ML20134F673
| ML20134F673 | |
| Person / Time | |
|---|---|
| Site: | 05200003 |
| Issue date: | 12/31/1996 |
| From: | Gresham J, Loftus M, Spencer J WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML20134F676 | List: |
| References | |
| WCAP-14812, WCAP-14812-R, WCAP-14812-R00, NUDOCS 9702100092 | |
| Download: ML20134F673 (112) | |
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3 0 4 5
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D E
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ORIGINATOR SIGyT
' ATE J. Woodcock
/
Md th D AP600 RESPONSIBLE MANAGER Si T
'IE' APP VA DATE J. GreSham Me
/ d(//f/7
" Approval of the responsible manager signifiesfiat document is complete, all required reviews are complete, electronic file is a!! ached and document is r: leased for use.
y t
oose_n.
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osa_n ta
WCAP-14812 Accident Specification and Phenomena Evaluation for AP600 Passive Containment Cooling System Mike Loftus Dan Spencer Joel Woodcock Nuclear Safety Analysis December 1996 Approved:
4 AM
). Gresham Westinghouse Electric Corporation Energy System Business Unit P.O. Box 355 Pittsburgh, PA 15230-0355 C 1997 Westinghouse Electric Corporation All Rights Resen'ed m:\\3429w.wpf:1b-01319','
lii I
l TABLE OF CONTENTS l
?
LIST OF ACRONYMS.................................................... viii 2
i
SUMMARY
............................................................. ix 3
1.0 INTRODUCTION
1-1 1.1 OBJECTIVE................................................
1-1 1.2 REPORT ORGANIZATION...................................
1-3
1.3 DESCRIPTION
OF AP600 PCS DESIGN AND OPERATION..........
1-3 1.4 OVERVIEW OF PHENOMENA................................
1-7 2.0 PROCESS FOR PHENOMENA IDENTIFICATION AND RANKING..........
2-1 2.1 PROCESS DESCRII'rION.....................................
2-1 2.2 TESTING PROGRAMS......................................
2-3 2.2.1 Heated Flat Plate Test..................................
2-5 2.2.2 Wind Tunnel Tests....................................
2-5 2.2.3 Condensation Tests....................................
2-5 2.2.4 Air Flow Path Flow Resistance Tests.......................
2-6 2.2.5 Water Distribution Tests................................
2-6 i
2.2.6 Small-Scale PCS Integral Tests............................
2-7 i
2.2.7 Large-Scale PCS Integral Tests.............................
2-7 2.3 SCALING ANALYSES.......................................
2-9 2.4 RANKING OF PHENOMENA.................................
2-9 2.5 PIRT CLOSURE..........................................
2-10 3.0 ACCIDENT SPECIFICATION.......................................
3-1 3.1 ISSUE AND SUCCESS CRITERIA...............................
3-1 3.1.1 Design Criteria.......................................
3-1 3.2 CONTAINMENT SYSTEMS AND STRUCTURES...................
3-2 3.2.1 Inside Containmen t....................................
3-4 3.2.2 Containment Shell.....................................
3-5 3.2.3 Outside Containrnent...................................
3-5
3.3 DESCRIPTION
OF TRANSIENTS...............................
3-8 3.4 EVENT SCEN ARIO.........................................
3-9 3.4.1 Initial and Boundary Conditions.........................
3-10 3.4.2 Loss-of-Coolant Accident................................ 3-15 3.4.3 Main Steamline Break.................................. 3-23 4.0 PHENOMENA IDENTIFICATION AND RANKING.....................
4-1 4.1 PHENOMENA OVERVIEW...................................
4-1 i
4.2 THE AP600 CONTAINMENT PHENOMENA IDENTIFICATION AND RANKING TABLE..........................................
4-8 4.3 RESULTS USED IN PHENOMENA RANKING....................
4-8 4.3.1 Test Results Summary.................................
4-12 Scaling Analysis Results Summary.......................
4-16 4.3.2 I
i i
i m:\\3429w.wpf:lt>.010997
iv l
TABLE OF CONTENTS (Cont.)
l 4.4 RANKING OF PHENOMENA LISTED IN PIRT...................
4-17 i
4.4.1. Break Source........................................
4-19 i
4.4.1A Mass and Energy Release.........................
4-19 4.4.1E Droplet / Liquid Flashing.........................
4-19
'j 4.4.2 Containment Volume 4-20 1
4.4.2A Mixmg/ Stratification in the Contamment Volume....... ~ 4-20 i
4.4.2B Intercompartment Flow in Containment Volume.......
4-21 j
4.4.2C Containment Volume Gas Compliance...............
4-21 l
4.4.2D Fog in the Containment Volume...................
4-22 i
4.4.2E Hydrogen Release 4-22 4.4.3 Containment Solid Heat Sinks...........................
4-25 4.4.3A Liquid Film Energy Transport on Containment Heat Sinks 4-25 4.4.3B Vertical Film Conduction on Contamment Heat Sinks...
4-26 4.4.3C Horizontal Film Conduction on Containment Heat Sinks.
4-26 4.4.3D Intemal Heat Sink Conduction.....................
4-26 I
4.4.3E Heat Capacity of Containment Heat Sinks............
4-27 4.4.3F Condensation on Containment Heat Sinks............
4-27 4.4.3G Convection From Containment Volume..............
4-28 1
4.4.3H Radiation From Containment Volume to Containment Heat Sid.s..........................
4-28 4.4.4 Initial Conditions Within Containment..................... 4-29 4.4.5 Break Pool..........................................
4-29 4.4.5A Break Pool Mixing / Stratification...................
4-29 l
4.4.5B Break Pool Condensation / Evaporation...............
4-29 i
4.4.5C Convection Heat Transfer with Containment Volume...
4-30 4.4.5D Radiation Heat Transfer with Containment Volume.....
4-30 4.4.5E Conduction in Break Pool........................
4-30 4.4.5F Flooding in Break Pool...........................
4-30 4.4.6 IRWST.............................................
4-31 4.4.7 Steel Shell..........................................
4-31 4.4.7A Convection Heat Transfer From Containment Volume...
4-31 4.4.7B Radiation Heat Transfer from Containment Volume to Steel Shell............................
4-32 4.4.7C Condensation on Inside Contamment Shell...........
4-32 4.4.7D Film Conduction on Inside of Steel Shell.............
4-32 4.4.7E Film Energy Transport on Steel Shell................
4-32 4.4.7F Conduction Through Shell........................
4-33 4.4.7G Heat Capacity of Shell...........................
4-33 4.4.7H Convection to Riser Annulus......................
4-33 4.4.7I Radiation to the Baffle...........................
4-33 4.4.7J Radiation to the Chimney........................
4-34 4.4.7K Radiation to the Fog / Air.........................
4-34 4.4.7L Outside Film Conduction.........................
4-34 4.4.7M Outside Film Energy Transport..................... 4-35 4.4.7N Evaporation to Riser Annulus.....................
4-35 m:\\3429w.wpf:1b-012797 4
=
v i
i TABLE OF CONTENTS (Cont.)
4 4.4.8 PCS Cooling Water..................................
4-35 4.4.8A Water Flow Rate...............................
4-35 4.4.8B Water Temperature.............................
4-35 4.4.8C Water Film Stability and Coverage..................
4-36 4.4.8D Film Stripping.................................
4-36 4.4.8E Film Drag.....................................
4-36 4.4.9 Riser Annulus and Chimney Volume.....................
4-36 4.4.9A PCS Natural Circulation..........................
4-36 4.4.9B Vapor Acceleration..............................
4-37 4.4.9C Fog..........................................
4-38 4.4.9D Flow Stability..................................
4-38 4.4.10 Baffle..............................................
4-39 4.4.10A Convection to Riser Annulus.....................
4-39 4.4.10B Convection to Downcomer Annulus................
4-39 4.4.10C Radiation to Shield Building.....................
4-40 4.4.10D Conduction Through Baffle......................
4-40 4.4.10E Condensation on the Baffle.......................
4-40 4.4.10F Heat Capacity of the Baffle.......................
4-40 4.4.10G Leaks Through Baffle...........................
4-41 4.4.11 Baffle Supports......................................
4-41 4.4.11A Convection to Riser Air.........................
4-41 4.4.11B Radiation from Shell...........................
4-42 4
4.4.11C Conduction from Shell into Baffle Supports..........
4-42 4.4.11D Heat Capacity of Baffle Supports..................
4-42 4.4.12 Chimney Structure...................................
4-43 4.4.13 Downcomer Annulus.................................
4-43 4.4.13A PCS Natural Circulation.........................
4-44 4.4.13B Downcomer Annulus Air Flow Stability.............
4-44 4.4.14 Shield Building......................................
4-44 4.4.14A Convection to the Downcomer...................
4-44 4.4.14B Conduction Through the Shield Building............
4-45 4.4.14C Convection to the Environment...................
4-45 4.4.14D Radiation to the Environment....................
4-45 4.4.15 External Atmosphere..................................
4-45 4.4.15A Temperature................................
4-45 4.4.15B H umidity....................................
4-45 4.4.15C Recirculation................................
4-46 4.4.15D Pressure Fluctuations...........................
4-46 1
5.0 CONCLU SIONS..................................................
5-1
6.0 REFERENCES
6-1 1
APPENDIX A Summary Table Showing Summary of Sources Supporting PIRT Ranking...................................... A-1 1
I I
l mA3429w.wpf:1bO12797 1
vi LIST OF TABLES Table 2-1 Containment Analysis Processes Used to Initially Define Test Program..
2-4 l
Table 3-1 PCS Compartment Gas and Heat Sink Volumes and Areas Considered in -
Safety Evaluations..........................................
3-7 Table 3-2.
Comparison of Key Containment Analysis Results..................
3-9 i
Table 3-3 Initial Conditions for AP600 Containment Pressure Calculations......
3-10 Table 3-4 Sequence of Events Leading to the Development of the PCS Cooling Film 3-21 Table 3-5 Large-Break LOCA Sequence of Events.........................
3-21 Table 4-1 Phenomena Identification and Ranking According to Effect on Containment Pressure.....................................
4-9 b
l 1
1 I
l I
i l
1 m:\\3429w.wpf:1M10997
1 vii j
LIST OF FIGURES 4
Figure 1-1 Relationship Between AP600 PCS PIRT, Testing, Analysis, i
and Evaluation Model......................................
1-2
}
Figure 1-2 Passive Containment Cooling System Arrangement.................. 1-4 Figure 1-3 Generalized Schematic Representation of Containment j
l Pressure Relationship to Heat Sinks and Momentum...............
1-5 Figure 1-4 Generalized Schematic Representation of Containment Pressure Relationship to Heat Sinks and Momentum................
1-9 Figure 2-1 PCS Phenomenon Identification and Ranking Confirmation Process.....
2-2 Figure 2-2 Section View of AP600 Large-Scale PCS Test Phase Two Configuration..
2-8 Figure 3-1 AP600 Containment Structures.................................
3-3 Figure 3-2 Simplified AP600 Containment Compartments.....................
3-6 Figure 3-3 AP600 DECLG Mass Release.................................
3-11 Figure 3-4 AP600 DECLG Energy Release...............................
3-12 i
Figure 3-5 MSLB Mass Release Rate..
.................................3-13 Figure 3-6 MSLB Energy Release Rate..........
.......................3-14 i
Figure 3-7 PCS Delivered and Applied Flow.............................
3-16 Figure 3-8 DECLG Containment Pressure vs. Time.........................
3-18 i
Figure 3-9 Four Time Phases for DECLG Event...........................
3-20 Figure 3-10 MSLB Containment Pressure vs. Time..........................
3-25 Figure 4-1 Overview of Containment Building Phenomena....................
4-2 i
l m:\\3429w.wpf:lb410097
viii
- LIST OF ACRONYMS CMT Core Makeup Tank DBA Design Basis Accidents DECLG Double-Ended Cold Leg Guillotine ECCS Emergency Core Cooling System In-Containment Refueling Water Storage Tank IRWST.
LOCA Loss-of-Coolant Accident LST Large-Scale Test
)
MSLB Main Steamline Break PCCWST Passive Containment Cooling Water Storage Tank
~
PCS Passive Containment Cooling System PIRT Phenomena Identification and Rankmg Table PRHR Passive Residual Heat Removal PWR Pressurized Water Reactor PXS Passive Core Cooling System RCS Reactor Coolant System STC Science and Technology Center l
T&AP.
Testing and Analysis Plan
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m:\\3429w.wpf:1t> 010997
U i
i I
SUMMARY
The purpose of this report is to identify the phenomena associated with the AP600 containment pressure response to a design basis accident (DBA) and document the basis for phenomena ranking. The most limiting DBAs are the double-ended cold leg guillotine loss-of-coolant accident (DECLG-LOCA) and main steamline break (MSLB), due to the large mass and energy releases to the containment and the resultant containment pressure increase. The timing and progression of these two events provides different challenges to the containment:
~
the LOCA is a relatively long transient compared to the MSLB. Characteristics of the mass and energy release such as superheat and momentum are also different for these two DBAs.
i The AP600 passive containment cooling system (PCS) was designed to maintain the AP600 containment below a pressure of 45 psig and reduce pressure over the long term. This function is accomplished primarily by absorption of energy by the volume and structures inside containment and by the evaporation of water that is applied via gravity directly to the outer containment shell surface.
The PCS consists of a 400,000-gallon water storage tank located above the containment shell, a set of weirs located on the containment shell to uniformly distribute the gravity-fed water from the storage tank onto the shell, and an air flow path to transfer energy from the shell to the environment.
A systematic process mvolving results from separate effects and integral effects tests, containment scaling analyses, first principles calculations and sensitivity studies and engineering judgement, was followed to identify and rank the respective containment cooling phenomena. Where useful, phenomenological reports have been issued to address specific items. This evaluation found the following:
The mass and energy release is important since it is the source for containment pressurization Gas compliance limits the rate of pressure change
~
Condensation mass transfer in the presence of noncondensibles inside containment is the most important phenomenon for pressure reduction inside containment Mixing / stratification and their effects on noncondensible distribution are important phenomena that affect the condensation mass transfer Evaporation mass transfer from the containment shell is the most important phenomena for energy transfer outside containment m:\\3C9w.wpf:1b-010997
l x
Natural circulation through the PCS air flow path is an important phenomenon that affects the evaporation mass transfer i
Film coverage and stability on the external surface are important for the heat removal by evaporation Convection and radiation heat transfer are much less important than condensation and evaporation h
m:\\M29w.wpf:lt410997
{
11 i
1.0 INTRODUCTION
1.1 OBJECTIVE The purpose of this report is to identify the phenomena associated with the AP600 containment pressure response to a design basis accident (DBA) and document the basis for phenomena ranking. The ranking is used to focus attention on the most significant phenomena and to identify aspects of the AP600 containment evaluation model which should be bounded or otherwise properly addressed to provide confidence in the predictions of the DBA.
The AP600 containment is a large, closed volume that undergoes pressurization during a postulated DBA. The containment shell serves as the boandary between the inside, pressurized region and the outside, atmospheric region. The passive containment cooling system (PCS) is used to transfer energy from the containment shell to the environment during a postulated DBA.
In addition to the PCS which transfers energy from the containment to the environment, the AP600 containment has several other pressure mitigation features. These include a large volume, a large amount of steel and concrete heat sinks, and non-safety grade containment fan coolers, that absorb the energy from breaks in the primary or secondary side, and mitigate containment pressurization.
This report is the starting point for the evaluation of the AP600 containment pressurization process and the PCS design. As shown in Figure 1-1, the Phenomena Identification and l
Ranking Table (PIRT) supports the other key containment evaluation activities and reports, since it identifies the containment phenomena and provides the basis for importance of the phenomena. This document draws on more detailed information provided in other containment reports (identified in Tables 1 and 2, of Ref.1). The primary purpose of this report is to describe the physics of the containment cooling process based upon realistic conditions and assumptions, and to provide supporting references.
This report focuses only on DBAs that may lead to containment over-pressurization. It excludes beyond-design-basis events, severe accidents, and under-pressure events as discussed in subsection 3.1.1. The design basis accidents considered in this evaluation are described in subsectior. 3.3 and were selected as those that pose the greatest challenge to containment design pressure.
Introduction 1
m:\\3429w.wpf:1b-010997
I 1-2 FIRT b
Test Reports Scaling Report Phenomenological WGOTHIC Reports Reports I
V Evaluation Model w zun 6 Figure 11 Relationship Between AP600 PCS PIRT, Testing, Analysis, and Evaluation Model Introduction m:\\3429w.wpf:1b-010997
J 1-3 f
1.2 REPORT ORGANIZATION The rest of the Introduction provides a description of the AP600 PCS design and operation and a brief overview of the most significant phenomena, to introduce the reader to the physical arrangement and important processes. Section 2 describes the process of developing and confirming the PCS DBA PIRT, summarizes results from testing and scaling, and defines criteria for ranking. Section 3 specifies the events to be evaluated and shows success criteria.
4 Section 4 contains the PIRT and a summary of the bases for ranking. Results of the following have been included in the bases where applicable:
Testing and test analyses Scaling 1
First principles calculations Sensitivity studies Phenomena evaluation reports Appendix A contains a table summarizing the sources of supporting information for PIRT ranking.
i
1.3 DESCRIPTION
OF AP600 PCS DESIGN AND OPERATION The PCS makes use of the steel containment vessel and the concrete shield buildmg 1
surrounding the containment. The major components of the PCS are. the passive containment cooling water storage tank (PCCWST), that is incorporated into the shield building above the containment; an air baffle located between the steel containment vessel and the concrete shield building that defines the c&ng air flowpath; air inlets and air exhaust, also incorporated into the shield building r. v>;ture; and a water distribution system, meunted on the outside surface of the steel containment vessel, that functions to distribute we'er flow on the containment. The PCS arrangement is shown in Figure 1-2. A water recirculation path is provided to control the PCS storage tank water chemistry and to provide heating for freeze protection, as shown in Figure 1-3. The major flow areas and heights are also shown in Figure 1-3. The intemal containment geometry is shown in Figure 3-2 and described in subsection 3.2.1.
Operation of the PCS is initiated upon receipt of two out of four Hi-2 containment pressure signals. System actuation consists of opening the PCS water storage tank isolation valves.
l This allows the PCCWST water to be delivered to the distribution bucket above the center of the containment dome. A weir-type water distribution system is used on tFe dome surface to maximize the wetted coverage of the dome and vertical sides of the containment shell. A l
corrosion-resistant paint on the containment shell enhances surface wettability and film 1
formation.
i Introduction mAM29w.wpf.1b 010997
1-4 i
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Figure 1-3 Schematic of Passive Containment Cooling System Showing Major Flow Areas and Heights Introduction m:\\3L%.wpf:1b-010997
1-6 The PCCWST (400,000 gallon volume) provides water for containment wetting for an appropriate period of time following system actuation. The flow of water is initially established at approximately 440 gpm for short term containment cooling. The flow rate is reduced over time to approximately 55 gpm as the decay heat decreases. The flow rate decreases as the water level in the storage tank decreases. A combination of different height standpipes in the storage tank is used along with orifices to adjust the delivered flow rate over time.
The cooling water not evaporated from the vessel wall flows down to the bottom of the inner annulus into floor drains. Floor drains with 100 percent redundancy route excess water to storm drains. The drain lines are always open (without isolation valves) and each is sized to accept maximum external PCS water flow. The interface with the storm drain is an open
~
connection such that any blockage in the storm drains would result in the annulus drains overflowing the connection, draining the annulus independently of the storm drain system.
The path for the natural circulation of air upward along the outside surface of the containment shell is always open. The natural circulation air flow path begins at the shield building inlet where atmospheric air enters through openings in the concrete structure. Air flows past a set of fixed louvers and is forced to turn 90 degrees into the downcomer. After flowing down the downcomer, curved vanes aid in turning the flow upward 180 degrees into the riser. The riser and downcomer are separated by the thin-walled baffle. Air flows up the riser to the top of the containment vessel and exhausts through the shield building chimney.
The important dimensions (Ref. 2,3, and 4) associated with the PCS air flow path are:
Containment inside diameter - 130 ft.
Containment shell thickness - 1.625 in.
Riser gap - 12 in. (approximate)
Baffle steel thickness - 0.12 in.
Shield building inside diameter - 139 ft.
Downcomer gap - 42 in. (approximate)
Shield building concrete thickness - 3 ft.
The inlets for the PCS air flow path are placed circumferentially at the top outside of the shield building. Wind tunnel tests (Ref. 5, Section 6) show that this provides a symmetrical air inlet to a plenum in which pressure is circumferentially equalized, minimizing the effect of wind speed and direction, and limiting the effects of terrain and nearby building interference and turbulence. There are 15 air inlets, each 16-ft. wide by 5-ft. high. Screens are provided at the air flow inlets and discharges to prevent debris from entering the annulus. A special process trace heating system provides for heating of the air inlet and chimney structures that may be sensitive to snow or ice buildup that could cause blockage of the air flow path. The chimney through which the air / water vapor exhausts is elevated Introduction m:\\M29w.wpf:1b-010997 l
1-7 above the air inlet to provide additional buoyancy and to minimize the potential for exhaust air being drawn into the air inlet (i.e., recirculation).
Since the PCS system design is relatively simple with few critical active components, and all components are accessible for inspection, maintenance, and repair, the PCS has been allotted zero hours per year of unavailability. The PCS cooling water conditions and the valve positioning on the water storage tank are governed by the technical specifications and are monitored. The PCS air flow paths are monitored via personnel inspection according to the technical specifications.
A description of the AP600 PCS design, operation, and maintenance is provided in the
~
System Specification Document (Ref. 5).
1.4 OVERVIEW OF PHENOMENA As the containment atmosphere heats and pressurizes in response to a postulated accident such as a high-energy line break, energy is removed from the containment atmosphere via several interrelated processes both inside and outside containment:
Circulation of the steam, water, and noncondensible mixture within the containment atmosphere Mass transfer via condensation on the initially " cool" heat sinks and inner shell surface inside containment Heat transfer via convection and radiation to initially " cool" heat sinks and inner shell surface inside containment Heat transfer via conduction through the steel containment shell Sensible heating of the relatively cool PCS water Mass transfer via evsporation on the outside shell Heat transfer via radiation and convection to the baffle and the environment Buoyancy-driven circulation in the extemal air flow path There are many parameters that can affect the above processes including the initial and boundary conditions, break size and location, thermal resistances, and the actual performance of the PCS such as the amount of water coverage on the outside of the containment shell or Introduction m:\\3429w.wpf:1b 010997 l
1-8 i
the air flow rate over the shell. An overview of these energy transfer processes is provided l
in subsection 4.1.
j i
The containment rate of pressure change is linked by mass and energy transfer resistances to the intemal heat sinks and through the containment shell to the PCS air flow path as shown l
schematically in Figure 1-4. The energy and mass transfer resistances depend on values of l
the local velocity, air / steam concentration, and temperature which are governed by the l
momentum inside containment and in the PCS air cooling path.
I The containment shell serves as both a heat sink and as a conductor of energy to the environment. As energy is transferred from the shell to the air in the riser, the air becomes less dense than the air in the downcomer. This density difference causes an increase in the f
natural circulation of air flow through the downcomer, up the riser, and through the chimney to the exit at the top of the shield building.
The relative importance of these processes changes as the transient proceeds. The transient
)
nature of these phenomena is described in subsection 4.4, " Ranking of Phenomena Listed m r
PIRT."
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1 2-1 i
2.0 PROCESS FOR PHENOMENA IDENTIFICATION AND RANKING This section describes the process for identifying and ranking the phenomena in the AP600 containment for a DBA event. The process that was followed provided for structure,
)
independence, and completeness; and was also iterative.
1 i
2.1 PROCESS DESCRIPTION The process for identifying and ranking the importance of the phenomena in the AP600 containment following a postulated accident was initiated in the late 1980s. This process started with the identification of accident scenarios that previously posed the greatest challenge to the containment and its respective cooling systems, i.e., large-break LOCA and MSLBs. These transients have traditionally provided the largest mass and energy release to the containment (see subsection 3.3 for discussion of transients considered). However, it was recognized that the AP600 containment design differs from those in existing pressurized water reactors. Since the AP600 containment DBA relies on energy transfer to the environment rather than to any active heat removal systems (containment fan coolers) it was necessary to extend the phenomena identification process outside the containment to include j
the environment. The identification of phenomena also helped to define what new tests needed to be performed.
The phenomena identification and ranking process involved the key steps showm in Figure 2-1. The phenomena that occurred during the most limiting scenarios were subsequently identified and documented by the Westinghouse engineers most familiar with the containment thermal hydraulic response. These personnel have extensive experience in analyzing fluid flow and heat transfer mechanisms, and evaluating the pressurization of nuclear power plant containment buildings. The identification and ranking of the containment phenomena was then subjected to test comparisons, scaling analyses, and sensitivity studies. These checks (described in subsections 2.2 and 2.3) verified that all containment phenomena were properly identified and ranked in importance. Appendix A contains a cross-reference table that summarizes the sources of information used to confirm PIRT rankings.
Several rounds of regulatory review have also been completed. The most recent include a preliminary containment PIRT report (Ref. 7), which was issued to the NRC in February, 1996. The PIRT was revised based upon the comments received and a presentation was made to the ACRS T/H subcommittee on May 10,1996. The presentation covered not only the PIRT, but also the containment scaling approach and test program.
Process for Phenomena identification and Ranking m:\\3429w.wpf;1t> 010997
2-2 l
l Define scenarios and success criteria V
Review phenomena and existing tests U
Document phenomena identification and ranking c
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1r j
SETS and LST data analysis 1r 1
i Scaling 1r U
AP600-specific En eerkg quantitative I
8'**"I phenomena reports 1r 1r u
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henomena lis No complete?
Yes V
Document PIRT and its bases
/desstece2/3M1Jak Figure 2-1 PCS Phenomenon Identification and Ranking Confirmation Process Process for Phenomena identification and Ranking m:\\3429w.wpf:1b-010997
2-3 2.2 TESTING PROGRAMS As a result of the process described above, several separate effects tests were identified to investigate specific phenomena such as the liquid flow over the outside of the containment shell and condensation and evaporation mass transfer. In addition, integral effects tests at two different scales were also identified to examine the integrated heat and mass transfer behavior of the PCS. The need for such tests was recognized and testing was initiated in the late 1980s. Table 2-1 was used to identify the contaimnent phenomena unique to AP600 and the tests required to validate models of those phenomena.
The results from the following tests were used to assess and validate the phenomena important in the AP600 containment:
Heated Flat Plate Test performed at Westinghouse Science and Technology Center (STC)
Wind Tunnel Tests performed at Boundary Layer Wind Tunnel Laboratory of the University of Western Ontario Condensation Tests performed at University of Wisconsin Air Flow Path Tests performed at Westinghouse STC Water Film Formation Tests Water Distribution Tests performed at Westinghouse Waltz Mill Small-Scale Integral PCS Tests performed at Westinghouse STC Large-Scale Integral PCS Tests (LST) performed at Westinghouse STC The first six test series represent the separate effects tests and the last two test series represent the integral effects tests. These tests are described briefly in the following subsections. A listing of test reports can be found in Ref. 8, for AP600 Design Certification.
In addition to the Westinghouse-sponsored tests, the evaluation of AP600 containment phenomena were supplemented with test data available in the open literature. These included the Hugot heated, parallel, vertical, isothermal plate tests; the Eckert and Diaguila heated vertical tube tests; the Siegel and Norris heated, parallel, vertical flat plate tests; and the Gilliland and Sherwood Evaporation tests. These tests, as described in Ref. 9, provided additional data to validate models for convective heat and mass transfer in AP600.
Process for Phenomena Identification and Ranking m:\\3429w.wpf:1b410997
2-4 1
Table 2-1 Containment Analysis Processes Used to Initially Define Test Program AP600 Uniqueness Cont.
Containment WRT W Validation AP600-Specific Process Plants Does it Exist Validation Needed Tests Identified Evaporative film Yes No Yes PCS tests,1/8-scale tes.'s, cooling heated plate tests Condensation, with No Yes, not Yes CVTR, U. of Wisconsin, noncondensables AP600-literature j
specific Air cooling of steel Yes No Yes A large-scale test to shell simulate air passage Internal circulation No Yes, not Yes A large-scale test to patterns in AP600-simulate containment containment specific Effect of hydrogen No Yes, not Yes A large-scale test to on containment heat AP600-simulate containment transfer specific Liquid film Yes No Yes Film flow experiments to distribution on investigate the water containment distribution.
. plate tests
. large-scale film flow tests Effects of buildings Yes No Yes Wind tunnel tests with and wind velocity on building effects and site air flow over steel effects shell Process for Phenomena Identification and Ranking m:\\3429w.wpf:11410997
2-5 2.2.1 Heated Flat Plate Test i
The Heated Flat Plate Tests were performed to generate heat and mass transfer data for evaporative cooling with parameters that bound the range of expected conditions on the AP600 containment shell (e.g., heat fluxes, liquid film flow rates, cooling air velocities). A secondary purpose was to observe the film hydrodynamics including possible formation of dry patches due to surface tension instabilities.
The test section was a vertical,6-ft. long,2-ft. wide,1-in. thick flat steel plate that was coated
)
with the highly wettable inorganic-zinc coating used for the AP600 containment shell. A i
clear acrylic cover provided a channel for the forced air flow and allowed observation of the applied liquid film. Preheated water was supplied at a metered rate to a simple distributor located at the upper end of the plate. To simulate heating of the containment wall, the test plate was heated from the back side using a high temperature fluid flowing through copper tubes welded into grooves in the back of the plate.
Tests were performed in two orientations: vertical to represent the containment sidewall and 15 degrees from horizontal to represent the upper portion of the dome. Tests were performed with no water on the plate and for a range of water film flow rates. Two of the tests were performed with very low film flow rates (as low as 15 lbm/hr-ft) at relatively high 2
heat flux (as high as 6000 BTU /hr ft ) to force the film to dry out before reaching the end of the test section. See Ref.10 for more information on the Heated Flat Plate Tests.
i e
2.2.2 Wind Tunnel Tests
]
The Wind Tunnel Tests were performed to test the aerodynamic response of air flow past the AP600 containment building and through the PCS air flow path under a variety of conditions. Three scale models (1:30,1:100, and 1:800) of the AP600 structures were used to simulate the shield building air inlet and exhaust, as well as the surrounding buildings and upwind terrain. Tests were performed to assess the response of the cooling air flow path to large external pressure fluctuations. See Section 6 of Ref. 5 for more information on analyses of the wind tunnel tests.
2.2.3 Condensation Tests A series of experiments to examine condensation of air / steam mixtures flowing over cold surfaces were performed. The test section was 6.25-ft. long with a 2.75-ft. entrance length and a 3.5-ft. long condensing surface. The channel-cross section war square with a flow area of 0.25 ft.2 The 3.5-ft. condensing surface length was coated with an inorganic zinc paint used on the AP600 containment shell to promote surface wetting and condensation.
i Process for Phenomena Identification and Ranking m:\\3429w.wpf;1b-010997
2-6 The condensing surface was held at a near-constant temperature of approximately 86'F by cooling plates located on the back side, while the steam flow rate and inlet temperature were varied. The effects of a noncondensible gas (helium) and the orientation of the surface (for simulation of inner shell surface orientation angle) were also examined. See Ref. 8 for more information on these tests.
2.2.4 Air Flow Path Flow Resistance Tests The Air Flow Path Flow Tests were performed to measure the hydraulic resistance in the PCS air flow path using a 1/6th-scale test (14-degree section). The test used a fan to force air l
through the flow path to characterize the pressure drop and flow resistance at approximately prototypic Reynolds numbers. The tests resulted in design changes to streamline the air flow path to reduce the pressure loss coefficient. See Ref.11 for more information on these tests.
i 2.2.5 Water Distribution Tests The Water Film Formation Tests (Ref.12) were performed to show the wettability of the i
selected inorganic zinc coating for the AP600 containment shell and to characterize general requirements for forming a water film over a large surface area. An unheated,6-ft. long,4-ft.
wide steel plate, painted with the selected inorganic zinc coating, was placed on a pivoting frame to simulate the various angles on the containment dome and sidewall. A stream of water was applied to the center top edge of the plate to see how it would spread to cover the surface.
With a flow rate of 1 gpm from a 0.5-in. diameter tube pointed perpendicular to the surface, the water spread to form a 1-foot wide stripe of film down the 8-ft. length. These same results were obtained at plate angles of 90 and 11 degrees from horizontal. The film thickness was not uniform near the point of application;it was thinnest just below the application point and thicker on both sides. The film stripe continued to spread (more slowly as the surface became more vertical) and a very thin, wet region was created at the edges as the film traveled downward.
~
i Various film spreading mechanisms were also investigated. A dam and weir system was found to be the most effective in distributing the water to create a wavy laminar film over the entire width of the plate.
The Water Distribution Tests were used to determine the water coverage as a function of flow rate on the containment outside surface and to determine the time to establish steady-state coverage on the AP600. A full-scale test section representing a 1/8th-sector of the containment dome and a portion of the vertical sidewall was built, and the performance of various weir distribution systems was tested. The tests were performed at ambient Process for Phenomena identification and Ranking m:\\3429w wpf:lt410997
1 2-7 conditions and included flow rates of 55 to 220 gpm equivalent flow on the AP600 containment. See Ref.13 for more information on the Water Distribution Tests.
2.2.6 Small-Scale PCS Integral Tests The small-scale tests were designed to provide heat and mass transfer data for both the inside and outside of the test vessel. The test apparatus consisted of a 3-ft. diameter,24-ft.
high, steel pressure vessel that was intemally heated by steam. The vessel was surrounded by a clear, plexiglass shield that formed a 15-in. wide annulus for either forced or natural circulation air flow. The tests were performed with varying steam flow rates, water film flow rates and temperatures, and inlet air flow rates, temperatures, and humidity.
Instrumentation was provided to measure internal steam condensation rates, external water evaporation rates, exit film temperatures, air velocity and temperature, and humidity. See j
Ref.14 for more information on small-scale tests.
2.2.7 Large-Scale PCS Integral Tests The large-scale PCS test (LST) facility was built to provide heat and mass transfer test data for a geometrically similar model of the AP600 containment vessel. The tests provided experimental data for evaluating the physics in containment, and for determining the relative importance of various parameters that affect heat and mass transfer on both the inside and outside containment surfaces.
The LST consisted of a 15-ft. diameter,20-ft. high pressure vessel that approximated the AP600 containment vessel at approximately 1/8th linear scale. A plexiglass cylinder was installed around the vessel to form the air cooling annulus (also called the riser in this 1
report). Air flows upward through the annulus via natural circulation to cool the vessel. A fan was located at the top of the annular shell to provide the capability to induce higher air velocities than can be achieved during natural circulation alone, so that Reynolds numbers in the range of AP600 could be simulated. A liquid film was applied to the outside of the test vessel by two rings of J-tubes to provide evaporative cooling. The J-tubes provided the capability to apply water in a manner similar to the water coverage observed in the water distribution tests. See Figure 2-2 for an overview of this test facility.
Test conditions were selected to provide steady-state heat and mass transfer validation over a range of conditions representative of a DBA. These conditions included pressure, steam flowrate, cooling air flowrate, and water coverage. The LST was designed to sufficiently encompass the parameters expected for the most limiting AP600 transients such as a large cold leg LOCA and MSLB. Due to practical limitations, the LST was not a simulation of a particular AP600 transient. Rather, the initial and boundary conditions were ranged to assess the impact on heat and mass transfer rates.
Process for Phenomena identification and Rankmg m:\\3429w.wpf:1b-010997
l 2-8
- Exhaust Fan Duttet T/C's Located by-.O 0 Equal Creumferentiat Areos g -Delta P Cell i
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Vater Dstesbution System fDffuser i
Portable Anemometer f--Baffte Temperatures for Traversing e
. i Internat Velocity Meters Heat Flux Meters sti ine mtrere r t Inside/Dutside Volt Ptexiglass Boffte Temperatures Traversing T/C Portable Anemometeeg for Traversing 4
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Fixed Inlet T/C N
open votume Gutters nmgweer veae Steen Injection Line Steam Flow Meter Helium Injection Figure 2-2 Section View of AP600 Large-Scale PCS Test Phase Two Configuration Process for Phenomena Identification and Ranking m:\\3429w.wpf:1b-010997
2-9 For most tests, steam was injected through a diffuser located under a simulated steam generator compartment below the operating deck (which simulated a LOCA). The steam rose as a plume, and air was entrained in the rising plume resulting in a natural circulation flow pattern and mixing within the simulated containment. Thermocouples located on the inner and outer surfaces of the vessel were used to determine the temperature and heat flux distributions. Tests were also performed with an elevated steam source to simulate an MSLB, with parametric variations to examine the effect of source direction and momentum.
See Ref.15,16, and 17 for more information on these tests.
Where the LST could not adequately represent AP600, phenomena evaluations and analyses have been used to assess appropriate methods for the PCS DBA evaluation model (Ref.18).
2.3 SCALING ANALYSES The scaling analysis results have been used to support quantification of the importance of various phenomena in the containment cooling process (Ref.18). The scaling analysis performed for the AP600 containment was submitted for review and revised to incorporate NRC comments (Ref.19,20).
Control volume equations were developed to describe the rate of change of the containment gas energy and pressure. These equations were coupled by conductances to energy equations for internal heat sinks and to the external PCS through the shell.
Scaling groups (pi groups) were developed by normalizing and nondimensionalizing the conservation equations, using initial and boundary conditions, in a form that shows the important dimensionless parameters in each group. Values were calculated for the pi groups during each time phase to quantify the relative importance of the transport processes and components. The evaluation of the pi groups assumed that the containment steam / air atmosphere was well-mixed. Nondimensional parameters and relevant test data were defined for assessing stratification and internal flow field stability.
The pi groups were evaluated for containment energy and pressurization, conductances to heat sinks and the shell, momentum in the air flow path, and momentum within the containment. The conclusions from the scaling analysis, which support the importance of the phenomena identified in the PIRT, are discussed in subsection 4.3.2.
2.4 RANKING OF PHENOMENA The purpose of ranking the phenomena is to identify the important phenomena that are to be bounded or otherwise properly addressed in the containment evaluation model. The criteria for ranking the phenomena for the AP600 containment cooling process was based upon a Process for Phenomena identification and Ranking m:\\3429wxpf:1b-010997
1 2-10.
t combination of test results, scaling analyses, sensitivity studies and engineering judgement from the containment analysis personnel. The ranking process is described in subsection 4.4.
f 2.5 PIRT CLOSURE The PCS DBA PIRT has been confirmed with various combinations of the following types of f
information:
i l
Scaling analyses Separate effects and integral systems tests Sensitivity studies First principles calculations i
Engineering judgement l
Test analyses Phenomena evaluations
(
The sources of information used to confirm ranking for each PIRT item are summarized in Appendix A.
f t
i s
i i
i Process for Phenomena Identification and Ranking m:\\3429w.wpf:1b-010997
~.
3-1 3.0 ACCIDENT SPECIFICATION i
4 The accident specification for the AP600 containment cooling process consists of a statement of the issue and success criteria, description of the containment systems and structures, identification of the transients considered, and description of the event scenarios.
3.1 ISSUE AND SUCCESS CRITERIA i
1 Most commercial nuclear reactor designs include containments to limit the release of radionuclides to the environment during a postulated breach in either the primary reactor -
coolant system (RCS), or the portions of the secondary cooling system inside containment.
The containment is a pressure vessel designed to the ASME Boiler and Pressure Vessel Code requirements for a specific design pressure. Vessel penetrations for entryways, piping, and instrumentation are sealed to limit leakage of the vessel atmosphere to the environment.
A group of design basis accidents (DBAs), known as high-energy line breaks, has the potential to release significant quantities of high-temperature, high-pressure steam / water inside containment, and may increase the internal pressure to values that challenge the design pressure. Both the primary and secondary coolant systems have large values of stored energy as a consequence of the large volume, high temperature, and high pressure of their steam / water coolant and the heat capacity and high temperature of the cooling system boundaries that include the reactor vessel, steam generators, turbines, pumps, and piping.
3.1.1 Design Criteria i
The AP600 PCS has been designed to maintain pressure below the containment vessel design pressure during a DBA with no credit for active containment heat removal systems and with no operator action. The only active part of the system is a one-time automatic valve opening to initiate PCS cooling water flow, based on a safety grade containment over-pressure signal.
For the PCS DBA, the PCS design will be judged to be successful if, for any postulated DBA, the PCS can:
Maintain the peak pressure difference across the containment shell below 45 psig Reduce the pressure difference at 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> to less than half of the design value Provide containment heat removal for a sufficient period with co operator actions Containment design criteria, contained in General Design Criteria for water-cooled nuclear power plants (10 CFR Part 50, Appendix A), are addressed in the SSAR (Ref. 21).
Accident Specification m:\\3429w.wpf:1b.010997
3-2 Criterion addressed by the PCS DBA PIRT:
The containment structure should be able to accommodate the caN1ated pressure and f
temperature conditions resulting from any LOCA. This is to be accomplished without exceeding a design leakage rate and with sufficient margin. The margin should reflect consideration of (1) potential energy sources such as energy in steam generators, limited metal-water reaction that might result from degradation but not failure of the ECCS, (2) limitations on the amount of information available on accident phenomena, and (3) conservatism in the calculations. (Criterion 50)
]
Criteria addressed elsewhere in the SSAR:
1 Containment will establish an essentially leak-tight barrier against the uncontrolled
[
release of radioactive material (Criterion 16)
Systems are required to be available to remove heat from the containment to negate pressure buildup that would otherwise result (Criteria So through 40) j A system is required to be provided to remove fission products from the containment atmosphere to reduce the consequences of ongoing leakage (Criteria 41 through 43)
Criteria for severe accidents addressed in the Probabilistic Risk Assessment:
Severe accidents, defined by 10 CFR 50.34(f) for near-term operating licenses, 10 CFR 52.47 for standard design certification, and 10 CFR 50.44 for combustible gas control are addressed in the Probabilistic Risk Assessment, and are excluded from the i
3.2 CONTAINMENT SYSTEMS AND STRUCTURES Temporal and spatial partitioning were used to organize the PIRT. Subsection 3.2.1 describes the spatial partitioning. Subsections 3.4.2.2 and 3.4.3.2 describe the temporal partitioning, which is event-specific.
The inside of containment is a large, closed volume (1.7Ex106 ft') which undergoes pressurization during the accident. The containment shell serves as the boundary between the pressurized region inside, and the ambient region outside. Therefore, the containment cooling system components were segregated into three global regions: internal, external, and shell. In order to more conveniently present the components in the PIRT, spatial partitioning of volumes inside and outside containment was used. Each global region was segregated into the major volumes shown in Figure 3-1. The components were considered in groups rather than individually for the PIRT; however, phenomena evaluation address specific Accident Specification m:\\3429w.wpf;1t>.010997
3-3 PCCWST chimney l
e i
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air inlet
/
/
inside
/
containment j
annulus annulus environ-riser
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downcomer ment
/
/
/
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operating intemal
/
deck
/ support i
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IRWST &
W Break Pool Figure 3-1 AP600 Containment Structures Accident Specification m:\\3429w.wpf;1b-010997
=
=
l l
l 3-4 components where appropriate. For further convenience, the initial and boundary conditions were considered as " components" that can affect the containment pressure response. Thus, 1
the PIRT can be used to assess the relative importance of factors that affect the AP600 pressure transient as well as the calculation of the DBA pressure response.
1 3.2.1 Inside Containment The internal containment includes the following key components:
Break source Containment volume
)
Containment solid heat sinks Initial conditions Break pool In-containment refueling water storage tank (IRWST)
The break source, which acts as the intemal boundary condition, includes the steam, water, and/or water drops depending on the mass and energy release rates. The water drops suspended in the steam initially flash a small fraction of their mass to steam to reach thermal equilibrium within the containment atmosphere. After flashing, the large surface area of l
these many tiny water drops maintains the atmosphere at or near saturation for up to thousands of seconds.
l The inner containment atmosphere includes the mixture of steam, water, and air contained within approximately 1.7x10' ft' of volume. The subcompartments below deck are large open volumes with relatively large interconnections that promote mixing throughout the below-deck volume. All compartments below-deck are provided with top openings to minimize the potential for a dead pocket of noncondensible concentration.
The containment solid heat sinks includes the steel hardware and concrete structures within the containment. The containment shell is listed separately from the internal heat sinks since the shell has a significantly different boundary condition due to the external evaporating film.
The initial conditions include the temperature, humidity, and pressure of the containment i
volume as well as the temperature of the solid and liquid heat sinks inside containment.
Liquid from the break that is not dispersed as drops with the steam accumulates in the bottom of the steam generator and reactor cavities to form the break pool. The break liquid is assumed to leave the break at the containment saturation pressure. Liquid from drop l
fallout and condensation on internal structures below the operating deck also drains into the break pool.
l Accident Specification m:\\3429w.vpf;1t410997
3-5 The IRWST collects the condensate that forms on the shell above the operating deck via gutters. After primary system depressurization, the IRWST provides a gravity flow of borated water into the reactor. While other condensate flows to the IRWST, the water may heat but only to the temperature corresponding to the steam partial pressure of the atmosphere at the operating deck. Consequently,it can not become a vapor source by heating from the atmosphere, either while above deck or after draining into the tank (IRWST water is assumed to be at an initial temperature of 120 F).
The distribution of gas volumes and internal heat sinks corresponding to each internal compartment are listed in Table 3-1. Figure 3-2 shows a cross-section of the containment with typical compartments. Additional information on the containment compartments can be obtained from Chapter 4 of Ref. 5.
3.2.2 Containment Shell The shell, which is 1.625-in. thick steel, is an important component because it stores energy and provides the path to transfer energy to the ultimste heat sink-the environment. An inorganic zinc coating is applied to both the inside and outside containment shell surfaces to promote wettability (and therefore, more efficient heat removal) and to provide corrosion resistance.
The PCS water is 3 plied directly to the top outside surface of the containment shell. The water is distributed across the surface by means of two sets of weirs. The water absorbs energy from the shell, heats up, and evaporates into the air flowing up the annulus riser and out the chimney.
3.2.3 Outside Containment The components outside the containment make up the PCS air flow path, where the evaporative, radiative, and convective energy transport processes occur that transfer the thermal energy from containment to the environment as described in subsection 1.3 and shown in Figures 1-2 and 1-3. The physical components of the PCS air flow path are the thin-walled steel baffle and diffuser, U-shaped baffle supports, chimney structure, and thick-walled concrete shield building. These outside components define the downcomer annulus, riser annulus, and the chimney volumes that make up the PCS air flow path. The PCS cooling air flows from the environment, through the inlet screen and downcomer, up the riser, through the diffuser, and out the chimney to the environment as shown in Figure 1-2.
Accident Specification m:\\3429w.wpf;1b-010997
. _ -. - = _
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[h sea SG REFUELING CANAL
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Figure 3-2 Simplified AP600 Containment Compartments Accident Specification m:\\3429w.wpf-1b-010997
B.. >
h Table 3-1 PCS Compartment Gas and lleat Sink Volumes and Areas Considered in Safety Evaluations *
- s h"*
Gas Concrete Jacketed Concrete Steel Control bb Volume A
A
Volume Vol Fract Area Fract Area Fract Volume Vol Fract Area Area Fract gg 8
Above Deck 1399500 0.80 22600 1.00 12600 0.18 1700 0.13 73800 0.34 (minus shell)
Above Deck 0.00 0.00 7100 0.55 52700 0.24 (shell)
IRWST 21100 0.01 0.000 11200 0.16 148 0.01 7100 0.03 Circulating Compartments SG East 29200 0.02 0.00 4500 0.06 110 0.009 6200 0.03 SG West 29100 0.02 0.00 4900 0.07 140 0.01 6300 0.03 CMT 157200 0.10 0.00 17300 0.24 2600 0.21 49300 0.23 Refueling 44300 0.03 0.00 5800 0.08 160 0.01 4300 0.02 Stairwell 16000 0.01 0.00 1400 0.02 29 0.002 2800 0.01 Dead-Ended Compartments Accum NE 13300 0.008 0.00 3800 0.05 140 0.01 4200 0.02 Accum SE 10200 0.006 0.00 2800 0.04 140 0.01 3200 0.01 CVCS 15700 0.010 0.00 3500 0.05 570 0.04 7000 0.03 Reactor 5300 0.003 0.00 3600 0.05 80 0.01 1000 0.005 Cavity TOTAL 1740900 1.00 22600 1.00 71400 1.00 12917 1.00 217900 1.00
- Safety evaluations neglect some of the smaller heat sinks available in containment. These values in Table 3-1 are consistent with Refs.17.
Y 9
3-8 As shown in Figure 3-1, the outside containment hardware is broken down into the following key components:
Riser annulus and chimney volume Baffle Baffle supports Chimney structure a
Downcomer annulus Shield building a
External atmosphere Initial conditions of structures (grouped under "Inside Containment")
The riser annulus (approximately 12 in. wide) is formed by the baffle and the outer surface of the containment shell. The chimney volume is located at the top of the shield building where the air and vapor mixture exit the riser annulus.
The baffle is the thin-walled steel plate that divides the annulus between the containment shell and the shield building into the riser and downcomer.
The baffle supports are U-shaped brackets that position the baffle at approximately 12 in.
from the containment shell.
The chimney structure consists of the concrete and steel structures at the top of the shield building.
The downcomer annulus (approximately 3.5 ft. wide) is formed by the inner surface of the shield building and the baffle.
The shield building is the 3-ft. thick concrete structure that surrounds the steel containment shell.
~
The external atmosphere acts as the AP600 plant ultimate heat sink,i.e., the external boundary condition. The atmospheric conditions which may affect the containment energy transfer process are the temperature, humidity, and wind conditions. The PCS air flow path interacts with the external atmosphere only at the inlet and outlet since the shield building concrete is so thick (3 ft.) that any thermal interaction through the concrete with the environment is insignificant.
3.3 DESCRIPTION
OF TRANSIENTS Representative results for two limiting containment overpressure DBAs typical of those provided in the SSAR (Ref. 21) have been used for phenomena evaluations. The transients Accident Specification m:\\3429w.wpf:1b-010997
3-9 are selected for use in identifying the phenomena which should be considered in PCS design basis analyses. Minor variations in transient progression or other similar transients were not expected to lead to additional DBA phenomena. The peak containment pressure and temperature for these events are provided in Table 3-2 below for comparison purposes:
1 Table 3-2 Comparison of Key Containment Analysis Results 1
Peak Containment Peak Containment Transient Pressure (psig)
Temperature (*F)
Double-ended hot leg guillotine 40.6 339 Double-ended cold leg guillotine 41.0 283 MSLB at 102% power 40.5 328 l
MSLB at 30% power 43.6 320 The results discussed in this section are sufficiently typical of AP600 PCS performance to allow identification and ranking of the physics of the containment cooling process, at an j
appropriate level of detail for the PIRT.
For the LOCA events, the hot leg break results in the highest blowdown peak pressure whereas the cold leg break results in the highest post-blowdown peak pressure. The cold leg break analysis includes the long-term contribution to containment pressure from stcred energy sources, such as steam generators. Since the peak containment pressure occurs during the blowdown phase of the hot leg break, the analysis of the hot leg break does not j
extend beyond blowdown.
Based upon peak containmen' pressure, the two most limiting transients are: the DECLG LOCA, and an MSLB at 30-percent power with delayed main steamline isolation valve closure. These two transients, which are described in subsections 3.4.2 and 3.4.3, provided the highest energy release and containment pressure. There are conservative assumptions built into these analysis results and for the purpose of this report, the values are shown only as a basis for selecting these two transients for further discussion of the containment cooling process. See Ref. 5, Section 14 for a discussion of conservatisms and assumptions in the containment evaluation model and Ref. 5, Section 2 for the tie between PIRT phenomena and the evaluation model.
3.4 EVENT SCENARIO This section provides a description of the two events under consideration based upon the representative evaluation model results. This includes the initial conditions, boundary conditions, and key assumptions in the modeling.
Accident Specification m:\\3429w.wpf;1b-010997
3-10 3.4.1 Initial and Boundary Conditions Both of the transient events under consideration (DECLG-LOCA and MSLB) are assumed to start from the same initial containment conditions. The initial conditions assumed for the safety analysis are consistent with the Technical Specification limits, and are presented below in Table 3-3. Additional discussion of the effects of these values is presented in subsections 4.4.4 and 4.4.15.
~
Table 3-3 Initial Conditions for AP600 Containment Pressure Calculations Parameter Reference Values Environmental temperature and pressure 115'F Environmental humidity 25%
- Shield building, baffle, and chimney temperature 115'F PCS Cooling water temperature 115' Containment air temperature and pressure 120"F 715.7 psia Containment air humidity 0%
Shell and heat sink temperature 120'F
- Based on 80 F wet bulb temperature Three transient boundary conditions are provided for each transient. The first two are the break mass release rates of liquid and vapor, and the break energy release rates of liquid and vapor. The mass and energy release rates for the LOCA are presented respectively in Figures 3-3 and 3-4. The mass and energy release rates for the MSLB are presented respectively in Figures 3-5 and 3-6.
The following mass and energy sources are accounted for in the long-term LOCA mass and energy release calculation:
Ccre power, temperature, and pressure increased to account for error and instrument dead band Decay heat (1979 ANS plus 2 sigma)
Core stored energy (+ 15 percent)
RCS fluid and metal energy (+ 3 percent RCS liquid volume)
Steam generator fluid and metal energy (+ 10 percent fluid mass)
Accumulators, core make-up tanks (CMTs), and IRWST Zirconium-water reaction (conservatively considers energy due to 1 percent of fuel cladding [see discussion in subsection 4.4.2E])
Accident Specification m:\\3429w.wpf;1b.010997
3-11 100000
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ht i - meemi 4
5
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1 10 100 1000 10000 100000 i
bMC (3822lWOS)
Figure 3-3 AP600 DECLG Mass Release Accident Specification m:\\M29w.wpf:11410997
I 3-12 i
100000000 1
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1000000 N.
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Figure 3-4 AP600 DECLG Energy Release Accident Specification m:\\3429w.wpf:1b-010997
3-13 i
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AP600 MSLB Peak Preu. Case. Mass Flow vs. Time i
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i Figure 3-5 MSLB Mass Release Rate Accident Specification m \\3429w.wpf:1b-010997
3-14 AP600 MSLB Peak Press. Case - Energy Flow vs. Time
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l Figure 3-6 MSLB Energy Release Rate Accident Spedfication m:\\3429w.wf :1b-010997 t
~
3-15 The energy release rates are calculated in the mass and energy model so that the energy is released quickly which results in a conservative containment pressure calculation. Other characteristics of the LOCA and MSLB are discussed in subsections 3.4.2 and 3A 3 respectively.
The third transient boundary condition is the PCS cooling water flow rate to the shell and a typical flow profile is shown in Figure 3-7. It is calculated assuming a single failure of one of two valves (located in parallel) to open. The delay in the flow to the containment shell is attributed to the time for filling the pipe, the distribution bucket, and the first weir. The J
largest flow resistance is in orifices in the standpipe so that the single valve failure assumption affects the gravity-driven flow by less than 2 percent. The reductions in the flow
~
rate at about 9,000 seconds and 80,000 seconds occur when the level falls below the first and second standpipes, respectively.
The environment is also a boundary condition and may change over the course of the transient. The effect of environment induced disturbances is addressed in subsection 4.4.15.
It is assumed in the safety analysis that the environmental conditions are constant at their initial (conservative) values.
3.4.2 Loss-of-Coolant Accident 3.4.2.1 Description of LOCA With the reactor core at 100 percent full power, a high-energy primary coolant line is postulated to break, releasing a combination of steam and water to the containment. It is assumed that the nonsafety grade containment fan coolers do not operate. The break is a DECLG rupture of the RCS piping in a steam generator compartment. The 22-in. inside diameter cold leg pipe is the second largest diameter pipe in the primary system, but produces a higher second pressure peak than a hot leg (31-in. inside diameter) break. The flow resistance from the reactor to the break is less for the hot leg break, so less of the reflood coolant goes through the steam generator. Consequently, for a hot leg break, the steam generator stored energy is released slower, over a period of hours, by convective heat transfer to the containment atmosphere. For a cold leg break, more of the steam generator stored energy is transferred earlier by means of passive reactor cooling system water leading to a higher peak containment pressure.
The DECLG blowdown releases approximately 6,000 ft.8 of water into containment.
Following initial reactor system depressurization, the accumulators force their remaining inventory of water into the primary system followed by the CMT delivering water to the primary system. The IRWST then provides a gravity flow of water into the core from its initial 70,850 ft.8 inventory. Approximately 40,000 ft.8 from these water sources will fill the Accident Specification mA3429w.wpf:lt>-010997
.=.
3-16 i
l i
I I
l l
70 60 w
I
-8 50 my Bucket a
Outflow
- 40-E Second Weir
[
Outflow
$ 30-l
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m
- 20 2
10 N
1 0
1 10 100 1000 10000 100000 1000000 i
Time (seconds) l l
l 4
Figure 3-7 PCS Delivered and Applied Flow i
l Accident Specification m:\\3429w.wpf:1b 010997 l
3-17 reactor cavity and lower portions of the steam generator compartments and flood the reactor hot and cold leg piping elevations, effectively flooding the core.
Water from the break that is not entrained into the atmosphere as drops drains into the reactor cavity and, as the break flow continues, the level rises to the bottom of the steam generator compartment. At approximately 15,000 seconds the break flow level rises to the level of the floor of the CMT room, the maximum flooding height.
Continued steam generation from the reactor into the atmosphere condenses on the externally cooled containment shell, and the condensate flows back into the IRWST through a system of gutters. The operating deck is sloped to return all other above-deck condensate and rain-out to the IRWST.
The continued release of both core decay heat and heat from structures with long thermal time constants, such as the reactor vessel, combined with water from the passive core cooling system (PXS), produces steam that causes the containment pressure to continue to increase following the blowdown, although at a slower rate. Initially, the steel and concrete heat sinks inside containment remove significant quantities of mass and energy from the containment atmosphere, but these approach thermal saturation after several minutes.
However, within a few minutes after the initiation of the accident, the PCS external cooling water and air flow become fully effective in removing energy from containment, and thereby limit the peak containment pressure.
The AP600 blowdown containment response to a LOCA is similar to that of existing two-loop plant designs. The blowdown phase of a large break lasts on the order of 30 seconds, after which the energy release rate remains below approximately 1 percent of the peak blowdown energy release rate. The release rates are determined by the size of the pipe break.
Generally, the larger the pipe, the more rapid the blowdown. Following the blowdown, the containment pressure typically drops as the steam flow rate rapidly decreases and the reactor lower plenum refills, while the containment shell and internal heat sinks absorb some of the energy released during the blowdown. As the internal heat sinks saturate, the continued lower release rate during the peak pressure phase causes the containment pressure to increase to a second peak until the source energy release rate reduces to values below the capacity of thm aeat removal systems and the containment begins a long-term depressurization.
A representative pressure history for a DECLG-LOCA in an AP600 plant is shown in Figure 3-8. The maximum pressure for a DBA is limited to 45 psig or less. For a more complete description of the PCS evaluation model and its results, including the respective assumptions and conservatisms, see Ref. 5.
Accident Specification m:\\3429w.wpf;1b-010997
3-18 i
l 1
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+
I f_ educed Subeootina
/
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.i PCS WaterflowChanen i
i t
i 1
10 100 1000 le+04 1e+06 Tline(Sec) 8" 34 DECLC Conhinment Pressure VS. Time Accident Specification m \\3429w.wpf 1b410997
3-19 3.4.2.2 Temporal Partitioning Temporal partitioning is used to help evaluate the transient by recognizing that the most important processes early in time may not remain important for all time. By separating the transient inf o tirne phases, the number of phenomena that are important at any one time phase is reduced, oereby reducing the complexity of each time phase.
The DECLG LOCA trarsient is partitioned into the following four phases as shown in Figure 3-9:
" Blowdown," which lasts approximately 30 seconds
" Refill," which lasts from 30 to 90 seconds
" Peak pressure," which lasts from 90 seconds to the time of pressure peak "Long-term depressurization," which lasts from the pressure peak and beyond The time phases are chosen to be useful for explaining the physics of the containment pressure transient and for scaling the various pressure phases. A useful partitioning for scaling is to choose inflection points to segregate the pressure curve. Such an approach is used for PCS analysis, as shown in Figure 3-9. The naming convention is to use the most prominent aspect occurring during each phase as the descriptor. Where the mass and energy release drives the shape of the curve, the phenomenon driving the mass and energy release is used. The mass and energy blowdown drives the initial pressure rise, so the first phase is named " blowdown." During the refill phase, while the reactor lower plenum is being filled, there are no releases, leading to a brief period of depressurization until releases begin again.
l Although refill lasts only until about 70 seconds, the second phase until the inflection point is i
simply called " refill." During the third phase the containment repressurizes up to the next inflection point at peak pressure, so the third phase is called " peak pressure." Continuing from the time of peak pressure, the containment depressurizes, other than for perturbations j
resulting from the degree of subcooling in the PXS and external PCS water flow rate changes.
Therefore, the final phase is named "long term depressurization," or simply "long term."
Although the temporal partitioning is based on the containment pressure transient characteristics, the extemal cooling water film flow is the major means of heat removal with its own time sequence. The time sequence of events leading to the development of the external film is shown in Table 3-4. The film coverage begins after only 37 seconds, although the time to reach a quasi-steady state of coverage on the side walls is estimated to be 337 seconds (based upon a flowrate of 440 gpm). Note that these values are based on 440 gpm; the timing of events would vary for other assumed flow rates.
Accident Specification m:\\3429w.wpf:1b-010997
3-20 "l
1 Peak Blowdown Refill Pressure Lorre Term I
B Reduced Su beoofine
/
L PCs WeierflowChanoe i
i t
i 1
10 100 1000 16+04 1e+06 Time (sec)
Figure 3-9 Four Time Phases for DECLG Event Accident Specification m:\\3429w.wpf:lt410997
3-21 Table 3-4 Sequence of Events Leading to the Development of the PCS Cooling Film Activity Time (sec)
Break triggers containment pressure setpoint 0
Valve opens (solenoid-actuated, air-operated) 20 Pipe and bucket fill 37 First weir fills 112 Second weir fills 187 Steady coverage is established 337 The external wetting processes, coupled with the slowly increasing exterr
-hell temperature, results in an increase over time in the wetted external shell surface area. As shown in Table 3-5, the flow of external cooling water onto the dome begins shortly after the j
start of refill, so during blowdown, the wetted areas are assumed to be zero and the entire shell surface area is dry. Furthermore, the shell time constant of approximately five minutes means the external shell surface temperature is too low to evaporate, radiate, or convectively transfer significant energy until after refill. Significant evaporation corresponds roughly to the time that steady water coverage on the shell is achieved. As a result, heat transfer and evaporation from the outside of the shell are not significant during the blowdown and refill time phases.
Table 3-5 Large-Break LOCA Sequence of Events Time (sec)
Event 0
Break occurs Blowdown begins 20 PCS valve opens 30 Blowdown ends Refill begins Water coverage begins on dome 37 i
90 Refill ends, peak pressure phase begins 337 Steady shell water coverage assumed to begin (440 gpm) (60.5 lb/sec) 1,500 Long-term containment depressurization begins 9,000 PCS water decreases to 120 gpm (16.5 lb/sec) 80,000 PCS water decreases to 55 gpm (7.6 lb/sec)
A discussion of the break source and external cooling water characteristics during the four LOCA time phases follows.
Accident Specification m:\\3429w.wpf:1b-010997
j 3-22 1
3 Blowdown Phase:
j The containment pressure increases from 1 atmosphere to approximately 3.5 atmospheres l
during the 30-second blowdown period. The mass released during blowdown is l
approximately 40 percent steam and 60 percent liquid, and the break flow is choked, or I
i nearly choked, throughout blowdown. The external water is not on the shell during blowdown.
1 Refill Phase:
Following blowdown, the accumulators refill the lower plenum of the reactor with a high flow rate of cold water so that releases from the break cease for about 60 seconds. As the l
~
i reactor water level rises through the core, typically termed reflood, water is turned to steam.
{
The resulting steam and water flow rates from the break are very low and increase with
{
time. The mass and energy release rates are two orders of magnitude less than the blowdown rates, and can be approximated as zero from 30 to 70 seconds. With a negligible j
steam source rate and high condensation rate the containment pressure drops by a few psi i
from its peak at the end of blowdown to the end of containment pressure refill phase at i
90 seconds. The external water film is not considered for heat removal during this phase
{
because the film is not well developed until after refill and the external shell temperature is too low for effective heat removal.
1 i
j Peak Pressure Phase:
i
)
The post-refill and peak pressure steam source velocity is low enough that a negligible i
amount of the break water will be entrained and dispersed as water drops. The external
]
wetted coverage increases from near zero at the beginning of the peak pressure phase to a
)
j maximum at about 337 seconds. The peak pressure phase ends when the PCS heat removal j
begins to exceed the heat source and pressure transient turns around.
b Long-Term Depressurization l
The long-term steam source velocity is low enough that no break water is entrained and dispersed as drops. The external wetted coverage remains at the coverage consistent with j
the source flow rate, liquid film stability, and the evaporation rate (evaluated more fully in i
Ref. 5, Section 7).
The sequence of events for the large-break LOCA is summarized in Table 3-5.
i I
i
. Accident Specification
'm:\\3429w.wpf:1b410997 1
1 I
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e
.. _ _. _ _, _ -,~
3-23 3.4.3 Main Steamline Break 3.4.3.1 Description of MSLB Steamline ruptures occurring inside containment may result in significant releases of high-energy fluid to the containment environment, resulting in increased containment temperatures and pressures. The quantitative nature of the releases following a steamline rupture is dependent upon the configuration of the plant steam system, the containment design, the plant operating conditions, and the size of the rupture.
The main steamline starts from the top of the steam generator, makes two 90-degree turns,
~
and passes vertically down through the operating deck into the CMT compartment. The line makes another 90-degree tum and runs along the top of the CMT compartment to the shell, where it passes outside containment. Part of the main steamline is shown in Figure 3-2. The break is assumed to occur at the top of the steam generator compartment, where the resulting flow pattern minimizes interactions with the belew-deck heat sinks in the CMT compartment. (A more complete evaluation of limiting source locations may be found in Ref. 5, Secdon 9).
For this evaluation, a double-ended guillotine rupture of a main steamline is postulated to occur wit.1 the reactor at 30 percent steady-state power. Since steam generator mass decreases with increasing power level, breaks occurring at lower power generally result in greater total mass release to the plant containment.
The main steamline isolation valve closure is assumed to be delayed permitting both loops to blowdown until closure (10 seconds after break). Offsite power is assumed to be available, since thi~ maximizes the mass and energy released from the break (due to continued s
operation of RCS pumps and feedwater pumps). The availability of ac power in conjunction with the passive safeguards system, CMT and passive residual heat removal (PRHR) maximizes the mass and energy releases via the break since this maximizes the reactor cooldown. When the PRHR is in operation, the core-generated heat is dissipated to the IRWST.
The blowdown flow may be superheated steam throughout the transient. Beyond the end of blowdown, the source energy release for an MSLB remains at zero. The peak pressure during an MSLB is determined by the containment volume, steam / air circulation to the heat sinks, and time response of the heat sinks. The external PCS water film is not well developed until late in the blowdown. The pressure history for a representative MSLB transient in the AP600 plant is shown in Figure 3-10.
Accident Specification m:\\3429w.wpf:1b 010997
3-24 3.4.3.2 Temporal Partitioning The blowdown for the MSLB transient lasts approximately 400 seconds, after which there is no additional flow. With no mass and energy source, the containment pressure decreases rapidly as the internal heat sinks absorb energy and external cooling is provided by the PCS.
Consequently, the MSLB evaluation does not extend beyond blowdown.
1 l
Accident Specification m:\\3429w.wpf:lt410997
3-25 g
1 a
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i Figure 3-10 MSLB Containment Pressure vs. Time Accident Specification m:\\3429w.wpf:1b 010997
l 4-1 4.0 PHENOMENA IDENTIFICATION AND RANKING This section provides the basis for the identification and ranking of the phenomena associated with the AP600 containment cooling process. An overview of the phenomena is described in subsection 4.1. Subsection 4.2 shows the PDRT and discusses how phenomena were grouped. Subsection 4.3 summarizes test and scaling results used to support ranking.
Subsection 4.4 provides the basis for ranking of each phenomenon, based upon a combination of test results, scaling analyses, engineering judgement, and sensitivity studies.
j 4.1 PHENOMENA OVERVIEW An overview of most phenomena involved in the containment cooling process is shown in Figure 4-1. This figure illustrates the energy transfer processes starting at the break source l
inside containment and leading to the environment. This figure shows the key hardware j
(shell, baffle, shield building) and volumes (containment) or flow areas (downcomer, riser)
I involved in the respective heat and mass transfer mechanisms.
The phenomena identification process was based upon identifying the "high-level" phenomena or parameters that may affect the containment cooling process, since more detailed phenomena enveloped by high-level phenomena are addressed in containment phenomenological reports. Examples of these high-level containment cooling phenomena include: mixing and stratification within the containment volume and the PCS cooling water film coverage on the containment shell, which have been addressed with specific evaluations (described in Sections 9 and 7 of Ref. 5).
A description of the more significant phenomena involved in the containment cooling process is presented below, starting with the break source and progressing to the environment. The phenomena are also illustrated in Figure 4-1. A more detailed listing of phenomena considered for the DBA PIRT is given along with the basis for ranking in subsection 4.4.
Inside Containment Flashing of high-pressure water (for LOCA) or release of superheated steam (for MSLB):
i The break source steam disrupts the initially quiescent containment atmosphere with a forced jet that may later transition to a buoyant plume. The pressure inside containment will increase as long as the source rate of mass and energy into the gas i
atmosphere exceeds the absorption rate of mass and energy from the atmosphere by i
liquid and solid heat sinks.
Containment Phenomena Identification and Ranking m:\\3429w.wpf:lt>010997 I
4-2 Extemal (air exit) condibons j
fHm conductance and capacitance air fkw
/
/
fim evaporation f
'I' "
gog film flow condensation
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drops annulus annulus nser
/
hmer film
\\
conductance &
/
capacitance p
/
radiation radiation 5
radiation
+
=
=
+
0" mpt a convecbon convection
/
[4 affle b
conduct. ion gucto,n g
' shel 4
\\ shield building sm
/m' s
N ing &
W
. = - - -
stratification natural convection Figure 4-1 Overview of Containment Building Phenomena Containment Phenomena Identification and Rankmg m:\\3429w.wpf;1t410997
]
4-3
)
The characteristic parameters of the break source mixture such as direction, momentum, and density cm affect circulation within containment. The density of the break source for the MSLB is lower than for the LOCA.
Saturated drops are formed during the blowdown process for a LOCA by entrainment into the high velocity steam and are transported with the steam. The drops flash initially to thermal equilibrium with the containment atmosphere, and later evaporate as the containment pressure decreases and, therefore, may be another source of energy release to the containment environment.
Natural circulation and mixing of the steam / water / air mixture inside containment:
The break source jet or plume causes convective transport of steam, water, and warm j
air inside containment. The subsequent heat transfer and condensation from the I
warm, steam-rich gas to the initially cold heat sinks produce additional body forces j
and wall plumes that induce further changes in the convective flow field.
j Natural circulation is composed of the overall convective flow patterns that occur on a compartraent scale and also on a large, containment-wide scale. The compartment scale circulation is due to wall layers, jets, plumes, and entrained flow. The large-scale circulation is flow between compartments induced by pressure, density, elevation, and momentum differences and results in intercompartment flow.
The pressure transient is mainly affected by parameters that influence mass transfer.
Mass transfer has as its primary parameters steam concentration and velocity, the latter only for the case of forced convection. Large-scale circulation and entrainment into jets i
or plumes can drive mixing and can affect local values of steam conceatration and velocity near the heat transfer surfaces. Jet and plume entrainment within compartments or the above-deck region can also result in stratification, or the existence of a vertical steam concentration gradient. The mixing and stratification that occurs in the AP600 have the potential to locally reduce heat and mass transfer rates by i
transporting and concentrating noncondensibles, or conversely, to improve heat and mass transfer by concentrating steam.
1 4
Segregation is the separation of steam and air into different compartments due to convective and boundary layer transport processes. For example, condensation in the j
below-deck compartments has the potential to segregate air and steam, creating an air-rich atmosphere below deck and a steam-rich atmosphere above deck. Such a state will increase mass transfer to the shell and heat sinks above deck and decrease mass transfer to the heat sinks below deck.
i 4
Containment Phenomena Identification and Rankmg
,]
m:\\3429w.wpf:lt410997
4-4 Fog inside containment:
Fog is formed inside containment by the break source steam / water interaction. During blowdown, the near-sonic velocities within the primary system pipe are expected to j
entrain a large fraction of the water. Post-blowdown, the break steam velocity is small j
enough that the entrainment rate is negligible. The minimum vapor fraction is 0.996, j
so drop coalescence is not expected. The fog is removed from containment primarily by " settling" and by phase change. The fog has a strong effect on the effective density of the mixture, and hence, on buoyancy-induced phenomena. Fog also increases the effective heat capacity of the gas mixture.
The effect of fog absorption on radiation can also be significant; however,inside containment fog will effectively enhance the absorption of radiant heat by the opaque gas, which is already a significant absorber.
Radiation heat transfer from high temperature mixture to the shell and internal heat sinks:
Energy is transferred by radi.ation between any two components-solid, liquid, or gas-that differ in temperature. The greater the temperature and the temperature difference, the greater the energy transfer.
Radiation is also enhanced by high emissivity surfaces and the zinc-coated steel surface emissivities are all near 1.0. The liquid films have emissivity of 0.95 to 0.96, the inorganic zinc paint emissivity is 0.90 to 0.95, and the concrete emissivity is approximately 0.90. Radiation between gases, and from gas to solid or gas to liquid, l
can be significant when the product of the steam partial pressure and radiation beam length are of the order of 1 ft-atmosphere. This is the case inside containment, where steam partial pressures may be as high as 3 atmospheres and beam lengths are l
frequently greater than 10 ft.
Convective heat transfer to internal heat sinks and containment shell:
Convective heat transfer is a boundary layer conduction process that is driven by a temperature gradient in the presence of a flowing bulk fluid. The greater the temperature difference, the greater the heat transfer. The bulk fluid motion may be due to a state of forced convection, free convection, or a combination of both, which results from convective flow driven by entrainment or density differences.
Contamment Phenomena Identification and Ranking m:\\3429w.wpElb-010997
4-5 Condensation of steam on inside of containment shell and internal heat sinks:
The condensation of steam is the convective transfer of mass to a liquid film on a heat sink. It is a boundary layer diffusion process that is driven by a steam partial density gradient. Condensation removes the gas enthalpy (h,) from the atmosphere, transfers the heat of formation of the gas (h,,) to the heat sink, and leaves behind the liquid enthalpy (h,) with the condensate.
i Liquid film thermal transport (heat capacity):
Liquid films form due to condensation inside containment and flow under the i
influence of gravity and shear forces due to external gas flows. The intemal liquid film carries away the liquid enthalpy (h,) that accounts for approximately 15 percent of the enthalpy of the condensed steam (h,). The liquid film that forms on the internal shell is collected at the crane rail, the stiffener ring, and at the deck elevation, and then i
drained into the IRWST.
j Flooding:
The condensation of steam from the heat sinks (other than the shell that drains into the IRWST), in addition to the release of water from the RCS and fog " settling," may cause flooding of some of the compartments below the operating deck. Flooding may restrict the natural circulation flow patterns through the lower compartments and affect the energy transfer process.
Liquid films conductance--vertical and horizontal:
The existence of the liquid phase (due to condensation) on the cooling surface offers a thermal resistance to the removal of energy from the vapor. The orientation of the surface affects the thickness of the liquid film.
Liquid films on surfaces inclined more than a few degrees flow fast enough to limit their thicknesses to approximately 0.005 in. or less. This results in a relatively high j
heat transfer coefficient so the rates of heat transfer to structures are limited by either the structure internal resistance, or the mass transfer coefficient, but not by the film.
The film resistance on inclined surfaces is always second order relative to the mass transfer and the structure internal resistance. This also remains true for concrete.
Liquid films on approximately horizontal surfaces facing up can develop rather thick films (greater than 0.05 in.) that may be more limiting to heat transfer than the heat sink internal resistance or the mass transfer coefficient. Horizontal surfaces facing down, with the inorganic zinc coating, experience film flow for slopes greater than Containment Phenomena Identification and Rankmg m:\\3429w.wpf.It>-010997
4-6 1 degree from horizontal. Slopes less than 1 degree were found to drip with water, and the heat transfer coefficients were greater than 1000 BTU /hr-ft - F. Less than 2
0.09 percent of the containment shell has a surface slope less than 1 degree.
Conduction heat transfer through internal heat sinks (see following discussion of conduction through steel shell).
Containment Shell Conduction heat transfer through steel shell:
The internal resistance of the shell and heat sinks must be considered in the transfer of energy from the containment atmosphere. The internal resistance can be scaled relative to the surface heat transfer using the Biot number, ht/k. Heat sinks with Biot numbers less than 0.1, such as steel with thickness less than approximately 0.5 in., may be simply modeled as lumped masses. Heat sinks with Biot numbers greater than 1, such as concrete and water pools, limit energy absorption by internal resistance rather than by surface transfer coefficients.
Stored energy in containment shell and internal heat sinks:
The containment volume, shell, and other sinks (concrete, steel, and water pools) provide a large capability to store energy from the break.
Outside Containment Radiation heat transfer in PCS air flow path:
In the riser, downcomer, and chimney, steam partial pressures are on the order of 0.1 atmosphere and beam lengths are on the order of a few feet, so radiation to or from gases in the PCS air flow path are relatively small. Drops or fog in the riser annulus may capture radiation. Radiant heat transfer from the shell to the baffle may keep the baffle at a temperature greater than the air in the annulus.
Convective heat transfer in PCS air flow path:
The convective heat transfer phenomena in the external annulus is similar to the internal containment convective heat transfer, but is driven by other parameters. These parameters include the environmental conditions, such as temperature and wind velocity.
Containment Phenomena Identification and Rankmg m:\\3429w.wpf:1b-010997
4-7 Liquid film thermal transport:
The external liquid film from the PCCWST is supplied at a subcooled temperature (maintained at 40 F to 120 F per Technical Specifications). The energy absorbed by the sensible temperature increase of the film, before it begins to evaporate significantly, accounts for approximately 10 percent of the energy transferred by evaporation. The energy absorbed by temperature increase is sometimes referred to as the subcooled heat capacity of the external film. Most of the external film is expected to evaporate, but the portion that does not evaporate runs down a drain and away from the PCS flow path.
Other characteristics of the liquid film such as stability discussed in Ref. 5, Section 7, stripping, and drag (discussed under subsection 4.4.8) can affect the energy transport process.
Evaporation of water on the outside of the contamment shell:
The evaporation of water is a convective mass transfer from a heat sink (containment shell). It requires a liquid source at (h,) that is supplied by a heat source with the enthalpy of formation of the gas, (h,,) and carries away the gas enthalpy (h,).
The evaporation rate is a function of the shell heat flux, wetted surface area, and film flow rate, all of which are interdependent and vary with time and position. The heat flux to the shell is dependent on the break mass and energy release rate and the condensation rate. The wetted area and the film flow rate are dependent on the applied PCS water flow rate that decreases with time, and are also affected by the film stability.
Fog generation in the riser annulus:
The external air flow path bulk gas is superheated, so fog absorption of radiation is not expected. When fog does exist, as in the large-scale test riser, its effect on radiation through the riser air is not significant (Ref. 22). Since fog can potentially impact the buoyancy of the annulus, it is evaluated for that effect.
Natural circulation in the PCS air flow path:
The shell heating and the cooling film evaporation on the outside of the shell induce a natural circulation air flow in the PCS air flow path (downcomer-riser-chimney, as shown in Figure 4-1). The resulting air flow rate affects the heat and mass transfer coefficients from all the bounding structures (shield, baffle, shell, and chimney) to the moving air.
Containment Phenomena Identification and Ranking MMC 9w.wpf:1t>-010997
1 4-8 The characteristics of the air flow on the liquid film (stripping and acceleration) may have an effect on the momentum and energy transport processes.
4.2 THE AP600 CONTAINMENT PHENOMENA IDENTIFICATION AND RANKING TABLE The PIRT rankmgs for the four LOCA and one MSLB time phases are presented in Table 4-1.
This table contains the component or volume in the first (left-hand) column and the phenomena or parameter most closely associated with that component or volume in the l
second column. The table is arranged to show the energy transfer processes starting from l
inside containment with the break source and ending with the ultimate heat sink, the I
environment.
For both simplicity and convenience in this PIRT, each energy transfer process has been assigned to the component or volume to which the process is most closely associated, either containment hardware such as the steel containment shell, or a gas mixture such as the containment volume, or a flow area such as the annulus riser. In particular, the heat transfer fluxes have been associated with the respective hardware surfaces and generally, with the energy source. For example, radiation heat transfer between the shell and the baffle was assigned to the shell.
The phenomena list includes parameters such as the initial and boundary conditions, which are important to the analysis of the containment cooling process.
Results from testing and scaling used to support PDRT ranking are summarized in subsection 4.3. A description of how each phenomena was ranked can be found in subsection 4.4. The numbers and letters for the components and phenomena in Table 4-1 refer to the specific paragraphs in subsection 4.4.X, where X represents the component or volume number in Table 4-1. For example, the basis for ranking break source mass and energy in the containment can be found in subsection 4.4.1A.
4.3 RESULTS USED IN PHENOMENA RANKING This section provides a summary of the results from the tests (subsection 4.3.1) and scaling analyses (subsection 4.3.2) used in ranking the phenomena. The results of the sensitivity studies are presented in Ref. 5. The specific ranking bases for each of the phenomena is provided in subsection 4.4.
Containment Phenomena Identification and Rankmg m:\\3429w.wpf:1b-010997
4-9 Tcbl2 4-1 Phenomena Identification and Ranking According to Effect on Containment Pressure LOCA MSLB Ccmponent or Blow Peak Long Blow Vclume Phenomenon or Parameter down Refill Pressure Term down Incid2 Containment:
i
,1.) Break Source A.) Mass and Energy H
N/A H
H H
l B.) Direction and Elevation H
N/A H
H H
C.) Momentum H
N/A H
H H
D.) Density H
N/A H
H H
E.) Droplet / liquid flashing H
H N/A N/A N/A 2.) Containment A.) Mixing / Stratification H
H H
H H
Volume B.) Intercompartment Flow L
H H
H H
C.) Gas Compliance H
H H
H H
D.) Fog L
H H
H N/A E.) Hydrogen Release L
L L
L N/A 3.) Containment A.) liquid Film Energy Transport L
M H
H M
Solid Heat Sinks B.) Vertical Film Conduction L
L L
L L
i (Steel and C.) Horizontal Film Conduction L
H H
H H
Concrete)
D.) Internal Heat Sink Conduction M
H H
H H
E.) Heat Capacity M
H H
H H
l F.) Condensation M
H H
H H
G.) Convection from containment L
M M
M L
H.) Radiation from containment L
M M
M L
4.) Initial A.) Initial temperature M
M H
H H
j Conditions B.) Initial humidity M
M H
H H
C.) Initial pressure M
M H
H H
5.) Break Pool A.) Mixing / Stratification in the pool L
L L
M L
B.) Condensation / evaporation L
L M
M L
C.) Convection with cnmt volume L
L L
L L
D.) Radiation with cnmt volume L
L L
L L
E.) Conduction in pool L
L L
M L
F.) Flooding in the pool L
L L
M L
4 i
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4-10 l
i Tchle 4-1 Phenomena Identification and Ranking According to Effect on Containment Pressure (Cont.)
LOCA MSLB C:mponent or Blow Peak Long Blow Volume Phenomenon or Parameter down Refill Pressure Term down Inside Containment:
6.) IRWST A.) Mixing / Stratification (gas & water)
L L
L L
L B.) Condensation L
L L
L L
C.) Convection L
L L
L L
D.) Radiation L
L L
L L
E.) Conduction in liquid L
L L
L L
F.) Liquid level changes L
L L
L L
Centainment Shell:
7.) Steel Shell A.) Convection from containment L
M M
M L
B.) Radiation from containment L
M M
M L
C.) Condensation L
H H
H H
D.) Inside film conduction L
L L
L L
E.) Inside film energy transport M
M M
M M
F.) Conduction through shell L
L H
H H
G.) Heat capacity of shell L
H H
L H
H.) Convection to riser annulus L
L M
M L
I.) Radiation to baffle L
L M
M L
J.) Radiation to chunney L
L L
L L
K.) Radiation to fog / air mixture L
L L
L L
L.) Outside film conduction N/A N/A L
L L
M.) Outside film energy transport N/A N/A M
M L
N.) Evaporation to riser annulus N/A N/A H
H M
8.) PCS Cooling A.) PCCWST flow rate N/A N/A H
H M
Water B.) PCCWST water temperature N/A N/A M
M L
C.) Water film stability and coverage N/A N/A H
H L
D.) Film stripping N/A N/A L
L L
E.) Film drag N/A N/A L
L L
Outside Containment:
9.) Riser A.) PCS Natural Circulation L
L H
H M
Annulus &
B.) Vapor acceleration N/A N/A L
L L
Chimney C.) Fog N/A N/A L
L N/A Volume D.) Flow stability L
L L
L L
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4-11 Tcbl2 4-1 Phenomena Identification and Ranking According to Effect on Containment Pressure (Cont.)
LOCA MSLB Ccmponent or Blow Peak Long Blow Vclume Phenomenon or Parameter down Refill Pressure Term down 10.) Baffle A.) Convection to riser annulus N/A N/A L
M N/A B.) Convection to downcomer N/A N/A L
M N/A C.) Radiation to shield building N/A N/A L
L N/A D.) Conduction through baffle N/A N/A L
M N/A E.) Condensation N/A N/A L
L N/A F.) Heat capacity N/A N/A L
L N/A G.) Leaks through baffle N/A N/A M
M N/A 11.) Baffle A.) Convection to riser air L
L L
L L
Supports B.) Radiation from shell L
L L
L L
C.) Conduction from shell L
L L
L L-D.) Heat capacity L
L L
L L
12.) Chimney A.) Conduction through chunney L
L L
L L
Structure B.) Convection from chimney air L
L L
L L
C.) Heat capacity of structure L
L L
L L
D.) Condensation on chimney L
L L
L L
l 13.) Downcomer A.) PCS Natural Circulation L
L H
H M
l Annulus B.) Air flow stability L
L L
L L
14.) Shield A.) Convection to downcomer N/A N/A M
M L
Building B.) Conduction through shield building N/A N/A L
L N/A l
C.) Convection to environment N/A N/A L
L N/A D.) Radiation to environment N/A N/A L
L N/A 15.) External A.) Temperature N/A N/A L
L L
Atmosphere B.) Humidity N/A N/A L
L L
C.) Recirculation N/A N/A L
L L
D.) Pressure Fluctuations N/A N/A L
L L
l l
l Containment Phenomena Identification and Ranking m:\\3429w.wpf:1t>010997 l
i
i i
4-12 i
4.3.1 Test Results Summary This section provides the key results from the containment tests which, in conjunction with the scaling analysis results, engineering judgement, and sensitivity studies were used to provide the final importance ranking of the phenomena. A list of documents associated with each test is provided in Ref. 28, including test specifications and test data reports. Some of the more relevant test references are provided in the following discussion.
Heated Flat Plate Test (Ref. 23) Results 1
I j
The following are the key observations and conclusions from the Heated Flat Plate Test analysis (Ref. 9):
A stable, wavy laminar water film formed on the hot, coated, steel surface in both j
orientations-vertical and 15 degrees from horizontal.
l
^
As the water flow rate was reduced, the waves in the film became smaller and l
t eventually disappeared.
l l
The water film was able to wet and rewet (after dryout) the hot, dry surface (at a surface temperature of 240 F).
i The two low-flow, high-heat flux tests showed that evaporation to dry-out of the a ater j
films on the heated surface produced stable film evaporation.
J The water film was not adversely affected by the countercurrent cooling air flow up to i
the maximum air velocity of the test (5.9 to 38.7 ft/sec.), i.e., there was no water-film stripping.
Heat transfer from the dry surfaces to the air (no water film) agreed very well with the Colbom heat transfer correlation.
Water film evaporation and resultant heat removal agreed with mass transfer correlation predictions.
i 3
i Contamment Phenomena Identification and Rankmg m:\\3429w.wpf:1t>-010997 I
-v w
4-13 Radiation to the air baffle wall and subsequent heat transfer to the cooling air occurred and accounted for heat transfer in addition to the convective heat transfer.
Wind Tunnel Test (Ref. 24, 25, 26, 27, 28) Results Based upon evaluation (Ref. 5, Section 6) of the wind tunnel test data, it was found that wind j
induces more containment heat and mass transfer than a quiescent atmosphere since wind drives more flow through the AP600 annulus (wind positive). Also, a literature review l
provided data to show that the effects of recirculation due to thermalinversions or strong winds have a negligible impact on the PCS heat removal.
i
~
Condensation Test (Ref. 29,30,31) Results The following are the key observations and conclusions from the Condensation Tests:
A slightly higher average heat transfer coefficient was observed on the horizontal condensing plate than on the vertical condensing plate.
The presence of helium affected the heat transfer coefficients the same as other noncondensibles at the same molar concentration.
Increased heat transfer coefficients were observed with the steam jet impinging directly on the horizontal plate (simulating steamline break).
Water Distribution Test (Ref. 32, 33, 34) Results The water coverage for an equivalent AP600 flow rate of 220 gpm was estimated to be 25 percent from the top of the dome down to the first weir. About 70 percer.t of the surface was wet between the first and second weirs and about 100 percent wet below the second weir. The coverage decreased from 100 percent as the applied water flow rate decreased.
At a flow rate equivalent to 220 gpm on the plant, water began to spill over the first weir at about 2.5 minutes and over the second weir at about 5 minutes. The time to completely fill the weirs to their steady-state level and to establish steady-state coverage of the dome and sidewall was conservatively estimated to be about 10 minutes. These tests reflected the plant S
Containment Phenomena Identification and Ranking mA3429w.wpf;1b.010997
4-14 design flow rate of 220 gpm at the time the tests were performed. Recent plant evaluations have been performed with a design flow rate of 440 gpm.
Small-Scale PCS Intecral Test (Ref. 35) Results The following observations and conclusions with respect to the water film were drawn from evaluation of these tests (Ref.14).
A stable, uniform, wavy laminar film was formed on the inorganic zinc-coated steel surface using simple weirs.
The film remained stable and uniform on the vertical sidewall of the vessel at average, evaporating heat fluxes in the range of those expected on the AP600.
The local heat removal rate at the top of the vessel where cool water was first applied was significantly higher than the vessel average heat removal rate.
The overall heat removal capability with a wetted surface and a well-mixed air / steam mixture inside the vessel agreed with analytical predictions.
9 Larce-Scale PCS Intecral Test (Ref. 36,12) Results The following important observations with respect to film behavior were made during the tests:
As the pressure and temperature increased inside the pressure vessel, dry spots first began to form in the wet, but low flow regions on the dome and sidewall.
The dry spots grew vertically, separating the original continuous film into several wavy laminar flow stripes.
At higher heat fluxes, dry spots also formed just below and in line with the J-tube location.
1 Containment Phenomena Identification and Rankmg m:\\3429w.wpf:lt@l0997
i l
4-15 The central wavy laminar flow region of the individual film stripes was surrounded by a region of laminar flow with no visible waves. The thickness of the laminar flow region appeared to continually decrease out to the edge (or bottom) of the film stripe.
The widths of both the wavy laminar and laminar flow regions of the stripe were observed to decrease with increasing heat flux.
The film stripes remained stable (i.e, they did not split or bunch up to form thick, narrow rivulets) as they evaporated on the vertical sidewall.
1 An evaluation (Ref. 37) of the LST data provided important conclusions on both water
~
coverage and heat removal:
Evaporation was the primary mode of heat removal from the outside of the vessel (approximately 75 percent) followed by the sensible heating of the subcooled film (approximately 17 percent). The remainder of the energy (8 percent) was transferred by convection and radiation.
Striped film coverage provided better heat removal than forced quadrant coverage for the same wetted perimeter.
The heat removal rate appeared to be more affected by ambient air temperature than by liquid film temperature.
The heat removal rate had a relatively weak dependence on annulus air velocity.
The highest heat flux ocurred near the top of the dome at the elevation where the external film was applied except for the horizontal, high-velocity steam jet injection test case.
Injection of a high-velocity steam jet (simulating a MSLB) resulted in a well-mixed vessel and thus, a relatively uniform wall temperature and heat flux over the evaporating surface.
l The heat removal rate increased as the steam concentration near the PCS increased (by i
raising the injection location).
Containment Phenomena Identification and Ranking
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1 4-16 i
4.3.2 Scaling Analysis Results Summary A scaling analysis (Ref.19 and 20) showed that the range of AP600 operation was adequately covered by the test data. The scaling analysis identified the relative importance of the t
phenomena under study, such as condensation mass transfer and evaporation mass transfer
[
via the appropriate non-dimensional parameters or pi groups. The evaluation of the i
pi group values provide the following conclusions.
r Inside containment:
The break source steam mass flow rate is important since it drives the pressurization.
The gas volume is important since it relates pressure to stored mass and energy (volumetric compliance or capacitance).
The liquid condensate is important since it carries away part of the energy of the condensed steam.
For mass transfer to heat sinks:
The internal steel, concrete, and steel-jacketed concrete heat sinks are important since they absorb energy and condense steam, thereby reducing pressure.
Intercompartment circulation affects the distribution of noncondensibles and velocity, which are important parameters for mass transfer.
Stratification is important since it can increase the concentration of dense non-condensibles and locally limit the utilization of heat sinks and conductors within a compartment.
The horizontal liquid films are important since they can produce low conductances that insulate upward-facing horizontal surfaces.
Containment Phenomena Identification and Ranking mM429w.wpf:lt>410997
4-17 Containment shell:
The shell is important since it is a major heat sink and the only energy transfer path out of containment to the riser.
The internal conductance of the shell is important since it limits energy absorption and transfer rates.
The condensation and evaporation mass transfer conductances are important since most of the energy transfer to and from the shell is by mass transfer.
~
The liquid film stability is important because it can limit the area for evaporation and evaporation is the dominant process for energy transfer from the shell.
Outside containment:
The buoyancy and flow resistance are important and have a strong effect on the evaporation rate.
The downcomer is not a significant contributor to air flow path energy or momentum.
The pi group results from the scaling analysis are provided in Ref. 20.
4.4 RANKING OF PHENOMENA LISTED IN PIRT Bases for Ranking The phenomena relevant to containment DBA are shown in the PIRT (Table 4-1). The PIRT has been structured into high level group'ngs consistent with the discussion in subsection 4.2.
The ranking of the importance of the containment phenomena was based upon a combination of engineering judgement and the results from the respective tests and scaling analyses as described in the above subsections. Sensitivity studies were also performed to gain additional insight into the performance of the PCS. Phenomena were ranked based upon their effect on the energy transfer process and containment pressure reduction.
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4-18 A summary of sources of information used to rank each phenomenon is provided in Appendix A. The phenomena were ranked either:
H - High importance to the energy transfer process and containment pressure reduction M - Medium importance to the energy transfer process and containment pressure reduction L - Low importance to the energy transfer process and containment pressure reduction N/A - Not applicable to the energy transfer process Purpose of PIRT Rankmg The ranking is used to help focus on the most important processes for containment pressure mitigation and to provide guidance on the appropriate level of detail in assessing uncertainties or developing bounding models, as follows:
Phenomena with a High or Medium ranking during any time period need to be considered in the evaluation model. Consideration may include showing that neglecting a phenomena is conservative for pressure predictions. The effect of important parameters should be assessed in developing uncertainties or bounding models.
Phenomena with a Low ranking during all time phases are those that can utilize an available model that captures the major features. Low ranked phenomena do not dominate pressure mitigation calculations and uncertainties in their analytical models do not dominate pressure uncertainty. It is generally not necessary to " fine-tune" or bound the Low ranked models. The phenomena may be neglected if their effect is small enough or if it would be conservative to neglect them.
Not all of the containment phe. omena described in subsection 4.1 are applicable to each of the hardware components identified in subsection 3.2. Also, some of the phenomena and components are not applicable for certain time phases of the transient, e.g., there is no break Containment Phenomena Identification and Ranking m:\\3429w.wpf:1b-010997
4-19 source during the lower reactor plenum refill process for the LOCA. The ranking for each of the phenomenon or parameters identified in the PIRT are provided in the subsections below.
The last digit of the section designator refers to the PIRT high-level grouping number and the letters correspond to specific phenomena entries in Table 4-1.
4.4.1 Break Source 4.4.1A Mass and Energy Release Scaling shows that the mass and energy releases from the break source are the driving forces for the containment response for both the LOCA and MSLB events, except during the refill portion of the LOCA when there is no break source, therefore these parameters were all ranked High. The LST covered a range of steam mass and energy input.
The following are ranked relative to their effect on internal circulation:
4.4.1B Break Source Direction and Elevation 4.4.1C Break Source Momentum 4.4.1D Break Source Density In addition to the bulk mass and energy released, other parameters associated with the boundary condition are scurce direction and elevation, momentum, and density. These parameters have the potential to strongly affect the mixing and stratification within containment (as evaluated in Ref. 5, Section 9), and thus are ranked the same as mixing and stratification (see subsection 4.4.2A) for all accident phases, except for refill, when there is no significant source.
4.4.1E Droplet / Liquid Flashing l
The blowdown liquid and entrained droplets enter the atmosphere saturated at the containment total pressure where they are exposed to the containment gas mixture of air and steam at the steam partial pressure. Since the liquid and drops are initially superheated, they evaporate quickly to reach thermal equilibrium with the gas mixture. Pressure scaling for the j
drops and pool flashing give a combined blowdown pressure pi value of approximately 10 percent, so droplet / liquid flashing is ranked High for LOCA blowdown and refill phases.
Containment Phenomena Identification and Rankmg m:\\3429w.wpf:1b-010997
)
l 4-20 Flashing does not occur after the LOCA blowdown, so post refill rankings are N/A. The MSLB releases superheated steam, so droplet /ligtJd flashing is ranked N/A.
4.4.2 Containment Volume 4.4.2A Mixing / Stratification in the Containment Volume Mixing and stratification are phenomena used to describe convective flow phenomena inside containment. Segregation is sometimes used to describe one of the possible effects of mixing and stratification on steam distribution. Pred.icting these phenomena rcquires the modeling of specific convection processes. Convection is the flow of fluids due to acceleration, pressure, shear and body forces acting locally on each microscopic fluid element. On a macroscopic scale, the collective motion of fluid elements organizes into recognizable fluid r
structures, such as jets, plumes, and wall plumes that interact with the bulk fluid by entrained flow. On a compartment scale, the combination of jets, plumes, wall plumes and entrained fluid produces circulation patterns. The strength of the compartment-scale circulation is responsible for the steepness of vertical density gradients, or stratification, within containment gas volumes. Some compartments may stably stratify and not experience circulation.
Inter-compartment convective transport produces a pattern of containment-scale circulation inside containment. The inter-compartment flow can occur due to one or more of the i
following four processes:
Pressurization of one compartment by the steam source, as during blowdown Momentum induced convective flow Condensation in a compartment creating a lower pressure Net buoyant force not counteracted by pressure or momentum Condensation dominates convection and radiation by more than an order of magnitude based upon the results from the scaling analyses. Since mixing / stratification are very important to condensation, these are ranked High for both the MSLB and LOCA (all time phases) in the PIRT. The test results from LST and sensitivity studies also support the High importance ranking, as discussed in Ref. 5, Section 9.
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4-21 4.4.2B Intercompartment Flow in Containment Volume Flows resulting from the driving forces discussed in 4.4.2A are affected by the geometry and loss characteristics of flow paths between compartments. Flow through compartments and the above deck region produces a pattern of containment-scale circulation inside containment.
l Intercompartment flow can tend to reduce stratification gradients, and affects the steam concentration near heat transfer surfaces, which affects mass transfer rates. Since intercompartment flow affects parameters, such as steam concentration and velocity, that are important to mass transfer, the ranking of intercompartment flow is the same as that for condensation on the steel shell (see subsection 4.4.7C) inside containment.
4.4.2C Containment Volume Gas Compliance The mass and energy that enter the internal containment volume are stored within the gas volume and cause the gas pressure, temperature, and density to increase. Storage within the gas volume is reduced by condensation and energy transfer to the heat sinks and to the containment shell. As internal gas pressure increases, the energy stored within the gas increases. The storage of a portion of the delivered work and energy within the gas increases its internal energy, and the energy change due to a pressure increase can be called " gas compliance." The term " gas compliance" is used to distinguish the concept from the well.
known gas compressibility discussed in most thermodynamic texts. Gas compliance is a capacitance term, and as such, does not increase or decrease pressure, but only reduces the rate of change of pressure in response to sources.
Gas compliance is related to the total volume and specific properties for the gas. Most of the uncertainty affecting gas compliance is the containment free volume, since properties of air and steam are well documented. During the LOCA blowdown period, pressure scaling shows that the energy stored by the gas accounts for a significant fraction of the break source. Although rates of pressure change, other than during LOCA blowdown, are much slower, the pressure x group from scaling is relatively constant. The pressure n group results show that the ratio of gas compliance to the product of the source work and time constant remains relatively constant for all accident phases. Therefore, the importance of gas compliance relative to the source and system time constant remains high for all accident l.
phases, and is ranked High.
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4-22 4.4.2D Fog in the Containment Volume The steam flow velocity in the cold leg pipe during the blowdown phase of a DECLG LOCA is high enough to entrain a large fraction of the available liquid and disperse it as fog, or very small drops. The velocity following blowdown is not high enough to create and entrain much liquid, so a negligible amount of the liquid will be converted to drops after blowdown.
The drops are expected to persist for as long as hours, so all time phases are affected, although drops delivered during blowdown are expected to settle out or be removed by other processes before 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> have elapsed.
During blowdown, the maximum mass flow rate for drop production is estimated to be
}
approxunately 0.5 times the steam mass flow rate, so the steam / drop mixture density can be j
approximated as 1.5 times the steam density, or p = 1.5 PM/RT = 1.5 x P x 18/(10.73 x T).
l The resulting mixture density is thus 1.5 x 18/29 = 0.93 times as dense as pure air at the j
same total pressure and temperature. Such a foggy source mixture may be higher in density j
than the mixture already in containment. Therefore, the blowdown source jet may not be as buoyant if it contains fog. However, LOCA blowdown flow rates are high enough to j
pressurize a compartment, so the flow distribution during blowdown will be governed by i
loss coefficients though openings exiting the break compartment, not the source density.
l Therefore, the break buoyancy does not drive circulation during blowdown.
t The heat capacity of the foggy source mixture is significantly increased over that of steam l
alone.
l Because of the potential for density to affect post blowdown natural circulation and the effect i
on gas heat capacity, it is judged that blowdown source fog has an importance with the same ranking as condensation on the steel shell (see subsection 4.4.7C) for blowdown, refill, peak-pressure, and long-term LOCA phases. For the superheated MSLB, fog is ranked as N/A l
since there is no fog.
4.4.2E Hydrogen Release l
Following a LOCA, hydrogen may be postulated to be released to the reactor containment atmosphere by:
Reaction of the zirconium fuel cladding with water
=
Corrosion of materials of construction i
i i
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4-23 Release of the hydrogen contained in the reactor coolant system Radiolysis of water The release of hydrogen has two effects on containment pressure. The partial pressure of hydrogen gives a direct increase in total pressure due to the addition of mass to the gas, and an indirect increase in pressure due to the noncondensible degradation of condensation rates.
Hydrogen production from zirconium-water production during a design basis LOCA or MSLB is not expected. Design basis analysis criteria for LOCA and MSLB preclude fuel failure, and limit worst case fuel cladding temperatures below that for which zirconium-water reactions would occur. For conservatism, LOCA M&E releases include the energy associated with zirconium-water reaction of 1 percent of the cladding material in the core; however, hydrogen is not assumed to be released into the containment gas volume.
Production of hydrogen from corrosion of aluminum and zinc occurs in the environment inside containment following a postulated LOCA, and is a function of the pH of the water contacting the metals. Corrosion is a relatively slow process of hydrogen generation at the marginally acid pH of the blowdown and safety injection water, requiring a period of days to generate significant quantities. Following a LOCA, the production of hydrogen from corrosion has been conservatively estimated to be 2990 standard cubic feed (scf) over the first 20 minutes, and 5380 scf over the first 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
Dissolved hydrogen is contained in the primary coolant and in the vapor space above the pressurizer liquid due to the hydrogen overpressure maintained by the chemical and volume control system. Dissolved hydrogen is conservatively estimated to be 1170 scf. Most, but not all, of the pre-accident liquid inventory is released during the LOCA blowdowm. A conservative evaluation assumes that all 1170 scf of the hydrogen dissolved in the primary coolant and contained in the pressurizer vapor space is released to containment during blowdown.
Hydrogen from radiolysis of water is considered to be generated due to the radiation field in the core. After the blowdown, the passive core cooling system (PXS) refills the reactor vessel and maintains the water level in the core. Radiolytic hydrogen generation continues in the core due only to gamma radiation from radioactive decay. A very conservative evaluation gives a hydrogen production of 280 scf over the first 20 minutes and 5400 scf during the first 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
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4-24 The above evaluations consider only conservatively high sources, and ignore significant sinks, which would reduce the amount of hydrogen actually delivered to the containment atmosphere, due to:
Solubility of radiolysis products in water in the break pool, liquid films, and drops Recombination or reaction of the chemically reactive hydrogen gas with the primary system piping and other containment materials and surfaces Recombiner operation The above conservative estimates of postulated sources of hydrogen during a DBA LOCA are summarized in the following table. Comparison is provided to the pressure increase represented by the mass addition, as well as the relative increase in number of moles of noncondensables to assess the potential impact on condensation rates.
Table Summary of Conservative Estimates of Postulated Sources of Hydrogen During a Containment Pressure DBA LOCA Integrated Total Hydrogen Release from Source Up to Indicated Time (scf) 20 minutes 3,
(peak pressure) 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> Corrosion 2990 5380 Initial solution 1170 1170 Radiolysis 280 5400 Totals 4440 11950 Assessment of Effect on Containment Pressure Predictions Time of Evaluation 20 minutes 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> Partial pressure increase due to mass addition at 0.05 psi 0.14 psi containment conditions 1
Increase in number of moles of noncondensibles 0.3 %
0.9 %
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4-25 The hydrogen sinks significantly reduce the amount of hydrogen actually delivered to the containment atmosphere. Because hydrogen sources are small and are further reduced by the sinks, it is concluded that hydrogen is not significant during any phase of a containment pressure design basis LOCA. Therefore, hydrogen is neglected in the mass releases, both for the mass effect on pressure and the noncondensible effect on condensation rates during a design basis containment pressure calculation.
It should be noted that the LOCA evaluation model energy released to containment includes, as a conservatism, an amount of energy equal to that resulting from reaction of 1 percent of the zirconium in the active fuel region, but no hydrogen mass is assumed to be added to containment from the break.
There is also no significant source of hydrogen from the secondary side during a MSLB.
Hydrogen from sources on the primary side has no significant path into the secondary side, so they are not considered significant sources for an MSLB. Since the secondary side does not utilize borated water, corrosion rates are less than for the LOCA, and the MSLB is over by 500 seconds, limiting the total production of hydrogen.
Hydrogen release is Low for all accident phases.
4.4.3 Containment Solid Heat Sinks 4.4.3A Liquid Film Energy Transport on Containment Heat Sinks i
Liquid films form due to the condensation on concrete and steel heat sinks inside containment. Liquid films flow under the influence of gravity and shear forces due to external gas flows. The internal liquid film carries away the liquid enthalpy (h,) that accounts for approximately 15 percent of the enthalpy of the condensed steam (h,) based on the convention of zero intemal energy at the triple point of water.
Based upon scaling analyses, the importance of the liquid film increases as condensation increases. The maximum energy input to films occurs during blowdown; the maximum heat input to heat sinks occurs during blowdown. However, relative to the blowdown source, these are small. During the blowdown portion of the LOCA, there is no film energy transport but by the peak-pressure period, there is a substantial amount of energy transfer via the film relative to the source. Therefore,it is ranked Low for blowdown, Medium for refill, and High for the peak-pressure and long-term phases. It is ranked Medium for the MSLB event since there is less time for film buildup from condensation.
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4-26 4.4.3B Vertical Film Conduction on Containment Heat Sinks Liquid films on concrete and steel surfaces inclined more than a few degrees flow fast enough to limit their thickness to approximately 0.005 in. or less. This results in a relatively high heat transfer coefficient so the rates of heat transfer to structures are limited by either the structure internal resistance or the mass transfer coefficient, but not by the film. The i
relative magnitude of energy transfer resistance on the shell are summarized in conductance scaling analysis (Ref. 20). The film resistance is always second order relative to the mass transfer and the structure internal resistance, therefore it is ranked Low for both the MSLB and LOCA events.
4.4.3C Horizontal Film Conduction on Containment Heat Sinks Liquid films on surfaces facing up that are horizontal, or nearly so, can develop rather thick films (greater than 0.05 in.) that may be more limiting to heat transfer than the heat sink internal resistance or the mass transfer coefficient. Horizontal surfaces facing down, with the i
inorganic zinc coating, experience film flow for slopes greater than 1 degree from horizontal.
Slopes less than 1 degree drip with water and have heat transfer coefficients greater than 2
1000 BTU /hr-ft..F. The condensation rate during blowdown will develop a film thickness of less than 0.01 in. Combined with the short drop settling time, the film thickness will remam i
less than 0.05 in., and thus is ranked Low for blowdown. At 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, energy scaling i
indicates the steel is a small heat source. As such, there is no condensation, so the film will either drain away or evaporate, in which case the film is nonexistent. During post blowdown LOCA, and MSLB, condensation and/or rainout of drops may cause the film to reach a thickness sufficient to degrade heat transfer into the surface, so the rankings are the same as those for condensation on the solid heat sinks (see subsection 4.4.3F), for post blowdown LOCA, and MSLB.
4.4.3D Internal Heat Sink Conduction Internal resistance to conduction into the heat sinks must be considered in the transfer of energy into the heat sinks. The resistance to internal conduction can be scaled relative to the surface heat transfer using the Biot number, hL/k. Heat sinks with Biot numbers less than 0.1, such as steel with thickness less than approximately 0.5 in., may be simply modeled as lumped masses. Heat sinks with Biot numbers greater than 1, such as concrete and water pools, limit energy absorption by internal resistance, rather than by surface transfer coefficients. Steel-jacketed concrete behaves initially as steel and, over the longer term, as concrete. The gap between the steel jacket and concrete can affect heat sink internal conduction and should be considered (see Ref. 5 Section 5.9 and Table 14-2). The calculation of energy into steel jacketed or concrete heat sinks requires more complicated means of i
accounting for the internal energy transfer and storage.
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4-27 During the LOCA blowdown, most of the mass and energy release is stored in the containment gas volume. As containment continues to pressurize prior to the initiation of significant cooling by external evaporation, pressure mitigation is primarily by heat transfer j
to internal heat sinks. Once evaporative cooling begins to be effective, the PCS heat removal becomes dominant and results in the pressure turning around :nt the time of peak pressure.
In the longer term, internal heat sinks saturate and are less effective than the PCS. Since internal heat sinks are one of the sources for removing energy from containment by condensation, and their internal conduction limits the amount of condensation that can occur on their surfaces, internal heat sink conduction is ranked the same as condensation on containment heat sinks (see subsection 4.4.3F) for all time phases.
From the point of view of containment gas, heat sink effects are small relative to the source and could effectively be neglected during LOCA blowdown. From the point of view of the heat sink, the heat sink temperature increases during b!owdown (about 15 percent of their temperature increase at peak pressure). Thus, the heat absorbed by heat sinks during blowdown must be tracked to provide an appropriate heat sink temperature during later phases.
i 4.4.3E Heat Capacity of Containment Heat Sinks The heat capacity of solid heat sinks within containment is a function of the material specific heat capacity, the mass of heat sinks, and the temperature increase in the source. The initial heat sink temperature affects the amount of total heat that can be removed by a heat sink for a given increase in containment temperature. Since internal heat sinks are one of the sources for removing energy from containment by condensation, and their heat capacity affects their temperature (and thus the amount of condensation on their surfaces), heat capacity of solid heat sinks is ranked the same as condensation on containment heat sinks (see subsection 4.4.3F) for all time phases.
4.4.3F Condensation on Containment Heat Sinks Condensation is a boundary layer diffusion process that is driven by a steam partial density gradient. Condensation removes the gas enthalpy (h,) from the atmosphere, transfers the heat of formation of the gas (h,,) to the heat sink, and leaves behind the liquid enthalpy (h,)
with the condensate.
l The validation of the condensation mass transfer correlation used on inner shell surfaces included an evaluation of LST data as described in Ref. 9, Section 3.9. Using measured values of total pressure, wall heat flux, wall surface and internal fluid temperatures, and air partial pressures, heat and mass transfer coefficients were derived and the results for condensation were compared to the correlation. Results showed that condensation accounted l
for about 80 to 95 percent of the wall heat flux. The test range covered internal conditions Containment Phenomena identification and Ranking m:\\3429w.wpf:lt>010997
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representative of AP600 post-blowdown quasi-steady heat and mass transfer conditions. In the AP600, solid heat sinks inside containment start out with the same initial temperature difference as the inner shell surface, and solid heat sink surface temperatures increase as the transient progresses. Since the LST had a higher degree of superheat in the source than AP600, sensible heat transfer is higher in LST than in AP600. Therefore, the LST results also show that the dominant heat removal process for AP600 internal solid heat sinks is condensation.
l The scaling analysis results and sensitivity studies showed that condensation on the internal heat sinks is a dominant process for containment pressure reduction for all time phases of the LOCA event although it was small relative to the break source during the blowdown period of the LOCA Therefore it was ranked as High except for LOCA blowdown period when it is ranked Medium. It was also ranked High for the MSLB event.
4.4.3G Convection From Containment Volume Convective heat transfer is a boundary layer conduction process that is driven by a temperature gradient in the presence of a flowing bulk fluid. The bulk fluid motion may be due to a state of forced convection, free convection (wall layers), or a combination of both.
Based on the scaling analysis results, sensitivity studies, and test results, it was determined that the convection from the containment air / steam mixture to the internal heat sinks had a i
much smaller effect on containment pressure reduction than condensation. However,it was not negligible for the later time phases of the LOCA event (refill through long-term).
Therefore, it was ranked as Low for the blowdown period of both LOCA and MSLB, and ranked as Medium for the other three LOCA time phases.
4.4.3H Radiation From Containment Volume to Containment Heat Sinks Radiation from the containment air / steam mixture to the solid heat sinks can be significant when the product of the steam partial pressure and radiation beam length are of the order of I ft. atmosphere. This is the case inside containment where steam partial pressures may be as high as 3 atmospheres and beam lengths are frequently greater than 10 ft. Radiation is enhanced by high emissivity surfaces: the containment surface emissivities are all near 1.0.
The liquid fihns have emissivity of 0.95 to 0.96, the inorganic zinc paint emissivity is 0.90 to 0.95, and the concrete emissivity is approximately 0.90.
Based on the scaling analysis results,it was determined that radiation to the internal heat sinks had a much smaller effect on containment pressure reduction than condensation.
However, it was not negligible for the later time phases of the LOCA event (refill through long term). Therefore, it is ranked Low for the blowdown period of both LOCA and MSLB, and ranked Medium for the other three LOCA time phases.
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4-29 4.4.4 Initial Conditions Within Containment 4.4.4A,B,C The initial conditions within the containment may have an important effect on the containment pressure response. These conditions, which include temperature, humidity, and pressure affect the capability of the solid, liquid, and gaseous heat sinks in containment to absorb energy from the break. Sensitivity studies showed that these three initial conditions I
had an effect on the peak and long-term containment pressure for a LOCA event, although a smaller effect on blowdown and refill phases (Ref. 5, Section 5).
i The MSLB is mitigated primarily by condensation on internal heat sinks, as well as the containment shell inner surface. Therefore, initial conditions for MSLB would have similar importance as for the LOCA peak pressure.
Initial conditions were ranked High for the peak-pressure and long-term phases of the LOCA and for the MSLB event. Initial conditions were ranked Medium for blowdown and refill.
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4.4.5 Break Pool i
4.4.5A Break Pool Mixing / Stratification For the LOCA event, the containment pressure interactions with the break pool do not
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become significant until the beginning of the long-term phase, therefore, mixing / stratification in the break pool is ranked Low in importance for the first three time phases, and ranked Medium for the long-term phase.
For the MSLB event, most of the released mass condensate drains into the IRWST. The break l
source is largely superheated, so the break liquid to the pool is not considered significant, J
and is therefore, ranked Low importance.
4.4.5B Break Pool Condensation / Evaporation The break pool which contains saturated water may function as either a heat sink that condenses some of the steam in containment, or as a heat and mass source that provides additional steam flow into the containment. Based upon the results from the scaling i
analyses, the break pool is a heat and mass source. The evaporation from the break pool provides a small increase in the containment pressure for most of the LOCA transient. Since it represents approximately a 5-to 7-percent increase in the containment pressure rate during j
the peak-pressure period, it was ranked as Medium for peak-pressure, long-term LOCA and was ranked Low for other time phases, as well as for the MSLB event.
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4-30 4.4.5C Convection Heat Transfer with Containment Volume The scaling analyses show that the convective heat transfer between the contamment steam / air mixture and the break pool is insignificant for all time phases of the LOCA event, therefore it was ranked as Low importance for the LOCA, as well as for the MSLB event.
4.4.5D Radiation Heat Transfer with Containment Volume Based upon results from the scaling analyses, radiation heat transfer between the containment steam / air mixture and the break pool is insignificant for all time phases of the LOCA event, therefore it was ranked as Low importance for the LOCA, as well as for the MSLB event.
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4.4.5E Conduction in Break Pool The internal resistance is a complicated function of the stratification and mixing of incoming fluid to the pool and of the interactions between the pool and its boundaries that are also heat sinks. Thus, conduction in the break pool was given the same ranking as pool mixing / stratification (see subsection 4.4.5A), i.e., Medium importance for the peak-pressure period and Low for other LOCA phases as well as for the MSLB event.
4.4.5F Flooding in Break Pool The break pool fills with break liquid and below-deck condensate. The break pool starts from the sump at the lowest region of the containment, and as the break continues, it floods more volume in additional compartments. Enough water is collected by 15,000 seconds to close the major flow path into the steam generators. Closing the steam generator opening may eliminate the large-scale circulation induced by the break source in the below-deck compartments. By this time, however the below-deck heat sinks (except concrete) are nearly saturated so circulation below-deck may not be very effective at condensing steam.
However, the lack of circulation permits air to concentrate below deck, creating a steam-rich mixture above-deck tv a reduces the condensation resistance to the shell.
The effect of break pool flooding on mixing / stratification in the containment is considered to be important for the long-term period and is ranked Medium for that phase and Low for other time phases, as well as for the MSLB event.
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4-31 4.4.6 IRWST l
4.4.6A,B,C,D,E,F The operating deck floor is sloped to drain into the IRWST. The above-deck condensate drains mostly into the IRWST, while that below deck drains mostly into the break pool. The IRWST water is initially assumed to be at a temperature of 120 F. A number of vents are provided in the roof of the IRWST, which provides a path to vent steam released by the spargers. The vents open on small pressure differentials to limit the differential.
The condensate that drains into the IRWST comes from the shell and from condensate on the above-deck heat sinks, at a temperature and steam partial pressure lower than that of the above-deck gas but hotter than the tank water. Consequently, the added liquid is stable, and the temperature of the gas over the tank will be cooler and more dense than the gas above.
J Any air / steam that enters from above will condense some of the steam on the water surface, leaving behind a more stable atmosphere that resists flow interactions with the gas above.
Late in the LOCA transient, the IRWST drains into the RCS to provide additional core cooling and therefore the liquid level decreases with time. The effects of IRWST level and subcooling of IRWST liquid on break source are addressed conservatively with the mass and energy release model. Consequently, the direct interactions between the containment volume and the IRWST volume by conduction, convection, or radiation are expected to be negligible, and were ranked Low for their effect on containment pressurization during all time phases of the LOCA and the MSLB.
4.4.7 Steel Shell 4.4.7A Convection Heat Transfer From Containment Volume Convective heat transfer is a boundary layer conduction process that is driven by a temperature gradient in the presence of a flowing bulk fluid. The bulk fluid motion may be due to a state of forced convection, free convection (wall layers), or a combination of both.
The scaling analysis shows that convection from the containment steam / air mixture to the steel shell has a smaller effect on containment pressure reduction than condensation.
l However, in combination with radiation, condensation had greater than a 5 percent effect on containment pressure for the later time phases of tne LOCA event (refill through long-term).
l Therefore, it was ranked as Low for the blowdown period of both LOCA and MSLB, and Medium for the other three LOCA time phases.
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- 4.4.7B Radiation Heat Transfer from Containment Volume to Steel Shell The scaling analysis shows that radiation from the containment steam / air mixture to the steel shell had a smaller effect on contamment pressure reduction than condensation. However, in combination with convection, radiation had greater than a 5 percent effect on pressure for the later time phases of the LOCA event (refill through long-term). Therefore, it was ranked as Low importance for the blowdown period of both LOCA and MSLB, and as Medium importance for the other three LOCA time phases.
4.4.7C Condensation on Inside Containment Shell The scaling analysis shows that condensation on the containment shell was a donunant effect on containment pressure reduction for the MSLB event and for all time phases of the LOCA event except during the LOCA blowdown period. Therefore, it was ranked as Low for the LOCA blowdown, and as High importance for all other time phases.
1 4.4.7D Film Conduction on Inside of Steel Shell Tests at the University of Wisconsin show that surfaces with slopes less than 1 degree may experience drops falling from the liquid film condensation, which has a higher heat transfer coefficient than film-wise cendensation without drops. Since only 0.09 percent of the shell heat transfer surface area has a slope less than 1 degree, horizontal films are not significant for the shell for any time phase.
On the majority of the inner shell surface, a flowing liquid film exists with a heat transfer 2
coefficient of approximately 1000 BTU /hr-ft _.F on the inorganic zinc coated surfaces. This 2
compares to a heat transfer coefficient of 50 to 100 BTU /hr-ft..Ffor condensation onto the film. Scaling of heat transfer conductances showed that the temperature drop through the film is small relative to the other series temperature drops, therefore, film conduction was ranked as Low importance for both the MSLB and LOCA events.
4.4.7E Film Energy Transport on Steel Shell Liquid films form due to condensation inside contamment. Liquid films flow under the influence of gravity and shear forces due to external gas flows. The liquid film that forms on the internal shell is collected at the crane rail, the stiffener ring, and at the deck elevation and drained into the IRWST. The internal liquid film carries away the liquid enthalpy (h,) that accounts for approximately 15 percent of the enthalpy of the condensed steam (h,) based on the convention of zero internal energy of the triple point of water. Since the effect of enthalpy transported by condensed liquid film should be considered in models, it is ranked Medium for all accident phases.
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4-33 4.4.7F Conduction Through Shell The time constant for the steel shell is approximately 5 minutes. For the short blowdown and refill phases, the shell remains nearly at its initial temperature and therefore the conduction was ranked as Low. Later in the LOCA transient (peak-pressure and long-term time phases), the conduction heat transfer through the shell accounts for approximately 1/3 of the containment to riser energy transfer resistance based upon scaling analyses, therefore the shell conduction was ranked High in importance. During an MSLB, the shell acts as a steel heat sink, so conduction is ranked High.
4 8.7G Heat Capacity of Shell The heat capacity of the coated steel shell is a function of the specific material heat capacity the mass of the shell, and the shell temperature rise. The initial shell temperature affects the amount of total heat that can be removed by heating the steel shell during the initial pressurization, while the outer surface of the steel shell can be considered adiabatic, for a given increase in containment temperature. Over the longer term, the steel shell heat capacity provides a significant heat storage capability, acting to dampen perturbations of short duration. The steel shell is the path for heat transfer between the internal condensing surface and the external evaporating surface. The heat capacity of the steel shell is important when the difference between the energy in and energy out are significant. Scaling analysis results show that the difference is significant during refill, peak-pressure, and MSLB when it is ranked High. It is ranked Low otherwise.
4.4.7H Convection to Riser Annulus Based upon the scaling analyses, convective heat transfer from the steel shell to the air in the riser annulus was insignificant until the time of peak pressure and beyond in the LOCA event, therefore it is ranked as Medium importance for the peak-pressure and long-term time phase and Low importance for the other LOCA phases as well as for the MSLB event.
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4.4.7I Radiation to the Baffle Based upon the scaling analyses, radiation heat transfer from the steel shell to the baffle was 4
j insignificant until the time of peak pressure and beyond in the LOCA event, therefore it is ranked as Medium importance for the peak-pressure and long-term time phase, and Low importance for the other LOCA phases as well as for the MSLB event.
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4-34 4.4.7J Radiation to the Chimney j
The dry upper dome surfaces of the steel shell can radiate to the missile shield which defines the bottom of the chimney volume. The low temperature of the liquid due to subcooled film reduces radiation from the wet upper dome surface to the missile shield. The radiant energy absorbed by the missile shield would be transferred to the chimney air volume by convection, adding to chimney buoyancy. This is judged to be a small effect and is thus ranked Low for all accident phases.
4.4.7K Radiation to the Fog / Air If drops are postulated to exist, they will capture radiation emitted by the shell that would otherwise be deposited in the baffle. Capture of the radiant energy will raise the temperature of the drops and riser gas flow, with the result that the saturation temperature increases and some of the drop vaporizes. The calculation performed to assess the effect of condensation from a supersaturated vapor (see subsection 4.4.9C) showed drop formation caused the net riser mixture density to decrease, so it might be expected that vaporization of drops leads to a density increase. Ihat is not the case, however, since the energy source for drop vaporization is not the riser gas mixture, but rather is the shell radiation. The drops only vaporize as required to balance the absorption of the radiant energy. Consequentb, the gas mixture temperature increases, causing the mixture density to decrease, the drop trass decreases, causing the mixture density to decrease, and the gas molecular weight decreases due to the increased gas fraction, causing the mixture density to decrease. All of these increase the buoyancy and the PCS natural circulation air flow rate. The effect of radiation j
capture by fog is therefore ranked Low during the peak-pressure and long-term time phases i
based on the above evaluation, and is ranked Low during blowdown and refill and MSLB when there is insufficient evaporation to form fog.
4.4.7L Outside Film Conduction A flowing liquid film exists with a heat transfer coefficient of approximately 2
1000 BTU /hr-ft - F on the inorganic zinc coated surfaces. This compares to a heat transfer 2
coefficient of up to about 40 BTU /hr-ft - F for evaporation from the film. Scaling of heat transfer conductances showed that the temperature drop through the liquid film is small relative to the other series temperature drops (shown in Figure 1-4), therefore, film conduction was ranked of Low importance for both the MSLB and LOCA events, except during blowdown and refill, when the ranking is N/A, since no external liquid film is present.
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4-35 4.4.7M Outside Film Energy Transport The extemal liquid film is supplied at a low temperature (115 F assumed for safety analyses).
The scaling analysis shows that energy absorbed by sensible temperature increase of the film, accounts for approximately 5-to 10-percent of the energy transferred by evaporation. The energy absorbed by temperature increase is sometimes referred to as the subcooled heat capacity of the external film. Most of the external film is expected to evaporate, but the portion that does not evaporate runs down a drain, away from the containment shell, carrying a small portion of containment energy.
During the blowdown and refill phases of the LOCA event there is no outside film, therefore
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film energy transport is N/A. During peak pressure and long term LOCA phases, film energy transport is ranked Medium based on the above discussion. Since PCS water is not rsumed effective until relatively late in the MSLB, a ranking of Low is used.
.4.7N Evaporation to Riser Annulus Based upon the scaling analyses and test results, evaporation is the dominant process for energy transfer from the shell and hence has a major effect on the rate of pressure change inside containment. However, evaporation does not take place until later in the LOCA transient after the shell heats up and full water coverage is achieved. Evaporation was ranked High for the peak pressure and long-term phases and N/A for earlier LOCA time i
phases. Evaporation is ranked Medium for the MSLB event since water is not available until later in the transient.
4.4.8 PCS Cooling Water 4.4.8A Water Flow Rate Water flow rate affects the amount of heat that can be removed due to subcooling (see subsection 4.4.8B). Water flow rate is also directly related to the film flow rate that influences film stability (see subsection 4.4.8C). Therefore, water flow rate is ranked the same as evaporation to riser annulus (see subsection 4.4.7N) for all time phases.
4.4.8B Water Temperature Water temperature applied from the PCCWST affects the amount of heat removed due to subcooling. Energy scaling shows that the amount of break energy removed from the vessel by subcooling is approximately 5 to 10 percent. Water temperature is ranked the same as outside film energy transport (see subsection 4.4.7M).
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4-36 4.4.8C Water Film Stability and Coverage The stability of the liquid film due to the effects of momentum, heat flux, effective thermocapillary forces, and surface tension has the potential to limit surface water coverage, and hence, the evaporative cooling (Ref. 5, Section 7). Film stability was ranked High whenever evaporative cooling was important, i.e., during the peak-pressure and long-term phases. Stability is N/A during earlier LOCA phases. Since PCS water is not available until later in the MSLB, it is ranked Low.
4.4.8D Film Stripping The riser gas interacts with the film surface by momentum transfer (shear stress). However, the maximum air velocity is approximately 17 ft./sec upward, and the film surface velocity is 3 ft./sec downward. The ratio of the gas-liquid shear stress to the liquid-shell shear stress is 0.04. Clearly, the gas has an insignificant effect on the liquid flow. In addition, the gas-liquid velocity difference is too low to induce liquid surface instabilities that would cause drops or spray to be torn from the film, based on observations in the STC flat plate tests at velocities up to 39 ft./sec.
During the blowdown and refill phases of the LOCA event, there is no outside film, therefore film stripping is N/A. For other time phases, film stripping was ranked Low in importance.
4.4.8E Film Drag The riser air flow friction resistance may be increased by the additional liquid film velocity.
However, the riser friction accounts for less than 50 percent of the total PCS air flowpath loss coefficient (Ref.19 and 20), based on hydraulic air flow path tests (Ref.11). Approximately 50 percent of the containment surface is wet at the peak pressure condition, the other 50 percent and the baffle are dry. Relating the effect of friction to the wet area fraction permits the estimate of 0.5 x 50 percent x 50 percent = 13 percent of the total drag to be affected by the film velocity. Relating this to the effective velocity increase from 17 to 20 ft./sec (see subsection 4.4.8D) permits the estimate that the film portion of the drag will increase by 38 percent. The net effect is a 5 percent increase in the total PCS loss coefficient from 2.5 to 2.6, which has a negligible effect on containment pressure. Consequently, this interaction is ranked Low for time phases where PCS water is available and N/A otherwise.
4.4.9 Riser Annulus and Chimney Volume 4.4.9A PCS Natural Circulation Natural circulation through the external PCS flow path (downcomer annulus, riser annulus, and chimney) results from the buoyancy introduced by heating the riser air. Flow rates are Containment Phenomena Identification and Rankmg m:\\3429w.wpf;1b-012297
4-37 established based on the balance of buoyancy and unrecoverable pmssure losses. Unrecoverable losses are taken from the 1/6 scale air flow tests as discussed in subsection 2.2.4.
The baffle geometry and baffle supports affect the unrecoverable losses in the riser, so the effect of baffle and supports is evaluated for PCS natural circulation in the riser. Baffle supports are a source of drag on the riser annulus air flow. Baffle support drag accounts for about 20 percent of the riser portion of the PCS air flow path loss coefficient. The baffle supports block less than 1 percent of the riser cross section area, and produce less drag than the geometry used to measure losses in the 1/6 scale loss coefficient tests.
Buoyancy in the riser is introduced by the convective heat and mass transfer off the shell and baffle surfaces. Because of the importance of velocity to mass transfer rates, and the importance of mass flow for effective removal of the evaporated water, PCS natural circulation is assigned the same ranking as evaporation to the riser annulus (see subsection 4.4.7N), except blowdown and refill, when it is ranked Low.
Sensitivities in Ref. 5, Section 5.8, show there is a low sensitivity of containment pressure to changes in loss coefficient. The lack of a high level of sensitivity is due to the self-correcting performance of the PCS, which results from the increase in buoyancy and heat transfer due to shell temperature increases. Increasing the loss coefficients tends to decrease the riser flow and energy removal, increasing the surface temperature. A surface temperature increase tends to increase the driving force for energy removal, and reduced flow rates tend to increase the annulus average temperature, and thus the buoyancy driving head.
Additionally, the evaporation rate increases nonlinearly (as saturation pressure as a function of temperature) with a shell temperature increase. Thus, buoyancy, AT increase, and evaporation rate increases dampen the impact on pressure of an increase in loss coefficient, resulting in a low sensitivity.
4.4.9B Vapor Acceleration The evaporating film produces a steam flux into the riser that has to be accelerated from the downward velocity of the film surface to the upward riser velocity. The effective drag can be 2
estimated as pv /2, where the density is that of steam in the riser and the velocity is the difference between the downward and upward velocities, or 20 ft./sec. The result must be 2
normalized to pv /2 for the riser mixture. The steam density at the top of the riser is less than 7 percent of the total density, so the steam will add approximately 0.07(20/17)2 = 0.10 to the total drag coefficient of 2.5. Thus the additional momentum is less than 4 percent and the net effect on containment is negligible, and was therefore ranked as Low.
Containment Phenomena Identification and Ranking m:\\M29w.wpf:1b-012297
4-38 4.4.9C Fog Fog can occur in the riser annulus if the evaporation of PCS ccoling water causes the partial pressure of the vapor to exceed the saturation pressure. If fog forms in the riser, it wi'l change the density of the riser gas / drop mixture. The density change results from three separate effects that accompany condensation:
The density increase due to the increased liquid mass The density increase due to the increased molecular weight of the gas, as air replaces the steam that condenses The density decrease due to the gas (air + steam) temperature increase when h,, is released by the condensate formation Relative values of these three effects were evaluated at conditions corresponding to those in the riser at the time of peak pressure Ref. 22. It was postulated that drops arise from supersaturated vapor and the relative magnitude of the three separate density change effects were calculated. The calculation showed the increased liquid mass produced a density increase that was approximately 10 percent of the net density change, the molecular weight increase produced a density increase that was approximately 10 percent of the net density change, and the release of h,, increased the mixture temperature and decreased the density by 120 percent of the net density change. The net change was a decrease in density with vapor condensation in the riser, that produces a net increase in the buoyancy and the PCS natural circulation air flow rate. Operation at temperatures other than design basis values are not expected to significantly affect the relative magnitudes.
The effect of fog formation in the riser gas is ranked Low during the peak-pressure and long-term time phases, based on the above evaluation, which shows that drop formation increases the PCS natural circulation flow rate. The effect of fog formation is also ranked N/A duririg blowdown refill, and MSLB since there is insufficient evaporation to cauce drop formation under the assumed em-ironmental conditions.
4.4.9D Flow Stability The chimney and upper part of the shield building are large concrete structures that can cool the PCS air flow before it exits from the chimney, producing a negatively buoyant wall boundary layer, thereby reducing the natural circulation buoyancy forces and potentially affecting equivalent chimney flow losses. The issues associated with wall boundary layer and the potential for reverse flow directions are considered relative to flow stability in the chirrt.cy Containment Phenomena Identification and Rankmg m:\\3429w.wpf:1b-012297
4-39 Calculations performed for the scaling analysis show the PCS air flow loses only 2 percent of its thermal energy and condenses less than 1 percent of the vapor while passing through the chimney at the time of peak pressure. Consequently, there is insufficient energy removal to develop significant instability in the chimney, and there is little effect on the net buoyancy.
Flow stability in the chimney is therefore ranked Low for all time phases of LOCA and MSLB.
4.4.10 Baffle The baffle receives radiant energy from the containment film and dry surface. Some of the energy is conducted through the baffle and rejected to the downcomer air by convection and to the shield building surface by radiation. The dry baffle has a time constant of approximately 30 minutes, so will significantly lag the shell (with a 5-minute time constant) during the initial transient of PCS annulus flow. The buoyant air flow in the riser end chimney will be well-developed long before the baffle begins to heat the downcomer air and the shield. Due to the significant time lag, the phenomena discussed in this section are negligible during LOCA blowdown and refill, and so are ranked N/A for those accident phases. Since extemal cooling begins to take effect so late in the MSLB transient, baffle phenomena are ranked Low for MSLB. Other time phase rankmgs are discussed below.
4.4.10A Convection to Riser Annulus The baffle transfers heat to the riser gas by convection. Energy pressure scaling shows the convective heat transfer from the baffle to the riser to account for less than 1 percent of the break energy during the long-term phase, which would qualify for a Low ranking. However, the heat transfer to the riser has an effect on the buoyancy that may account for a significant part of the natural circulation driving force, so the importance is ranked Medium during long-term time phases and Low during the peak-pressure phase.
4.4.10B Convection to Downcomer Annulus The baffle transfers heat to the downcomer gas (air and steam) by convection. Energy pressure scaling shows the convective heat transfer from the baffle to the downcomer to account for less than 2 percent of the containment pressure change during the long-term time phases. This would qualify for a Low ranking. However, external momentum scaling shows that the downcomer has an effect on the buoyancy that may account for about 6 percent or less of the natural circulation driving force, so the importance is ranked Medium during the long-term time phases and Low during the peak-pressure phase and MSLB.
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4.4.10C Radiation to Shield Building The baffle transfers energy to the shield building by radiation. Energy pressure scaling shows that the effect is less than 1 percent of the break energy. Tl e bng time constant of the baffle, and the small heat transfer rate results in a rankmg of Low for all time phases.
4.4.10D Conduction Through Baffle The baffle is 1/8-in. thick steel with a very small Biot number of 0.0004. Thus the baffle behaves as a lumped mass with negligible Siternal resistance for all time phases. Since conduction through the baffle supplies the heat that is convected to the downcomer, conduction through the baffle is ranked the same as convection to the downcomer annulus
~
(see subsection 4.4.10B) for peak-pressure and long-term accident phases.
4.4.10E Condensation on the Baffle Under conditions where the baffle temperature is below the dewpoint of the riser annulus air, condensation on the baffle could occur. Condensation on the baffle would decrease the baffle time constant by a factor of 20 (t would reduce from about 1000 to about 50), which potentially could impact the relative importance of some baffle phenomena. An evaluation performed to support scaling analysis and using annulus average conditions shows that, for the limiting condition of high ambient temperatures, condensation on the baffle is not expected during any time phase of a LOCA. An examination of evaluation model results shows that WGOTHIC predicts condensation on the baffle only during about 3 seconds after PCS water is assumed to be applied, and only at the top where riser annulus steam
4.4.10F Heat Capacity of the Baffle At quasi-steady annulus conditions typical of peak pressure and long term LOCA phases, heat into the baffle is about equal to heat c<at of the baffle, so its heat capacity has no effect.
Heat capacity of the baffle is ranked Low during the peak-pressure and long-term LOCA.
Dunng transients in the annulus, the heat capacity of the baffle could affect the rate of heating of the riser, and thus affect the initial annulus flow start-up transient. The riser annulus is in fully developed turbulent flow by the time the containment shell is greater than 2 F above ambient. The time constant of the wet shell is about 500 seconds, as compared to the baffle time constant of 1000 seconds, and baffle heatup is driven by radiation from the shell, so the transient development of external flow is complete well before the baffle heats Contamment Phenomena Identification and Rankmg m:\\3429w.wpf:lt412297
4-41 up significantly. (See also the discussion in subsection 4.4.10.) Heat capacity of the baffle is ranked Low for peak-pressure and long-term accident phases.
4.4.10G Leaks Through Baffle Baffle leaks due to postulated missing or misaligned baffle plates, may permit some of the air flow from the downcomer to "short circuit" through the baffle to the riser, rather than following the normal air flow path to the bottom of the downcomer, up the riser and out the chimney. In a forced flow system, the effect of short circuiting could be significant, depending on the relative area and pressure losses of the flow paths. However, the process that causes natural circulation is the relative density of the gas in the downcomer and riser
~
flow paths. Short circuiting through missing baffle panels at the top of the downcomer will not reduce the buoyancy in the riser below the leakage point. In fact, if it is postulated that the riser flow decreases, the gas temperature and buoyancy will increase, at least partially compensating for the leak.
It is well known that a top-to-bottom pattern of circulation develeps in rectangular cross section channels with a heated vertical wall, even without a partition (or baffle). Test data from Siegel and Norris (Ref. 38) show a reduction of less than 50 percent in the heated wall Nusselt number when a rectangular channel with heated pan,llel sides have their open bottom closed (sides closed, L/D = 20). Thus, it is expected that even the worst case of baffle leakage has a minor effect.
Based on engineering judgement, the effect of baffle leaks is ranked medium for the peak pressure and long term time phases.
4.4.11 Baffle Supports 4.4.11A Convection to Riser Air The baffle supports are approximately 3/8-in. thick,18-in. high, U-shaped steel brackets with the two ends of the U welded to the shell. After the external surface of the shell heats up,it will transfer heat to the baffle supports that behave as fins to transfer heat to the riser gas. A comparison of the hA values (heat transfer coefficient times area) shows the relative magnitude of heat loss through the surface and the bracket. The cross section of the 2
2 supports is approximately 13 in, and there is one support for every 11,088 in of shell surface. The heat transfer coefficient is approximately 3 for the dry shell and 50 for the support, giving a ratio of 0.02 for support / dry shell. This ratio is much less for the wet shell.
Furthermore, the dry shell contributes only a small fraction to shell heat rejection.
Consequently the ability of the support to influence the shell temperature is so limited that baffle support energy transport is estimated to be insignificant. Hence, this process is ranked Low during all time phases.
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4-42 4.4.11B Radiation from Shell After the external surface of the shell heats up, it will radiate to the baffle supports which will convect heat to the riser air and conduct heat to the baffle. Since the area of a baffle support that is exposed to radiation is normal to the shell surface, and the shell surface curves away from the supports, a conservative assessment of the potential effect of radiation to a baffle can be based on the areas of the two sides. The area of the two sides of the 2
2 supports is approximately 216 in, and there is one for every 11,088 in of shell surface. Since the radius of the shell and baffle differ by only one foot, the corresponding baffle surface is also approximately 11,088 in. Since a conservative assessment of the area blocked by the 2
supports is less than 2 percent of the area for radiation, the supports block a negligible amount of the energy transferred by radiation from the shell to the baffle. As determined from scaling, radiation from the shell is less than 2 percent of the break source energy. Thus, radiation from shell to supports is ranked Low for all accident phases.
4.4.11C Conduction from Shellinto Baffle Supports A small amount of heat is conducted from the shell to the baffle supports from which the heat can be convected into the riser air or conducted into the baffle. Convection from baffle supports to riser air has been evaluated and ranked Low during all accident phases as discussed above. The resistance to heat transfer from the shell through supports to the baffle by conduction is much higher than the resistance from the shell to riser annulus gas by convection. Since convection from the baffle supports to riser gas is ranked Low, conduction from the shell through baffle supports is also judged to be Low for all accident phases.
4.4.11D Heat Capacity of Baffle Supports The heat capacity of baffle supports is evaluated for the potential to store energy leaving the shell by conduction or radiation. From scaling analysis, the amount of energy leaving the shell by conduction or radiation is less than 4 percent of the break scurce energy, while convective mass and energy from the shell represents more than 50 percent. A conservative assessment of the heat capacity of the baffle supports relative to the baffle can be made by
~
comparing the mass of supports to the mass of the baffle, neglecting the mass of baffle stiffeners, which shows the support mass is less than about 13 percent of the baffle mass.
Since the amount of mass is not negligible, the following provides an assessment.
At quasi-steady conditions, the heat into the baffle supports by conduction or radiation is approximately equal to the heat removed by the baffles by convection, so that there would be ro net energy storage in the supports. During the initial startup transient, heat absorption by ti baffle supports could delay the rate of baffle temperature increase or reduce the rate of 2r air temperature increase. A delay in baffle heat-up would put less heat into the 1
dawncomer during the initial transient, which would be a benefit for startup of PCS annulus Containment Phenomena Identification and Rankmg l
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4-43 flow. The effect can be quantified by comparing the baffle time constant ( T = p c 6 / h ) to p
the time constant assuming the support mass is added to the baffle mass as increased thickness. The time constant for baffle heating, about 1000 seconds, would be increased by 13 percent. Since flow is fully developed, turbulent forced convection in the annulus by the time the contairanent external temperature reaches 2 F above ambient, the increase in baffle time constant would not adversely affect air flow startup.
A delay in the rate of riser air te.nperature increase could reduce the rate at which buoyancy driven annulus flow develops, thus reducing external heat removal. From the discussion in subsection 4.4.11B, the supports would absorb less than 2 percent of the radiant energy leaving the shell. An upper bound can be based on assuming that all the energy into the
~
support comes from radiation, which shows that the supports would absorb less than (2 percent x 5 percent ), or 0.1 percent of the energy leaving the shell. Relative to the energy delivered directly to the riser gas by heat and mass transfer, the energy absorbed by supports has a negligible effect on the development of the initial transient. Therefore, the heat capacity of baffle supports is ranked Low for all accident phases.
4.4.12 Chimney Structure The following phenomena are ranked on the basis of energy scaling:
A - Conduction through chimney B - Convection from chimney air C - Heat capacity of structure D - Condensation on chimney The chimney and upper part of the shield building are large concrete structures that can cool the PCS air flow before it exits from the chimney, producing a negatively buoyant wall boundary layer, thereby reducing the natural circulation buoyancy forces and potentially affecting chimney flow. Calculations performed for the scaling analysis show the PCS air flow loses only 2 percent of its thermal energy and condenses less than 1 percent of the vapor while passing through the chimney at the time of peak pressure. Consequently, there is little energy removal within the chimney, and there is little effect on the net buoyancy.
Chunney structure heat and mass transfer phenomena are ranked Low for all accident time phases.
4.4.13 Downcomer Annulus Containment Phenomena Identification and Rankmg m:\\3429w.wpf:1t>412297 I
4-44 I
4.4.13A PCS Natural Circulation A general description of this phenomenon is provided in subsection 4.4.9A. The unrecoverable pressure losses in the downcomer region include the inlet and turning loss and frictional losses on the baffle and shield building surfaces. Also considered in the downcomer are the losses due to the flow path which connects to the riser annulus, which have been min mized by the use of a curved vane based on separate effects tests of the flow losses.
The baffle is heated by radiant energy from the shell. The downcomer air may be heated by convective heat removal from the baffle, as well as by cooled heat convected to the shield building surface. The impact of those heat sources on downcomer buoyancy should be considered. The ranking is the samc as for the riser annulus natural circulation (see subsection 4.4.9a).
4.4.13B Downcomer Annulus Air Flow Stability Since the baffle is heated by radiation from the shell, the downcomer air will convect energy from the baffle, giving rise to a buoyant boundary layer. Based on results of scaling energy and momentum in the external flow path, the downcomer accounts for a negligible fraction of energy and a small part of momentum in the PCS flow path. Therefore, the buoyant boundary layer does not significantly impact downcomer air flow. Downcomer air flow stability is ranked Low for all accident phases.
4.4.14 Shield Building 4.4.14A Convection to the Downcomer A maximum heat load from the shield to the downcomer can be estimated by assuming the radiation to the shield from the baffle ic all transferred by convection to the downcomer.
Energy scaling shows that the radiati.in i om the baffle to the shield building is less than 1 percent, which qualifies for a Low rankmg. However, the heat transfer to the downcomer has an effect on the buoyancy that may account for a small but not insignificant part of the natural circulation driving force, so the importance is ranked Medium during the peak-pressure and long-term LOCA time phases. Since PCS water flow is not available until later in the transient for MSLB, a Low ranking is used. Since the external shell surface does not reject significant heat until after n ill due to its thermal capacity and relatively long-time constant, convection to downcomer is ranked N/A for those phases.
{
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4-45 4.4.14B Conduction Through the Shield Building The shield building is 3 ft. thick and has a time constant of 40 hours4.62963e-4 days <br />0.0111 hours <br />6.613757e-5 weeks <br />1.522e-5 months <br />. Consequently, the inside surface will not experience the day-night temperature fluctuations or effects of sun exposure. Thus, heat transfer to the environment through the shield building is ranked Low for the peak pressure and long term time phases, and is N/A during blowdown, refill, or MSLB.
4.4.14C Convection to the Environment Due to the time constant of the shield building, the outside surface of the shield building is
~
expected to remain at or near the environmental temperature. Therefore convection to the environment was ranked Low for both the MSLB and LOCA events. The day-night average outside surface tempe:ature will be less than the maximum technical specification value assumed in analyses.
4.4.14D Radiation to the Environment Due to the time constant of the shield building, the outside surface of the shield building is expected to remain at or near the environmental temperature, therefore radiation to the environment was ranked Low for both the MSLB and LOCA events.
4.4.1S External Atmosphere 4.4.15A Temperature I
The initial condition for the atmospheric temperature is assumed for safety analyses to be at the maximum Technical Specification value. The sensitivity of peak containment pressure to atmospheric air temperature is low because the pressure is primarily limited by mass transfer rates that are functions of steam partial pressure differences and not temperature.
Temperature was ranked Low during the peak-pressure and long-term phases, and was N/A for blowdown and refiJl because the PCS has no effect. Since PCS operation is not significant until later in the MSLB, a Low ranking is used.
4.4.15B Humidiy Th,e peak containment pressure is limited by mass transfer rates that are functions of steam partial pressure differences. The peak internal steam pressure is approximately 40 psia, while the ambient steam partial pressure is limited to the saturation pressure of less than i
1.5 psia. In terms of steam pressure difference between the inside and outside of containment and riser, tht. maximum variation expressed as a fraction of the total can be used as a first order estimate of the containment pressure sensitivity to humidity. The Containment Phenomena Identification and Rankmg m:\\3429w.wpf:1b 012297
1 i
4-46 resulting variation is less than 3 percent, so the rankmg is Low during the peak pressure and long term phases. A sensitivity to assumed inlet humidity shows relatively low effect on the assumed value in the calculation (see Ref. 5, Section 5.6 and Table 14-3). The phenomena was N/A for blowdown and refill because the PCS has no effect that early in the transient.
Since PCS operation is not significant until later in the MSLB, a Low rankmg is used. See also subsection 4.4.9C for the effect of fog formation in the annulus.
4.4.15C Recirculation The downcomer draws air from the environment through 16 discrete openings near the top of the shield building. Environmental disturbances, such as gusts, wakes, or "downwash" from upwind structures, and the downwash of chimney outflow to inlets on the downwind side (recirculation) can potentially affect PCS performance.
It was shown (Ref. 5, Section 6) that the worst recirculation will cause less than 15 percent of the chimney outflow to be drawn through the inlets. The resulting effect on PCS performance was calculated and found to produce a negligible increase in containment pressure. The effect of recirculation is therefore ranked Low during the peak pressure and long term time phases. The phenomena was N/A for blowdown and refill, because the PCS has no effect that early in the transient. Since PCS operation is not significant until later in the MSLB, a Low rankmg is used.
4.4.15D Pressure Fluctuations The air flow in the environment can affect the PCS behavior by inducing pressure fluctuations on the PCS inlet and outlet due to large-scale vortex shedding from such upwind obstructions as buildings, cooling towers, and terrain. The effect of the worst case inlet-outlet pressure fluctuations from the wind tunnel tests were evaluated and determmed to have (conservatively) no effect, and most realistically, a beneficial effect on heat rejection. The reason is that the wind positive nature of the external PCS design results in more positive benefit due to increased flow rates than the negative effect of the few-second cycles of zero to negative flow rates. The long time constant of the shell (about 5 minutes) buffers the containment pressure from such fluctuations. The fluctuations are wind induced, so only occur when the wind is blowing, which is also the time when the wind positive design assists heat and mass transfer. Thus the importance of terrain, structures, and wind induced pressure fluctuations on the AP600 containment pressure was ranked Low during the peak-pressure and long-term phases and was N/A during the short blowdown and refill phases. Since PCS operation is not significant until later in the MSLB, a Low ranking is used.
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5.0 CONCLUSION
S The phenomena identification and importance ranking of the phenomena for the AP600 containment were completed for the LOCA and MSLB transients. These transients were considered to be the most limiting events due to their effect on containment pressure.
It is concluded that the mass transfer processes of condensation inside containment and evaporation outside containment are the most important phenomena for reducing the containment pressure and transferring energy to the environment especially for the LOCA event. The large heat capacity of the steel, concrete, and containment shell are also significant in reducing the containment pressure. The MSLB event is not significantly affected by the phenomena outside containment due to the rapid pressure transient.
The process for making these conclusions was based on results from test programs, scaling analyses, sensitivity studies, and engineering judgement.
The results of this evaluation have been used to focus on the phenomena and models which have the most significant impact on containment pressure. The phenomena identified have been addressed in the evaluation model for the AP600 containment, as discussed in Ref. 5, Section 2.
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7
6-1
6.0 REFERENCES
- 1.
NSD-NRC-96-4876, AP600 Passive Containment Cooling System Design Basis Analysis Reports, November 1,1996.
2.
1000-P2-901, Nuclear Island General Arrangement, Rev. 8.
3.
1100-CC-902, Containment / Shield Buildings - Section B, Rev.1.
4.
PCS-M3-001, Passive Containment Cooling System-System Spectpcation Document (SSD),
Rev.3.
- 5.
WCAP-14407, WGOTHIC Application to AP600, September 10,1996.
6.
PCS-M3-001, Passive Containment Cooling System - System Specification Document (SSD),
Rev. 3, October 1995.
7.
" Accident Specification and Phenomena Evaluation for AP600 Passive Containment i
Cooling System," February 12,1996.
8.
NTD-NRC-94-4138, AP600 Design Cert:pcation Test Program Overview, Rev. 6, May 17,1994.
- 9.
WCAP-14326, Experimental Basisfor the AP600 Containment Vessel Heat and Mass Transfer Correlations, March 1995.
10.
WCAP-12665, Rev.1, Tests of Heat Transfer and Water Film Evaporation on a Heated Plate Simulating Cooling of the AP600 Reactor Containment, April 1992.
11.
WCAP-13328, Tests of Air Flow Pathfor Cooling the AP600 Reactor Containment.
12.
WCAP-13884, Water File Formation on AP600 Reactor Containment Surface, February 1988.
13.
WCAP-13960, PCS Water Distribution Phase 3 Test Data Report, December 1993.
14.
WCAP-14134, AP600 Passive Containment Cooling System Integral Small-Scale Tests, August 1994.
15.
WCAP-14135, Final Data Reportfor PCS lArge-Scale Tests, Phase 2 and Phase 3, July 1994.
'One or more sections of report will be revised as a result of outstanding NRC open items.
References m:\\3429w.wpf:1b-020197
6-2 16.
WCAP-13566, AP6001/8th large-Scale Passive Containment Cooling System Heat Transfer Test Baseline Data Report, October 1992.
17.
NTD-NRC-95-4489 (WCAP-14382), WGOTHIC Code Description and Validation, June 20, 1995.
18.
NTD-NRC-95-4561, Sca!ine Role in AP600 PCS DBA Analysis, September 19,1995.
- 19.
NSD-NRC-96-4790, Scaling Analysisfor AP600 Containment Pressure During Design Basis Accidents, August 8,1996.
- 20. WCAP-14783, Scaling of the PCS DBA, (to be issued).
- 21.
SSAR 6.2.1.1.3, NTD-NRC-95-4504, " Containment Structure Design Evaluation, Proposed Draft / Markups of SSAR, Sections 6.2 and 6.4", July 10,1995.
- 22.
NTD-NRC-94-4100, Enclosure 1, Radiation Heat Transfer Through Fog in the PCCS Air Gap, April 18,1994.
23.
WCAP-12665, Tests of Heat Transfer and Water Film Evaporation on a Heated Plate Simulating Cooling of the AP600 Reactor Containment, Rev.1, April 30,1992.
24.
WCAP-14048, Passive Containment Cooling System Bench Scale Wind Tunnel Test, April 29,1994.
25.
WCAP-13294, Phase 1 Wind Tunnel Testingfor the Westinghouse AP600 Reactor, April 30,1992.
26.
WCAP-13323, Phase II Wind Tunnel Testingfor the Westinghouse AP600 Reactor, October 2,1992.
27.
WCAP-14%8, Phase IVA Wind Tunnel Testingfor the Westinghouw AP600 Reactor, June 6,1994.
28.
WCAP-14091, Phase IVB Wind Tunnel Testingfor the Westinghouse AP600 Reactor, July 19,1994.
29.
WCAP-13307, Condensation in the Presence of a Noncondensible Gas - Experimental Investigation, April 30,1992.
'One or more sections of report will be revised as a result of outstanding NRC open items.
- WCAP-14783 will supercede Reference 19 l
References m:\\3429w.wpf;1M20197 l
6-3 j
30.
- 1. K. Huhtiniemi, Condensation in the Presence of Noncondensible Gas: The Effect of Surface Orientation, Preliminary Thesis (1990), August 16,1993.
31.
A. P. Fernsteiner, Condensation in the Presence of Noncondensible Gas: Effect of Helium Concentration,1993, University of Wisconsin Thesis, November 12,1993.
- 32. WCAP-13353, Passive Containment Cooling System Water Distribution Phase I Test Data Report, Rev. O, April 30,1992.
33.
WCAP-13296, PCS Water Distribution Test Phase II Test Date Report, April 30,1992.
34.
WCAP-13960, PCS Water Distribution Phase 3 Test Data Report, Rev. 0,
~
February 2.1994.
35.
WCAP-12667, Tests of Heat Transfer and Water Film Evaporationfrom a Simulated Containment to Demonstrate the AP600 Passive Containment Cooling System, Rev.1, April 30,1992.
36.
WCAP-13566, AP6001/8th lArge Scale Passive Contu.tment Cooling System Heat Transfer Test Baseline Data Report, Rev. O, January 1,1993.
- 37.
PCS-T2R-050, Large-Scale Test Data Evaluation, May 1995.
38.
R. Siegel and R.H. Norris, ' Tests of Free Convection in a Partially Enclosed Space Between Two Heated Vertical Plates," Transactions of the.,74E, April 1957, pp. 663-673.
- 39.
NTD-NRC-94-4100, " Enclosure 2, Liquid Film Model Validation," April 18,1994.
- 40.
NTD-NRC-95-4397, " Supporting Information for the Use of Forced Convection in the AP600 PCS Annulus," February 16,1995.
41.
NTD-NRC-94-4166, "AP600 Containment Plume Investigation," June 10,1994.
42.
NTD-NRC-96-4467 (PCS-T2C-059), " Analysis of PCS Wind Tunnel Testing for PCS Heat Removal, June 2,1995.
43.
NTD-NRC-94-4174, "AP600 PCS Design Basis Analysis (DBA) and Margin Assessment,"
June 30,1994.
'One or more sections of report will be revised as a result of outstanding NRC open items.
References m:\\3429w.wpf:1b-020197
A-1 APPENDIX A
SUMMARY
TABLE SHOWING
SUMMARY
OF SOURCES SUPPORTING PIRT RANKING Summary Tc. le Showing Closure Basis for PIR1 m:\\3429w.wpf:1b-020107
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e 1
1 9m ib p
p g APPENDIX A -
SUMMARY
OF BASES FOR PIRT CLOSURE
{q q
Ranking Basis for Phenomena h;r Test Phenomena 3 [m ScaEng Sensitivity First Analysis Evaluation G
Component Analyses Testing Studies Principles Engineering Report Report
- (
or Volume Phenomena / Parameter (Ref. 20)
Results Ref. No.
Calc Judgment Ref. No.
Ref. No.
5' Inside Containment:
O S
- 1) Break Source A Mass and Energy yes yes 5 Section 10 17 B Direction / Elevation yes 5 Section 9 17 5 Sedron 9 C Momentum yes 17,9 Section 3.9 5 Section 9
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D Density 5 Section 9 g-E Droplet / liquid flashing yes yes k
- 2) Containment Volume A Mixing /Stratificatkm yes 17,5 Section 9 yes 17 5 Sectioa 9 3
B Intermmpartment Flow yes 17,5 Section 9 17 5 Sectian 9 lc C Gas Compliance yes H
D Fog yes yes 22 E Ilydrogen Release yes yes yes
- 3) Containment Solid A Liquid Film Energy Tran.[mrt yes yes yes Ileat Sinks B Vertical Film Conduction yes yes yes yes 29 39 (Steet & Concrete)
C 11orizontal Film Condudion yes yes yes 5 Section 9 D Internal IIcat Sink Conduction yes 5 Sectkin 5 E Ileat Capacity 5 Section 5 yes F Condensation yes yes 5 Section 10 9
G Convection from cnmt volume yes 9
11 Radiation from cnmt volume yes yes
- 4) Initist Conditions A initial Temperatum 5 Section 5 yes B Initial Ilumidity 5 Section 5 yes C Initial Pressure 5 Section 5 yes
- 5) Break Pool A Mixing / Stratification yes B Condensation /Evaporatkm yes yes C Convectkm from cnmt volume yes yes D Radiatkm from cnmt volume yes yes E Pool Conduction yes yes F Thx= ling yes
E*
APPENDIX A -
SUMMARY
OF BASES FOR PIRT CLOSURE Ranking Basis for Phenomena
..,E Test Phenomena Scaling Sensitivity First Analysis Evaluation
%m Component Analyses Testing Studies Principles Engineering Report Report
{
4 or Volume Phenomena / Parameter (Ref. 20)
Results Ref. No.
Calc Judgment Ref. No.
Ref. No.
E' og
- 6) IRWST A Mixing / Stratification (gas & water) yes yes n
B Condensation yes yes
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C Convectmn yes yes c
D Radiation yes yes 5
E Conduction in liquid yes yes a
F Liquid Level yes yes
&T Containment Shell:
o
- 7) Steel Shell A Convection from enmt volume yes yes 9
5 B Radiation from cnmt volume yes W
C Condensation yes 9
D Inside Film Conduction yes 39 E Inside Film Energy Transport yes F Conduction through shell yes yes 5 Secti(m 10 C lleat Capacity yes
.yes 5 Section 10 11 Convection to riser annulus yes yes 9
40 I Radiation to baffle yes yes yes 22 l Radiation to chimney yes yes yes yes K Radiation to fog / air yes yes yes yes L Outside Film Conduction yes
>n 9
39 M Outside Film Energy Transport yes yes 43 17 N Evaporation to riser annulus yes yes 5 Section 10 9
40
- 8) PCS Cooling Water A PCCWST Water Flowrate yes yes 17 5 Section 7 B PCCWST Water Temperature yes yes 43 17 5 Section 7 C Film stability and coverage yes 5 Section 7 yes 5 Sectitm 7 5 Section 7,39 D Film Stripping yes yes yes E Film Drag yes yes Outside Containment::
- 9) Riser Annulus &
A PG Natural Circulation yes yes 5 Section 5 17,9,11 40 Chimney Volume B Vapor Acceleration yes yes C Fog yes yes D Flow Stability in Chimney yes yes yes
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1 g3 APPENDIX A -
SUMMARY
OF BASES FOR PIRT CLOSURE I
i h
Ranking Basis for Phenomena a>
((
Test Phenomena 8m Scaling Sensitivity First Analysis Evaluation gg Component Analyses Testing Studies Principles Engineering Report Report 4
or Volume Phenomena / Parameter (Ref. 20)
Results Ref. No.
Calc Judgment Ref. No.
Ref. No.
5' og
- 10) Baffle A Convection to riser annulus yes yes yes 17 n
B Convection to downmmer yes yes yes 5"
C Radiation to shield building yes yes yes
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D Conduction through baffle yes yes yes e
E Condensation yes yes yes 17 to F IIeat Capacity yes yes G Leaks yes yes yes ep
- 11) Baffle Supports A Convection to riser air yes yes B Radiation from shell yes yes 3
C Conduction from shell to baffle yes yes D 11 eat Capacity yes yes
- 12) Chimney Structure A Conduction into chimney yes yes yes B Convection from chimney air yes yes yes C 11 eat capacity of structure yes yes D Condensation on chimney yes yes yes
- 14) Shield Building A Convection to downcomer yes yes yes B Conduction thruegh shield bldg yes yes yes C Convection to environment yes yes D Radiation to environment yes yes
- 15) External Atmosphere A Temperature yes 5 Section 5 14 B liumidity yes 5 Sedian 5 14 C Recirculation yes 41,5 Section 6 D Pressure Fluctuations yes 42,5 Section 6 42,5 Section 6
.