ML052560138

Redacted Version of Proprietary Submittal Dated February 15, 2005 Regarding Safety Significance Evaluation of ECCS Containment Sump Voided Piping

ML052560138
Person / Time
Site:	Palo Verde
Issue date:	07/05/2005
From:	Overbeck G Arizona Public Service Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
102-05303-GRO/TNW/GAM
Download: ML052560138 (53)

Text

ZAMS Palo Verde Nuclear Generating Station Gregg R. Overbeck Senior Vice President Nudear Tel (623) 393-5148 Fax (623) 393-6077 e-mail: GOVERBEC@apsc.com Mail Station 7602 PO Box 52034 Phoenix, Arizona 85072-2034 102-05303-GRO/TNW/GAM July 5, 2005 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 v

Dear Sirs

Subject:

Palo Verde Nuclear Generating Station (PVNGS)

Units 1, 2 and 3 Docket Nos. STN 50-528, 50-529, and 50-530 Redacted Version of Proprietary Submittal Dated February 15, 2005 Regarding Safety Significance Evaluation of ECCS Containment Sump Voided Piping In letter no. 102-05213, dated February 15, 2005, Arizona Public Service Company (APS) submitted to the NRC the safety significance evaluation of emergency core cooling system (ECCS) containment sump voided piping. APS requested that Enclosure 2 and Attachments 2-A, 2-B, 2-C, 2-D, and 2-E of that submittal be withheld from public disclosure under 10 CFR 2.390(a)(4) because they contained information considered to be proprietary to APS. Since that time, NRC Region IV personnel have requested that APS submit redacted versions of and Attachments 2-A, 2-B, 2-C, 2-D, and 2-E of the February 15, 2005 submittal.

The requested redacted versions of the enclosure and attachments are enclosed.

There are no commitments in this letter. Should you have any questions, please contact Mr.

Thomas N. Weber at (623) 393-5764.

Sincerely, tf/K GROITNW/GAM/ca

Enclosure:

Redacted Versions of Proprietary Enclosure 2 and Attachments 2-A, 2-B, 2-C, 2-D, and 2-E of APS Letter No. 102-05213, dated February 15, 2005, Regarding Safety Significance Evaluation of ECCS Containment Sump Voided Piping cc:

T. W. Pruett B. S. Mallett M. B. Fields G. G. Warnick NRC Region IV NRC Region IV Regional Administrator NRC NRR Project Manager NRC Senior Resident Inspector for PVNGS (wI Enclosure)

(w/o Enclosure) a' a'

A member of the STARS (Strategic Teaming and Resource Sharing) Alliance Callaway

• Comanche Peak

• Diablo Canyon

• Palo Verde

• South Texas Project

• Wolf Creek

Redacted Versions of Proprietary Enclosure 2 and Attachments 2-A, 2-B, 2-C, 2-D, and 2-E of APS Letter No.

102-05213, dated February 15, 2005, Regarding Safety Significance Evaluation of ECCS Containment Sump Voided Piping

ENJCLOSURE 2 OF THIS LETTER AND ITS ATACI IIMENTS (EXCEPT ATTACI IMENT 2 F)

GONT AINS PROPRIETARY INreRMATIeN AND SieULD BE WITH HELD FROM PUBLIC I DISCLOSURE UNDER 10 CrR 2.190 REDACTED VERSION.

ENCLOSURE 2 SAFETY SIGNIFICANCE EVALUATION OF ECCS CONTAINMENT SUMP VOIDED PIPING

-(Proprietary)

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PePRIETARY INlFRmATiew

. Page 1 SIGNIFICANT CRDR 2726509 SAFETY SIGNIFICANCE EVALUATION OF ECCS CONTAINMENT SUMP VOIDED PIPING REDACTED VERSION Satet Signiricance Determination I,,

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PROPRIETARY INFORMATION Page 2 Executive Summary In July, 2004, Engineering personnel determined that a section of Emergency Core Cooling System (ECCS) piping leading from the containment recirculation sump, in both ECCS trains in each of the three Palo Verde Units, was left in an unfilled condition during normal plant operation. The resultant volume of air could potentially be ingested into the ECCS pumps suction following a Recirculation Actuation.

Signal (RAS). A review of design basis information determined that this condition was not consistent with the design intent of the ECCS and not consistent with the analyses that demonstrate the ability of the ECCS to perform its design basis safety functions. Coiiditioni Repoit/Disposition Request (CRDR)'

2726509 was initiated to document and evaluate the condition.

The purpose of this report is to describe and provide the results of a comprehensive testing and analysis program performed to evaluate the ECCS system response to the voided piping condition. The results of the evaluation are then used in a risk assessment to determine the safety significance of the discovered condition.

Scale model tests were performed at Fauske and Associates which simulated the system response during and following a RAS with the affected section of piping initially unfilled. The scale tests were conducted in phases. The purpose of the first phase (typically referred to as Phase I) was to demonstrate the ability to simulate the transient and measure the important parameters' such as void fraction, pressure, and flow rate. [

]

Full-scale pump tests were performed [

pump performance under the projected air ingestion conditions. [

] to determine the impact on I

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Safety Significanc~e Determnination REDACTED VERSION PrOPMlETAV

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I In addition to the testing program, a computer hydraulic transient analysis of the ECCS voided pipe condition was performed. [

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Ultimately, the analysis results are compared to the testing program and shown to be complimentary.

Given the results of the tests and analyses, the risk significance was determined by making appropriate adjustments to the Palo Verde Probabilistic Risk Assessment (PRA) model. [

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1.1 Background/Purpose of Report In July, 2004, Engineering personnel determined that a section of Emergency Core Cooling System (ECCS) piping leading from the containment recirculation sump, in both ECCS trains in each of the three Palo Verde Units, was left in an unfilled condition during normal plant operation. The resultant volume of aircould potentially be ingested into the ECCS pumps suction following a Recirculation Actuation Signal (RAS). A review of design basis information determined that this condition was not consistent w'ith the design intent of the ECCS and not consistent with the analyses that demonstrate the ability of the ECCS to perform its design basis safety functions. Condition Report/Disposition Request (CRDR) 2726509 was initiated to document and evaluate the condition.

The purpose of this report is to describe and provide the results of a comprehensive testing and analysis program'perforimed to evaluate the ECCS response to the voided piping condition. The results of the evaluation are then used in a risk assessment to determine the safety significance of the discovered condition.

1.2 Description of Condition The Palo Verde ECCS design employs recirculation from'the containment sump after the contents of the Refueling Water Tank (RWT) have been injected into the reactor vessel and containment building. Upon receipt of a RAS, automatic valve actuations result in suction of the ECCS pumps being transferred from the RWT to the containment sumps. Two completely redundant and separated ECCS trains are utilized. Figure 1-1 illustrates a typical ECCS suction piping and component layout.

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REDACTED VERSION PROPRIETARY INFORMATION Page 5 Emergency Core Cooling and Containment Spray System Suction Piping - Train A.

Refueling Water Tank Minimum elev. 94 ft. 4 in.

Containment Recirculation Sump A Minimum elev. 84 ft. 6 in.

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-sUV-674 V-2051 HPSI Pump A SIA-P02 HV-683 Ebz V-306

-Not to scale-Figure 1-1 Typical Palo Verde ECCS Suction Layout As illustrated in Figure 1-1, the containment sump outlet pipe contains an in-board and an out-board containment isolation valve, and a downstream check valve. Engineering personnel determined that this section of the ECCS suction piping, between the two containment isolation valves and between the out-board valve and the downstream check valve, had been routinely left in an unfilled condition during plant operation.

In the unlikely event of a Loss-of-Coolant Accident (LOCA), the contents of the Reactor Coolant System (RCS) will leak into containment and flow into the containment sumps. Automatic ECCS actuation would occur causing the contents of the RWT to be injected into the RCS and the containment building to maintain core cooling and containment pressure and temperature control.

Ultimately the basement of the containment building, including the containment sumps, would become flooded. Once the contents of the RWT are depleted, a RAS would be automatically generated causing both containment sump isolation valves in each train to open, resulting in closure REDACTED VERSION Safety Significance Determination PRAPziFF RN'fNFRnhfA T40 a.-.8........

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..PROPRIETARY INFORMATION Page 6 of the RWT isolation 'check valves. The RAS would also cau'sie, by design, the Low Pressure Safety Injection (LPSI) pumps to be turned off.ECCS'suction, consisting of a HPSI pump and a CS pump in each train, would thus be transferred to the containment sump.

With the containment sumps flooded and the section of containment sump piping not filled with water, air would be trapped in the piping. As flow is initiated from the sump, this air could be entrained and/or transported into the ECCS suction piping and potentially into the ECCS pump inlets. Industry literature and operating experience indicates that pump performance could be severely degraded, or even result in air binding or pump failure, if the resultant air volume fraction ingested by the pump exceeds the pump's tolerance for air ingestion. Industry literature (Ref. I NUREG/CR 2792) indicates that a pump's tolerance for air ingestion varies by design and fluid conditions, but at air volume fractions above approximately 3%, pump degradation can be experienced.

Therefore, in order to determine the safety significance of this condition, the air volume fraction that could be ingested by the HPSI and CS pumps would need to be determined. Once the air volume fraction is determined, each pump's tolerance for the projected air ingestion can be assessed, and ultimately the impact on the ECCS safety functions.

1.3 Significance Determination Approach The assessment of voided and two-phase fluid behavior is complex. A comprehensive scale model testing program was employed to develop a full understanding of the system response to the void and the resulting air/fluid conditions that would be delivered to the pumps' suction inlet. The impact to pump performance was then assessed via full-scale testing, given the projected air/fluid inlet conditions.

The scale model tests were performed at Fauske and Associates, and simulated the system response during and following a RAS with the affected section of piping initially voided. The scaled tests were conducted in phases. The first phase modeled the RWT and associated piping, and the sump and associated piping down through and including the long vertical run of pipe. The purpose of the first phase (typically referred to as Phase 1) was to demonstrate the ability to simulate the transient and measure the important parameters such as void fraction, pressure, and flow rate. A series of tests were performed to test important scaling parameters to ensure the results of the test could be confidently applied to the full scale Palo Verde units. A series of phenomenological tests using a larger scale model was incorporated into the test plan to verify that the flow regime in the vertical section of the scaled piping configuration was representative of large pipe behavior.

The second phase extended the scale model to include the individual pump suction piping up to each pump inlet. An extensive series of tests under varying flow and pressure conditions were performed.

These results established the inlet conditions for the subsequent full-scale pump performance tests.

Full-scale pump performance tests were performed at Wyle Labs utilizing a spare Palo Verde High Pressure Safety Injection (HPSI) pump and a representative Containment Spray (CS) pump to determine the impact on pump performance under the projected air ingestion conditions. The HPSI pump was of the same make and model as those installed at Palo Verde. A spare CS pump of the S

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'I~NFRmAATiOM Page 7 same make and model as the Palo Verde CS pumps was not readily available; therefore a spare CS pump from a cancelled WPSS plant was utilized for the test. This pump is the same make and model as the Palo Verde LPSI pumps and is very similar in design and size to the Palo Verde CS pumps.

The impact on performance for equivalent fluid conditions is expected to be representative. Tests were performed for a spectrum of flow rates and air ingestion rates based on the results of the scale model test program. Pump performance was measured as a function of air volume fraction. A maximum degraded pump performance curve was (lhen constructed using the test results for the tests performed at maximum air volume fractions.

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] For those system conditions in which the required head do not exceed the degraded pump performance capability, continued degraded ECCS delivery (i.e. continued pump flow) is assumed until the air inventory available for ingestion into the pump is consumed, at which time restoration of full pump performance is assumed.

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REDACTEDVERSION' Page 8 PROPRIETARY W$NFMATION 2 Scale Model Testing 2.1 Phase I Test Program and Results 2.1.1 Experimental Obiectives and Physical Arraneement The objective of the Phase 1 testing was to investigate the potential for the air initially resident in the horizontal piping section from the containment sump to be forced into the vertical downward piping section. Phase [tests included the transient effects of switching the supply from the simulated RWT to the simulated containment sumii by simultaneously opening the sump suction isolation valves.

Clear piping was used for the horizontal and vertical segments of the simulated suction line to observe and record the flow pattern and the behavior of the initial air filled void. A complete report on the conduct and results of the Phase I test program is attached as Attachment 2-A to this report.

The test facility that was used was comprised of two tanks with water inventories, a centrifugal pump, piping, valves, and associated instrumentation. The piping and valves used to establish and visualize the flow pattern development from the initial location between the valves and into the downcomer piping were all 4 inch in diameter. Clear plastic piping facilitated observation of the initial air inventory behavior during the opening of the motor operated valves. The vertical segment was also'clear plastic piping that allowed for the observation [

] in the downward vertical flow.

2.1.2 Scaling Considerations As indicated, 4 inch diameter piping was used to simulate the sump horizontal and vertical downward sections of piping. Since actual Palo Verde piping is 24 inch in diameter, this results in a 1/6"'

geometric scaling factor. This geometric (lengths and diameters) scaling factor was maintained through out the Phase I tests to the extent possible.

Previous tests and experiments described in the literature have demonstrated that maintenance of the Froude number, particularly for horizontal flow regimes, will result in prototypical behavior in scaled experiments. As such, flow rates were scaled in the Phase I tests so as to maintain the same dimensionless Froude Number parameter as would exist in the Palo Verde units.

2.13 Phase 1 Results and Observations A series of twelve tests were performed with varied [

-REDACTED VERSION Safety Significance Determination PROPRIETARY IN':ORMATWO.:

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-PRePRIET-ARY !NFeRMAFE)N-Page 9 2.2 Phenomenological Testing Program 2.2.1 Experimental Obiective and Physical Arraneement Design reviews conducted before and after the Phase I tests and an independent review [

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REDACTED VERSION PROPRIETARY.ItFORMA TIONF Page 10 The test arrangement also provided the opportunity to observe the flow patterns and influence of the HPSI and CS branch connections off the lower header piping.

2.I2 Phenomenolozical Testing Results and Observations An extensive series of tests using the [ ] scale test apparatus were performed. Key observations from these tests were I

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REDACTED VERSION PROPRIETARY INFORMATION 2.3 Phase 2 Test Program and Results 2.3.1 Experimental Objectives and Physical Arrangement The test facility for Phase 2 was similar to that of Phase I [

I Page 11 Figure 2-1 Phase 2 Test Arrangement.

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' PROPRIETARY 'ieFORMATION Page 12 I

] In the plant system under accident conditions, air transported through the 1-PSI line would influence the pump performance and cause a decrease in the flow rate being pumped. Reduced flow rate would cause a corresponding reduction in the rate of air ingestion.

Thus, the air intrusion rate deduced from these scaled experiments provides a conservative representation of the plant response.

The test instrumentation is also illustrated in Figure 2-l:fA computer with a CIO-DAS008 data acquisition card was used to collect the data. Key pieces of instrumentation included Various pressure, level, and flow meters

[

I During the Phase 2 tests, the flow rate through the CS pump was again held constant at the maximum predicted flow rate equivalent to 4885 gpm, except for several tests in which CS flow was set to zero to simulate a HPSI flow only scenario. HPSI flow rate was varied ranging from the equivalent to 200 gpm to an equivalent maximum run-out flow of 1310 gpm. I

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U 2.3.2 ScalinE Considerations The same 1/6' geometric scaling used in Phase I was used for the Phase 2 experiments. Flow rates were scaled to maintain the same Froude number that would exist at Palo Verde. The Froude number relationship was maintained for both the total flow and the individual flow rates to the simulated HPSI, CS, and LPSI pumps.

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- RmpR ietiO Page 13 In this horizontal orientation, the principal scaling parameter has been well established previously (References 3 and 4) to be the Froude number which is a ratio of the inertial and buoyancy forces, i.e.

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U" gD (p. - pg where:

• D is the diameter of the horizontal piping,

• g is the acceleration of gravity,

• U is the one-dimensional velocity of the flow in this line,

• Pg is the air density, and

• Pw is the water density.

Since Pw >> pg, this reduces to the familiar form N F =

U

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Eq. (2)

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U 2.3.3 Phase 2 Results and Observations A series of twenty-eight tests were initially performed with varied flow rates, containment level, and containment pressure conditions. Additional tests were later performed to investigate the air transport process during potential LPSI pump start scenarios. Key observations from the tests were:

Flow Pallerns Digital movie cameras were used to record the flow patterns in all the Phase 2 tests. Each test was initiated by simultaneously opening the sump containment isolation valves. As the valves open, water REDACTED VERSION

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The air is swept out of the horizontal segment and into the vertical piping segment. I HPS1 Air Ingestion Rates E

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These results show that the air flow ingestion rates increase to their maximum value within approximately [ I seconds for the scaled experiments and then subsequently decay towards zero as the air inventory in the horizontal suction header becomes insufficient to enter the HPSI line. Similar evaluations for scaled HPSI flow rates [

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With a 116th linear scale, the respective volumes are determined by the cube of this linear scale, i.e.

the scaled up quantities are defined by the volume multiplied by 216. More simply put, the area is scaled by the square of the diameter times the length. Thus six cubed equals 216. Since mass is directly proportional to volume at a given pressure and temperature, mass quantities are also scaled by a factor of 216.

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UM Using the results from the Phase 2 tests, these scale factors are applied and the results illustrated in Figure 24 for the case of a HPSI flow rate of 1310 gpm. As shown, the meaningful delivery period for the air flow is approximately [

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Since Reference 1, and other pump performance tests described in the literature, indicates that pump performance is typically assessed as a function of air volume fraction, the peak mass flow rate data obtained during the Phase 2 tests was converted to air volume fractions for use in the full-scale pump tests. [

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REDACTED VERSION PrEO)rtsFAR[ llNFeRJvAT!E A od Page 23 3.1 Description of Analysis and Computer Model A hydraulic computer model of a typical Palo Verde ECCS system was developed [

I 3.2 Analvsis Results REDACTED VERSION Safety Significance Determination

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4.1 Description of Test Facility The pump performance tests were conducted at Wyle Labs in Huntsville, AL. The test facility consisted of two closed pump loops each drawing suction from, and discharging to, a common 30,000 gallon pressure vessel. One loop was constructed to provide for testing of the spare HPSI pump. Suction and discharge pipe sizes were selected to correspond to the actual pipe sizes at Palo Verde. The specific suction piping configuration leading into the HPSI suction nozzle was explicitly reproduced. The second loop was provided for testing of the representative CS pump.

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'.1 4.2 Test Conduct

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For each base case, tests were performed at incrementally increasing air injection mass flow rates.

The resulting air volume fraction, defined as the ratio of volumetric air flow rate to total volumetric air flow rate, was then determined. [

-Figure 4-1 illustrates the final test for the [

] base case.

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U During every test, the duration of air injection was specified to assure that the total volume of air I

I exceeded the total volume of air predicted by the scale model tests. Pump performance data was taken during each test for subsequent assessment of the air ingestion on pump performance.

Visual observations, and digital camera recordings, were made for all HPSI test cases.

4.3 Test Results Visual observations through the clear spool piece on the HPSI suction line confirmed I similar in nature to that observed during the scale model Phase 2 tests. The visual observations confirmed the proper scaling of the Phase 2 tests and gives reasonable confidence that the Phase 2 and Phase 3 tests closely approximate the full-scale plant conditions. Pump performance data was taken using a data acquisition system that recorded each data point 10 times per second. The recorded data was then inserted into Excel spreadsheets to facilitate calculation of pump developed The data represents the calculated developed head (TDH) from the recorded pump inlet and outlet pressure data taken every 0.1 seconds, and the corresponding flow rates as measured on the pump discharge line. The data represents that obtained over a specific time period during which the air injection rate was at its maximum steady state value and the corresponding peak air volume fractions were obtained. The data points, as expected, fall along the test loop system curve.

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PREPRIETAR.W' I FORMATieN Page 35 I

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U As illustrated in the preceding three figures, and as would be expected, pump performance progressively degrades as inlet air volume fraction increases: This progressive degradation is consistent with data reported in NUREG/CR 2792 (Reference 1). The following figure 4-S is taken from Reference 32 hs cited in the NUREG.

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REDACTED VERSION Page 36 PRO)PRIETAR'. INFeRMATION-A maximum bounding degraded pump curve is then constructed as shown in Figure 4-6. As illustrated, the maximum degraded pump curve conservatively bounds all recorded data for the peak air volume fraction cases tested. The use of this maximum degraded pump curve results in additional conservatism since the Phase 3 tests conditions in some cases exceeded the specified air volume fraction from the Phase 2 scale model tests.

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5.1 REDACTED VERSION Page 38 PROPRIETARY INFORMATION Safety Function impact Thermal Hydraulic Analysis of Spectrum of LOCA Break sizes A series of thermal hydraulic analyses of the Palo Verde ECCS system were performed using the I

] These analyses established the expected I

] conditions that would exist at the time of RAS for a spectrum of LOCA break sizes. Operator actions as prescribed in the Palo Verde Emergency Operating Procedures (EOPs) to initiate a cool down and depressurization of the RCS upon diagnosis of a LOCA were explicitly considered in the analyses. In this way, best-estimate parameters [

at time of RAS were established.

descriptions of the [

codes are presented, followed by I Detailed descriptions of the codes and their applications and limitations are within References These references also provide detailed descriptions of the individual transient results.

5.1.1 NIAAP4 Analysis Code Description MAAP is a computer code that simulates light water reactor system response to accident initiation events. The Modular Accident Analysis Program (MAAP), an integral systems analysis computer code for assessing severe accidents, was initially developed during the industry-sponsored IDCOR Program. At the completion of IDCOR, ownership of NfAAP was transferred to Electric Power Research Institute (EPRI). Subsequently, the code evolved into a major analytical tool (MAAP 3B) for supporting the plant-specific Individual Plant Examinations (IPEs) requested by NRC Generic Letter 88-20. Furthermore, MAAP 3B was used as the basis to model the Ontario Hydro CANDU designs. As the attention of plant-specific analyses was expanded to include accident management evaluations, the scope of MAAP (its design basis) was expanded to include the necessary models for accident management assessments. MlAAP4 is the first archived code that contains a graphical representation of the reactor and containment response. MAAP4, like MAAP 313, is currently being maintained by Fauske & Associates, LLC (FAI) for EPRI and the MAAP User's Group (MUG).

MAAP4 is an accident analysis code that provides results with confidence in all phases of severe accident studies, including accident management, for current PWR reactor/containment designs and for ALWRs. MAAP4 includes models for the important accident phenomena that might occur within the primary system, in the containment, and/or in the auxiliary/reactor building. For a specified reactor and containment system, MAAP4 calculates the progression of the postulated accident sequence, including the disposition of the fission products, from a set of initiating events to either a safe, stable state or to an impaired containment condition (by overpressure or over-temperature) and the possible release of fission products to the environment.

Since the beginning of the MAAP code development, the codes have represented all of the important safety systems such as emergency core cooling, containment sprays, residual heat removal, etc.

MAAP4 allows operator interventions and incorporates these in a flexible manner, permitting the user to model the operator response and the availability of the various plant systems in a general way.

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REDACTED VERSION Page 39 PROPRIETARYINO AIN The user can represent operator actions by specifying a set of values for variables used in the code and/or events, which are the operator intervention conditions. There is a large set of actions that the operator can take in response to the intervention conditions.

MAAP4 has been developed under the FAI Quality Assurance Program, in conformance with 10CFR50 Appendix B and with the International ISO 9000 Standard. Furthermore, the new software has been subjected to review by a Design Review Committee, comprised of senior members of the nuclear community, in a manner similar to that exercised for MAAP 3B.

MAAP4 has been benchmarked against plant experience and large-scale integral experiments and also against one integral computer code. Most of the plant experience and experiment benchmarks are documented in the MAAP4 User's Manual [EPRI, 2003a].

The USNRC reviewed and approved MAAP 3.0B for support of probabilistic risk assessment (PRA) activities at licensed power reactors in the U.S., particularly the 1PE's that occurred in the late 1980's and early 1990's. While MAAP4 has not undergone a formal review process by the NRC, the code owner, EPRI, Fauske & Associates, and the MAAP User's Group previously engaged in MAAP4 familiarization activities with the NRC when MAAP4 was first released. Recently, a MAAP4 Information Exchange between these parties has been undertaken in view of the expanding scope of MAAP4 application and MAAP4-supported submittals to the NRC.

MAAP4 has been used previously for safety analyses outside of the risk arena with NRC approval.

For example, an NRC Safety Evaluation Report (SER) was written for the D.C. Cook plant in its assessment of minimum safe sump level in the containment recirculation sump during a small LOCA event. This assessment involved small LOCA scenarios that are similar to those in the present analysis for PVNGS.

The MAAP4 RCS model uses momentum equation selectively for sub-models that demand a momentum equation for model integrity. One of the aspects for which a full-fledged momentum equation is not implemented is water flow. Consequently, MAAP4 cannot void the core by reversing flow from the core lo the downcomer and loop piping during a large LOCA event. However, small breaks of the size being analyzed for this analysis do not engage in such significant flow reversal, so this limitation is not relevant to this analysis.

The MAAP4 containment model can accommodate most physical phenomena that would occur.

However, since it does not entrain pre-existing liquid and condensate from heat sink surfaces, it does not mechanistically bring suspended water droplets into the containment atmosphere (although the model could accommodate droplets if such liquid entrainment was added). Consequently, it conservatively predicts excess gas-phase superheat and pressurization during the blowdown stage of a large LOCA event. Since small breaks of the size being analyzed for this analysis do not engage in this phenomenon, this limitation is not relevant to this analysis. Documented containment benchmarks are testament to the adequacy of the containment model for predicting short-term and long-term containment pressurization under small and medium LOCA conditions, which is necessary for an accurate depiction of containment spray actuation signal (CSAS) timing in this analysis.

The latest MAAP4 archived revision, MAAP 4.0.5 [EPRI, 2003b], was used with the latest PVNGS-specific plant model (a.k.a., parameter file).

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REDACTED VERSION PRGPRIETFARY INFO)RmA-TieH Page 42 The analyses provide three key results. The first result is the RCS pressure that would exist at the time of RAS for various size breaks. These results are provided in Figure 5-1.

.Break RCS Pressure at RCS Pressure at Size RAS (psia)

RAS (psia) Suction Discharge Leg Leg Breaks Breaks 1"

1386 1384 2"

546 438 3"

222 233 4"

213 155 5"

132 148 6"

102 79 7"

77 74 8"

47 53 9"

49 46 10" 37 38 Table 5-1 RCS Pressure at RAS for Various Break Sizes from CENTS This parameter is used in the following section to [

] assess ECCS performance (i.e. HPSI flow) under the maximum predicted air ingestion conditions.

The second result from these analyses is that break sizes of 2" diameter or smaller [

1 alternate method of core cooling is available should the HPSI pump fail due to air ingestion. The current PVNGS Emergency Operating Procedures fully implement this recovery strategy.

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, il~AT The resulting HPSl system performance or operating points, given the degraded pump performance and the system resistance curves developed above, can be determined and illustrated graphically as shown in Figure 5-2. The developed head and flow rate of the degraded pump is determined by the intersection of the system curves and the degraded pump curves.

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U As indicated in Figure 5-2, the static head associated with the 1" diameter small break LOCA at the time of RAS is well above the developed head of the degraded HPSI pump under maximum air ingestion.

For break sizes 2" diameter and larger, Figure 5-2 indicates the degraded HPSI pump has sufficient developed head to continue delivering ECCS flow to the RCS for the short time until the volume of air originally resident in the voided piping is exhausted. After the total air volume is ingested, the Phase 3 pump performance tests demonstrated the HPSI pump would recover and return to its normal non-degraded performance.

5.3 HPSI Pump (Emergency Core Cooling) Safety Function Impact Conclusion From the Phase 3 pump performance tests under air ingestion, a bounding degraded FIPSI pump performance curve was developed. The bounding degraded performance curve envelopes the maximum predicted air volume fractions ingested by the HPSI pump, based on Phase 2 scale-model testing. This study then compared the resulting degraded pump performance with the calculated system resistance that would exist at the time of RAS, for the spectrum of break sizes. The comparison indicates the degraded HPSI pump would develop sufficient discharge head to maintain flow to the RCS for all break sizes except for the smallest breaks less than 2". The degraded flow rate delivered to the RCS would only exist [

] until the air inventory available to be ingested is exhausted, at which time pump performance can be assumed to return to normal. The analyses performed using the CENTS and MIAAP codes determined that for the full spectrum of REDACTED VERSION satet y.SigniTicance Determination

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Tests were conducted on the representative CS pump by injecting air at rates up to approximately

[

] air volume fraction. This air volume fraction conservatively bounds the amount of air predicted by scale model testing for all scenarios tested. The pump experienced a reduction in flow during the period of air ingestion, and then returned to normal baseline performance after air injection was suspended. It is concluded that the voided pipe condition does not have a significant impact on Containment Spray pump functionality.

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REDACTED VERSION PnrREIETAR' INFeRMATIeN Page 46 6.1 Waterhammer The ECCS voided piping condition did not present any negative impacts stemming from waterhammer. Numerous analyses and experiments (References 12 through 14) have been performed to evaluate the influence of air in a system during a strong hydraulic transient such as a pump start.

As stated by Martin (Ref. 12):

The effect of the presence of entrapped air on transient pressures of a liquid pipeline can either be beneficial or detrimental, depending on the amount of air, the nvo-phase flow regime of the mixture (whether homogeneous or slug), and the nature and cause of the transient.

Of particular importance are those situations which could be detrimental to the piping system.

Generally these are conditions in which a significant coherent gas volume has formed on the discharge side of the pump. Significant means a volume that is comparable to or larger than the integrated volumetric flow discharged from the pump during the time that it comes up to speed.

Given these conditions the pump can accelerate to essentially runout flow conditions with the only resistance being the frictional forces generated by the moving water column between the pump discharge and the air pocket. Subsequent to this, the moving water column will begin to compress the air volume and the gas pressure will increase dramatically as volume is reduced.

For example, under these conditions, the gas bubble pressure more than doubles when the gas volume is reduced by one half and similarly more than doubles again when it is reduced again by one half, etc. Hence, with a low pressure gas volume on the discharge side of the pump, the compression of the gas bubble will eventually absorb the kinetic energy of the water column. For this to occur, the gas volume pressure can increase to values much greater than the maximum pump discharge pressure.

Conversely, if the air volume is on the suction side of the pump such as in the case of the Palo Verde ECCS voided piping, [

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REDACTED VERSION PROPRIETARY INFORMATiON Page 48 6.2 Net Positive Suction Head NUREG/CR-2792 (Ref. I) provides discussion and guidance regarding the affect of pump air ingestion on NPSH considerations. For example, Section 3.2.3 states that "the presence of air at the inlet.increases the limiting NPSH required for satisfactory operation. The increased degradation at the pump inlet, as inlet NPSH or pressure is lowered, results from the increased volumetric expansion of air between the pump inlet flange and the impeller inlet. Thus pumps operating with air ingestion will have higher NPSH requirements than those required in single-phase operation."

Section 4.2 goes on to establish an "arbitrary relationship" for the purpose of minimizing this volumetric expansion that occurs between the inlet and the impeller eye. The relationship is:

NPSHRi,rwatr = NPSIIRWat, + (I + 0.5 AF)

Where AF is the air volume fraction in percent. It is noted that this relationship is only intended for use with air volume fractions less than 2%

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REDACTED VERSION PWC)PPIPTAIY IPFNFZMATl-Page 50 l7 Probabilistic Risk Assessment Probabilistic Risk Assessment Conclusion From the CENTS thermal-hydraulics analyses and the Phase 3 pump performance tests, modifications to the Palo Verde Probabilistic Risk Assessment (PRA) model were made to assess the risk significance of the voided pipe condition. The Palo Verde model contains an event tree for small break LOCAs of 2.3 inch diameter and smaller. The model was revised by inserting a failure of the HPSI pumps at RAS (failing the high pressure recirculation function) for small-break LOCA due to air binding, and modeling the subsequent plant cool down and depressurization and LPSI alignment for low pressure recirculation. Consideration was also given to small LOCA events that are induced through the lifting of a PSV and the subsequent failure to reseat. An estimate of the risk increase due to small LOCAs resulting from seismic events was also calculated. Since the pump performance tests indicate that for breaks 2 inches in diameter and larger failure of the H-PSI pump is not likely, medium and large LOCA events were unaffected by the voided condition. Thus the small LOCA event would be the dominant contributor to the risk increase due to the voided pipe condition.

I I calculated the increase in risk associated with the unfilled containment sumps suction lines. The following table shows the overall impact of loss of High Pressure Recirculation (l-IPSR) for break sizes of two inches or less.

Initiator Delta-CDF (per year)

Small LOCA 4.5E-6 PSV - Internal Events Plus Fire 2.0E-6 Seismic 4.7E-7 Total 7.OE-6 Table 7-1 Over-all Risk Associated with Loss of HPSR The above described model adjustments were applied to the entire range of small break LOCA events (i.e. 2.3 " diameter and smaller). The pump testing and analysis program described in the previous sections of this report demonstrate that continued functionality of the HPSI pump for the upper end of the SBLOCA range (those breaks approaching 2" in diameter and larger) would be expected. For the small end of the SBLOCA range of approximately 0.5" in diameter or less, analyses using the CENTS and MAAP code demonstrate that complete depressurization of the RCS to shutdown cooling conditions would be achieved prior to RAS. Therefore, no additional risk is associated with REDACTED VERSION PPAOPmETA Pl? It rrflMA TAINI:

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r XRI ET-ARY IWeRmAT-Ife Page 51 the breaks on the small end of the SBLOCA range. Therefore, the above result provided in Table 7-1 is considered to be a conservative estimate of the incremental risk associated with the ECCS voided piping condition.

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REDACTED VERSION PRE)P4rnEAn* NFORMATION Page 52 A comprehensive testing and analysis program was conducted to conservatively estimate the risk significance of the ECCS voided piping condition. The scale model testing program simulated bounding conditions and parameters to provide high confidence the air ingestions rates obtained from the tests exceeded the air ingestion rates the ECCS pumps would have actually experienced had an accident requiring containment recirculation actually occurred. Subsequent pump performance tests were conducted under conditions considered to be more severe than would have been experienced during an actual emergency. The results of the pump performance tests were then used in a set of thermal hydraulic analyses of the Palo Verde Reactor Coolant System and Containment. The analyses determined that performance of the ECCS and containment and temperature control functions would have been maintained. For most postulated accidents scenarios, the ECCS safety function would have been maintained by the HPSI pumps. For a subset of SBLOCA scenarios, the ECCS function would have been maintained by the use of any available CS or LPSI pump following RCS cooldown and depressurization by the Plant Operators, if the HPSI pumps were to have failed due to air ingestion. Utilizing the results of the testing and analysis program in a conservative manner, the incremental risk associated with the ECCS voided piping condition is estimated to be 7.0 x 10 6.

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REDACTED VERSION OPREPRllET-ARY INFEORMATINM Page 53 References

1. NUREG/CR-2792 "An Assessment of Residual Heat Removal and Containment Spray Pump Performance Under Air and Debris Ingesting Conditions". Published September 1982.

2. Paranjape, S. S. et al. 2003. "Interfacial Structures in Downward Two-Phase Bubbly Flow". 1V, International Conference on Nuclear Engineering (ICONEI 1). Tokyo, Japan.

6. Wallis, G.B., 1969. One Dimensional Two-Phase Flow McGraw-Hill, New York.

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12. Martin, C. S., 1976, "Entrapped Air in Pipelines," Second Int'l Conf. on Pressure Surges, Sept. 22-24, London, England, pp. F2-15 to F2-28.

13.Chaiko, M. A. and Brinckman, K. W., 2002, "Model for Analysis of Waterhammer in Piping with Entrapped Air," Transactions of the ASME, Journal of Fluids Engineering, 124, pp. 194-204.

14. Lee, N. H. and Martin, C. S., 1999, "Experimental and Analytical Investigations of Entrapped Air in a Horizontal Pipe," Proceedings of the Third ASME/JSME Joint Fluids Engineering Conference, July 18-23, San Francisco, California.

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