ML20202E044: Difference between revisions

From kanterella
Jump to navigation Jump to search
(StriderTol Bot insert)
 
(StriderTol Bot change)
 
Line 17: Line 17:


=Text=
=Text=
{{#Wiki_filter:}}
{{#Wiki_filter:--
Crunswkk Mont it32 ECCS Sueden Strainer Replacement Project NRCBulletin 96-03 FinalReport P
4 Brunswick Plant, Unit 2 ECCS Suetion Strainer Replacement Project NRC Bulletin 96-03 Final Report i
e r'
9712050227 971106 PDR 0      ADOCK05000324k
          \y                        PDR    r 4\'T
{ _:                                                                          .
 
Brunswkk Plant I):lt 2 ECCSS ctica Strainer Replacement Project N.'CC:lletin WO3 FinalReport
\
Table of Contents 1.0 Introduction.........................................................................................................................................1 2.0 Potential ECCS Suction Stralner Blockage issue Resolution Approach ........................................1 2.1 S u ctio n S t raln e r Re place m e n t.................. .                                                      .... . ..... .... ... ..... ................. . ............................ .... . .'.......... 2 2.2 Su ppression Pool aad ECCS Strainer Clesniiness ................. ...................................................... 3 2.3FMEProgram.................................................................................................................................5 2.4 ECCS Pump Technical Specification Surveillance Data Review................................................. 4 i                                      3.0 Plaat Co n figuratio n and Design Pa ra m ete rs .................. .................................................................. 4
                                                                                                                                                                                                                                                          ..............4 3.1 Co n t a i n m e n t Desig n . . . . .. . . ........ . .. ..... .. . . .. .. .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . ..
3.2ECCSDesign.................................................................................................................................5 3.3 D ryw e ll I a s ula t io n . .. . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . . .. . . ..... .. ... . .. ... . .. . . . . . . . .. . . . . . . . . . . . . . . . . .
3.4 O t her Pot e n tial D rywell Deb ris So u rc es .. ....... ..... ............... .. .................. ............. . .... ..... ..10 3.5 Polen tial S u pp ression Pool Debris Soi s rces .............................. ...... ......... .............. .................. 11 4.0 Replacement St rainer Design Requlrerr ents .. .... . ................. ....... . ............ ...... ............. .. ......11 4.1 St rain e r Fu n etio n al Req ulre m e nts .. ......... . . ....... . ....... .. .. ...... . ... ... ...... ..... ....... . .... .......11 4.2 S t mi n e r Co d e R eq u i re m e n ts . . .. . .. .. . . . .. . . . . . . ... . .. . .. . . . . . ... .. . .. . . . . . . . . .. .. .... . .. . . . .. . .... ... . .... 12 4.3 Strainer and Strainer Support Strv etural Requirements . .... .. . ... ...... ...... .. ........................13 5.0 Replacement Strainer /ECCS Long Term Recirculation Evaluation Methodology.... . .. . . . ...13 5.1 D rywell Deb ri' G eneratio n.......                                                    . . ... .... .... . . ... ....                    . . . . . . . . . . . . . . . . . .  ..........................13 5.2 Dryw ell Debri Transport .... ... . ...... .. ............ .. . .                                                                                ...........            .....................................15 5.3 Suppression Dol Debris Transport.... . ........                                                                                    . . . . . . . . . . .        .........................................15 5.4 Suction St raln er Bloc ka ge and 11end Loss .. . ...... . . ...... . ...... ... ............. . ...... ......... ....... .15 5.5 ECCS Pumps NPSil Margin Assessment ................ . . ... ... ........................................16 5.6 Suction Strainer St ructu ral Evaluation . ..... .... . ....... . ... . .. . .. .......... .... . . . ... ............ .17 5.7 Other Plant Structural, Systems and Components Evaluation .. .                                                                                                                    ..........................19 6.0 Licensing Coasiderations..... . . . . ..                                                          . . . . . . . . . . . . . . . . . . . . . .        . . . . . . ....            ..........................19 APPENDICES A - Modified NUREG/CR-6224 IIcad Loss Correlation B - Sure-Flow" Strainer Performance Test Reports C - BSEP Unit No. 2 Replacement Suction Strainers Design Drawings D - BSEP Unit No. 2 Torus Design Drawings E - Strainer Ilydrodynamic Mass Test Reports 11
 
Bru:;swick Plut Unit 2 ECCS S:ction Ste:Iner Rept: cement Project NRCBulletin 9M3 FinalReport                                              _
1.0      Introduction This document constitutes Carohna Power and Light (CP&L) Company's final report in response to the NRC Bulletin 96 03," Potential Plugging of Emergency Core Cooling Suction Strainers by Debris in Boiling Water Reactors," dated May 6,1996, for the Brunswick Steam Electric Plant (BSEP), Unit No. 2. An initial response to Bulletin 96-03 was provided in CP&L Letter, No.
BSEP 96 0364, dated November 1,1996. A supplemental response was provided with CP&L Letter, No. BSEP 97-0209, dated June 5,1997. This final report provides the details of: 1) the approach CP&L has implemented to resolve the potential Emergency Core Cooling System (ECCS) suction strainer blockage issue identified in Bulletin 96-03; 2) the criteria CP&L has imposed in the design and evaluation of the replacement strainers; and 3) the methodology CP&L has employed in the evaluation of the replacement strainers and demonstration of the long term ECCS recirculation capability. This final report is submitted to the NRC as required by Bulletin 96-03.
As described in Section 2, CP&L's primary action for resolution of the potential ECCS suction strainer blockage issue was the replacement of the previously mstalled suction strainers with large, passive strainers. CP&L, together with Duke Engineering and Services, Inc. (DE&S),
innovative Technology Solutions Corporation (ITS) and Performance Contracting, Inc. (PCI),
has completed the design, engineering, fabrication, installation and testing of the replacement strainers for BSEP Unit No. 2.
This report also serves as an interim report for BSEP Unit 1. The strainer design for Unit I is planned to be similar to the design for Unit 2. Most of the engineering analyses performed for Unit 2, as documented in the design calculations, are applicable to Unit 1. Any differences in the strainer design or design parameters for Unit I will be reconciled in the calculations, as appropriate. The engineering and fabrication activities for Unit I will be initiated in the fourth quarter of this year. The refueling outage for Unit 1 is scheduled to start on May 2,199f..
2.0      Potential ECCS Suction Strainer Blockage Issue Resolution Approach CP&L's approach for resolution of the potential ECCS suction strainer blockage issue involves four actions:
* Replace the existing ECCS suction strainers on the Residual Heat Removal (RHR) and Core Spray (CS) systems, with larger, passive strainers.
* Perform periodic suppression pool and ECCS suction strainer inspections and cleanings, based on the prelicted sludge generation rate.
* Maintain an effective foreign materials exclusion (FME) program to ensure the cleanliness of the suppression pool,
* Perform reviews of data obtained during the quarterly Technical Specification surveillance of the ECCS pumps.
These four actions are presented in more detail below.
1
 
Brunswkk Plant Un 2 ECCSSuction Streiner Replacement Project NRCRulletin W03 FinalReport 2.1    Suetion Strainer Replacement CP&L has replaced the existing suction strainers on the RHR and CS Systems with large, passive                '
suction strainers, a stacked disk strainer design, for strainers. CP&L selected PCI's Sure-Flow the replacement strainers (see Appendices B and C). The main design features of the Sure-Flow" suction strainers are as follows:
* The disks are fabricated from stainless steel, perforated plate. The holes in the perforated plate are selected such that a maxin.um particle size that can pass through the strainer is less than the minimum orifice size in the RHR and CS systems. The perforated plate can be cleaned under vater. Stainless steel is utilized to prevent degradation of the strainers due to corrosion.
* The disks are attached to an internal core tube that provides flow control capabilities. Holes are cut in the core tube and are designed such that uniform flow is achieved along the entire strainer length and the flow across the perforated plate is laminar, thereby reducing head loss.
* The core tube also acts as a structural backbone, capable of resisting the large submerged structure hydrodynamic loads that are postulated in Boiling Water Reactor (BWR) suppression pools. Use of the core tube allows relatively long strainer lengths to be installed. Supports are provided to increase the load carrying capacity of the strainers. The supports attach directly to the core tube.
* The strainers are designed to prevent vortexing. This has been confirmed by prototype testing.
f        The Sure Flow" strainers have been tested to demonstrate their hydraulic performance. These
!        tests were conducted at the Electric Power Research Institute (EPRI) Non-Destructive Engineering (NDE) Center in Charlotte, NC. The tests included low and high fiber quantities, with and without particulates. Testing of mixed fiber and reflective metal insulation beds was also performed. These tests demonstrate the ability of the Sure-Flow m trainers    s        to perform l        under various debris loadings, and also demonstrate that the analytical methods developed to evaluate strainer perbrmance provide acceptable results. The reports documenting the tests performed and the results obtained are included in Appendix B.
The replacement strainers have been sized to provide sufficient surface area to ensure that the l
head loss through the fouled strainers, for the design basis events, is acceptable. The strainer sizes for each system loop, in terms of strainer diameter, length and surface area, are provided in the table shown below. Also shown is the approximate increase in surface area, as compared to the existing strainers.
er l
l 2
 
Crunswkk Mont Unk 2 ECCSSuction Strainer Replacement Project NRCBelletin %-M FinalReport Approx.                  Approx.              Approx. Increase System          Replacement Strainer      Replacement Strainer      in Surface Ares
* Diameter x Length
* Surface Ares
* RHR            45" cia. x 16' 9"long                529 ft3                    1550 %
CS              45" dia. x 6'-0"long                245 ft2                    1530 %
          * - length is the total values per system loop suppression pool penetration 2.2    Suppression Pool and ECCS Strainer Cleanliness in response to the NRC Bulletin 95-02, " Unexpected Clogging of Residual Heat Removal (RHR)
Pump Strainer While Operating in Suppression Pool Cooling Mode," CP&L committed to develop a long term program for inspecting and cleaning tne Brunswick suppression pools and ECCS suction strainers. This program has been oeveloped and is documented in Preventive Maintenance (PM) Routes APUll 001 and API'L 001, for Unit Nos. I and 2, respectively. These routes require periodic inspection' of the suppression pool for sludge accumulation. If the sludge accumulation limit specified in the routes is . cached, then suppression pool cleaning is scheduled and performed in accordance with guidelines given in the routes. The routes also require periodic physical condition inspections of the ECCS suction strainers, at the same interval as the suppression pool inspections. The strainers are cleaned, if required by the route criteria.
The sludge accumulation limit specified in the PM routes was selected to ensure that the maximum sludge accumulation in the suppression pool does not exceed the design value used in the strainer performance evaluation (refer to Section 3.5). The sludge accumulation limit was established based on sludge generation rates determined from data obtained from previous BSEP suppression pool inspections /cleanings and from review of other industry data. These sludge generation rates will be reviewed as additional data is obtained from upcoming suppression pool l
l        inspections and the inspection intervals and sludge accumulation limit may be adjusted accordingly.
l 2.3    FME Program
;        Also as discussed in CP&L's response to Bulletin 95-02, CP&L maintains FME and cleanliness l        administrative procedures to ensure the cleanliness of the suppression pool. These procedures are 0Al-125, " System Cleanliness /FME," and 0Al-127, " Primary Containment Inspection and Closcout." These procedures control materials in the drywell, suppression pool, and systems that interface with the suppression pool, and ensure that materials that could potentially impact ECCS operations are properly controlled and prevented from entering the suppression pool.
  '      Currently performed each refueling outage I
3
 
    . - .            . _ - , _ _    .      =.. _ _ _                --    -          ~    .        .  -      . - . - -
1 Brunswick Plant U:st 2 ECCSSuction Strainer Replacement Pro}ect                                                      l NRC Bulletin WN FinalReport 2.4    ECCS Pamp Technical Specification Surveillance Data Review As discussed in the response to Bulletin 95-02, CP&L will continue to collect and review pump suction data obtained during the quarterly Techaical Specification surveillances. This collection and review of the Technical Specification surveillance data will identify potential degradation of the ECCS systems during the period between the inspections.
3.0    Plant Configuration and Design Parameters 3.1    Containment Design 3.1.1  Containment System Design Both units have Mark I containment designs. The Mark I containment consists of the following:
* Drywell e    Pressure suppression chamber (torus) e    Vent system between the drywell and torus e    Isolation valves e    Vacuum breakers in the event of a process system piping failure within the drywell, reactor water and steam are released into the drywell atmosphere. The resulting increased drywell pressure t. , forces a mixture of drywell atmosphere, steam, and water through the vent system, which opeas beneath the surface of the suppression pool water in the torus. The water quenches the steam, thereby controlling the pressure rise in the drywell. The drywell steam-air-nitrogen atmosphere, which is transferred to the torus, pressurizes the torus, in the event that the drywell pressure drops below the torus pressure, the torus is vented to the drywell through vacuum breakers to equalize the pressure between the two vessels.
Cooling systems are provided to remove heat from the reactor core, the drywell, the water in the torus, and from the atmosphere in the vapor space above the water. This provides continuous cooling of the primary containment under accident conditions to maintain the pressure suppression capabilities and containment integrity.
3.1.2  Pressure Suppression Chamber (Torus) Mark I Design The pressure suppression chamber (torus) is a hollow, reinforced concrete shell encircling the lower portion of the drywell containment structure. The concrete shell encloses a continuous 16-sided steel tcrus liner of circular cross sections. The major centerline d4 meter of the torus is 109 feet and the cross sectional diameter of the circular liner is 29 feet. A paper joim is provided between the bottom of the torus and the mat foundation, to allow radial expansion of the torus. Vertical keys are provided along the outside perimeter of the drywell pedestal to allow independent,' unrestrained radial expansion of the torus, when subjected to symmetric loadings.
Under asymmetric loads, the keys force the drywell and torus to respond as a single unit.
l l                                                                4 l
 
  - Brunswick Plant U:lt 2 ECCSSection Strainer Replacement Project NRC Bullerhe 96-N FinalReport The pressure suppression vent system connects the drywell and torus together. Eight circular vent pipes, equally spaced around the periphery of the drywell, connect the drywell to a vent header contained within the air space of the torus. Projecting downward from the header are 96 downcomer pipes, which terminate approximately 3 feet below the water level of the torus Jet deflectors are provided at the inlet of each vent pipe to prevent possible damage to the i ent line and header due to jet forces and/or missiles which might accompany a pipe break inside the drywell. Steam dischs:ged into the torus causes the pool to swell and surge upward. Vent header deflectors are provided to deflect this surge away from the vent header toward the torus liner.
Steam discharged through the safety relief valves (SRVs) enters the torus through T-quenchers.
The T quenchers contain numerous orifices through which the steam exits into the suppression pool.
A drawing i,howing section views through the torus is provided in Appendix D.
3.2      ECCS Design The ECCS include the RHR system (operating in the low pressure coolant injection mode), CS, High Pressure Core injection (HPCI), and the Automatic Depressurization System (ADS). The combination of these systems is designed to satisfy the 10CFR50.46 ECCS Acceptance Criteria.
The RHR and CS systems are the ECCS utilized for long term cooling. These systems take suction from the suppression pool, and, as such, are potentially impacted by the suction strainer plugging issue. Therefore, the RHR and CS suctions strainers have been planned to be replaced.
As the HPCI system is not utilized for long term cooling, its suction strainer will not be replaced.
A brief description of the RHR and CS Systems is provided below.
3.2.1    RHR System The RHR system consists of two essentially complete and independent loops, identified as Loop A (Division I) and Loop B (Division 11), with each loop containing two pumps and the necessary piping, valves and controls to support the various modes of operation. The suction piping of each pump on a loop ties into a common header (24" diameter pipe) which singly penetrates the torus. The suction strainer is attached to a flanged connection immediately adjacent to the torus penet ation.
,            The RHR system is designed for the following six modes of operation:
              .        Low Pressure Coolant injection (LPCI)
* Shutdown Cooling
* Containment (Drywell and Suppression Pool) Spray
* Suppression Pool Cooling (SPC)                    ,
* RHR Service Water injection .
* Fuel Pool Cooling Assist 5
 
                                              -    .            .                - _ _ - _.      .      .=
Brunswick Plant Uit 2 ECCSSuction Strniner Replacement Project NRC Bulletin 96-03 FinalReport The LPCI mode of operation of the RHR system is the only mode providing an emergency core cooling function, 'Ihis mode of operation provides a low pressure source of core cooling for the entire range of postulated bitak sizes. It'the break is small, yet high pressure systems are unable to recover level, ADS will depressurize the Reactor Vessel, thus allowing LPCI and CS injection.
For larger breaks, the Reactor Vessel depressurizes via the break which allows almost irrmediate (approximately 20 sec) LPCI injection. The LPCI mode injects via the Reactor Recirculation system discharge piping to provide a volume of water to flood the Reactor core. Automatic initiation of the system occurs from low reactor vessel water level or high drywell pressure -
(approximately 1.8 psig) " coincident" with low ienctor vessel pressure (approxirnately 410 psig).
After the core is flooded to at least two-thirds core height, one RHR or CS pump is normally required to make up for Jet Pump throat to diffuser slip joint leakage. The other pumps are stopped so that the emergency power (if there is a loss of offsite power) that was being used by these pumps may be shifted to other plant loads including RHR Service Water booster pumps.
One RHR pump and heat exchanger are normally placed in the Containment Cooling Mode aftet a line break in the drywell as per the Emergency Operating Procedures (EOPs).
Upon automatic initiation of the A(B)2 Loop of LPCI, the associated pumps should start approximately 10 seconds after the initiation signal is received. If a loss of power has occurred resulting in the Emergency Diesel Generators powering the pumps, the pumps will start 10 seconds after 'he associated diesel ties onto the emergency bus.
The A(B) and C(D)' RHR pumps take suction from the Suppression Pool through the normally open motor operated Suppression Pool Suction Valve, Ell F020A(B), and the associated motor-operated RHR Pump Suppression Pool Suction Valve, Ell-F004A(B) or Ell-F004C(D),
and discharge through the Pump Discharge Check Valve, El1-F031 A(B) or El1 F031C(D). The discharge check valves are designed to prevent backflow through the p' imp and to maintain a water leg in the discharge piping.
With the RHR and CS pumps running on minimum flow or dead headed, indicated pump discharge pressure on CS should increase to approximately 305 psig and RHR should be 202 psig. As reactor pressure decreases to approximately 410 psig, the LPCI Inboard Injection Valve, Ell F015A(B), should automatically open. As reactor pressure continues to decrease, the discharge of the RHR pumps should overcome reactor pressure below approximately 200 psig, allowing the flowpath to continue from the RHR pumps' discharge check valve directly into the Reactor Vessel through the normally open LPCI Outboard injection Valve, Ell-F017A(B), the LPCI Inboard injection Valve, El1-F015A(B), the LPCI Injection Line Check Valve, Ell-F050A(B), the locked open LPCI Manual injection Valve, Ell-F060A(B), and into the Reactor Recirculation system discharge lines. Once reactor pressure is reduced to approximately 20 psig, RHR flow should reach approximately 17,000 gpm per operating loop with two pumps.
The LPCI Outboard injection Valve, F017A(B), is a thrattle valve which may be adjusted to control flow into the vessel, whereas the Inboard Injection Valve, Ell-F015A(B), is designed for either full open or full close service. El1-F017A(B)is normally open, but with a LPCI initiation 2
A(B) Loop signifies Loop A and/or Loop B.
8 A(B) and C(D) pump signifies Pump A and'or Pump C on Loop A, and Pump B and'or Pump D on Loop B. A similar identification applies to valves.
6
 
Crunswkk Plant U22 ECCSSuction Strainer Replacement Project NRCBalletin W0.1 FinalReport signal present, this valve cannot be closed or throttled for 5 minutes to ensure a discharge path exists from the pumps to the vessel. Ell F015A(B), cannot be closed as long as the LPCI initiation signal is present. In addition, the RHR heat exchanger is automatically bypassed via the RHR Heat Exchanger Bypass Valve, El1.F048A(B), for the first three minutes to ensure that flow gets to the reactor through the most direct route. During the mterval of time when the RHR pumps are operating to restore the reactor vessel level, suppression pool kat removal is not necessary.
Each LPCI loop is provided with a minimum flow line to the Suppression Pool to protect the pumps from damage due to overheating as a result of low or no flow operation. This feature allows the pumps to operate with a closed discharge valve, without overheating, by recirculating Suppression Pool water through the minimum flow bypass line.
Each of the two separate loops of RHR, in the (SPC) mode, provide the primary source 0!
containment cooling. Following a Loss of Coolant Accident (LOCA), SPC is manually initiebd to limit Suppression Pool temperature and Containment pressure within design limits.
The RHR pumps receive power from the 4160 volt AC emergency auxiliary buses. For each loop, the RHR pump motor and associated automatic motor valves receive AC power from different buses.
3.2.2  Core Spray System The CS system provides a low pressure source of core cooling for the entire range of postulated break sizes. If the break is small, yet high pressure systems are unable to recover level, ADS will depressurize the reactor vessel, thus allowing CS injection. For larger breaks, the reactor vessel depressurizes via the break which allows immediate CS injection. The CS system injects via a dedicated set of spray spargers (one per loop) in a spray pattern directly over the reactor core.
The CS system consists of two redundant and independent loops. Each loop contains one 100%
capacity pump and the necessary piping, valves, and controls. The suction piping (14" diameter pipe) on each loop independently penetrates the torus. The suction strainer attaches to a flanged connection on the torus penetration.                                                              .
The CS system controls support vessel injection when reactor pressure is reduced below the injection permissive. Automatic initiation of the system occurs from low reactor vessel water level or high drywell pressure coincident with low reactor vessel pressure.
The two leops are designated as "A" and "B", with the "A" ooop assigned to Division I and the "B" Loop assigned to Division 11. Water from the suppression chamber is discharged into the reactor pressure vessel via the CS spargers. An alternative water supply to the CS loops is available from the Condensate Storage Tank, via a manual valve.
The CS system relies upon station AC power sources to power the pumps. Pump power is supplied from the as ociated station emergency 4160 VAC Bus. Each pump is powered from an independent bus with each emergency bus capable of being powered from one of three sources of l
7
 
Brunswick Nant U2!2 ECCSSuction Strainer Replacement Project hRC Rulleren 9643 FinalSeport AC power. Normally dering plant operation, the buses are powered from the Main Generator            l output via the Unit Auxiliary Transformer (UAT). During plant shutdown conditions, and in the event this power source is lost, the emergency buses are automatically or manually transferred to the unit's Startup Auxiliary Transformer (SAT). Should both of the above fail, the station Emergency Diesel Generators automatically power the buses.
Failure of a single loop of the CS system is backed up by the other loop. Failure of one or both loops is also backed up by the LPCI mode of RHR.
3.2.3    Limiting Conditions for Strainer Performance De specific, limiting (i.e., worst case) conditions that are to be used in the evaluation of C.a strainer performance are provided below. These conditions are associated with a LOCA and loss of one emergency power system. For the governing case for RHR, this results in two RHR pumps on one loop operating at run-out conditions, and one CS pump operating at minimum Technical Specification flow. For the governing case for CS, one CS pump is operating at run-out conditions, with one RHR operating at minimum Technical Specification flow. Run-out flow is assumed to continue until up to ten minutes into the accident, when coolant level in the reactor is reestablished. Operator action is taken at that time to reduce RHR and/or CS flows, as any single RHR or CS pump is able to maintain water level in the reactor. This action is required to change the RHR mode of operation to containment cooling and/or containment spray and also assures adequate steam condensation capability by limiting suppression pool temper m et
                . Worst case RHR flow until ten minutes into the accident is either: 1) 21,000 gpm 'hrough one loop, coincident with total CS flow of 4,625 gpm; or 2) 18,750 gpm through one loop, coincident with total CS flow of 4,625 gpm. At ten minutes, the total RHR flow is reduced to 11,550 gpm, which is the nominal, single pump RHR flow.
* Worst case CS flow of 6,700 gpm, coincident with total RHR flow of 7,700 gpm, until ten minutes into the accident. At ten minutes, the CS flow is reduced to 4,725 gpm, which is the nominal CS flow.
These conditions are based on a Design Basis Accident (DBA) LOCA. The DBA conditions, in coiunction with the debris loading generated as a result of a DBA, represent the governing suction strainer design conditions.
3.2.4    Net Positive Suction Head Margins The Net Positive Suction Head (NPSH) margins (NPSH available minus NPSH required) for the replacement strainers for the RHR Loops A and B pumps (2 pumps and one strainer per loop) and CS Loops A and B pumps (one pump and one strainer per loop) were determined. The pump NPSH is affected by the suppression chamber pressure, suppression pool temperature and line losses. The losses through the previously installed strainers were removed in determining these margins. He BSEP NPSH evaluation does not take credit for suppression chamber pressure increases, so the pressure is held at 14.7 psia, for the design basis events.
8
 
Brunswick Plant itAt 2 ECCS Suction Strainer Repiecement Project NRCBulletin WO3 FinalReport At ten minu'.es into the accident, the NPSH margins are as follows:
* RHR
              -    @ 21,000 gpm,161.8'F,14.7 psia:                  9.4 ft. head
              -    @ l8,750 gpm,162.5'F,14.7 psia:                  12.0 ft. head I
e  CS @ 6,700 gpm,162.5'F,14.7 psia:                    10.9 ft. head .
After ten minutes into the accident, the NPSH margins are as follows:                              l
* RHR @ l1,550 pm,189'F,14.7 psia:                      9.0 ft head                            l e  CS @ 4,725 gpm,189'F,14.7 psia:                        9.6 ft. head These NPSH margins are based on the maximum predicted suppression pool temperatures. As the head less through the fibrous debris increases with decreasing temperature, the NPSH margins at minimum predicted tempe-atures and corresponding debris loadings have been reviewed and show that the limiting condition is at the maximum predicted suppression pool temperatures.
3.3    Drywell Insulation The insulation systems used in the drywell include both fibrous and reflective metal insulation.
The specific types and approximate quantities are summarized below:
Approx. Quantity Type                              Unit 1              l      Unit 2 Fibrous Insulation (quantities in ft')
NUKON - Jacketed                                    1.406                    1.232 NUKON - Unjacketed                                  1.145                    1.036 Temp Mat                                                46                        58 Reflective Metal Insulation (quantities in it')
Trai4sco RMI(1 mit Al)                              33.021                    44.609 Diamond Power RMI(2 mil SS)                        44.320      .            44.320 Other (quantities in ft')
l                                                                    4                        4 Calcium Silicate Micro Therm                                              2                        2 Vention (fiberglass fabric)                              0.2                      0.2 9
 
Brunswkk Plant U:st 2 EC CSSuction Streiner Replaceneent Pro}ect NRC Sulletin W0.1 FinalReport 3.4        Other Potential Drywell Debris Sources The other potential drywell debris sources can be divided into three categories. These categories are defined in Section 3.2.2 of NEDO 32686, " Utility Resolution Guidance for ECCS Suction Strainer Blockage,"(URG), Revision 0, dated November 1996.                                                                  j 3.4.1        Fixed Debris Fixed debris is material that is pan of the permanent plant that becomes a debris source only after exposure to the effects of a LOCA. This in:ludes material that is blown or stripped off as a l
result of impingement forces. Material in this category are listed in Section 3.2.2.2.2 of the URG.
The other potential drywell debris applicable to BSEP are as follows.
Quantity Debris Type                      (ibm)                          Reference LOCA generated dirt / dust                        150        URG, Section 3.2.2.2.3.2 Qualified paint / coatings                          71        URG, Section 3.2.2.2.2.1.1 (100% epoxy)
Rust flakes from unpainted steel                    50        URG, Section 3.2.2.2.3.6 Other Drywell fixed debris                          15        Contingency (assumed to be paint)
Total                                              286 3.4.2        Latent Debris Latent debris is debris that would not be present until later in the LOCA event progression after prolonged exposure to a LOCA environment. For example, unqualified coatings (not directly impacted by the LOCA jet) have the potential to detach from the surface where applied, but only after prolonged exposure to the LOCA environment and after containment pressure is reduced later in the event. This type of debris is discussed in detail in Section 3.2.2.3 of the URG.
For strainer design purposes,6.67 ft' of unqualified paint / coatings was utilized. This equates to 628 lbm of debris.
3.4.3      . Transient Debris Transient debris is non-permanent plant material brought into the drywell, typically during an outage (e r., tools, rags, sheets, plastic bags, temporary filters, dirt / dust, etc.). Transient debris is principally controlled through FME and housekeeping programs. This type of debris is described in more detail in Section 3.2.2.2.1 of the URG.
10
 
    .                      _-                        .                    ~
Brunswick Plant U:lt 2 ECCSSection Streiner Replacement Project-NRC Bulletin 96-0.1 FinalReport CP&L maintains an effective FME and housekeeping program. Dirt / dust debris is included in          l the fixed drywell debris and suppression pool sludge quantities. A contingency of an additional      I 4 ft' of fibrous debris is included in the strainer design to account for fibrous transient debris.
3.5    Potential Suppression Pool Debris Sources Potential suppression pool debris sources are rust particles (sludge) from the carbon steel, including the torus liner and interior surfaces of torut attached piping systems, and unqualified paint.                                                                                              l Based on limited BSEP specific data, with congarisons to other industry data, the sludge generation rate is currently being conservatively set at 100 lbm (dry) per year. A total sludge quantit, of 600 lbm (dry) is to be used in the strainer evaluation, based upon an essumption of three,24 month cycles of sludge generation before cleaning. It should be noted that the last sludge measurement at BSEP Unit Nos. I and 2 showed a sludge generation rate of 33 lbm (dry) per year.
The quantity of unqualified paint in the suppression chamber is included in the quantity of
: t. w.ualified paint specified in Section 3.4.2.
4.0    Replacement Strainer Design Requirements 1
4.1    Strainer Functional Requirements The functional requirement for the suction strainers are as follow:
* The suction strainers shall screen out debris particles greater than 0.095".
This partcle size limit will ensure that any particle passing through the strainer will be smaller than the orifices associated with the cyclone separaicis, and a.1 other small flow restrictions in the pump seal flush piping. This requirement is consistent with the General Electric Company Service Information Letter, SIL No. 323, " Suppression Pool Suction Strainer Mesh Size Mismatch with Emerpncy Cooling System (ECCS) Pump Seal Orifices." which provided recommendations for sizing the strainer hole size.
* The suction strainers, including the collected debris and the interconnecting ramsheads, shall produce a total head loss not to exceed:
                -    For the first ten minutes into the LOCA:
                    -    8.4 ft water at 21,000 gpm at 161.8'F, and 11.0 ft. water at 18,750 gpm at 162.5*F for RHR;
                      -  9.9 ft, water at 6,700 gpm at 162.5'F for CS I1
 
                                                  .          _~    . - . _ - -    .  -    _.      . . _ _ ~
Brunswick Plant Ucht 2 ECCSSuction Strainer Replacement Pro,%ct NRC Rulletin Wo.t finalReport
            -    After ten minutes:
                -    8.0 ft. water at 11,550 gpm at 189.4'F for RHR
                -    8.6 ft. water at 4,725 gpm at 189.4'F for CS These total head lois values will provide an available NPSH that exceeds the required NPSH for the ECCS modes of operation for RHR and CS. The debris generation, transport and head loss determinations shall be in accordance with Bulletin 96-03 and hegulatory Guide 1.82, Revision 2," Water Sources for Long-Term Recirculation Cooling Following a Loss-of Coolant Accident," unless otherwise justined. The bounding head losses for the strainer elements and the interconnecting rarr.shead are 1.06 feet of water for the RHR strainer assembly, and 0.99 feet of water for the CS strainer assembly.
4.2    Strainer Code Requirements The strainer code requirements shall be in accordance with the requirements listed below. These code requirements are considered appropriate for the intended applications and function of the suction strainers. The code edition and addenda to be utilized are to be consistent with those s*>ecified in the UFSAR.
e  Quality Assurance
            -    10CFR50 Appendix B
            -    ASME CertiGcate not required e  Materials
            -    Conform to ASTM material specifications
            -    Certified Material Test Reports (CMTRs) are to be provided for all materials (except where ASME Section 111 Code would permit Certincates of Compliance)
* Design
            -    Quali0ed to ASME Section Ill, Nuclear Power Plant Components, Subsection NC e  Welding
            -    Weld procedures and personnel qualified to ASME Section IX.
* NDE
            -    Critical structural welds examined by PT or MT per ASME Section 111
            -    All other welds visually examined per ASME Section Ill
* Stamping
            -    NPT Stamp is not required The RHR and CS piping systems are classified as ASME Class 2 piping systems. The suction strainers are components which attach to these piping systems. As the suction strainers are non-pressure boundary components, the suction strainers are not required to be constructed to the requirements of the ASME Code (refer to Paragraphs NCA-ll30(b) and NC-2121,1985 12
 
I Bru'>swick Plant Unit 2 ECCSSuction Strainer Replacement Project                                                  1 NRC Bulletin W0.1 FinalReport Edition). In fact, no code and standards have been developed or adopted that specifically define the requirements for the construction of the strainers for this application. Therefore, the above        i requirements have been developed and are specified for the construction of ths strainers. These requirements rely on good engineering practice and have adopted appropriate sections of the ASME Code to assure the fabricated strainers are robust and of high quality.                              !
Suction strainers constructed to the above requirements will be of a quality and reliability that are appropriate and commensurate with their intended application and function.
4.3    Strainer and Strainer Support Structural Requirements The strainers and strainer supports shall be qualified for the loads, :oad combinations and acceptance criteria established under the Mark 1 Containment Reevaluation Program. The governing documents for these requirements are as follows:
* NEDO-21888, " Mark 1 Containment Program Load Definition Report," (LDR) Revision 2, November 1981, including Addenda, Sheet 1, April 1982.
* NEDO-245831, " Mark i Program, Structural Acceptance Criteria, Plant Unique Analysis Application Guide,"(PUAAG), October 1979.
          . NUREG 0661, " Safety Evaluation Report, Mark i Containment Long-Term Program, Resolution of Generic Technical Activity 4-7," July 1980, including Supplement 1, August 1982.
5.0    Replacement Strainer /ECCS Long Term Recirculation Evaluation Methodology This section presents the methodology used to evaluate the replacement strainers and ECCS l
long term recitculation. This methodology meets the replacement strainer design requirements discussed in Section 4. This methodology is based an information contained in NUREG/CR-6224, " Parametric Study of the Potcntial for BWR ECCS Strainer Blockage Due to LOCA Generated Deoris", in the URG and in other published literature, in some instances, the information in these publications is modified to provide appropriate application to this evaluation for BSEP.
i 5.1 -  Drywell Debris Generation The types and quantities of debris in the drywell generated as a result of a LOCA is a function of:
          . Postulated pipe break locations,
          . Break blast / jet shapes, sizes and intensities (i.e., Zone ofinfluence),
13
 
Brunswick Plant itit 2 ECCS Srcrion Streiner Replacement Project NRCBulletin WO.f FinalReport
* Insulation debris types and quantities within the Zone ofInfluence, and
* Other debris types and quantities.
Each of these items can be evaluated explicitly for each postulated pipe break location to determine the quantity of debris generated by that break. Alternately, simplified, but conservative evaluations can be made that provide bounding debris quantities to be utilized for the strainer and ECCS long-term recirculation evaluation.
CP&L performed a more simplified evaluation cf the bounding debris quantities generated as a result of a LOCA. This simplified evaluation approach is described below.
* Determine the type, thickness and application (i.e., the piping sy:tems and pipe sizes) of the insulatioa systems utilized in the drywell. (Note that this information, as well as the general condhior. of the insulation, has been confirmed through the performance of field walkdown conducted during the last two refueling outaEes.)
* Determine the Zone ofinfluence for each insulation type by pipe break size using Method 3 of the URG (refer to Section 3.2.1.2.3.3).
The URG uses a spherical Zone of Influence (hemispherical for single-ended guillotine breaks), which is consistent with guidance given in Re@ tory Guide 1.82, Revision 2. As defined in the URG, the volume of the sphere is determined for each insulation type based on the pipe diameter where the break is postulated, the amount of break separation, and the destruction pressure for that insulation type. All breaks are conservatively assumed to be unrestrained (i.e., having c. radial offset greater than 3D/2, where D = inside pipe diameter :n feet). Destruction pressures are provided for the required insulation types. Once the volume of the sphere is known, the equivalent sphere radius is calculated.
* Determine the routing of the high energy piping and their location relative to the majority of the piping insulation in the drywell using the plant composite piping drawings.
l
* Identify potentially controlling pipe breaks using the above information.
l l
This step invc,1ves inspecting the composite piping drawings, and with the knowledge of the i              Zones of influence for various pipe break sizes, selecting break locations which would generate large volumes of debris. Tha location of the insulation debris relative to platforms and grating is also accounted for due to different transport factors being applied (see Section 5.2). As BSEP is mostly homogeneous in its fiber-type insulation, this identification of l
potentially controlling pipe breaks from visual inspection of the composite drawings is possible.
* Determine the insulation debris types and quantities associated with each potentially controlling pipe break, and associated RHR/CS demand (i.e., flow rates).
* Select the break location (s) that results in the bounding debris volumes to be utilized in the strainer performance evaluation. More than one break location may be required to be evaluated to address all insulation types and corresponding RHR/CS demand.
l 4
14
 
_      -                            .    .                -_        .. .            =-      -        . . .    --
Brunswick Plant UD 2 ECC3 Section Strniner Replacement Project NRCBulletin W0.1 TinstReport The rea; tor vessel is insulated with reflective metal insulation (RMI). Any pipe breaks inside the blot nield wall would be bounded by the drywell pipe break quantities of RMI, since there are
            - guard pipes installed on the reactor vessel nozzles which direct 85% of the pressure out through the boshield walls.
Other non-insulation drywell debris is generated as a result of the LOCA. This debris and the potential quantities generated for each are provided in Section 3.4.
5.2    Drywell Debris Transport The drywell insulation debris transported as a result of a LOCA were determined using the combined generation and transport factors given in Section 3.2.3.2.5, Tables 5 and 6, of the URG. The bases for these combined generation and transport factors are also provided in the URG, and provide the,iustification for utilizing transport factors less than 100%.
All particulate debris generated in the drywell was assumed to be transported to the suppression pool with 100% efficiency, and in the suppression pool at time zero into the accident.
l 5.3      Suppression Pool Debris Transport The suppression pool is highly turbulent during the blowdown phase of a LOCA. However, after the first several minutes of the transient, the degree of turbulence is significantly reduced. The key phenomenon that can occur at that time is settling of debris to the bottom of the suppression pool. The nbious debris is typically too light for settling to be important, and thus fiber settling was not considered.
particulate debris, especially larger particulate species, such as paint chips and rust flakes, have been shown to settle very rapidly. The key parameters for calculating settling, other than the particulate sizes and densities, are the suppression pool volume, the volumetric flow rate through the suppression pool, and the effective particulate settling velocities (based on degree of pool turbulence). For conservatism, this settling was not included in the strainer head loss analysis.
110 wever, it was considered in the strainer margia assessment performed as described in Section 5.5.
5.4    Suction Strainer Blockage and Head Loss The strainer head loss, given the deposition of a certain amount (and type) of Gbrous uebris and a certain amount (and type) of particulate debris on the strainer, was calculated using a modified NUREG/CR-6224 head loss model. The head loss model in NUREG/CR-6224 was modified by our cor.sultants, ITS, to provide a correlation for s a aers having different geometries and for strainers with a heavy fiber loading. This modifie      ,ad loss model explicitly treats the etTects of:
15
 
Cruns wkk Plant linit 2 ECCS Suction Strainer Replacement Pro}ect NRCBulletin WO3 FinalReport                                                                                _
e    stacked disk strainer geometry for both light fiber loads (when the entire stacked disk surface area is accumulating debris) and for heavy fiber loads (when the fibrous debris is simply _
building up on the outside of a cylindrical shape);
e    debris deposition on the outside of a cylinder rather than a flat surface (resulting in a reduced bed thickness);
e    different fibrous debris constituents;
* difTerent types and quantities of particulate debris constituents; e    overall bed porosity (e.g., compression); and e    the effect of different fluid temperature and flow rate.
It should be noted that the last four items listed above are also explicitly treated in the original NUREG/CR-6224 head loss model.
The model was developed to represent the physics and fluid flow behavior expected for thick fiber beds (with particulates) on large passive strainers. A comparison of model predictions to the results of the PCI strainer tests performed at EPRI has been performed. This comparison showed agreement to within ~10% for a wide range of fiber quantities, and sludge to fiber mass ratios.
A complete description of the modified head Icss model is presented in Appendix A. This modified head loss model and a comparison of the model predictions with results from tests performed at EPRI were presented to the NRC Staff by ITS on February 18,1997.
A copy of the PCI Test Reports is included in Appendix B.
As discussed in Appendix A, the strainer head loss was calculated considering the time-dependent debris build-up on the strainer. This buildup of debris on the strainers, which occurs over a finite time interval, is primarily determined by the volumetric flow rate through the strainer as compared to the suppression pool volume.
The strainer head loss, due to the deposition of RMI across the strainer surface, was calculated using RMI head loss model presented in Appendix B to the URG.
5.5    ECCS Pumps NPSH Margin Assessment The existing NPSH margin cakulations were revised to incorporate the head loss through the            .
debris, calculated as described above, the head loss through the strainer itself, and the head 40ss through any interconnecting piping components required to connect the strainer to the existing ECCS pipe connections. The revised NPSH margins were then determined and shown to be positive.
16
 
Brunswick Plant Unk 2 ECCS Suction Strainer Replacement Pro}ect MtCBulletin W0.1 FinalReport An assessment was also made as to the conservatisms contained in the NSPH margins calculated above and the additional margins that would be shown to exist, without these consenatisms included, nis included consideration of other time-dependent aspects of strainer performance, including:
* the efficiency with which the fibrous debris can trap particulate debris while the debris build-up is occurring and prior to the development of very thick beds; and
* the settling of particulate hr the suppression pool, which occurs during the finite time of debris build up on the strainer.
+
The BLOCKAGE computer program was utilized to quantify this debris bed efficiency and particulate sedimentation his assessment provides additional assurance that RHR and CS systems can perform their intended functions.
He potential for air / steam ingestion into the strainers during an SRV T-Quencher discharge and the potential for vonexing was assessed and was determined to be acceptable. Minimum water levels for varicus RilR and CS flow conditions specified in the EOPs were revised for the replacement strainer design ar.d performance requirements, without reliance on drywell pressure.
5.6    -oction Strainer Structural Evaluation The replacement suction strainers and supports were qualified for the Mark ! submerged structure loads defined in the Load Definition Report (LDR), and other applicable loads acting on the strainers (i.e., deadweight, thermal expansion and seismic). This qualification utilized methods and parameters that are the same as described in the BSEP Plant Unique Analysis Report (PU AR) and in the UFSAR.
In the implementation of these methods, specific analytital techniques were employed to reduce unnecessary conservatism that was included in the qualiiication of other BSEP submerged structures during the Mark I Program. These analytical techniques are discussed below.
              +    in the determination of the acceleration drag volumes for the hydrodynamic loads acting on ue repla:ement strainer assemblies, a hydrodynamic mass coefficient, C., was determined for the strainers taking into account their geometry and perforated nature. As the Mark I LDR Application Guides do not prodde a coefficient for stf. merged structures with geometries similar to the replace.nent strainers, one had to be developed to eliminate excessive conservatism. The Mr.k I LDR Application Guides do provide the methodology that was used for determinatica of the C, coefficient for other submerged structures. This same methodology was employed in the development of the C, coefficient for the replacement strainers.
I                  Testing of prutotypical strainers was performed to measure C, coefficients for the Sure-Flow" stacked disk strainer design. The C, coefficients obtained from the testing of the 17
 
Brunswkk Plant UD 2 ECCSSuction Strainer Replacement Project NRC Bulletin %-0.1 FinalReport prototype strainer were adjusted to account for differences between the prototype design and the BSEP-specific design. This methodology used to determine C, assures no reduction in the safety margin for the BSEP containment and ECCS design basis.
Non proprietary copies of the test .eports on the prototype strainers, " Test Report for liydrodynamic inertial Mass Testing of ECCS Suction Strainers," No. TR-ECCS-GEN-01, Revision 2, and " Supplement I to Hydrodynamic Inertial Mass Testing of ECCS Suction Strainers - Free Vibration Analysis," No. TR ECCS-GEN-05, Revision 0, are included in
.            Appendix E. Report TR ECCS-GEN-01 justified that, for a strainer with perforated plate with 1/8" and 40% open area, the C, coefficient could be conservatively taken as 62% of the coemeient for the same " strainer without holes"(i.e., C,=1.24). Report TR ECCS-GEN-05 further examined the test results and concluded that a C, coemcient of 50% of the coemcient for the same " strainer without holes" (i.e., C,=1.00) could also be justified and shown to be conservative.
The BSEP strainers utilize perforated plate with 3/32" holes and 33% open area. The effect of the reduced hole size and flow area will tend to increase the differential pressure across the perforated plate, thereby potentially increasing the C, coefficient. This effect is equivalent to the phenomenon commonly referred to as a vena contracta. A ratio of the areas of the vena contracta was used to conservatively account for the change in the reduction in hole size and flow area. This evaluation determined a C, coefficient for the 3/32" perforated plate to conservatively be 80% of the coefficient that would be applied to the same " strainer without holes"(i.e., C,=:.60).
Giher design differences, i.e., the disk and gap sizes, and the escentricity of the disks, were conserva8vely not included in the adjustment, as these items, considered together, were judged to rence the C, factor.
* In the development of specific load combinations, the BSEP PUAR typically used conservative, bounding combinations to simplify the analysis processes. Typically, jet loads and drag loads (due to either LOCA and CRV discharge) were conservatively added together.
These components ofload do not occur simultaneously. Therefore, this practice was overly conservative. The analysis of the replacement strainer assemblies more appropriately considers the maximum of the jet load or the drag load when determining the maximum loads due to a specific phenomena.
Additionally, the analysis for the replacement strainer assemblies combined independent loads using a square-root sum-of the-squares (SRSS) combination method. Although this combination method was not used in the analysis of the previously installed strainers, it was rsed on certain components in the original Mark I design at BSEP, when it was required to remove excessive conservatism. It was part of the original Mark I design basis and its use was documented in the PUAR.
* For the Chugging load case, the original analysis conservatively combined the effects of each harmonic absolutely, regardless of sign or phase. The analysis for the replacement strainer assemblies takes credit for the random nature of the phasing of each of the harmonics using a combination method developed during the Boiling Water Reactor 18
 
a i
Brunswick Plant Unit 2 ECCSSuction Strainer Replacement Pro}ect NRCBulletin Wo.1 FinalReport Owners' Group (BWROG) efforts to resolve the Mark I Program. His combination method            I i
has been shown to meet NUREG-0661 generic acceptance criteria, thus maintaining a l
consistent margin of safety with the original Mark I program.
5.7    Other Plant Struc6ral, Systems and Components Evaluation De installation of the replacement strainers affected the qualification of other existing structures, systems and components. The affected items were:
            +  the RHR and CS torus penetrations;
* the strainer support attachment points on the torus linen, e  other torus internal structures and systems in close proximity to the replacement strainers; and
            +  torus water volume, ne requalifiestion of torus penetration, liner, internal structures and piping components utilized the same methods as described in the BSEP PUAR and UFSAR. In some cases, the specific analytical techniques described above (i.e., SRSS of dynamic loads) for the strainer qualification were applied in the requalification of some of these components.
The torus water volume is decreased less than 100 ft', which is approximate!y 0.11% of the total l
water volume in the torus, due to the installation of the replacement strainers. This change has been evaluated and shown to have a negligible effect in the torus temperatures during accident l                                                                    -
conditions.
l l
l  6.0    Licensing Considerations CP&L has implemented the strainer replacement, and evaluated the acceptability of the strainers l          and the ECCS long term recirculation capability, under the 10CFR50.59 process.
A Technical Specification change, to address the change in suppression pool water volume due the installation of the repucement strainers, was submitted by letters dated July 8,1997 and August 22,1997.
No other Technical Specification change is contemplated at this time. CP&L does not plan to include torus and suction strainer surveilla: cc requirements in the Technical Specifications. As discussed in Section 2, these surveillance requirements are already contain 1 in the appropiate BSEP preventive maintenance procedures (routes). Placing these surveillance requirements in the maintenance procedures provides the appropriate level of control to insure that adequate inspection and cleaning is performed.
19
 
    . . .    .                  .. _ .    .      ._. ,- ..  . - . - . - . ~ . _ . _ _ . _ . ..  - - .
J Crunswkk Mont U:k 2 ECCSSuctkur kraker Replacement Pro}ect NRCRollerk %03 ThalRepvt i                                  ,        -            .
i APPENDICES i
e f
4
  .        -  -          , m ,
 
    . Crunswkk U:k 2 ECCSSection Strainer Replacement Project NRCBullerin %03 FinalSeport
                                                                                                                                              ~!
Appendix A Modified NUREG/CR-6224 Head Loss Correlation t
4 4
1
                                          ~ *~                            . , , . . _
 
_    , _ .              .                                      _                  .                .                          .~ .
Crunswick linit 2 ECCS Suction Strainer Replacement Project                                                                                Appendk A NRCBulletin WO3 FinalReport Table of Contents A1.0          Background and Objectives .........................................................
A1.1 Background.................................................................................................................................1 A1.2 Objectives of th e Analys i s ..... .................. ........ .... . ....................................................................... 2 A1.3 Assumptions.................................................................................................................................3 A2.0          M ethodol ogy Descripti on ....... ...... .................... ..... ................................... ....... ............................ 3 A2.1 P asic H ead Loss Correlation . ..... ................ ....... .......... ..... ............ ........................................ ...... 3 A2.2 Calculation of Effective Strainer Surfa- Area and Approach Velocity..................................... 5 A2.3 Calculation of Debris Deposition on the Strainer.. ...................... ............................................... 6 A2.4 Debris Thickness Calculation - Efrective Surface Area Consideration............................ ........ . 6 A2.5 Debris Bed Thickness Calculation - Surface Geometry Considerations..................................... 7 A2.6 Consideration ofimpact of Strainer Supports .................. ......................                                              ................8 A2.7 Calculation of Average Debris Surface to Volume Ratio................ ........... ............................... 8 A3.0          References................................................................,................................................,,,....,,,....,10 l
l i
l t
(
l l
l-                  .
ii f
 
Brunswkk Unit 2 ECCSSuction Strainer Replaceneent Project                                      AppendeA NRC Bulletin W03 FinalReport                                                                                  } ;
A1.0 Background and Objectives There is a concern that during e design basis loss of coolant accident (LOCA) in a Boiling Water Reactor (BWR), the strainers at the suction inlet to the Emergency Core Cooling System (ECCS) could become sufficiently clogged widi debris generated during the LOCA as to cause cavitation and failure of the ECCS pumps. This debris consists of drywell piping insulation (fibrous or metallic) loosened as a result of the LOCA forces and transported to the suppression pool, sludge that has built up in the suppression pool during the reactor's steady-state operation, and other particulate debris sources such as dirt and dust, loosened paint chips, and loosened rust. In order to assess the performance and adequacy of such strainers, one must be able to predict the head
              !oss across those strainers during the accident as a function of the time dependent debris buildup and the time-depeadent ECCS flow and coolant temperature. At the present time no single tool exists that can predict such head loss under the full range of potential strainer debris loading conditions.
A1.1  Background Ihe most comprehensive assessment of the phenomenological issues that impact strainu head loss and the poti ntial computational models that could be used to predict such head loss was conducted by the Nuclear Regulatory Commission (NRC) and documented in the NUREG/CR-6224 report entitled " Parametric Study of the Potential for BWR ECCS Strainer Blockage Due to LOCA Generated Debris" [Zigler,1995). The correlations presented therein were shown to be applicable to a wide variety of situations involving head loss due to flow through a fibrous debris layer. The BLOCKAGE computer code [Rao,1996] was subsequently developed to implement the modeling approaches recommended in NUREG/CR-6224.
The BLOCKAGE code provides a very comprehensive assessment of
              . the time-dependence of debris transport from the BWR drywell to the suppression pool, e    the buildup of debris on the strainers as a function of pump flow rate and pool water vdume, e    the potential reduction in debris buildup as a result of sedimentation to the floor of the suppression pool, e    the potential reduction in the buildup of particulate debris as a result of less than perfect filtration of such particulate by the fibrous debris, and
* the head loss resulting from the flow through the deposited debris.
Ilowever, the BLOCKAGE code was developed under the assumption that the surface area of the strainer could be treated as a constant, user supplied input to the analysis, with the Febris buildup being calculated as though the strainer could be represented as a flat surface with the same surface area. This simplifying assumption is sery valid in the case where one has a large surface area relative to the debris volume, such that only a thin debris layer would be calculated.
Ilowever, in the case where one has a large volume of debris, with a complex strainer geometry involving s.acked disks and curved surfaces, the BLOCKAGE approach to debris deposition is no longer valid. There are two principal reasons for this:
1
 
l Appendte A BrunswicA Unis HCCSSuction Strainer Replacement Project NRC Bulletin WO3 nnel Report
: 1. A stacked disk straiott has a very large surface area relative to the overall strainer volume.
With large volumes of fibrous debris, the interstitial gaps between the disks can become filled with debris. When that occurs, the effective surface area of the strainer for additional debri s deposition is reduced to the circumscribed area of the strainer.
: 2. For thick layas of debris on the outside of a cylindrical shape, the debris thickness relative to the debris volume is a function of the surface cuevature, and is less than tne thickness that would result from deposition on a flat surface of the sarne area.
The NUREG/CR 6224 methodology accounts for the head loss due to particulate material trapped within a fibrous debris bed through a decrease in the bed porosity as well as through the use of a modified (" average") debris bed surface to volume ratio. A suggested approach to determining the average surface to volume ratio is not provided.1lowever, the BLOCKAGE code allows for the use of either a simple volume averaged value, or simply the fiber value. For a paniculate material such as sludge, whose surface to volume ratio is not too difTerent from that of Nukon fiber, these two approximations do not have a large impact on the calculated result.
This is not the case, however, for a small surface to volume ratio particulate species such as paint chim. In tHs case, the approximation of keeping the surface to volume ratio fixed at the value e mer has the effect of treating tu paint chips as though it were simply more fiber. This tends to signi"cartly overpredict the impact of paint chips on head loss. Even wor >e, however, is the use of a simple volume average. It can be shown that because of the form of the NUREG/CR-6224 head loss correlation, the use of simple volume averaging actually leads to the non physical result that the addition of a small surface to volume ratio particulate such as paint chips actually reduces the calculated head loss. This is clearly an unacceptable, non conservative result. It had not been identified as a problem previously, because the NUREG/CR 6224 correlations had not previously ocen applied to paint chip particulate.
A1.2    Objectives of the Analysis in light of the limitations in BLOCKAGE identiGed above, and the fact that no near term revision of the BLOCKAGE code has beeri publicized, a modified NUREG/CR 6224 head loss correlation was developed that could be used to assess stacked disk strainer performance under heavy fiber load with signiGcant quantities of arbitrary paniculate (including small surface to volume ratio material such as paint chips). This m:thodology incorporates the following features:
e      head loss estimates based on the identical basic head loss correlations used in DLOCKAGE, e      time dependent debris build-up on the strainers based on strainer flow rate and pool water volume as in DLOCKAGE (with all debris assumed to be suspended in the suppression pool at time zero),
e    use of the full strain:r surface area for debris deposition until the gaps between the stacked disks are filled with debiis, e      use of the strainer circumscribed area for further debris depasition after the gaps are filled, e      a calculation of debris thickness on the outside of the circumscribed area that accounts for the surface cun ature, and 1
l
 
Brunswick l! nit 2 ECCSSuction Stoainer Replacement Project                                                Appendit A NRC Rulletin 96-0.1 Final Report
* implementation of an averaging algorithm for the debris surface to volume ratio that is consistent with the basic head loss correlations.
A1.3          Assurnptions The methodology relics on the same basic head loss correlation documented in NUREG/CR-6224 and implemented in the BLOCKAGE code. Validation of that correlation as documented in NUREG/CR 6224 and in the 11 LOCKAGE Code validation report [Shaffer,1996) is sssumed to be applicable herein.
A detailed discurslon of stacked disk strainers is not provided herein. The reader who is not familiar with the basics of that strainer geometry is referred to the Performance Contracting, Incorporated (PCI) report " Summary Report on Performance of Performance Contracting, incorporated's Sure Flow Suction Strainer with Various Mixes of Simulated Post LOCA Debris,"in Appendix B.
A2,0 Methodology Description As discussed in the section on objectives, this methodology is based on the identical head loss correlation described in NUREG/CR 6224. Any enhancements are implemented exclusively in the calculation of certain terms in that correlation. The methodology is set up to perform the strainer performance assessment for one strainer at a time. Thus, if multiple strainer designs are being evaluated (such as Residual llent Removal and Core Spray, for example) a separate analysis would need to be performed for each one.
A2.1            Itasie llead less Correlatlan The NUREG/CR 6224 head loss correlation is described in detail in Appendix B to that PCI report and is a semi theoretical head loss model. The correlation is based on the theoretical and experimental research for the pressure drops across a variety of fibrous porous media carried out since the 1940s. This head loss model, proposed for lamina , transient and turbulent flow regimes through mixed debris beds (i.e., debris beds composed of fibrous and particulate matter) is given by:
All =A [3.5 S,8 u, " (1 + 57 u,' ) U + 0.66 S, n /(l a.) p U2 ) AL, w here.
All is the head loss, S, is the average surface to volume ratio of the debris, p is the dynamic viscosity of water, U is the fluid approach velocity, p is the density of water, n, is the mixed debris bed solidity (one minus the porosity),
3
 
i Brunswkk Unk 2 ECCS Suction Streker Replacenwnt Pro}rct                                                                    Appendk A NRC Buneten N-M That Report M is the mixed debris bed thickness, and i
A is a unit conversion factor (A = 1 for SI units),
The mixed debris bed solidity is given by:                                                                                                        j r
f                    3 a* a              1+b y
                                        <          Pr > a* AL,                                                                                                            ,
where, a, is the w fabricated fiber bed solidity, M, is the theoretical fibrous debris bed thickness, 9 = m,/m ris the paniculate to fiber mass ratio in the debris bed, pf is the fiber density, and p, is the average paniculate material density.
For N, classes of particulate materials, m, and p, are defined by:
x, m, = [ m, and x
0o l's p,=
I's where m,, p, and V, are the mass, density and volume of a particulate material i.
Compression of the fibrous bed due to the pressure gradient across the bed is also accounted for.
The relation that accounts for this effect, which must be satisfied in parallel to the previous equation for the head loss, is given by (valid for (MUM,) > 0.5 ft water / inch insulation):
M, = 1.3 M (.;
* 1/ M.)* "
For very large pressure gradients, the compression is limited such that a maximum solidity is not exceeded. In the NUREG/CR-6224, this maximum solidity is defined to be n, = 65 IWft'/p, 4
      . _ . -        .              _                      _ - . _ .  . _ .      . _ _ _ _- _                  ~.. _ . - . _  _ _ _ _ _ . . _ _ _ _ _ . . _ .
 
Brunswkk Unsr 2 ECCS Section Stenker Replacenunt Pro}ect                                                            AppendixA NRC Bulletin W0.t finalReport I
S which is equivalent to having a debris layer with a density of 65 lb/A . Note that 65 lb/ft'is the macroscopic density of a granular media such as sand or gravel and clay [Baumeister,1958). In this methodology, ot.is considered to be a user specified input parameter, it should be noted that there are indications that this fonnulation for debris bed compression may overpredict compression significantly in the case of very thick debris layers. As such, it is possible to ignore compression in this analysis if so desired.
The NUREO/CR 6224 model assumes that the debris is uniformly distributed on the strainer surface. For flat disk strainers and thin layer beds (M s 0.125 inches), that correlation is known to over predict the results [Zigler, et al,1995). Thus, a minimum debris thickness equal to the strainct perforated plate hole diameter is assumed to be necessary to result in any measurable head loss.
in the above formulation, three parameters have as of y6t not been r.pecified; the approach velocity, U, the theoretical (uncompressed) fibrous debris bed thickness, M,, and the average surface to volume ratio, S,. The approach velocity is a function of the user specified volumetric flow rate through the strainer ofinterest and the effective strainer surface area. The debris bed thickness is a function of the volume of debris on the strainer, the effective strainer surface area, and the strainer surface curvature.
A2.2          Calculation of Effective Strainer Surface Area and Approach Veloelty The fluid approach velocity, U, is given simply in tenns of the volumetric flow rate and the effective surface area as U=S  A w here, Q is the volumetric lbw rate through the strainer, and A is the effective strainer surface area.
The effective area. A, is a function of the debris bed thickness, M . While the debris bed thickness is calculated to be less than half the width of the gaps between the disks (a user-defined input parameter), the effective area is simply set equal to the full (perforated) surface area of the strainer, A,. This area is calculated based on simple geometrical considerations of the strainer dimensions. Once the debris thickness exceeds half the gap width, the effective surface area is reduced to the circumscribed area of the strainer, A,(i.e., ignoring the surface area within the gaps between the disks). Note that in both cases, this area calculation accounts for the perforated surfaces on the ends of the strainer.
S
 
Bruzswk A U@ 2 ECCS S:ction Stederr Replacement Project                                              Appendle A
(                                                    NRC B:lletin 96-03 Find Report A2.3    Calculation of Debris Deposition on the Strainer                                                          f w
The total quantities of Gbrous debris and particulate assumed to be suspended in the pool at time zero must be specified. In order to calculate the rate at which this debris accumulates on the          '
J strainer being analyzed, one has to specify the volumetric flow rate through the strainer, Q, the        f total volumetric flow rate through all strainers, Q,, and the total pool water inventory, V g.
E I
If the flows are constant, the fractional debris deposition on the strainer at any given time, t (user input), into the accident can be calculated assuming that
* all debris in the pool stays uniformly distributed in the pool, e    debris is removed from the pool (deposited on all strainers) in proportion to the rate at which water is removed from the pool, and
* debris is deposited on the strainer being analyzed in proportion to the flow rate through that strainer versus the total Dow rate.
This results in the following equation to describe the fractional debris buildup on the strainer of interest:
r g 3f 3                                                    j FRAC =                                  g""
* I)                                                      ^
                                                                            \ 0m >              <1 - exp( l,e,a >
This fraction is applied uniformly to the total quantity of each debris constituent (Ober and all particulate species) initially in the pool.
In the case where a time-dependent (but step-wise constant) flow is specined in the input, each time interval of constant flow (out to the user specined problem time) must be treated separately.
The fractional deposition during an interval is applied to the total debris quantity in the pool at the start of the interval. These deposition quantities can then be summed to yield the total deposition for the problem time ofinterest.
A2.4    Debris Thickness Calculation - Effective Surface Arca Consideration As a rough approximation, it would be expected that the theoretical Gbrous debris bed thickness on the strainer would be given by the volume of fibrous debris divided by the strainer surface area. This is in fact the approach taken in the DLOCKAGE code. In the case of a stacked-disk strainer, however, it is clear that the debris thickness would be underpredicted by such an approximation once the gaps between the ciisks are 611ed. Thus, the calculation of theoretical Obrous debris bed thickness is secomplished in two steps.
Sitn.1 it is first necessary to calculate a theoretical debris thickness using the entire strainer surface area, A,. Thus, for a total fiber volume on the strain:.r of V,, the thickness would be calculated as AL, = V,/ A,.
6
 
i O            Brunswica Unk 2 ECCSSuction Streker Coplaceneent Prefect                                                      AppendkA V            '; tc saueth nn rhetReport                                                      - .
i I                                                                                                                                          '
If following the correction for bed compression, the debris bed thickness, M., is less than half the width of the gaps between the disks, the calculation of theoretical debris bed thickness is complete.
Stp.2 If the thickness calculated in step i exceeds the maximum allowable, a correction must be made. To do this it is necessary to first calculate the volume of fiber in the gaps between the disks. This is simply given by                                                                                        j V' = V ,,,* M / M ,,                                          g where, V,,, is the total volume associated with the interstitial gaps between the disks. This quantity is calculated explicitly based on the strainer geometry.
The theoretical '.hickness of the fibrous debris on the outside of the strainer circumscribed surface is then given by M. = (V, V') / A, where, A, is the circumscribed surface area of the strainer.
A2.5    Debris Hed Thickness Calculation - Surface Geometry Considerations The discussion on fibrour debris bed thickness above is only qualitatively correct. Simply dividing the fiber volume by the strainer surface area would accurately predict theoretical thickness if the surface geometry were planar. This is a good approximation when one is dealing with the entire strainer surface area (step 1 above), since much of the disk area is associated with the Cat surfaces of the disks. !!owever, once the gaps are filled, most of the deposition effectively occurs on the outside of a cylinder, and the debris bed thickness is less than that predicted above.
To account for this effect, the debris bed thickness can be calculated assuming that the thickness of the Ober is the same on the ends of the strainer (Dat surfaces) as on the outside of the circumscribed cylindrical surface. With this assumption, it is a simple matter to calculate the total volume of a debris layer of thickness M., equate this to the known volume of the fiber (reduced by the fiber in the interstitial gaps), and solve the resulting quadratic equation for the debris bed thickness This result is given by:
                                ](nDH + Af ,)' + 4(V, - V') n# -(nDH + A,,)
                              ~
2nH where, 7
 
BrunswkA Unk 2 ECCS Suction Stenener Replacement Pro}ect                                            AppendkA NRCBuHerin W93 fkalReport D is the outer diameter of the strainer disks (assumed to be uniform axially),
11 is the active length of the strainer, and Au is the flat surface area of the ends of the strainer (calculated automatically).
This equation is used in lieu of the simpler expression previously presented in Section 2.4 under step 2.
A2.6  Consideration ofImpact of Strainer Supports For relatively small strainets, the only connection to the strainer is the inlet tube to the strainer.
liowever, for larger strainers, it may be necessary to provide additional supports for structural reasons. Depending on the details of these supports, they can have a sman impact an the effective strainer total surface area, AA , circumscribed area, AA,, and interstitial gap volume, AV,,,, To account for this effect, it is necessary to quantify these reductions. These reductions are then taken into account in the calculation of theoretical debris bed thickness by setting A, -+ A, - M, in the equation describing the debris thickness over the entire strainer surface area (Section 2.4, Step 1), and setting nDH H -+ H ' nDH - M, y,,    y ,, , l're  b l's.<-
an in the equation describing the debris thickness once the interstitial gaps have been filled (Section 2.5).
!        A2.7    Calculation of Average Debris Surface to Volume Ratio l
The intuitive choice for the average surface to volume ratio is to use a volume weighted average, l
which is equivalent to defining the average by the total surface area divided by the total volume.
Unfortunately, such a choice is inconsistent with the formulation of the basic head loss correlation and can lead to non physical results. Presented below is a simple derivation of an alternate averaging scheme that provides consistency with the basic form of the correlation.
Consider a fixed volume V that contains a mixture of two types of fiber, I and 2. We take S, = the surface to volume ratio of fiber type i, and q, = the quantity (microscopic volume) of fiber type i.
8
    --                                          - .                                      . ..      . . .        . . . _    _ - ~      _
 
I Smnswich Unir 2 ECCSSection Streiner Replacenennt Project                                                                                                                  AppendteA NRC RnHerin Wo.1 FinalReport                                                                                                                                                                l If we consider the dominant terms in the relationship for the pressure drop, AP, we have                                                                            i I
AP ~ <S>' ' a''                                        ,
where,
                              <S> is the average surface to volume ratio, and u is the total solidity which is (qi + q:)/V.                                                                                                                        .
We now consider the following argument for determining the average <S>. Rather than having the two fiber types well mixed in region V, consider that the fibers are separated into adjacent regions V, and V,(with the constraint that the sum of the two volumes be V), with part of the flow going through each region (in parallel rather than in series). We now postulate that the flow velocity through each of these two regions is the same, and that the pressure drop scross the two regions is also the same. This allows us to solve for the relative volumes V                                                        i and V . One form of this relationship can be expressed as r    ) 4/3 l'
                                  =1+b                      -
6 e, s S, s If we once again consider the two fiber constituents to be well mixed, and again postulate the pressure drop to be unchanged and the flow velocity to be unchanged, we can equate the pressure drops calculated from the two different viewpoints to yield r 5r (S)" = S "        A h,3 r9 > \l).      s Substituting for the volume ratio, we are left with 1/4 (S) =
b S,"+bSl''                      .
9          9              .
It is obvious how this result could be extended to more than two such fiber species. We further make the assumption that the same formulation can be applied when some of the species are particulate rather than fiber.
l l
l 9
(  - -                                    _
 
Brunswka Unk 2 ECCSSuction Streker Replacenunt Pre}ect                                                          AppenneA NRC BnHemin WO3 That Report A3,0 References Baumeister T., " Mechanical Engineers' Handbook," Sixth Edition, McGraw ifill Book Company,1958.                                                                                                    ;
w 11stt, G., "De Development and Testing of Performance Contracting. Inc.'s Sure Flow Stocked Dis A Suction Straincr," Performance Contacting, Inc., February 1,1996.
Rao, D. V., et. al., " BLOCKAGE 2.5 User's Manual," NUREG/CR 6370, U.S. Nuclear Regulatory Commission, December,1996.
i Shaffer, Clint, et, al., " BLOCKAGE 2.5 Reference Manual," NUREG/CR 6371, U.S. Nuclear Regulatory Commission, December,1996.
Zigler, G., J. Brideau, DN. Rao, C. Shaffer, F. Souto, and W. Thomas, " Parametric Study of the Potentialfor BWR ECCS Strainer Blockage Due to LOCA Generated Debris," NUREGICR.
6224, U.S. Nuclear Regulatory Commission, October 1995.
10 t
 
Brunswick Unk 2 ECCS Seenm .kreiner R:;'::enunt Pre} ret                                                                        4penk R                  I NRCBulletin W03 FinalReport 1
Appendix B                                                                                  <
Sure-Flowm Strainer Performance Test Reports Reports:                                                                                                                                                f
: 1.              Performance Contracting Inc. Report," Summary Report on Performance of Performance Contracting Inc.'s Sure-Flow Suction Strainer with Various Mixes of Simulated Post.
LOCA Debris," Revision 1 dated September 19,1997.
: 2.              Cuatinuum Dynamics, Inc. Report," Performance Contracting, Inc. ECCS Sure-Flow" Strainer Data Report," No. WO4536-01, Revision 0, dated December 1996.
I
--,,.- .            -        -  .-.. ,              ..                                                                                . . - -  - . _ _ _ _ _ .}}

Latest revision as of 00:01, 2 January 2021

ECCS Suction Strainer Replacement Project,Nrc Bulletin 96-003,Final Rept
ML20202E044
Person / Time
Site: Brunswick Duke Energy icon.png
Issue date: 11/06/1997
From:
CAROLINA POWER & LIGHT CO.
To:
Shared Package
ML20198Q855 List:
References
IEB-96-003, IEB-96-3, NUDOCS 9712050227
Download: ML20202E044 (35)
v • d • e


Text

--

Crunswkk Mont it32 ECCS Sueden Strainer Replacement Project NRCBulletin 96-03 FinalReport P

4 Brunswick Plant, Unit 2 ECCS Suetion Strainer Replacement Project NRC Bulletin 96-03 Final Report i

e r'

9712050227 971106 PDR 0 ADOCK05000324k

\y PDR r 4\'T

{ _: .

Brunswkk Plant I):lt 2 ECCSS ctica Strainer Replacement Project N.'CC:lletin WO3 FinalReport

\

Table of Contents 1.0 Introduction.........................................................................................................................................1 2.0 Potential ECCS Suction Stralner Blockage issue Resolution Approach ........................................1 2.1 S u ctio n S t raln e r Re place m e n t.................. . .... . ..... .... ... ..... ................. . ............................ .... . .'.......... 2 2.2 Su ppression Pool aad ECCS Strainer Clesniiness ................. ...................................................... 3 2.3FMEProgram.................................................................................................................................5 2.4 ECCS Pump Technical Specification Surveillance Data Review................................................. 4 i 3.0 Plaat Co n figuratio n and Design Pa ra m ete rs .................. .................................................................. 4

..............4 3.1 Co n t a i n m e n t Desig n . . . . .. . . ........ . .. ..... .. . . .. .. .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . ..

3.2ECCSDesign.................................................................................................................................5 3.3 D ryw e ll I a s ula t io n . .. . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . . .. . . ..... .. ... . .. ... . .. . . . . . . . .. . . . . . . . . . . . . . . . . .

3.4 O t her Pot e n tial D rywell Deb ris So u rc es .. ....... ..... ............... .. .................. ............. . .... ..... ..10 3.5 Polen tial S u pp ression Pool Debris Soi s rces .............................. ...... ......... .............. .................. 11 4.0 Replacement St rainer Design Requlrerr ents .. .... . ................. ....... . ............ ...... ............. .. ......11 4.1 St rain e r Fu n etio n al Req ulre m e nts .. ......... . . ....... . ....... .. .. ...... . ... ... ...... ..... ....... . .... .......11 4.2 S t mi n e r Co d e R eq u i re m e n ts . . .. . .. .. . . . .. . . . . . . ... . .. . .. . . . . . ... .. . .. . . . . . . . . .. .. .... . .. . . . .. . .... ... . .... 12 4.3 Strainer and Strainer Support Strv etural Requirements . .... .. . ... ...... ...... .. ........................13 5.0 Replacement Strainer /ECCS Long Term Recirculation Evaluation Methodology.... . .. . . . ...13 5.1 D rywell Deb ri' G eneratio n....... . . ... .... .... . . ... .... . . . . . . . . . . . . . . . . . . ..........................13 5.2 Dryw ell Debri Transport .... ... . ...... .. ............ .. . . ........... .....................................15 5.3 Suppression Dol Debris Transport.... . ........ . . . . . . . . . . . .........................................15 5.4 Suction St raln er Bloc ka ge and 11end Loss .. . ...... . . ...... . ...... ... ............. . ...... ......... ....... .15 5.5 ECCS Pumps NPSil Margin Assessment ................ . . ... ... ........................................16 5.6 Suction Strainer St ructu ral Evaluation . ..... .... . ....... . ... . .. . .. .......... .... . . . ... ............ .17 5.7 Other Plant Structural, Systems and Components Evaluation .. . ..........................19 6.0 Licensing Coasiderations..... . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... ..........................19 APPENDICES A - Modified NUREG/CR-6224 IIcad Loss Correlation B - Sure-Flow" Strainer Performance Test Reports C - BSEP Unit No. 2 Replacement Suction Strainers Design Drawings D - BSEP Unit No. 2 Torus Design Drawings E - Strainer Ilydrodynamic Mass Test Reports 11

Bru:;swick Plut Unit 2 ECCS S:ction Ste:Iner Rept: cement Project NRCBulletin 9M3 FinalReport _

1.0 Introduction This document constitutes Carohna Power and Light (CP&L) Company's final report in response to the NRC Bulletin 96 03," Potential Plugging of Emergency Core Cooling Suction Strainers by Debris in Boiling Water Reactors," dated May 6,1996, for the Brunswick Steam Electric Plant (BSEP), Unit No. 2. An initial response to Bulletin 96-03 was provided in CP&L Letter, No.

BSEP 96 0364, dated November 1,1996. A supplemental response was provided with CP&L Letter, No. BSEP 97-0209, dated June 5,1997. This final report provides the details of: 1) the approach CP&L has implemented to resolve the potential Emergency Core Cooling System (ECCS) suction strainer blockage issue identified in Bulletin 96-03; 2) the criteria CP&L has imposed in the design and evaluation of the replacement strainers; and 3) the methodology CP&L has employed in the evaluation of the replacement strainers and demonstration of the long term ECCS recirculation capability. This final report is submitted to the NRC as required by Bulletin 96-03.

As described in Section 2, CP&L's primary action for resolution of the potential ECCS suction strainer blockage issue was the replacement of the previously mstalled suction strainers with large, passive strainers. CP&L, together with Duke Engineering and Services, Inc. (DE&S),

innovative Technology Solutions Corporation (ITS) and Performance Contracting, Inc. (PCI),

has completed the design, engineering, fabrication, installation and testing of the replacement strainers for BSEP Unit No. 2.

This report also serves as an interim report for BSEP Unit 1. The strainer design for Unit I is planned to be similar to the design for Unit 2. Most of the engineering analyses performed for Unit 2, as documented in the design calculations, are applicable to Unit 1. Any differences in the strainer design or design parameters for Unit I will be reconciled in the calculations, as appropriate. The engineering and fabrication activities for Unit I will be initiated in the fourth quarter of this year. The refueling outage for Unit 1 is scheduled to start on May 2,199f..

2.0 Potential ECCS Suction Strainer Blockage Issue Resolution Approach CP&L's approach for resolution of the potential ECCS suction strainer blockage issue involves four actions:

  • Perform periodic suppression pool and ECCS suction strainer inspections and cleanings, based on the prelicted sludge generation rate.
  • Maintain an effective foreign materials exclusion (FME) program to ensure the cleanliness of the suppression pool,
  • Perform reviews of data obtained during the quarterly Technical Specification surveillance of the ECCS pumps.

These four actions are presented in more detail below.

1

Brunswkk Plant Un 2 ECCSSuction Streiner Replacement Project NRCRulletin W03 FinalReport 2.1 Suetion Strainer Replacement CP&L has replaced the existing suction strainers on the RHR and CS Systems with large, passive '

suction strainers, a stacked disk strainer design, for strainers. CP&L selected PCI's Sure-Flow the replacement strainers (see Appendices B and C). The main design features of the Sure-Flow" suction strainers are as follows:

  • The disks are fabricated from stainless steel, perforated plate. The holes in the perforated plate are selected such that a maxin.um particle size that can pass through the strainer is less than the minimum orifice size in the RHR and CS systems. The perforated plate can be cleaned under vater. Stainless steel is utilized to prevent degradation of the strainers due to corrosion.
  • The disks are attached to an internal core tube that provides flow control capabilities. Holes are cut in the core tube and are designed such that uniform flow is achieved along the entire strainer length and the flow across the perforated plate is laminar, thereby reducing head loss.
  • The core tube also acts as a structural backbone, capable of resisting the large submerged structure hydrodynamic loads that are postulated in Boiling Water Reactor (BWR) suppression pools. Use of the core tube allows relatively long strainer lengths to be installed. Supports are provided to increase the load carrying capacity of the strainers. The supports attach directly to the core tube.
  • The strainers are designed to prevent vortexing. This has been confirmed by prototype testing.

f The Sure Flow" strainers have been tested to demonstrate their hydraulic performance. These

! tests were conducted at the Electric Power Research Institute (EPRI) Non-Destructive Engineering (NDE) Center in Charlotte, NC. The tests included low and high fiber quantities, with and without particulates. Testing of mixed fiber and reflective metal insulation beds was also performed. These tests demonstrate the ability of the Sure-Flow m trainers s to perform l under various debris loadings, and also demonstrate that the analytical methods developed to evaluate strainer perbrmance provide acceptable results. The reports documenting the tests performed and the results obtained are included in Appendix B.

The replacement strainers have been sized to provide sufficient surface area to ensure that the l

head loss through the fouled strainers, for the design basis events, is acceptable. The strainer sizes for each system loop, in terms of strainer diameter, length and surface area, are provided in the table shown below. Also shown is the approximate increase in surface area, as compared to the existing strainers.

er l

l 2

Crunswkk Mont Unk 2 ECCSSuction Strainer Replacement Project NRCBelletin %-M FinalReport Approx. Approx. Approx. Increase System Replacement Strainer Replacement Strainer in Surface Ares

  • Diameter x Length
  • Surface Ares
  • RHR 45" cia. x 16' 9"long 529 ft3 1550 %

CS 45" dia. x 6'-0"long 245 ft2 1530 %

Pump Strainer While Operating in Suppression Pool Cooling Mode," CP&L committed to develop a long term program for inspecting and cleaning tne Brunswick suppression pools and ECCS suction strainers. This program has been oeveloped and is documented in Preventive Maintenance (PM) Routes APUll 001 and API'L 001, for Unit Nos. I and 2, respectively. These routes require periodic inspection' of the suppression pool for sludge accumulation. If the sludge accumulation limit specified in the routes is . cached, then suppression pool cleaning is scheduled and performed in accordance with guidelines given in the routes. The routes also require periodic physical condition inspections of the ECCS suction strainers, at the same interval as the suppression pool inspections. The strainers are cleaned, if required by the route criteria.

The sludge accumulation limit specified in the PM routes was selected to ensure that the maximum sludge accumulation in the suppression pool does not exceed the design value used in the strainer performance evaluation (refer to Section 3.5). The sludge accumulation limit was established based on sludge generation rates determined from data obtained from previous BSEP suppression pool inspections /cleanings and from review of other industry data. These sludge generation rates will be reviewed as additional data is obtained from upcoming suppression pool l

l inspections and the inspection intervals and sludge accumulation limit may be adjusted accordingly.

l 2.3 FME Program

Also as discussed in CP&L's response to Bulletin 95-02, CP&L maintains FME and cleanliness l administrative procedures to ensure the cleanliness of the suppression pool. These procedures are 0Al-125, " System Cleanliness /FME," and 0Al-127, " Primary Containment Inspection and Closcout." These procedures control materials in the drywell, suppression pool, and systems that interface with the suppression pool, and ensure that materials that could potentially impact ECCS operations are properly controlled and prevented from entering the suppression pool.

' Currently performed each refueling outage I

3

. - . . _ - , _ _ . =.. _ _ _ -- - ~ . . - . - . - -

1 Brunswick Plant U:st 2 ECCSSuction Strainer Replacement Pro}ect l NRC Bulletin WN FinalReport 2.4 ECCS Pamp Technical Specification Surveillance Data Review As discussed in the response to Bulletin 95-02, CP&L will continue to collect and review pump suction data obtained during the quarterly Techaical Specification surveillances. This collection and review of the Technical Specification surveillance data will identify potential degradation of the ECCS systems during the period between the inspections.

3.0 Plant Configuration and Design Parameters 3.1 Containment Design 3.1.1 Containment System Design Both units have Mark I containment designs. The Mark I containment consists of the following:

  • Drywell e Pressure suppression chamber (torus) e Vent system between the drywell and torus e Isolation valves e Vacuum breakers in the event of a process system piping failure within the drywell, reactor water and steam are released into the drywell atmosphere. The resulting increased drywell pressure t. , forces a mixture of drywell atmosphere, steam, and water through the vent system, which opeas beneath the surface of the suppression pool water in the torus. The water quenches the steam, thereby controlling the pressure rise in the drywell. The drywell steam-air-nitrogen atmosphere, which is transferred to the torus, pressurizes the torus, in the event that the drywell pressure drops below the torus pressure, the torus is vented to the drywell through vacuum breakers to equalize the pressure between the two vessels.

Cooling systems are provided to remove heat from the reactor core, the drywell, the water in the torus, and from the atmosphere in the vapor space above the water. This provides continuous cooling of the primary containment under accident conditions to maintain the pressure suppression capabilities and containment integrity.

3.1.2 Pressure Suppression Chamber (Torus) Mark I Design The pressure suppression chamber (torus) is a hollow, reinforced concrete shell encircling the lower portion of the drywell containment structure. The concrete shell encloses a continuous 16-sided steel tcrus liner of circular cross sections. The major centerline d4 meter of the torus is 109 feet and the cross sectional diameter of the circular liner is 29 feet. A paper joim is provided between the bottom of the torus and the mat foundation, to allow radial expansion of the torus. Vertical keys are provided along the outside perimeter of the drywell pedestal to allow independent,' unrestrained radial expansion of the torus, when subjected to symmetric loadings.

Under asymmetric loads, the keys force the drywell and torus to respond as a single unit.

l l 4 l

- Brunswick Plant U:lt 2 ECCSSection Strainer Replacement Project NRC Bullerhe 96-N FinalReport The pressure suppression vent system connects the drywell and torus together. Eight circular vent pipes, equally spaced around the periphery of the drywell, connect the drywell to a vent header contained within the air space of the torus. Projecting downward from the header are 96 downcomer pipes, which terminate approximately 3 feet below the water level of the torus Jet deflectors are provided at the inlet of each vent pipe to prevent possible damage to the i ent line and header due to jet forces and/or missiles which might accompany a pipe break inside the drywell. Steam dischs:ged into the torus causes the pool to swell and surge upward. Vent header deflectors are provided to deflect this surge away from the vent header toward the torus liner.

Steam discharged through the safety relief valves (SRVs) enters the torus through T-quenchers.

The T quenchers contain numerous orifices through which the steam exits into the suppression pool.

A drawing i,howing section views through the torus is provided in Appendix D.

3.2 ECCS Design The ECCS include the RHR system (operating in the low pressure coolant injection mode), CS, High Pressure Core injection (HPCI), and the Automatic Depressurization System (ADS). The combination of these systems is designed to satisfy the 10CFR50.46 ECCS Acceptance Criteria.

The RHR and CS systems are the ECCS utilized for long term cooling. These systems take suction from the suppression pool, and, as such, are potentially impacted by the suction strainer plugging issue. Therefore, the RHR and CS suctions strainers have been planned to be replaced.

As the HPCI system is not utilized for long term cooling, its suction strainer will not be replaced.

A brief description of the RHR and CS Systems is provided below.

3.2.1 RHR System The RHR system consists of two essentially complete and independent loops, identified as Loop A (Division I) and Loop B (Division 11), with each loop containing two pumps and the necessary piping, valves and controls to support the various modes of operation. The suction piping of each pump on a loop ties into a common header (24" diameter pipe) which singly penetrates the torus. The suction strainer is attached to a flanged connection immediately adjacent to the torus penet ation.

, The RHR system is designed for the following six modes of operation:

. Low Pressure Coolant injection (LPCI)

  • Containment (Drywell and Suppression Pool) Spray
  • Suppression Pool Cooling (SPC) ,
  • Fuel Pool Cooling Assist 5

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

Brunswick Plant Uit 2 ECCSSuction Strniner Replacement Project NRC Bulletin 96-03 FinalReport The LPCI mode of operation of the RHR system is the only mode providing an emergency core cooling function, 'Ihis mode of operation provides a low pressure source of core cooling for the entire range of postulated bitak sizes. It'the break is small, yet high pressure systems are unable to recover level, ADS will depressurize the Reactor Vessel, thus allowing LPCI and CS injection.

For larger breaks, the Reactor Vessel depressurizes via the break which allows almost irrmediate (approximately 20 sec) LPCI injection. The LPCI mode injects via the Reactor Recirculation system discharge piping to provide a volume of water to flood the Reactor core. Automatic initiation of the system occurs from low reactor vessel water level or high drywell pressure -

(approximately 1.8 psig) " coincident" with low ienctor vessel pressure (approxirnately 410 psig).

After the core is flooded to at least two-thirds core height, one RHR or CS pump is normally required to make up for Jet Pump throat to diffuser slip joint leakage. The other pumps are stopped so that the emergency power (if there is a loss of offsite power) that was being used by these pumps may be shifted to other plant loads including RHR Service Water booster pumps.

One RHR pump and heat exchanger are normally placed in the Containment Cooling Mode aftet a line break in the drywell as per the Emergency Operating Procedures (EOPs).

Upon automatic initiation of the A(B)2 Loop of LPCI, the associated pumps should start approximately 10 seconds after the initiation signal is received. If a loss of power has occurred resulting in the Emergency Diesel Generators powering the pumps, the pumps will start 10 seconds after 'he associated diesel ties onto the emergency bus.

The A(B) and C(D)' RHR pumps take suction from the Suppression Pool through the normally open motor operated Suppression Pool Suction Valve, Ell F020A(B), and the associated motor-operated RHR Pump Suppression Pool Suction Valve, Ell-F004A(B) or Ell-F004C(D),

and discharge through the Pump Discharge Check Valve, El1-F031 A(B) or El1 F031C(D). The discharge check valves are designed to prevent backflow through the p' imp and to maintain a water leg in the discharge piping.

With the RHR and CS pumps running on minimum flow or dead headed, indicated pump discharge pressure on CS should increase to approximately 305 psig and RHR should be 202 psig. As reactor pressure decreases to approximately 410 psig, the LPCI Inboard Injection Valve, Ell F015A(B), should automatically open. As reactor pressure continues to decrease, the discharge of the RHR pumps should overcome reactor pressure below approximately 200 psig, allowing the flowpath to continue from the RHR pumps' discharge check valve directly into the Reactor Vessel through the normally open LPCI Outboard injection Valve, Ell-F017A(B), the LPCI Inboard injection Valve, El1-F015A(B), the LPCI Injection Line Check Valve, Ell-F050A(B), the locked open LPCI Manual injection Valve, Ell-F060A(B), and into the Reactor Recirculation system discharge lines. Once reactor pressure is reduced to approximately 20 psig, RHR flow should reach approximately 17,000 gpm per operating loop with two pumps.

The LPCI Outboard injection Valve, F017A(B), is a thrattle valve which may be adjusted to control flow into the vessel, whereas the Inboard Injection Valve, Ell-F015A(B), is designed for either full open or full close service. El1-F017A(B)is normally open, but with a LPCI initiation 2

A(B) Loop signifies Loop A and/or Loop B.

8 A(B) and C(D) pump signifies Pump A and'or Pump C on Loop A, and Pump B and'or Pump D on Loop B. A similar identification applies to valves.

6

Crunswkk Plant U22 ECCSSuction Strainer Replacement Project NRCBalletin W0.1 FinalReport signal present, this valve cannot be closed or throttled for 5 minutes to ensure a discharge path exists from the pumps to the vessel. Ell F015A(B), cannot be closed as long as the LPCI initiation signal is present. In addition, the RHR heat exchanger is automatically bypassed via the RHR Heat Exchanger Bypass Valve, El1.F048A(B), for the first three minutes to ensure that flow gets to the reactor through the most direct route. During the mterval of time when the RHR pumps are operating to restore the reactor vessel level, suppression pool kat removal is not necessary.

Each LPCI loop is provided with a minimum flow line to the Suppression Pool to protect the pumps from damage due to overheating as a result of low or no flow operation. This feature allows the pumps to operate with a closed discharge valve, without overheating, by recirculating Suppression Pool water through the minimum flow bypass line.

Each of the two separate loops of RHR, in the (SPC) mode, provide the primary source 0!

containment cooling. Following a Loss of Coolant Accident (LOCA), SPC is manually initiebd to limit Suppression Pool temperature and Containment pressure within design limits.

The RHR pumps receive power from the 4160 volt AC emergency auxiliary buses. For each loop, the RHR pump motor and associated automatic motor valves receive AC power from different buses.

3.2.2 Core Spray System The CS system provides a low pressure source of core cooling for the entire range of postulated break sizes. If the break is small, yet high pressure systems are unable to recover level, ADS will depressurize the reactor vessel, thus allowing CS injection. For larger breaks, the reactor vessel depressurizes via the break which allows immediate CS injection. The CS system injects via a dedicated set of spray spargers (one per loop) in a spray pattern directly over the reactor core.

The CS system consists of two redundant and independent loops. Each loop contains one 100%

capacity pump and the necessary piping, valves, and controls. The suction piping (14" diameter pipe) on each loop independently penetrates the torus. The suction strainer attaches to a flanged connection on the torus penetration. .

The CS system controls support vessel injection when reactor pressure is reduced below the injection permissive. Automatic initiation of the system occurs from low reactor vessel water level or high drywell pressure coincident with low reactor vessel pressure.

The two leops are designated as "A" and "B", with the "A" ooop assigned to Division I and the "B" Loop assigned to Division 11. Water from the suppression chamber is discharged into the reactor pressure vessel via the CS spargers. An alternative water supply to the CS loops is available from the Condensate Storage Tank, via a manual valve.

The CS system relies upon station AC power sources to power the pumps. Pump power is supplied from the as ociated station emergency 4160 VAC Bus. Each pump is powered from an independent bus with each emergency bus capable of being powered from one of three sources of l

7

Brunswick Nant U2!2 ECCSSuction Strainer Replacement Project hRC Rulleren 9643 FinalSeport AC power. Normally dering plant operation, the buses are powered from the Main Generator l output via the Unit Auxiliary Transformer (UAT). During plant shutdown conditions, and in the event this power source is lost, the emergency buses are automatically or manually transferred to the unit's Startup Auxiliary Transformer (SAT). Should both of the above fail, the station Emergency Diesel Generators automatically power the buses.

Failure of a single loop of the CS system is backed up by the other loop. Failure of one or both loops is also backed up by the LPCI mode of RHR.

3.2.3 Limiting Conditions for Strainer Performance De specific, limiting (i.e., worst case) conditions that are to be used in the evaluation of C.a strainer performance are provided below. These conditions are associated with a LOCA and loss of one emergency power system. For the governing case for RHR, this results in two RHR pumps on one loop operating at run-out conditions, and one CS pump operating at minimum Technical Specification flow. For the governing case for CS, one CS pump is operating at run-out conditions, with one RHR operating at minimum Technical Specification flow. Run-out flow is assumed to continue until up to ten minutes into the accident, when coolant level in the reactor is reestablished. Operator action is taken at that time to reduce RHR and/or CS flows, as any single RHR or CS pump is able to maintain water level in the reactor. This action is required to change the RHR mode of operation to containment cooling and/or containment spray and also assures adequate steam condensation capability by limiting suppression pool temper m et

. Worst case RHR flow until ten minutes into the accident is either: 1) 21,000 gpm 'hrough one loop, coincident with total CS flow of 4,625 gpm; or 2) 18,750 gpm through one loop, coincident with total CS flow of 4,625 gpm. At ten minutes, the total RHR flow is reduced to 11,550 gpm, which is the nominal, single pump RHR flow.

  • Worst case CS flow of 6,700 gpm, coincident with total RHR flow of 7,700 gpm, until ten minutes into the accident. At ten minutes, the CS flow is reduced to 4,725 gpm, which is the nominal CS flow.

These conditions are based on a Design Basis Accident (DBA) LOCA. The DBA conditions, in coiunction with the debris loading generated as a result of a DBA, represent the governing suction strainer design conditions.

3.2.4 Net Positive Suction Head Margins The Net Positive Suction Head (NPSH) margins (NPSH available minus NPSH required) for the replacement strainers for the RHR Loops A and B pumps (2 pumps and one strainer per loop) and CS Loops A and B pumps (one pump and one strainer per loop) were determined. The pump NPSH is affected by the suppression chamber pressure, suppression pool temperature and line losses. The losses through the previously installed strainers were removed in determining these margins. He BSEP NPSH evaluation does not take credit for suppression chamber pressure increases, so the pressure is held at 14.7 psia, for the design basis events.

8

Brunswick Plant itAt 2 ECCS Suction Strainer Repiecement Project NRCBulletin WO3 FinalReport At ten minu'.es into the accident, the NPSH margins are as follows:

- @ 21,000 gpm,161.8'F,14.7 psia: 9.4 ft. head

- @ l8,750 gpm,162.5'F,14.7 psia: 12.0 ft. head I

e CS @ 6,700 gpm,162.5'F,14.7 psia: 10.9 ft. head .

After ten minutes into the accident, the NPSH margins are as follows: l

  • RHR @ l1,550 pm,189'F,14.7 psia: 9.0 ft head l e CS @ 4,725 gpm,189'F,14.7 psia: 9.6 ft. head These NPSH margins are based on the maximum predicted suppression pool temperatures. As the head less through the fibrous debris increases with decreasing temperature, the NPSH margins at minimum predicted tempe-atures and corresponding debris loadings have been reviewed and show that the limiting condition is at the maximum predicted suppression pool temperatures.

3.3 Drywell Insulation The insulation systems used in the drywell include both fibrous and reflective metal insulation.

The specific types and approximate quantities are summarized below:

Approx. Quantity Type Unit 1 l Unit 2 Fibrous Insulation (quantities in ft')

NUKON - Jacketed 1.406 1.232 NUKON - Unjacketed 1.145 1.036 Temp Mat 46 58 Reflective Metal Insulation (quantities in it')

Trai4sco RMI(1 mit Al) 33.021 44.609 Diamond Power RMI(2 mil SS) 44.320 . 44.320 Other (quantities in ft')

l 4 4 Calcium Silicate Micro Therm 2 2 Vention (fiberglass fabric) 0.2 0.2 9

Brunswkk Plant U:st 2 EC CSSuction Streiner Replaceneent Pro}ect NRC Sulletin W0.1 FinalReport 3.4 Other Potential Drywell Debris Sources The other potential drywell debris sources can be divided into three categories. These categories are defined in Section 3.2.2 of NEDO 32686, " Utility Resolution Guidance for ECCS Suction Strainer Blockage,"(URG), Revision 0, dated November 1996. j 3.4.1 Fixed Debris Fixed debris is material that is pan of the permanent plant that becomes a debris source only after exposure to the effects of a LOCA. This in:ludes material that is blown or stripped off as a l

result of impingement forces. Material in this category are listed in Section 3.2.2.2.2 of the URG.

The other potential drywell debris applicable to BSEP are as follows.

Quantity Debris Type (ibm) Reference LOCA generated dirt / dust 150 URG, Section 3.2.2.2.3.2 Qualified paint / coatings 71 URG, Section 3.2.2.2.2.1.1 (100% epoxy)

Rust flakes from unpainted steel 50 URG, Section 3.2.2.2.3.6 Other Drywell fixed debris 15 Contingency (assumed to be paint)

Total 286 3.4.2 Latent Debris Latent debris is debris that would not be present until later in the LOCA event progression after prolonged exposure to a LOCA environment. For example, unqualified coatings (not directly impacted by the LOCA jet) have the potential to detach from the surface where applied, but only after prolonged exposure to the LOCA environment and after containment pressure is reduced later in the event. This type of debris is discussed in detail in Section 3.2.2.3 of the URG.

For strainer design purposes,6.67 ft' of unqualified paint / coatings was utilized. This equates to 628 lbm of debris.

3.4.3 . Transient Debris Transient debris is non-permanent plant material brought into the drywell, typically during an outage (e r., tools, rags, sheets, plastic bags, temporary filters, dirt / dust, etc.). Transient debris is principally controlled through FME and housekeeping programs. This type of debris is described in more detail in Section 3.2.2.2.1 of the URG.

10

. _- . ~

Brunswick Plant U:lt 2 ECCSSection Streiner Replacement Project-NRC Bulletin 96-0.1 FinalReport CP&L maintains an effective FME and housekeeping program. Dirt / dust debris is included in l the fixed drywell debris and suppression pool sludge quantities. A contingency of an additional I 4 ft' of fibrous debris is included in the strainer design to account for fibrous transient debris.

3.5 Potential Suppression Pool Debris Sources Potential suppression pool debris sources are rust particles (sludge) from the carbon steel, including the torus liner and interior surfaces of torut attached piping systems, and unqualified paint. l Based on limited BSEP specific data, with congarisons to other industry data, the sludge generation rate is currently being conservatively set at 100 lbm (dry) per year. A total sludge quantit, of 600 lbm (dry) is to be used in the strainer evaluation, based upon an essumption of three,24 month cycles of sludge generation before cleaning. It should be noted that the last sludge measurement at BSEP Unit Nos. I and 2 showed a sludge generation rate of 33 lbm (dry) per year.

The quantity of unqualified paint in the suppression chamber is included in the quantity of

t. w.ualified paint specified in Section 3.4.2.

4.0 Replacement Strainer Design Requirements 1

4.1 Strainer Functional Requirements The functional requirement for the suction strainers are as follow:

  • The suction strainers shall screen out debris particles greater than 0.095".

This partcle size limit will ensure that any particle passing through the strainer will be smaller than the orifices associated with the cyclone separaicis, and a.1 other small flow restrictions in the pump seal flush piping. This requirement is consistent with the General Electric Company Service Information Letter, SIL No. 323, " Suppression Pool Suction Strainer Mesh Size Mismatch with Emerpncy Cooling System (ECCS) Pump Seal Orifices." which provided recommendations for sizing the strainer hole size.

  • The suction strainers, including the collected debris and the interconnecting ramsheads, shall produce a total head loss not to exceed:

- For the first ten minutes into the LOCA:

- 8.4 ft water at 21,000 gpm at 161.8'F, and 11.0 ft. water at 18,750 gpm at 162.5*F for RHR;

- 9.9 ft, water at 6,700 gpm at 162.5'F for CS I1

. _~ . - . _ - - . - _. . . _ _ ~

Brunswick Plant Ucht 2 ECCSSuction Strainer Replacement Pro,%ct NRC Rulletin Wo.t finalReport

- After ten minutes:

- 8.0 ft. water at 11,550 gpm at 189.4'F for RHR

- 8.6 ft. water at 4,725 gpm at 189.4'F for CS These total head lois values will provide an available NPSH that exceeds the required NPSH for the ECCS modes of operation for RHR and CS. The debris generation, transport and head loss determinations shall be in accordance with Bulletin 96-03 and hegulatory Guide 1.82, Revision 2," Water Sources for Long-Term Recirculation Cooling Following a Loss-of Coolant Accident," unless otherwise justined. The bounding head losses for the strainer elements and the interconnecting rarr.shead are 1.06 feet of water for the RHR strainer assembly, and 0.99 feet of water for the CS strainer assembly.

4.2 Strainer Code Requirements The strainer code requirements shall be in accordance with the requirements listed below. These code requirements are considered appropriate for the intended applications and function of the suction strainers. The code edition and addenda to be utilized are to be consistent with those s*>ecified in the UFSAR.

e Quality Assurance

- 10CFR50 Appendix B

- ASME CertiGcate not required e Materials

- Conform to ASTM material specifications

- Certified Material Test Reports (CMTRs) are to be provided for all materials (except where ASME Section 111 Code would permit Certincates of Compliance)

  • Design

- Quali0ed to ASME Section Ill, Nuclear Power Plant Components, Subsection NC e Welding

- Weld procedures and personnel qualified to ASME Section IX.

- Critical structural welds examined by PT or MT per ASME Section 111

- All other welds visually examined per ASME Section Ill

  • Stamping

- NPT Stamp is not required The RHR and CS piping systems are classified as ASME Class 2 piping systems. The suction strainers are components which attach to these piping systems. As the suction strainers are non-pressure boundary components, the suction strainers are not required to be constructed to the requirements of the ASME Code (refer to Paragraphs NCA-ll30(b) and NC-2121,1985 12

I Bru'>swick Plant Unit 2 ECCSSuction Strainer Replacement Project 1 NRC Bulletin W0.1 FinalReport Edition). In fact, no code and standards have been developed or adopted that specifically define the requirements for the construction of the strainers for this application. Therefore, the above i requirements have been developed and are specified for the construction of ths strainers. These requirements rely on good engineering practice and have adopted appropriate sections of the ASME Code to assure the fabricated strainers are robust and of high quality.  !

Suction strainers constructed to the above requirements will be of a quality and reliability that are appropriate and commensurate with their intended application and function.

4.3 Strainer and Strainer Support Structural Requirements The strainers and strainer supports shall be qualified for the loads, :oad combinations and acceptance criteria established under the Mark 1 Containment Reevaluation Program. The governing documents for these requirements are as follows:

  • NEDO-21888, " Mark 1 Containment Program Load Definition Report," (LDR) Revision 2, November 1981, including Addenda, Sheet 1, April 1982.
  • NEDO-245831, " Mark i Program, Structural Acceptance Criteria, Plant Unique Analysis Application Guide,"(PUAAG), October 1979.

. NUREG 0661, " Safety Evaluation Report, Mark i Containment Long-Term Program, Resolution of Generic Technical Activity 4-7," July 1980, including Supplement 1, August 1982.

5.0 Replacement Strainer /ECCS Long Term Recirculation Evaluation Methodology This section presents the methodology used to evaluate the replacement strainers and ECCS l

long term recitculation. This methodology meets the replacement strainer design requirements discussed in Section 4. This methodology is based an information contained in NUREG/CR-6224, " Parametric Study of the Potcntial for BWR ECCS Strainer Blockage Due to LOCA Generated Deoris", in the URG and in other published literature, in some instances, the information in these publications is modified to provide appropriate application to this evaluation for BSEP.

i 5.1 - Drywell Debris Generation The types and quantities of debris in the drywell generated as a result of a LOCA is a function of:

. Postulated pipe break locations,

. Break blast / jet shapes, sizes and intensities (i.e., Zone ofinfluence),

13

Brunswick Plant itit 2 ECCS Srcrion Streiner Replacement Project NRCBulletin WO.f FinalReport

  • Insulation debris types and quantities within the Zone ofInfluence, and
  • Other debris types and quantities.

Each of these items can be evaluated explicitly for each postulated pipe break location to determine the quantity of debris generated by that break. Alternately, simplified, but conservative evaluations can be made that provide bounding debris quantities to be utilized for the strainer and ECCS long-term recirculation evaluation.

CP&L performed a more simplified evaluation cf the bounding debris quantities generated as a result of a LOCA. This simplified evaluation approach is described below.

  • Determine the type, thickness and application (i.e., the piping sy:tems and pipe sizes) of the insulatioa systems utilized in the drywell. (Note that this information, as well as the general condhior. of the insulation, has been confirmed through the performance of field walkdown conducted during the last two refueling outaEes.)
  • Determine the Zone ofinfluence for each insulation type by pipe break size using Method 3 of the URG (refer to Section 3.2.1.2.3.3).

The URG uses a spherical Zone of Influence (hemispherical for single-ended guillotine breaks), which is consistent with guidance given in Re@ tory Guide 1.82, Revision 2. As defined in the URG, the volume of the sphere is determined for each insulation type based on the pipe diameter where the break is postulated, the amount of break separation, and the destruction pressure for that insulation type. All breaks are conservatively assumed to be unrestrained (i.e., having c. radial offset greater than 3D/2, where D = inside pipe diameter :n feet). Destruction pressures are provided for the required insulation types. Once the volume of the sphere is known, the equivalent sphere radius is calculated.

  • Determine the routing of the high energy piping and their location relative to the majority of the piping insulation in the drywell using the plant composite piping drawings.

l

  • Identify potentially controlling pipe breaks using the above information.

l l

This step invc,1ves inspecting the composite piping drawings, and with the knowledge of the i Zones of influence for various pipe break sizes, selecting break locations which would generate large volumes of debris. Tha location of the insulation debris relative to platforms and grating is also accounted for due to different transport factors being applied (see Section 5.2). As BSEP is mostly homogeneous in its fiber-type insulation, this identification of l

potentially controlling pipe breaks from visual inspection of the composite drawings is possible.

  • Determine the insulation debris types and quantities associated with each potentially controlling pipe break, and associated RHR/CS demand (i.e., flow rates).
  • Select the break location (s) that results in the bounding debris volumes to be utilized in the strainer performance evaluation. More than one break location may be required to be evaluated to address all insulation types and corresponding RHR/CS demand.

l 4

14

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

Brunswick Plant UD 2 ECC3 Section Strniner Replacement Project NRCBulletin W0.1 TinstReport The rea; tor vessel is insulated with reflective metal insulation (RMI). Any pipe breaks inside the blot nield wall would be bounded by the drywell pipe break quantities of RMI, since there are

- guard pipes installed on the reactor vessel nozzles which direct 85% of the pressure out through the boshield walls.

Other non-insulation drywell debris is generated as a result of the LOCA. This debris and the potential quantities generated for each are provided in Section 3.4.

5.2 Drywell Debris Transport The drywell insulation debris transported as a result of a LOCA were determined using the combined generation and transport factors given in Section 3.2.3.2.5, Tables 5 and 6, of the URG. The bases for these combined generation and transport factors are also provided in the URG, and provide the,iustification for utilizing transport factors less than 100%.

All particulate debris generated in the drywell was assumed to be transported to the suppression pool with 100% efficiency, and in the suppression pool at time zero into the accident.

l 5.3 Suppression Pool Debris Transport The suppression pool is highly turbulent during the blowdown phase of a LOCA. However, after the first several minutes of the transient, the degree of turbulence is significantly reduced. The key phenomenon that can occur at that time is settling of debris to the bottom of the suppression pool. The nbious debris is typically too light for settling to be important, and thus fiber settling was not considered.

particulate debris, especially larger particulate species, such as paint chips and rust flakes, have been shown to settle very rapidly. The key parameters for calculating settling, other than the particulate sizes and densities, are the suppression pool volume, the volumetric flow rate through the suppression pool, and the effective particulate settling velocities (based on degree of pool turbulence). For conservatism, this settling was not included in the strainer head loss analysis.

110 wever, it was considered in the strainer margia assessment performed as described in Section 5.5.

5.4 Suction Strainer Blockage and Head Loss The strainer head loss, given the deposition of a certain amount (and type) of Gbrous uebris and a certain amount (and type) of particulate debris on the strainer, was calculated using a modified NUREG/CR-6224 head loss model. The head loss model in NUREG/CR-6224 was modified by our cor.sultants, ITS, to provide a correlation for s a aers having different geometries and for strainers with a heavy fiber loading. This modifie ,ad loss model explicitly treats the etTects of:

15

Cruns wkk Plant linit 2 ECCS Suction Strainer Replacement Pro}ect NRCBulletin WO3 FinalReport _

e stacked disk strainer geometry for both light fiber loads (when the entire stacked disk surface area is accumulating debris) and for heavy fiber loads (when the fibrous debris is simply _

building up on the outside of a cylindrical shape);

e debris deposition on the outside of a cylinder rather than a flat surface (resulting in a reduced bed thickness);

e different fibrous debris constituents;

  • difTerent types and quantities of particulate debris constituents; e overall bed porosity (e.g., compression); and e the effect of different fluid temperature and flow rate.

It should be noted that the last four items listed above are also explicitly treated in the original NUREG/CR-6224 head loss model.

The model was developed to represent the physics and fluid flow behavior expected for thick fiber beds (with particulates) on large passive strainers. A comparison of model predictions to the results of the PCI strainer tests performed at EPRI has been performed. This comparison showed agreement to within ~10% for a wide range of fiber quantities, and sludge to fiber mass ratios.

A complete description of the modified head Icss model is presented in Appendix A. This modified head loss model and a comparison of the model predictions with results from tests performed at EPRI were presented to the NRC Staff by ITS on February 18,1997.

A copy of the PCI Test Reports is included in Appendix B.

As discussed in Appendix A, the strainer head loss was calculated considering the time-dependent debris build-up on the strainer. This buildup of debris on the strainers, which occurs over a finite time interval, is primarily determined by the volumetric flow rate through the strainer as compared to the suppression pool volume.

The strainer head loss, due to the deposition of RMI across the strainer surface, was calculated using RMI head loss model presented in Appendix B to the URG.

5.5 ECCS Pumps NPSH Margin Assessment The existing NPSH margin cakulations were revised to incorporate the head loss through the .

debris, calculated as described above, the head loss through the strainer itself, and the head 40ss through any interconnecting piping components required to connect the strainer to the existing ECCS pipe connections. The revised NPSH margins were then determined and shown to be positive.

16

Brunswick Plant Unk 2 ECCS Suction Strainer Replacement Pro}ect MtCBulletin W0.1 FinalReport An assessment was also made as to the conservatisms contained in the NSPH margins calculated above and the additional margins that would be shown to exist, without these consenatisms included, nis included consideration of other time-dependent aspects of strainer performance, including:

  • the efficiency with which the fibrous debris can trap particulate debris while the debris build-up is occurring and prior to the development of very thick beds; and
  • the settling of particulate hr the suppression pool, which occurs during the finite time of debris build up on the strainer.

+

The BLOCKAGE computer program was utilized to quantify this debris bed efficiency and particulate sedimentation his assessment provides additional assurance that RHR and CS systems can perform their intended functions.

He potential for air / steam ingestion into the strainers during an SRV T-Quencher discharge and the potential for vonexing was assessed and was determined to be acceptable. Minimum water levels for varicus RilR and CS flow conditions specified in the EOPs were revised for the replacement strainer design ar.d performance requirements, without reliance on drywell pressure.

5.6 -oction Strainer Structural Evaluation The replacement suction strainers and supports were qualified for the Mark ! submerged structure loads defined in the Load Definition Report (LDR), and other applicable loads acting on the strainers (i.e., deadweight, thermal expansion and seismic). This qualification utilized methods and parameters that are the same as described in the BSEP Plant Unique Analysis Report (PU AR) and in the UFSAR.

In the implementation of these methods, specific analytital techniques were employed to reduce unnecessary conservatism that was included in the qualiiication of other BSEP submerged structures during the Mark I Program. These analytical techniques are discussed below.

+ in the determination of the acceleration drag volumes for the hydrodynamic loads acting on ue repla:ement strainer assemblies, a hydrodynamic mass coefficient, C., was determined for the strainers taking into account their geometry and perforated nature. As the Mark I LDR Application Guides do not prodde a coefficient for stf. merged structures with geometries similar to the replace.nent strainers, one had to be developed to eliminate excessive conservatism. The Mr.k I LDR Application Guides do provide the methodology that was used for determinatica of the C, coefficient for other submerged structures. This same methodology was employed in the development of the C, coefficient for the replacement strainers.

I Testing of prutotypical strainers was performed to measure C, coefficients for the Sure-Flow" stacked disk strainer design. The C, coefficients obtained from the testing of the 17

Brunswkk Plant UD 2 ECCSSuction Strainer Replacement Project NRC Bulletin %-0.1 FinalReport prototype strainer were adjusted to account for differences between the prototype design and the BSEP-specific design. This methodology used to determine C, assures no reduction in the safety margin for the BSEP containment and ECCS design basis.

Non proprietary copies of the test .eports on the prototype strainers, " Test Report for liydrodynamic inertial Mass Testing of ECCS Suction Strainers," No. TR-ECCS-GEN-01, Revision 2, and " Supplement I to Hydrodynamic Inertial Mass Testing of ECCS Suction Strainers - Free Vibration Analysis," No. TR ECCS-GEN-05, Revision 0, are included in

. Appendix E. Report TR ECCS-GEN-01 justified that, for a strainer with perforated plate with 1/8" and 40% open area, the C, coefficient could be conservatively taken as 62% of the coemeient for the same " strainer without holes"(i.e., C,=1.24). Report TR ECCS-GEN-05 further examined the test results and concluded that a C, coemcient of 50% of the coemcient for the same " strainer without holes" (i.e., C,=1.00) could also be justified and shown to be conservative.

The BSEP strainers utilize perforated plate with 3/32" holes and 33% open area. The effect of the reduced hole size and flow area will tend to increase the differential pressure across the perforated plate, thereby potentially increasing the C, coefficient. This effect is equivalent to the phenomenon commonly referred to as a vena contracta. A ratio of the areas of the vena contracta was used to conservatively account for the change in the reduction in hole size and flow area. This evaluation determined a C, coefficient for the 3/32" perforated plate to conservatively be 80% of the coefficient that would be applied to the same " strainer without holes"(i.e., C,=:.60).

Giher design differences, i.e., the disk and gap sizes, and the escentricity of the disks, were conserva8vely not included in the adjustment, as these items, considered together, were judged to rence the C, factor.

  • In the development of specific load combinations, the BSEP PUAR typically used conservative, bounding combinations to simplify the analysis processes. Typically, jet loads and drag loads (due to either LOCA and CRV discharge) were conservatively added together.

These components ofload do not occur simultaneously. Therefore, this practice was overly conservative. The analysis of the replacement strainer assemblies more appropriately considers the maximum of the jet load or the drag load when determining the maximum loads due to a specific phenomena.

Additionally, the analysis for the replacement strainer assemblies combined independent loads using a square-root sum-of the-squares (SRSS) combination method. Although this combination method was not used in the analysis of the previously installed strainers, it was rsed on certain components in the original Mark I design at BSEP, when it was required to remove excessive conservatism. It was part of the original Mark I design basis and its use was documented in the PUAR.

  • For the Chugging load case, the original analysis conservatively combined the effects of each harmonic absolutely, regardless of sign or phase. The analysis for the replacement strainer assemblies takes credit for the random nature of the phasing of each of the harmonics using a combination method developed during the Boiling Water Reactor 18

a i

Brunswick Plant Unit 2 ECCSSuction Strainer Replacement Pro}ect NRCBulletin Wo.1 FinalReport Owners' Group (BWROG) efforts to resolve the Mark I Program. His combination method I i

has been shown to meet NUREG-0661 generic acceptance criteria, thus maintaining a l

consistent margin of safety with the original Mark I program.

5.7 Other Plant Struc6ral, Systems and Components Evaluation De installation of the replacement strainers affected the qualification of other existing structures, systems and components. The affected items were:

+ the RHR and CS torus penetrations;

  • the strainer support attachment points on the torus linen, e other torus internal structures and systems in close proximity to the replacement strainers; and

+ torus water volume, ne requalifiestion of torus penetration, liner, internal structures and piping components utilized the same methods as described in the BSEP PUAR and UFSAR. In some cases, the specific analytical techniques described above (i.e., SRSS of dynamic loads) for the strainer qualification were applied in the requalification of some of these components.

The torus water volume is decreased less than 100 ft', which is approximate!y 0.11% of the total l

water volume in the torus, due to the installation of the replacement strainers. This change has been evaluated and shown to have a negligible effect in the torus temperatures during accident l -

conditions.

l l

l 6.0 Licensing Considerations CP&L has implemented the strainer replacement, and evaluated the acceptability of the strainers l and the ECCS long term recirculation capability, under the 10CFR50.59 process.

A Technical Specification change, to address the change in suppression pool water volume due the installation of the repucement strainers, was submitted by letters dated July 8,1997 and August 22,1997.

No other Technical Specification change is contemplated at this time. CP&L does not plan to include torus and suction strainer surveilla: cc requirements in the Technical Specifications. As discussed in Section 2, these surveillance requirements are already contain 1 in the appropiate BSEP preventive maintenance procedures (routes). Placing these surveillance requirements in the maintenance procedures provides the appropriate level of control to insure that adequate inspection and cleaning is performed.

19

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

J Crunswkk Mont U:k 2 ECCSSuctkur kraker Replacement Pro}ect NRCRollerk %03 ThalRepvt i , - .

i APPENDICES i

e f

4

. - - , m ,

. Crunswkk U:k 2 ECCSSection Strainer Replacement Project NRCBullerin %03 FinalSeport

~!

Appendix A Modified NUREG/CR-6224 Head Loss Correlation t

4 4

1

~ *~ . , , . . _

_ , _ . . _ . . .~ .

Crunswick linit 2 ECCS Suction Strainer Replacement Project Appendk A NRCBulletin WO3 FinalReport Table of Contents A1.0 Background and Objectives .........................................................

A1.1 Background.................................................................................................................................1 A1.2 Objectives of th e Analys i s ..... .................. ........ .... . ....................................................................... 2 A1.3 Assumptions.................................................................................................................................3 A2.0 M ethodol ogy Descripti on ....... ...... .................... ..... ................................... ....... ............................ 3 A2.1 P asic H ead Loss Correlation . ..... ................ ....... .......... ..... ............ ........................................ ...... 3 A2.2 Calculation of Effective Strainer Surfa- Area and Approach Velocity..................................... 5 A2.3 Calculation of Debris Deposition on the Strainer.. ...................... ............................................... 6 A2.4 Debris Thickness Calculation - Efrective Surface Area Consideration............................ ........ . 6 A2.5 Debris Bed Thickness Calculation - Surface Geometry Considerations..................................... 7 A2.6 Consideration ofimpact of Strainer Supports .................. ...................... ................8 A2.7 Calculation of Average Debris Surface to Volume Ratio................ ........... ............................... 8 A3.0 References................................................................,................................................,,,....,,,....,10 l

l i

l t

(

l l

l- .

ii f

Brunswkk Unit 2 ECCSSuction Strainer Replaceneent Project AppendeA NRC Bulletin W03 FinalReport } ;

A1.0 Background and Objectives There is a concern that during e design basis loss of coolant accident (LOCA) in a Boiling Water Reactor (BWR), the strainers at the suction inlet to the Emergency Core Cooling System (ECCS) could become sufficiently clogged widi debris generated during the LOCA as to cause cavitation and failure of the ECCS pumps. This debris consists of drywell piping insulation (fibrous or metallic) loosened as a result of the LOCA forces and transported to the suppression pool, sludge that has built up in the suppression pool during the reactor's steady-state operation, and other particulate debris sources such as dirt and dust, loosened paint chips, and loosened rust. In order to assess the performance and adequacy of such strainers, one must be able to predict the head

!oss across those strainers during the accident as a function of the time dependent debris buildup and the time-depeadent ECCS flow and coolant temperature. At the present time no single tool exists that can predict such head loss under the full range of potential strainer debris loading conditions.

A1.1 Background Ihe most comprehensive assessment of the phenomenological issues that impact strainu head loss and the poti ntial computational models that could be used to predict such head loss was conducted by the Nuclear Regulatory Commission (NRC) and documented in the NUREG/CR-6224 report entitled " Parametric Study of the Potential for BWR ECCS Strainer Blockage Due to LOCA Generated Debris" [Zigler,1995). The correlations presented therein were shown to be applicable to a wide variety of situations involving head loss due to flow through a fibrous debris layer. The BLOCKAGE computer code [Rao,1996] was subsequently developed to implement the modeling approaches recommended in NUREG/CR-6224.

The BLOCKAGE code provides a very comprehensive assessment of

. the time-dependence of debris transport from the BWR drywell to the suppression pool, e the buildup of debris on the strainers as a function of pump flow rate and pool water vdume, e the potential reduction in debris buildup as a result of sedimentation to the floor of the suppression pool, e the potential reduction in the buildup of particulate debris as a result of less than perfect filtration of such particulate by the fibrous debris, and

  • the head loss resulting from the flow through the deposited debris.

Ilowever, the BLOCKAGE code was developed under the assumption that the surface area of the strainer could be treated as a constant, user supplied input to the analysis, with the Febris buildup being calculated as though the strainer could be represented as a flat surface with the same surface area. This simplifying assumption is sery valid in the case where one has a large surface area relative to the debris volume, such that only a thin debris layer would be calculated.

Ilowever, in the case where one has a large volume of debris, with a complex strainer geometry involving s.acked disks and curved surfaces, the BLOCKAGE approach to debris deposition is no longer valid. There are two principal reasons for this:

1

l Appendte A BrunswicA Unis HCCSSuction Strainer Replacement Project NRC Bulletin WO3 nnel Report

1. A stacked disk straiott has a very large surface area relative to the overall strainer volume.

With large volumes of fibrous debris, the interstitial gaps between the disks can become filled with debris. When that occurs, the effective surface area of the strainer for additional debri s deposition is reduced to the circumscribed area of the strainer.

2. For thick layas of debris on the outside of a cylindrical shape, the debris thickness relative to the debris volume is a function of the surface cuevature, and is less than tne thickness that would result from deposition on a flat surface of the sarne area.

The NUREG/CR 6224 methodology accounts for the head loss due to particulate material trapped within a fibrous debris bed through a decrease in the bed porosity as well as through the use of a modified (" average") debris bed surface to volume ratio. A suggested approach to determining the average surface to volume ratio is not provided.1lowever, the BLOCKAGE code allows for the use of either a simple volume averaged value, or simply the fiber value. For a paniculate material such as sludge, whose surface to volume ratio is not too difTerent from that of Nukon fiber, these two approximations do not have a large impact on the calculated result.

This is not the case, however, for a small surface to volume ratio particulate species such as paint chim. In tHs case, the approximation of keeping the surface to volume ratio fixed at the value e mer has the effect of treating tu paint chips as though it were simply more fiber. This tends to signi"cartly overpredict the impact of paint chips on head loss. Even wor >e, however, is the use of a simple volume average. It can be shown that because of the form of the NUREG/CR-6224 head loss correlation, the use of simple volume averaging actually leads to the non physical result that the addition of a small surface to volume ratio particulate such as paint chips actually reduces the calculated head loss. This is clearly an unacceptable, non conservative result. It had not been identified as a problem previously, because the NUREG/CR 6224 correlations had not previously ocen applied to paint chip particulate.

A1.2 Objectives of the Analysis in light of the limitations in BLOCKAGE identiGed above, and the fact that no near term revision of the BLOCKAGE code has beeri publicized, a modified NUREG/CR 6224 head loss correlation was developed that could be used to assess stacked disk strainer performance under heavy fiber load with signiGcant quantities of arbitrary paniculate (including small surface to volume ratio material such as paint chips). This m:thodology incorporates the following features:

e head loss estimates based on the identical basic head loss correlations used in DLOCKAGE, e time dependent debris build-up on the strainers based on strainer flow rate and pool water volume as in DLOCKAGE (with all debris assumed to be suspended in the suppression pool at time zero),

e use of the full strain:r surface area for debris deposition until the gaps between the stacked disks are filled with debiis, e use of the strainer circumscribed area for further debris depasition after the gaps are filled, e a calculation of debris thickness on the outside of the circumscribed area that accounts for the surface cun ature, and 1

l

Brunswick l! nit 2 ECCSSuction Stoainer Replacement Project Appendit A NRC Rulletin 96-0.1 Final Report

  • implementation of an averaging algorithm for the debris surface to volume ratio that is consistent with the basic head loss correlations.

A1.3 Assurnptions The methodology relics on the same basic head loss correlation documented in NUREG/CR-6224 and implemented in the BLOCKAGE code. Validation of that correlation as documented in NUREG/CR 6224 and in the 11 LOCKAGE Code validation report [Shaffer,1996) is sssumed to be applicable herein.

A detailed discurslon of stacked disk strainers is not provided herein. The reader who is not familiar with the basics of that strainer geometry is referred to the Performance Contracting, Incorporated (PCI) report " Summary Report on Performance of Performance Contracting, incorporated's Sure Flow Suction Strainer with Various Mixes of Simulated Post LOCA Debris,"in Appendix B.

A2,0 Methodology Description As discussed in the section on objectives, this methodology is based on the identical head loss correlation described in NUREG/CR 6224. Any enhancements are implemented exclusively in the calculation of certain terms in that correlation. The methodology is set up to perform the strainer performance assessment for one strainer at a time. Thus, if multiple strainer designs are being evaluated (such as Residual llent Removal and Core Spray, for example) a separate analysis would need to be performed for each one.

A2.1 Itasie llead less Correlatlan The NUREG/CR 6224 head loss correlation is described in detail in Appendix B to that PCI report and is a semi theoretical head loss model. The correlation is based on the theoretical and experimental research for the pressure drops across a variety of fibrous porous media carried out since the 1940s. This head loss model, proposed for lamina , transient and turbulent flow regimes through mixed debris beds (i.e., debris beds composed of fibrous and particulate matter) is given by:

All =A [3.5 S,8 u, " (1 + 57 u,' ) U + 0.66 S, n /(l a.) p U2 ) AL, w here.

All is the head loss, S, is the average surface to volume ratio of the debris, p is the dynamic viscosity of water, U is the fluid approach velocity, p is the density of water, n, is the mixed debris bed solidity (one minus the porosity),

3

i Brunswkk Unk 2 ECCS Suction Streker Replacenwnt Pro}rct Appendk A NRC Buneten N-M That Report M is the mixed debris bed thickness, and i

A is a unit conversion factor (A = 1 for SI units),

The mixed debris bed solidity is given by: j r

f 3 a* a 1+b y

< Pr > a* AL, ,

where, a, is the w fabricated fiber bed solidity, M, is the theoretical fibrous debris bed thickness, 9 = m,/m ris the paniculate to fiber mass ratio in the debris bed, pf is the fiber density, and p, is the average paniculate material density.

For N, classes of particulate materials, m, and p, are defined by:

x, m, = [ m, and x

0o l's p,=

I's where m,, p, and V, are the mass, density and volume of a particulate material i.

Compression of the fibrous bed due to the pressure gradient across the bed is also accounted for.

The relation that accounts for this effect, which must be satisfied in parallel to the previous equation for the head loss, is given by (valid for (MUM,) > 0.5 ft water / inch insulation):

M, = 1.3 M (.;

  • 1/ M.)* "

For very large pressure gradients, the compression is limited such that a maximum solidity is not exceeded. In the NUREG/CR-6224, this maximum solidity is defined to be n, = 65 IWft'/p, 4

. _ . - . _ _ - . _ . . _ . . _ _ _ _- _ ~.. _ . - . _ _ _ _ _ _ . . _ _ _ _ _ . . _ .

Brunswkk Unsr 2 ECCS Section Stenker Replacenunt Pro}ect AppendixA NRC Bulletin W0.t finalReport I

S which is equivalent to having a debris layer with a density of 65 lb/A . Note that 65 lb/ft'is the macroscopic density of a granular media such as sand or gravel and clay [Baumeister,1958). In this methodology, ot.is considered to be a user specified input parameter, it should be noted that there are indications that this fonnulation for debris bed compression may overpredict compression significantly in the case of very thick debris layers. As such, it is possible to ignore compression in this analysis if so desired.

The NUREO/CR 6224 model assumes that the debris is uniformly distributed on the strainer surface. For flat disk strainers and thin layer beds (M s 0.125 inches), that correlation is known to over predict the results [Zigler, et al,1995). Thus, a minimum debris thickness equal to the strainct perforated plate hole diameter is assumed to be necessary to result in any measurable head loss.

in the above formulation, three parameters have as of y6t not been r.pecified; the approach velocity, U, the theoretical (uncompressed) fibrous debris bed thickness, M,, and the average surface to volume ratio, S,. The approach velocity is a function of the user specified volumetric flow rate through the strainer ofinterest and the effective strainer surface area. The debris bed thickness is a function of the volume of debris on the strainer, the effective strainer surface area, and the strainer surface curvature.

A2.2 Calculation of Effective Strainer Surface Area and Approach Veloelty The fluid approach velocity, U, is given simply in tenns of the volumetric flow rate and the effective surface area as U=S A w here, Q is the volumetric lbw rate through the strainer, and A is the effective strainer surface area.

The effective area. A, is a function of the debris bed thickness, M . While the debris bed thickness is calculated to be less than half the width of the gaps between the disks (a user-defined input parameter), the effective area is simply set equal to the full (perforated) surface area of the strainer, A,. This area is calculated based on simple geometrical considerations of the strainer dimensions. Once the debris thickness exceeds half the gap width, the effective surface area is reduced to the circumscribed area of the strainer, A,(i.e., ignoring the surface area within the gaps between the disks). Note that in both cases, this area calculation accounts for the perforated surfaces on the ends of the strainer.

S

Bruzswk A U@ 2 ECCS S:ction Stederr Replacement Project Appendle A

( NRC B:lletin 96-03 Find Report A2.3 Calculation of Debris Deposition on the Strainer f w

The total quantities of Gbrous debris and particulate assumed to be suspended in the pool at time zero must be specified. In order to calculate the rate at which this debris accumulates on the '

J strainer being analyzed, one has to specify the volumetric flow rate through the strainer, Q, the f total volumetric flow rate through all strainers, Q,, and the total pool water inventory, V g.

E I

If the flows are constant, the fractional debris deposition on the strainer at any given time, t (user input), into the accident can be calculated assuming that

  • all debris in the pool stays uniformly distributed in the pool, e debris is removed from the pool (deposited on all strainers) in proportion to the rate at which water is removed from the pool, and
  • debris is deposited on the strainer being analyzed in proportion to the flow rate through that strainer versus the total Dow rate.

This results in the following equation to describe the fractional debris buildup on the strainer of interest:

r g 3f 3 j FRAC = g""

  • I) ^

\ 0m > <1 - exp( l,e,a >

This fraction is applied uniformly to the total quantity of each debris constituent (Ober and all particulate species) initially in the pool.

In the case where a time-dependent (but step-wise constant) flow is specined in the input, each time interval of constant flow (out to the user specined problem time) must be treated separately.

The fractional deposition during an interval is applied to the total debris quantity in the pool at the start of the interval. These deposition quantities can then be summed to yield the total deposition for the problem time ofinterest.

A2.4 Debris Thickness Calculation - Effective Surface Arca Consideration As a rough approximation, it would be expected that the theoretical Gbrous debris bed thickness on the strainer would be given by the volume of fibrous debris divided by the strainer surface area. This is in fact the approach taken in the DLOCKAGE code. In the case of a stacked-disk strainer, however, it is clear that the debris thickness would be underpredicted by such an approximation once the gaps between the ciisks are 611ed. Thus, the calculation of theoretical Obrous debris bed thickness is secomplished in two steps.

Sitn.1 it is first necessary to calculate a theoretical debris thickness using the entire strainer surface area, A,. Thus, for a total fiber volume on the strain:.r of V,, the thickness would be calculated as AL, = V,/ A,.

6

i O Brunswica Unk 2 ECCSSuction Streker Coplaceneent Prefect AppendkA V '; tc saueth nn rhetReport - .

i I '

If following the correction for bed compression, the debris bed thickness, M., is less than half the width of the gaps between the disks, the calculation of theoretical debris bed thickness is complete.

Stp.2 If the thickness calculated in step i exceeds the maximum allowable, a correction must be made. To do this it is necessary to first calculate the volume of fiber in the gaps between the disks. This is simply given by j V' = V ,,,* M / M ,, g where, V,,, is the total volume associated with the interstitial gaps between the disks. This quantity is calculated explicitly based on the strainer geometry.

The theoretical '.hickness of the fibrous debris on the outside of the strainer circumscribed surface is then given by M. = (V, V') / A, where, A, is the circumscribed surface area of the strainer.

A2.5 Debris Hed Thickness Calculation - Surface Geometry Considerations The discussion on fibrour debris bed thickness above is only qualitatively correct. Simply dividing the fiber volume by the strainer surface area would accurately predict theoretical thickness if the surface geometry were planar. This is a good approximation when one is dealing with the entire strainer surface area (step 1 above), since much of the disk area is associated with the Cat surfaces of the disks. !!owever, once the gaps are filled, most of the deposition effectively occurs on the outside of a cylinder, and the debris bed thickness is less than that predicted above.

To account for this effect, the debris bed thickness can be calculated assuming that the thickness of the Ober is the same on the ends of the strainer (Dat surfaces) as on the outside of the circumscribed cylindrical surface. With this assumption, it is a simple matter to calculate the total volume of a debris layer of thickness M., equate this to the known volume of the fiber (reduced by the fiber in the interstitial gaps), and solve the resulting quadratic equation for the debris bed thickness This result is given by:

](nDH + Af ,)' + 4(V, - V') n# -(nDH + A,,)

~

2nH where, 7

BrunswkA Unk 2 ECCS Suction Stenener Replacement Pro}ect AppendkA NRCBuHerin W93 fkalReport D is the outer diameter of the strainer disks (assumed to be uniform axially),

11 is the active length of the strainer, and Au is the flat surface area of the ends of the strainer (calculated automatically).

This equation is used in lieu of the simpler expression previously presented in Section 2.4 under step 2.

A2.6 Consideration ofImpact of Strainer Supports For relatively small strainets, the only connection to the strainer is the inlet tube to the strainer.

liowever, for larger strainers, it may be necessary to provide additional supports for structural reasons. Depending on the details of these supports, they can have a sman impact an the effective strainer total surface area, AA , circumscribed area, AA,, and interstitial gap volume, AV,,,, To account for this effect, it is necessary to quantify these reductions. These reductions are then taken into account in the calculation of theoretical debris bed thickness by setting A, -+ A, - M, in the equation describing the debris thickness over the entire strainer surface area (Section 2.4, Step 1), and setting nDH H -+ H ' nDH - M, y,, y ,, , l're b l's.<-

an in the equation describing the debris thickness once the interstitial gaps have been filled (Section 2.5).

! A2.7 Calculation of Average Debris Surface to Volume Ratio l

The intuitive choice for the average surface to volume ratio is to use a volume weighted average, l

which is equivalent to defining the average by the total surface area divided by the total volume.

Unfortunately, such a choice is inconsistent with the formulation of the basic head loss correlation and can lead to non physical results. Presented below is a simple derivation of an alternate averaging scheme that provides consistency with the basic form of the correlation.

Consider a fixed volume V that contains a mixture of two types of fiber, I and 2. We take S, = the surface to volume ratio of fiber type i, and q, = the quantity (microscopic volume) of fiber type i.

8

-- - . . .. . . . . . . _ _ - ~ _

I Smnswich Unir 2 ECCSSection Streiner Replacenennt Project AppendteA NRC RnHerin Wo.1 FinalReport l If we consider the dominant terms in the relationship for the pressure drop, AP, we have i I

AP ~ ' ' a ,

where,

is the average surface to volume ratio, and u is the total solidity which is (qi + q:)/V. .

We now consider the following argument for determining the average . Rather than having the two fiber types well mixed in region V, consider that the fibers are separated into adjacent regions V, and V,(with the constraint that the sum of the two volumes be V), with part of the flow going through each region (in parallel rather than in series). We now postulate that the flow velocity through each of these two regions is the same, and that the pressure drop scross the two regions is also the same. This allows us to solve for the relative volumes V i and V . One form of this relationship can be expressed as r ) 4/3 l'

=1+b -

6 e, s S, s If we once again consider the two fiber constituents to be well mixed, and again postulate the pressure drop to be unchanged and the flow velocity to be unchanged, we can equate the pressure drops calculated from the two different viewpoints to yield r 5r (S)" = S " A h,3 r9 > \l). s Substituting for the volume ratio, we are left with 1/4 (S) =

b S,"+bSl .

9 9 .

It is obvious how this result could be extended to more than two such fiber species. We further make the assumption that the same formulation can be applied when some of the species are particulate rather than fiber.

l l

l 9

( - - _

Brunswka Unk 2 ECCSSuction Streker Replacenunt Pre}ect AppenneA NRC BnHemin WO3 That Report A3,0 References Baumeister T., " Mechanical Engineers' Handbook," Sixth Edition, McGraw ifill Book Company,1958.  ;

w 11stt, G., "De Development and Testing of Performance Contracting. Inc.'s Sure Flow Stocked Dis A Suction Straincr," Performance Contacting, Inc., February 1,1996.

Rao, D. V., et. al., " BLOCKAGE 2.5 User's Manual," NUREG/CR 6370, U.S. Nuclear Regulatory Commission, December,1996.

i Shaffer, Clint, et, al., " BLOCKAGE 2.5 Reference Manual," NUREG/CR 6371, U.S. Nuclear Regulatory Commission, December,1996.

Zigler, G., J. Brideau, DN. Rao, C. Shaffer, F. Souto, and W. Thomas, " Parametric Study of the Potentialfor BWR ECCS Strainer Blockage Due to LOCA Generated Debris," NUREGICR.

6224, U.S. Nuclear Regulatory Commission, October 1995.

10 t

Brunswick Unk 2 ECCS Seenm .kreiner R:;'::enunt Pre} ret 4penk R I NRCBulletin W03 FinalReport 1

Appendix B <

Sure-Flowm Strainer Performance Test Reports Reports: f

1. Performance Contracting Inc. Report," Summary Report on Performance of Performance Contracting Inc.'s Sure-Flow Suction Strainer with Various Mixes of Simulated Post.

LOCA Debris," Revision 1 dated September 19,1997.

2. Cuatinuum Dynamics, Inc. Report," Performance Contracting, Inc. ECCS Sure-Flow" Strainer Data Report," No. WO4536-01, Revision 0, dated December 1996.

I

--,,.- . - - .-.. , .. . . - - - . _ _ _ _ _ .

Retrieved from "https://kanterella.com/w/index.php?title=ML20202E044&oldid=3072472"