value of kere remains less than 1.0.

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Attachment 7 Page 3 of 35 The overall governing methodology for crediting soluble boron j is described in WCAP-14416-NP-A (Reference 1). This approach requires that a boron dilution analysis be performed to ensure that sufficient time is available to detect and mitigate the l dilution before the 0.95 kerr design basis criterion is l exceeded. This approach further states that the dilution analysis should include an evaluation of the following plant- 1 l

specific features:

1. Spent Fuel Pool and Related System Features  !

1

• Dilution Sources

• Dilution Flow Rates

• Boration sources I

• Instrumentation l l

• Administrative Procedures i

• Piping

• Loss of Off-Site Power Impact

2. Boron Dilution Initiating Events (including operator error)

3. Boron Dilution Times and Volumes The staff has concluded that the new methodology in WCAP-14416 can be used in licensing actions. All licensees proposing to use the new method for soluble boron credit should identify potential events which could dilute the spent fuel pool boron to the concentration required to maintain the 0.95 kee r limit and should quantify the time span of these dilution events to show that sufficient time is available to enable adequate detection and suppression of any dilution event. The effects of incomplete boron mixing should be considered.

":e methodology employed uses four basic steps:

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1. Develop Preliminary List of Potential Events

2. Screen Events that are not Credible or are Irrelevant

3. Evaluate Events for Dilution Times and Volumes

4. Summarize Results and Conclusions A preliminary list of events for review was developed through the review of several industry studies and review of the design l i

of the McGuire Spent Fuel Pool and related systems. A plant  !

i walkdown was conducted to examine SFP structural features and l l

the spatial relationships between the SFP and related plant  !

systems. Furthermore, a review of industry operating experience was conducted to check for possible failures modes not previously considered. Many types of postulated events were screened out because they lead to consequences different than deboration, and others were screened out because they are not credible with the McGuire pool design.

Events which were not initially screened out were evaluated further to determine the potential impact of those events on pool boron concentration. In some cases, the accident source of unborated water comes from a finite source that is relatively small compared to the volume of the pool. These events were evaluated to show the resulting boron concentration if the entire source were added to the pool. On the other hand, some sources of unborated water could come from continuously flowing systems. These " infinite" water sources were evaluated for the highest flow rate as the bounding case.

Events involving continuously flowing systems are also evaluated to determine the available time for operator action l to show that sufficient time is available to terminate the flow into the pool.

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r Attachment 7 Page 5 of 35 A number of important assumptions were made to perform this assessment. Most of the major assumptions are discussed below.

2.0 ASSUMPTIONS 2.1 Unit 1 and Unit 2 Spent Fuel Pool Similarity The layout and overall dimensions of the Unit 1 and Unit 2

! Spent Fuel Pools are the same except that each is a " mirror image" of the other. As a result, the estimated volumes are i also the same. No significant differences were found in design parameters between the interfacing systems for each unit.

Although there were some differences in piping layout around the pool areas, no differences in the piping system were found that would have any obvious effect on the rate or magnitude of dilution in either pool. Therefore, only one set of calculations is made and the results are applicable to both McGuire Spent Fuel Pools.

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2.2 Boron Concentration l The initial pool boron concentration is conservatively assumed to be 2475 ppm. This corresponds to the COLR limit for McGuire Unit 1 Cycle 12 which is the lowest limit currently in use at McGuire. However, the Unit 1 Cycle 13 limit is scheduled to be raised to 2675 ppm matching the current limit for McGuire Unit

2. Choosing the lower value provides some additional safety i i

margin as well as allows the COLR limit to be lowered for future reactor designs (if needed or desired) without impacting this analysis. Based on the double contingency pri ciple, it is not necessary to postulate that the pool boron c..tcentration l

is below its TS minimum concentration concurrently with a 1

second evuat that puts a large volume of unborated water into the pool (Reference 1).

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Attachment 7 Page 6 of 35 2.3 Spent Fuel Pool Water Level The initial pool water level is assuced to be at the normal level at elevation 771' + 4.75". The volume of water in the Spent Fuel Pool at this water level is 43,108 cubic feet, which corresponds to approximately 322,450 gallons. This value will be used for the initial water volume in dilution calculations.

This volume includes the cask Jo Jing pit and the fuel transfer canal, but excludes the volume of water within the fuel pin area. It also excludes the volume in the gate openings between ,

the main pool and the transfer canal and between the main pool i and the cask loading pit. The Tech Spec minimum level is 23 feet above the fuel, which corresponds to an elevation of 769'.

Again due to the double contingency principle, it is not necessary to postulate that the spent fuel pool level is below its normal level concurrently with a second event that puts a l large volume of unborated water into the pool. Furthermore, the additional volume of water in the fuel pin area should more i than account for an' slight level variations that might occur prior to a postulated boron dilution event. Thus it is l concluded that the assumption of normal pool level with 322,450 j gallons of water volume is acceptable.

Note that SFP level is not measured using the control room instrument but rather by a physical marking on the pool wall for the purposes of normal routine surveillance and normal makeup to the SFP for evaporation. The control room SFP level instrument instead serves to provide a high and low level alarm function. Given that such a physical marking is not subject to

" instrument drift" and that the water volume estimate is l

conservative, it is unnecessary to account for " instrument error" in the water volume estimation.

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i Attachment 7 Page 7 of 35 This analysis is only addressing dilution events where there is the potential to add large amounts of unborated water to the SFP. Events involving a large loss of SFP coolant inventory are not evaluated for boron dilution from emergency makeup used to restore SFP level. Certain catastrophic failures of the pool ~could result in a large loss of SFP inventory that could cause a zircaloy cladding fire. However, it is assumed that plant procedures will address boron addition as a part of the emergency makeup response. In addition, the new SFP criticality analysis will examine a case where there is no soluble boron in the SFP. Emergency makeup without boration could lead to a loss of all boron and thus a loss of the 5%

safety margin; however, the "no boron" case will show that keer

- will remain still less than 1.0.

2.4' Mixing Factors It is conservatively assumed that any unborated water that enters the pool will mix completely with the existing water in the pool. Complete mixing generally maximizes the rate of boron c',ilution . This assumption is consistent with the approach used in Reference 2 and in similar licensing I submittals made by other licensees. l Good mixing is expected for the dilution events of interest. ,

1 Operation of the KF system in conjunction with thermal mixing

. of warmer water rising from the fuel help ensure good mixing in i the pool. Specifically, the KF pumps continuously recirculate  !

l i approximately 1000 gpm from the South end of the main pool to i the North end. Also the Spent Fuel Pool Skimmer Pump provides l an additional 100 gpm of flow from the South end of the pool i back to the opposite ends of the main pool, fuel transfer canal and cask loading pit. Partial mixing may occur in cases where a pipe breaks in the pool area and causes the pool to overflow.

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Attachment 7 Page 8 of 35 In this case, the water entering the pool may not fully mix with the rest of the pool inventory before exiting the pool.

Partial mixing in this case would serve only to slow-down the dilution of the rest of the pool. The potential for " pockets" of lower boron concentration are bounded by the "no boron" criticality case and do not need to be considered further.

2.5 Piping Break Sizes For random piping breaks, the break size is determined using the method in FSAR Section 3.6.2.2. While high-energy systems must consider double-ended pipe breaks, moderate energy systems are only required to assume through-wall cracks. The through-wall crack break area considered for this event is based on a length equal to one-half the nominal inside diameter and a width equal to one-half the minimum wall thickness of the system piping material.

For this~ assessment, piping breaks caused by seismic or tornado events are also considered for non-seismic piping or piping not protected from tornado winds or missiles. For these breaks a larger through-wall crack size was assumed than for random break events. The through-wall crack break area assumed for these events ic based on a length equal to the circumference of the pipe at its inside diameter and a width equal to one-half the minimum wall thicknees of the system piping material.

3.0 Identification and Screening of Dilution Initiating Events A preliminary list of events for review was developed through  !

! the review of several industry studies (References 2 and 3) and l

l review of the McGuire Spent Fuel Pool and related systems.

Table 1 provides a listing of the types of events considered and how these events were dispositioned. Many types of postulated events were screened out because thcy lead to i

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Attachment 7 Page 9 of 35 l

l-consequences different than boron dilution, and others were screened out because they are not credible with the McGuire pool design.

l 4.0 Evaluation of Boron Dilution Times and Volumes l

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'In order to determine the boron concentration for various flow

! rates and volumes it is necessary to examine the dimensions and configuration of the spent fuel pool. A sketch of the Unit 1 SFP is provided in Figure 1. The pool consists of three j connected compartments: the main pool area where fuel is stored, the cask loading area, and the transfer canal area.

Normally all three areas are connected, but gates can be installed for infrequent activities such as maintenance on the "upender" in the Transfer Canal, or the loading or unloading of a cask in the cask loading area. For the base case analysis, the initial pool volume is 322,450 gallons which includes all three areas (i.e., gates removed). Other modes are evaluated separately.

All of the events to be evaluated involve the addition of unborated water to the existing water volume. It is important to note that the normal water level (771' + 4.75") is well below the top of Spent Fuel Pool operating floor (Elevation 778'+10"). Since no water is assumed to flow out of the pool at the initiation of a dilution event, unborated water enters the pool and [ fills the pool continuously until it reaches the top of the pool and overflows. Figure 1 provides an illustration of the various water and pool elevations.

Three stages of boron dilution flow are examined. The first stage involves filling up the pool to the top of the Transfer Canal wall at elevation 773' + 6". The second stage involves filling the pool from the top of the Transfer Canal wall up to

Attachment 7 Page 10 of 35 the top of the pool operational deck at elevation 778' + 10".

The third stage involves the flow of unborated water into the

, pool with an equal amount of the diluted mixture flowing out of l \

l the pool into the lower areas of the Spent Fuel Pool Building.

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l The vclume of water required to fill the pool up to the top of the Transfer Canal wall is 23,995 gallons. The volume of water required to fill the pool from tho top of the Transfer Canal wall up to the top of the pool is 68,029 gallons.

The pool boron concentration at the end of stage 1 (C1 ) is J found using:

Co* Vo ,

Cn = '

Vo + Vc where Co = Initial Pool Boron Concentration (2475 ppm),

Vo = Initial Pool Water Volume (322,450 gallons), and Vc = Volume of water to fill to top of Transfer Canal Wall (23,995 gallons)

This yields a value for C3 ef 2304 ppm. The lengt;h of time to reach this concentration is dependent on the dilution flow rate into the pool. This length of time can be found by dividing Ve by the flow rate. Table 2 provides a listing of times required to fill the pool to the top of the Transfer Canal Wall for various flow rates. To find the pool concentration at any specific time during stage 1, the following equation is used:

Co* Vo C=

V, + (Q*60* t) where Co = Initial Pool Boron Concentration (2475 ppm),

Vo = Initial Pool Water Volume (322,450 gallons),

f Q = Flow rate into Pool (gpm),

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Attachment 7 Page 11 of 35 l

t = Length of time af ter initiation of dilution flow (hours), and 60 = Conversion factor for converting hours to  !

i minutes.

The pool boron concentration at the end of stage 2 (C2) is I found using:

Co*%

C2 = \G + % + VT where C6 = Initial Pool Boron Concentration (2475 ppm), I V6 = Initial Pool Water Volume (322,450 gallons),

Ve = Volume of water to fill to top of Transfer Canal Wall (23,995 gallons)

Vr = Volume to fill from Canal "all to Top of Pool (68,029 gallons)

This yields a value for C2 of 1925 ppm. The length of time to reach this concentration is dependent on the dilution flow rate into the pool. This length of time can be found by dividing the sum of Vc and V7 by the flow rate. Table 1 provides a listing of times required to fill the pool to the )

1 top for various flow rates. To find the pool concentration i at any specific time during stage 2, the following equation is used:

Co*\G C=

% + % + (Q*60* (t - tc)) )

where O = Flow rate into Pool (gpm),

te = Length of time to fill to top of Transfer Canal Wall (hours),

l t = Length of time after initiation of dilution flow (hours), and

Attachment 7 Page 12 of 35 60 = Conversion factor for converting hours to I I

minutes.

• By definition, t must be greater than te and less than tr. Value of tc and tr are prt ded in Table 2.

After the pool reaches stage 3 where the pool is overflowing, the boron concentration is found using, i

C = C2 e(~o* X'-") j where C2 = equals the pool concentration at the end of stage 2 (1925 ppm)  ;

O = Flow rate into Pool (gpm),

Vu = Total SFP Mixing Volume (Vo+Ve+Vr=414,474 gal) I tr = Length of time to fill to top of pool (hours),

t = Length of time after initiation of dilution flow ,

1 (hours), and Using the equations above, the pool boron concentration was estimated for a range of flow rates for various times from 1 to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> with the results presented in Table 1.

In some of the events evaluated, the source of dilution flow l is defined by a fixed volume instead of a continuous dilution flow. If the total volume added to the pool does not overflow the pool (less than 92 024 gallons), the pool boron concentration is found using-C,* %

C = V<, + V where Cc = Initial Pool Boron Concentration (2475 ppm),

V = Water Volume added to the pool (gallons), and Vo = Initial Pool Water Volume (322,450 gallons).

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Attachment 7 Page 13 of 35 If the total volume added to the pool does overflow the top of the pool (greater than 92,024 gallons), then the pool boron concentration is found using:

v C = C2 edv-92a24

= 24

)

where C2 = equals the pool concentration at the end of stage 2 (1925 ppm),

Vo = Initial Pool Water Volume (322,450 gallons),

V = Water Volume added to pool (gallons), and 92024 = number of gallons to fill pool to overflowing (Vc+Vr).

5.0 Evaluation of SFP Dilution Events  ;

5.1 Pipe Breaks Both McGuire Spent Fuel Pools are located at an elevation above all adjacent buildings. Pipe breaks in adjacent buildings or areas can not flow into the pool and are excluded. Through the review of plant drawings and a plant walkdown, piping for the following systems was identified in the SFP area that, if broken, could flow into the SFP:

YM - Deminerali:ced Water 2 5 inch 120 psig Supply YD - Drinking Water 1 inch 100 psig Supply WE - High Pressure Decon

• System Abandon In Place

• Water Note: KF system piping in the SFP area is excluded because it contains borated water. 1 i

Attachment 7 Page 14 of 35 l

i Besides being the largest and highest pressure line in the SFP area, the RF header is supplied by the RF pumps taking suction l from Lake Norman (an " infinite" source) For this reason, the RF line is taken to be the worst line break.

l The RF system is classified as a moderate energy system (FSAR Table 3-19). For random piping breaks of moderate energy systems, the size of the break is determined per the criteria provided in UFSAR Section 3.f For the 4" RF line, the piping material is Schedule 40 C trbon Steel (McGuire Piping Specification 154.1) which has a thickness of 0.~237" and an inside diameter of 4.026" For the RF line, the equivalent diameter is 0.551 inches and the system pressure is 150 psig. This results in a break flowrate of 111.1 gallons per minute.

Since the RF line is not seismically qualified, it is also evaluated for a larger through-wall crack size. Using a pipe thickness 0.237" and an inside diameter of 4.026", the break area is 1.50 square inches.

For the RF line seismic break, the equivalent diameter is 1.382 and the system pressure is 150 psig. This results in a break flowrate of approximately 700 gallons per minute.

Table 2 provides a tabulation of the resulting boron concentration over time from a 700 gpm dilution flow rate.

5.2 Misalignment of Systems Interfacing with KF System The potential exists for systems that interface (directly or l indirectly) with the KF system to become misal'igned due to operator errors or component malfunction or failure causing

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unborated water to be added to the Spent Fuel Pool. These interfacing systems are the Refueling Water (FW) System, Boron j Recycle (NB) System, Liquid Waste Recycle (WL) System, Chemical and Volume Control (NV) System, Makeup Demineralized Water (YM)

System, Filtered Water (YF) System, Drinking Water (YD) System, j Fire Protection (RF) System, Nuclear Service Water (RN) System, and Component Cooling Water (KC) System. The potential impact

! of these systems is evc.luated below. Attachment 2 provides additional information on the flow paths between these systems.

The SSF Standby Makeup Pump also connects to the SFP through the Fuel Transfer Tube; however, the impact of SSF operation will be examine later (Loss of Off-site Power discussion).

l 5.2.1 Dilution From Reactor Makeup Water Storage Tank While normal makeup to the Spent Fuel Pool is provided by the Refueling Water Storage Tank, an alternate makeup source is provided by the Boron Recycle (NB) System. This is l accomplished by aligning the Reactor Makeup Water (RMW) Pumps from the Reactor Makeup Water Storage Tank (RMWST) to discharge directly into the pool. The RMWST has a usable volume of 112,000 gallons and the RMW pumps have a capacity of 150 gpm each.

If an error occurred that inadvertently caused the entire volume of unborated water in the RMWST to be pumped into the SFP, the resulting boron concentration is 1834 ppm.

5.2.2 Dilution From The Recycle Holdup Tanks Another portion of the Boron Recycle (NB) System contains the Recycle Evaporator Feed Pumps and the Recycle Holdup Tanks I (RHT). There are two pumps (30 gpm each) and two tanks with a l

usable volume of 112,000 gallons each. There is not a direct connection between this source and the KF system or the SFP; I

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Attachment 7 Page 16 of 35 however, it is possible to pump this water into the pool indirectly by misaligning the Refueling Water (FW) system makeup line to the SFP through manual valves KF-81 and KF-83.

However, another path from the RHTs to the SFP would be to align the Recycle Evaporator Feed Pumps to the RMWST and to l " piggy-back" the RMW pumps into the SFP. This path is potentially worse because of the greater combined volume of l both Recycle Holdup Tanks and the RMWST. However, the flow I' rate is limited to 60 gpm by the two Recycle Evaporator Feed Pumps. The total volume of these tanks is 336,000 gallons l

(112,000+112,000+112,000). The maximum pool dilution resulting from this event is 1068 ppm.

5.2.3 Dilution From Domineralized Water (YM) System f While the normal makeup to the pool comes from the FWST, makeup water can also be added to the pool from the Demineralized i

! Water' (YM) System. There is not a direct connection between this source and the KF system or SFP; however, there are two indirect paths which could be used to add YM to the SFP.

First, it is possible to attach a hose to a YM connection in

the pool area.and run the hose a few feet over into the pool.

However, the flow rate is somewhat limited due to the smaller piping size. The second path is considered to be the worst case event in which the YM system is aligned through the RMWST.

This event conservatively assumes that a misalignment occurs in which YM is " piggy-backed" on the RMW pumps putting water into l' the pool. The volume of water is assumed to be the sum of all l the water available in . the YM system plus the volume of the RMWST. The volume of water available in the YM system is assumed to include both Demineralized Water Storage Tanks (1000 gallons each) and both Filtered Water Tanks (42,500 gallons each). The total volume of the all these tanks is 199,000 l

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T Attachment 7 Page 17 of 35 gallons (2,000+85,000+112,000). The maximum pool dilution L resulting from this event is 1489 ppm.

5.2.4 Dilution Frost The Recycle Monitor Tank Another source of makeup water to the RMWST comes from the

! Liquid Waste Recycle (WL) System. The Recycle Monitor Tank l

Pumps can be connected to pump the Recycle Monitor Tank (RMT) (

inventory into the RMWST. Since there is not a direct connection between this source and the KF system or SFP, it is l assumed to be misaligned where both RMT Pumps are ' piggy-backed" on the RMW pumps putting water into the pool. For this event the volume of water is assumed to be the sum of both RMTs (5,000 gallons each) and the volume of the RMWST (112,000).

1 The total volume of the all these tanks is 122,000 gallons (10,000+112,000). The maximum pool dilution resulting from this event is 1791 ppm.

5.2.5 Dilution Frosa Nuclear Service Water System The KF System is designed with a connection to the RN System "A" Header and a separate connection to the RN "B" Header.

This is considered to be the safety-related " assured" makeup source to the Spent Fuel Pool,which would only be used if no other da. mineralized water were available. Each connection is designed to provide 500 gallons per minute of makeup flow.

Each line is isolated from the SFP by two " locked-closed" manual valves in series. The postulated dilution event is the unintentional opening of one of these lines resulting in an assumed dilution flow rate of 500 gpm. Table 2 provides a tabulation of-the resulting boron concentration over time from a 500 gpm flowrate.

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Attachment 7 Page 18 of 35 i-l 5.2.6 KC/KF Heat Eacchanger Leak j The Component Cooling Water (KC) System provides cooling water to the KF heat exchangers for decay heat removal. There is no l

direct connection between the KC system and KF system.

However, a connection would occur if a leak were to develop in a KF heat exchanger that is in service. In case of a leak, KC water would be expected to flow into the KF system since KC is l at a slightly higher pressure. It is expected that the flow rate from such leakage would be very small due to the very l small difference in system operating pressures.

Even if a significant flow rate resulted from a leak, the impact on the SFP boron concentration would be very small due l to the limited volume of water available in the KC systen. The i

total volume of water in the KC system is 31,214 gallons.

Operator response to a loss of KC inventory includes manually

! . aligning a demineralized water makeup source (YM) or using the

" assured" makeup source from the RN system. The alarms from l the KC surge tank and the SFP high level alarm would alert control room operators of the lost inventory and the source of the leak.

The boron concentration resulting from a dilution volume of 31,214 gallons is found to equal 2257 ppm, a change of only 218 ppm. ,

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! i l -Because of the limited amount of water available for the KC L

l- system and the mechanisms available to operators to identify such leakage, a KF heat exchanger leak can not result in any i significant dilution of the SFP and is not considered further, q

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Attachment 7 Page 19 of 35 5.2.7 Dilution From Drinking T(ater System There is a Drinking Water (YD) System supply line located in the SFP area to dispense potable water for va aus cleaning and decontamination activities that take place in this area. Water for this system is supplied from the local Charlotte /Mecklenburg County water system. It is postulated that this source could be misaligned or inappropriately used causing unborated water to enter the pool. It is assumed that this source could not produce more than 50 gpm of flow from this connection. Table 2 provides a tabulation of the resulting boron concentration over time from 50 gpm of dilution flow. However, this dilution source is not a concern due to the much greater flow rates estimated for piping breaks for the RF System.

5.2.8 Boron Removal By Spent Fuel Pool Domineralizer When the spent fuel pool demineralizer is first placed in service after being recharged with fresh resin it can initially remove boron from the water passing through it. The demineralizer normally utilizes a mixed bed of anion and cation resin which would remove only a small amount of boron before saturating. Because of the small amount of boron removed by the demineralizer, it is not considered a limiting dilution event for the purposes of this evaluat-ion.

5.2.9 Dilution From Fire Protection System The Fire Protection (RF) System is not directly connected to the pool. However, two fire protection hose stations located in the SFP area could be used to manually add water to the SFP.

l Each hose station has the capacity to deliver approximately 100 gpm of unborated water. Use of RF for this purpose would be as a last resort to restore pool inventory following the failure L I

Attachment 7 Page 20 of 35 or depletion of all normal makeup sources to the pool as well as both trains of the RN " assured" makeup source. The impact of this dilution source is bounded by the consideration of a pipe break in the 4" RF supply header which feeds both hose stations. In addition, station procedures for emergency makeup to the SFP are as.cumed to address the addition of boron to the i pool regardless of which makeup source is used. Therefore, this source will be addressed under " Pipe Breaks" in Section 5.1 and will not be considered further in the context of

" Interfacing System". l 5.3 Loss of Off-Site Power l l

Of the dilution sources considered, only the RN assured makeup, j fire protection system, and drinking water system are capable l of providing non-borated water to the spent fuel pool during a loss of off-site power. Each fuel pool cooling (KF) pump is supplied backup power by its corresponding emergency diesel ]

generator at one hour after the loss of normal station power,  !

however, the pumps must be manually started. The Fire Protection (RF) pumps are also supplied with emergency diesel power which must be manually connected. The Fuel Pool Skimmer Pump is not provided with a backup source of power. The spent l fuel pool level instrumentation is powered from a battery- l backed source which can be manually aligned to receive l emergency diesel generator backed power if normal power can not be promptly restored.

Due to the low probability of a loss of power event concurrently with a pipe break or a misalignment of the RN, RF, i or YD water sources, an accidental dilution of the spent fuel pool water is not considered credible. However, there is a scenario involving operation of the Standby Shutdown Facility (SSF) where the pool boron concentration may be intentionally

p Attachment 7 Page 21 of 35 lowered. The SSF includes an independent diesel generator ac power source and the Standby Makeup Pump which takes suction from the spent fuel pool to provide seal injection flow for the Reactor Coolant (NC) Pumps. The SSF was designed to respond to

! security events or Appendix R fire events, but is also credited j for responding to station blackout scenarios if emergency diesel power fails.

Operation of the SSF is postulated for up to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. During this 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, the Standby Makeup' Pump draws approximately 26 gpm of flow from the pool. Plant procedures have provisions to provide makeup to the pool during SSF operation. The maximum volume of borated water taken from the pool is estimated to be

! (2 6. gpm x 60 min /hr x 72 hr) 112,320 gallo.is . If this water i i

volume is replaced with non-borated water, the maximum dilution is calculated to be 1255 ppm.

5.4 Evaluation of Infrequent Spent Fuel Pool Configurations Two corifigurations were identified that are significantly different than the normal SFP configuration. These would be if either the fuel transfer canal or cask loading pit were isolated from the main pool. l l

The purpose for isolating the transf r canal would be to drain the canal to gain access to the fue) handling equipment used to transport fuel assemblies between the SFP and the Refueling Canal. Under current policies and practices, the transfer canal is not drained unless the fuel handling equipment can not be repaired by using diving equipment. The use of high-quality underwater color television cameras at McGuire has also elimirated the need to drain the transfer canal to perform

! visual inspections of this equipment. Pool high-level alarms and plant personnel involved in the equipment repair would

Attachment 7 Page 22 of 35 ensure very prompt detection prior to a significant amount of unborated water being added to the SFP. In fact, the pool would actually spill over into the fuel transfer canal and stop any work taking place there. Piping breaks in the pool area would also be obvious to crews working there. Also, the borated water drained from the transfer canal would be stored in the Recycle Holdup Tanks, effectively eliminating one of the more significant dilution sources. Because of the very low frequency of this configuration, the enormous volume of water required to significantly dilute the pool, and the effective means of early detection of an event, this configuration is not considered to be a part of a credible boron dilution accidant scenario and is not considered further in this analysis.

The purpose of isolating and draining the cask loading pit is to prepare for the loading of fuel into a cask or for the actual movement of a cask into or out of the pit. While this activity has been very rare in recent past experience, some cask loading activities are planned for the future. Isolation of the cask loading pit removes approximately 46,423 gallons from the total volume of borated water available in the pool.

For this special cuse, a new set of parameters is derived that exclude water volume in the cask loading area.

Using these new parameters, the previous dilution calculations for the worst case bounding events (the 700 gpm RF line break and the RHT/RMWST misalignment event) were performed again.

For the 700 gpm RF lino break, the results for this alternate configuration are provided in Table 3 which shows that it would take more than 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> for this dilution event to lower pool boron concentrations below the non-accident conditions minimum horon credit of 440 ppm (Attachment 6). For the RHT/RMWST misalignment event, the final pool boron concentration is 937

Attachment 7 Page 23 of 35 ppm which would require this dilution event continue unnoticed for more than 3 days. Neither of these events are likely since they would be detected by a spent fuel storage pool level alarm or by plant operations personnel walki: A through the area before the required volumes of water were added to a fuel pool.

6.0 Results A summary of dilution event results is provided in the table below.

Summary of Dilution Event Results Pipe Break (4" RF 700 gpm Time Dependent Header) (See Table 2) 112,000 1834 ppm IDilutionFromRMWST gallons 336,000 1068 ppm lDilutionFromRHT&

RMWST gallons 122,000 1791 ppm lDilutionFromRMT&RMWST Igallons 500 gpm Time Dependent lDilutionFromRNSystem (See Table 2) 50 gpm Time Dependent IDilutionFromYDSystem (See Table 2)

Attachment 7 Page 24 of 35 SSF Operation (Refill to 112,320 1613 ppm Normal) gallons removed and 112,320 added back SSF Operation (Refill to 112,320 1255 ppm Overflow) gallons removed and 204,344 added back Infrequent Configuration 700 gpm Time Dependent (Cask Loading Pit (See Table 3)

Isolated) (4 " RF Pipe Break)

Infrequent Configuration 336,000 937 ppm (Cask Loading Pit gallons 1 Isolated) (RHT & RMWST Mialigned) l l

Table 2 also provides an estimate of the length of time )

required for various flow rates to fill the pool to the high level alarm setpoint and to reach the pool overflow level.

7.0 Conclusions Potential deboration accident scenarios in the SFP have been evaluated over a range of possible conditions. These postulated events involve combinations of multiple human

Attachment 7 Page 25 of 35 errors, medium to large pipe breaks, or infrequent SFP configurations that make a significant loss of boron in the SFP very unlikely. The impact of these accidents result in a range of values cf boron concentration depending on dilution flow rates and pool volumes. The results also show that the dilution process requires many hours to significantly reduce pool boron concentration even under the most limiting conditions and provides sufficient time for operator actions to terminate the accident. Based on the analysis presented above, it is concluded that the worst case unplanned or inadvertent dilution events are not credible since, in the unlikely event they occurred, they would be detected by plant operators walking through the spent fuel pool areas or by spent fuel pool level alarms before sufficient water could be added to a pool to lower its soluble boron concentration to levels approaching the minimum non-accident conditions boron credit of 440 ppm.

8.0 References

1. WCAP-14 416-NP-A, Revision 1, " Westinghouse Spent Fuel Rack Criticality Analysis Methodology, Westinghouse Electric Corporation, November 1996.

2. NUREG-13 53, "Beyond Design Basis Accidents in Spent Fuel Pools", U.S. Nuclear Regulatory Commission, April 1989.

3. WCAP-14181, " Westinghouse Owners Group Evaluation of the Potential For Diluting PWR Spent Fuel Pools," Westinghouse Electric Corporation, July, 1995.

Attachment 7 Page 26 of 35 1

1 Table 1 -

Preliminary List of Dilution Initiating Events

. s .. .

Structural Failure - Screened Postulated missiles causing damage Missilas to the pool structure could lead to a loss of inventory and zircaloy cladding fire but can not cause a I dilution event.

Structural Failure - Screened Postulated damage to the pool Aircraft Crashes structure from an aircraft crash could lead to a loss of inventory and zircaloy cladding fire but can i

not cause a dilution event. (See i also below - " Piping Damage caused by Airplane Crashes")

Structural Failure - Screened Postulated heavy load drop events Heavy Load Drops causing damage to the pool structure l could lead to a loss of inventory and zircaloy cladding fire but can not cause a dilution event.

Seismic Structural Screened Seismic structural failure is Failure postulated to cause an unrecoverable loss of water in the SFP, and leads to a zircaloy cladding fire and cannot cause a dilution event.

l l

l I

Attaclunent 7 Page 27 of 35 Table 1 -

Preliminary List of Dilution Initiating Events Reactor Cavity Seal Screened The design of the McGuire Reactor Failure and/or Nozzle Cavity Seals nakes a catastrophic Dam Failure failure of the seals extremely unlikely. Such failures would be quickly isolated by procedure by closing valve KF122 (Fuel Transfer Tube Isolation Valve). In addition, a catastrophic failure would result in a loss of SFP inventory that could cause a zircaloy cladding fire and is not a boron dilution initiating event. The same conclusion applies to other failures of the reactor coolant system piping during refueling operation (including nozzle dams).

Loss of Cooling / Makeup Screened Loss of cooling / normal makeup is not considered a deboration event since the loss of inventory through evaporation and/or boil off does not remove boron from the pool.

l 1

Attacrun<ent 7 Page 28 of 35 Table 1 -

Preliminary List of Dilution Initiating Events Inadvertent Screened Most loss of inventory events are Drainage / Loss of expected to be small. Design Inventory features of the KF system (e.g.,

siphon breaks) purposely limit the amount of water that could be removed from the pool due to KF system pipe breaks, system malfunctions, or operator errors. A boron dilution event could occur if unborated water is used to refill the pool. However, these events are not generally expected to remove enough water to deborate the pool significantly. Plant procedures will address the addition of boron )

to the pool in response to a l l

significant loss of inventory which I requires emergency makeup water.

Fires (at or near the Screened Typically, combustible loadings ,

pool) around the pool area are relatively l small. If the fire hose stations were used to extinguish a fire, the volume of water required to extinguish a local fire is not expected to be of sufficient magnitude to cause a significant change in pool boron concentration.

pg

Attaclunent 7 Page 29 of 35 Table 1 -

Preliminary List of Dilution Initiating Events e e se e

' A External Floods Scre;ned This type of event is not credible for McGuire Nuclear Station. FSAR analysis of potential external flood sources showed that the station embankment will protect the plant from worst case flooding scenarios.

In addition, the elevation of the top of the pool is an add' 'onal 18 feet above grade.

Attacrunent 7 Page 30 of 35 Table 1 -

Preliminary List of Dilution Initiating Events Storms Causing Runoff Screened The location of the spent fuel pool into the Spent Fuel is high enough to preclude storm I Pool water from entering the pool due to flooding of the site. However, the roof drains for the Spent Fuel Pool Building are located directly above the pool. This piping is Class B (OA-1) seismically designed, although the portion of this piping over the railroad bay is not tornado wind or missile protected. However, wind or missile damage to this piping is considered very unlikely in a tornado strike event on the plant site and is not considered further. The McGuire 'lAR does not postulate piping break. t lines fed by gravity such as this <ne. Also, with a probable maximum precipitation (PMP) event (30" rain in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />), the 8825 sq. ft area on l

the roof would only generate 165,000 i gallons. Even with a significant crack in the piping, most of the water flow would go down the drain (path of least resistance). Thus l even a PMP event could not produce a dilution event greater than other postulated events. This event is screened.

Attachment 7 Page 31 of 35 Table 1 -

Preliminary List of Dilution Initiating Events Pipe Breaks caused by Evaluate Some piping in the SFP area (RF, YM, seismic events, or YD, etc.) is not seismically tornadoes qualified and is not specifically protected from tornadoes.

Realistically the probabilities of these failure events is lower than from random pipe breaks. In particular, the probability of tornado wind or missile damage is judged to be extremely low and do not need to be considered further.

Sir. .:e non-seismically qualified piping has been identified in the SFP area, this type of piping damage will be evaluated in Section 5.1.

Randoin Pipe Breaks Evaluate Piping in the vicinity of the pool will be e"aluated for dilution accidents.

Other Damage caused by Screened The likelihood of an aircraft crash Airplane Crashes on either of the McGuira Spent Fuel Pools is extremely remote and is dismissed as a credible boron dilution initiating event Tank Ruptures near the Screened Review of plant drawings and a plant SFP walkdown determined that no tanks in or around the plant could flow into i i

the SFP if the tank ruptured. i Dilution Events Screened No credible pathways could be Initiated in the identified for this type of event.

Reactor Coolant System Misalignment of Evaluate There are several interfacing Systems Interfacing systems that will be evaluated.

with KF system

Attachment 7 )

Page 32 of 35 Table 1 -

Preliminary List of Dilution Initiating Events reifiating Event' . )ispositjos Loss of Off-site Power Evaluate The impact of loss of ac power events will be reviewed and evaluated including possible SSF scenarios.

Loss of Boron Due To Evaluate The potential impact of the Demineralizers or purification system will be other Purification evaluated.

Equipment Infrequent SFP Evaluate Potential alternative configurations Configurations will be evaluated.

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Attachment 7 Page 34 of 35 Table 3 - RF Line Break With Cask Loading Pit Isolated 1 2190 2148 -42 2 1963 1897 -66 l l l l l 4 1603 1500 -103 l l l l l 6 1309 1186 -123 l l l l l 8 1069 937 -132 l l l l l 10 873 741 -132 l l l l l 11 789 659 -130 l l l l 12 713 586 -127 l l l l 16 475 366 -109 l l l l l 24 211 143 -68 l l l l l SS 63 35 -28 l l l l l 48 10 9 -10 l l l l l 56 8 3 5 l l l l l 64 4 1 -3 l l l l l 72 2 1 -1 l l l l l

l Attachment 7 Page 35 of 35

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Figure 1 - McGuire Spent Fuel Pool Elevations T ent Fuel Pool (Overflow Level) = 778'+10' I

Volume V,

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"g.y." ,j jj'"*',yy:."%: - n =:=:=: . :=:=:=.

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Normal Pool Level = 771'+4.75* i _ _ _ _ . . _

TS Minimum level _ = 769' . , . , , , , , , , , , , , , , , , . , . , , , , , , , , . . , , , , , , , , , . j Grade Elev. 760' Main Pool Transkr (Volume V,) Canaf Top of Fuel = 746' llllllllllllll l Fuel Racks l Fuel Transfer Tube

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KF122 Drawing Not To Scale l

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