YoungJong Chung

1
Two Phase Natural Circulation Analysis of
Passive Residual Heat Removal System

• Young-Jong Chung
• Korea Atomic Energy Research Institute

2
CONTENTS

• INTRODUCTION
• SMART Plant and Characteristics
• VISTA Experimental Facility
• ANALYSIS CODE
• INITIAL and BOUNDARY CONDITIONS
• RESULTS and DISCUSSIONS
• CONCLUTIONS

3
INTRODUCTION

• SMART (System-integrated Modular Advanced
  ReacTor)
• Integral type pressurized water reactor
• Maximum core power of 330 MWt
• Inherent safety characteristics Passive safety
  systems
• In order to confirm the enhanced safety of SMART,
  systematic safety analyses have been performed
• Total loss of flow (TLOF)
• Steam line break (SLB)
• Feedwater line break (FLB)
• Small break loss of coolant accidents (SBLOCA)
• In order to evaluate the safety and to optimize
  the design, probabilistic safety assessment (PSA)
  for the SMART basic design has been performed

4
INHERENT SAFETY CHARACTERISTICS

• The integral arrangement
• Eliminates the possibility of LBLOCA.
• Canned motor MCPs
• Eliminates MCP seal leak SBLOCA
• SGs location
• Maintains natural circulation up to the 25 power
• Helical coiled steam generator
• A large volume of passive PZR
• The system pressure is self-controlled using N2
  gas
• Soluble boron-free operation with the low core
  power density
• Large negative MTC

5
INTRODUCTION

• SAFETY SYSTEM
• Passive residual heat removal system
• Emergency core cooling system
• Over pressure protection system (POSRV)
• Shutdown cooling system
• Component cooling water system

6
INTRODUCTION
7
INTRODUCTION
PRHRS

• Consist of Hx, compensating tank, ECT, check
• and isolation valve
• For normal operating condition
• This system is isolated from the secondary system
• For accident condition
• Close MFIV and MSIV
• Open PRHRS isolation valves
• Established a natural circulation flow by gravity
• Characteristics
• Remove decay heat for 72 h without operator
  action

8
INTRODUCTION VISTA Facility

• VISTA DESIGN
• Test facility to simulate the integral type
  reactor
• Scale ratio with respect to the reference plant
• 1/1 for height scale
• 1/96 for volume scale
• Major components
• Vessel Height-4.0m, Diameter-0.17m
• 1 SG cassette,
• 1 MCP
• 1 train of PRHRS
• Primary components are simplified to be a loop
  type in order to perform easily maintenance and
  instrumentation
• Understand the thermal hydraulic responses for
  the integral type reactor

9
INTRODUCTION VISTA Facility

10
ANALYSIS MODEL

• MARS 3.0 code
• 1-D and 3-D system analysis code for thermal
  hydraulic analysis of the light water reactor
  transients
• Developed at the KAERI by consolidating and
  restructuring the RELAP5/MOD3.2 and COBRA-TF
  codes
• SMART specific models
• Helically coiled SG
• Pressurizer with non-condensable gas
• Performed verification and validation using
• Comparison of RELAP5/MOD3 results
• Data of VISTA experiment

11
ANALYSIS MODEL

• Heat transfer model for helical SG
• For tube side of helically coil (Mori-Nakayama)
• For shell side of helically coil (Zukauskas)
• For nucleate boiling (Chen)
• For natural convection (Churchill-Chu)

Vertical
Horizontal
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ANALYSIS MODEL

• Primary coolant flow passage
• Core
• Upper core region
• Main coolant pump
• Shell side of SG cassette
• Downcomer
• Lower header
• Pressurizer
• Gas cylinder

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ANALYSIS MODEL

• Secondary and PRHRS flow passage

• Secondary flow passage
• FW pump
• FW pipe
• SG secondary side
• Steam pipe
• Atmosphere
• PRHR flow passage
• FW pipe
• SG secondary side
• Steam pipe
• PRHR steam pipe
• Heat exchanger
• PRHR liquid pipe

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ANALYSIS MODEL

• Summary of input model
• Volume and Node
• 1-dimensional 233 nodes, 252 junctions
• RCS 0.13 m3
• secondary system 0.035 m3
• ECT tank 9 m3
• Heat structure of the primary system
• Heater rods
• SG helical tubes
• Wall heat structure including heat loss
• Heat structure of the secondary system
• PRHR heat exchanger tubes
• Not model other structure

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ANALYSIS MODEL

• Nodalization of MARS 3.0

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INITIAL/BOUNDARY CONDITIONS

• Assumptions for realistic calculation
• Initial conditions 100 conditions of power and
  flow rate
• Initial Condition for steady state
• Liquid temperature End cavity-14 oC,
  Intermediate cavity-24 oC, Annular cavity-74.5
  oC, PRHR pipe- 8 oC, ECT tank and Hx pipe-27 oC
• Gas temperature Gas cylinder-14 oC,
  Compensating tank-5 oC
• Pressure PRHRS-1bar
• Boundary Conditions for transient
• Isolation valves at compensating tank and gas
  cylinder are always closed
• Power of heater rods is 0 kw
• Chilled water of 0.29kg/s(17.5lpm) is supplied
  the ECT
• Isolation Valve stroking time 4 sec

17
INITIAL/BOUNDARY CONDITIONS
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Event Description

• Initiating Event
• Full power
• Loss of power occurs at 140.0 sec
• Opening the PRHRS valves and closing the
  MFIV/MSIV at 140.0 sec
• General Phenomena
• When the electric power is lost,
• MCP begins to coast-down ? decrease primary and
  secondary flow rate ? formed natural circulation
  ? decrease average pressure and temp. in the
  primary and secondary ? stabilize primary and
  secondary systems
• PZR water level decrease and end and intermediate
  cavities are empty
• Latent heat removes thru the natural circulation
  of primary and PRHRS
• Then, system temperature/pressure/level become to
  stabilize

19
RESULTS and DISCUSSIONS-Experiment

• Power changes with constant FW flow
• System ave. temp. is similar for these cases
• Result is nearly the same
• From these results,
• initial power is not influenced the N.C.
• initial heat capacity is important

20
RESULTS and DISCUSSIONS
Primary Natural Circulation The primary
flow decrease 12 of initial flow early stage
Then, the flow decreases gradually with time
This flow does not measure in the
experiment Secondary Natural Circulation The
flow decrease 10 of initial flow early stage
The natural circulation loop is formed within a
few minutes Cal. result is predicted well
the experimental result Periodic
oscillation (60sec) is appeared in the cal.
21
RESULTS and DISCUSSIONS
Primary Liquid temperature Calculated result
shows to predict well overall trend of
the primary fluid temperature Temp. diff of
calculation is smaller than the exp. and
SG outlet temperature is higher at latter stage
gt calculation seems to under-predict
slightly heat transfer at the SG
under natural circulation
Primary Pressure Primary pr. depends on the
coolant temperature under natural
circulation Primary pr. becomes to decrease
with transient due to the cut-off of the
heat source Depressurization rate of
calculated pr. is higher than the exp.
pressure at the early stage
22
RESULTS and DISCUSSIONS
Secondary pressure Secondary pr. becomes to
decrease with transient this pressure is
function of fluid temperature Peak
calculated pr. is higher than the exp. Pr. gt
to find discrepancy, parametric study is
performed it seems to not vent
completely the PRHR loop in the
facility
Secondary temperature Fluid temp. in the
steam line is a saturated steam Fluid temp.
in the liquid line id a subcooled liquid
Cal. fluid temp. is 20 K higher than the
experiment gt the code seems to under
predict heat transfer at the heat
exchanger or SG
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RESULTS and DISCUSSIONS
PRHRS pressure Trend at PRHRS pr. is the
same as the secondary pr. Liquid temperature in
the ECT Fluid temperature in the upper part
maintains a constant value and that in the
lower part increase with time Cal.
Liquid temp. in the lower part is same with the
exp. result Cal. liquid temp. in the
upper part is slightly lower than the exp.
because heat transfer is small from Hx
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RESULTS and DISCUSSIONS
Surface temperature in the Hx tube Surface
temperature in the upper part is always above
100 ? (106 ?) gt local boiling occurs
at the upper part The lower part maintains a
sub-cooled liquid condition Calculated
temp. at the lower part is over-predicted
since the cal. heat transfer in the Hx is smaller
than the exp.

• In the experiment, quantity of the heat transfer
  converts
• using the enthalpy change between the SG
  inlet and outlet,
• and the secondary flow rate

25
RESULTS and DISCUSSIONS

• Parametric Study Do not model wall heat
  structure

• To find boundary condition effects
• Dont simulate wall heat structure
• Cooldown rate of the pri. system is higher due to
  small initial heat capacity
• Fluid temp. in the ECT is lower than the
  reference case
• Wall heat capacity is important

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RESULTS and DISCUSSIONS

• Parametric Study Chilled water supply or not in
  the ECT

• To find boundary condition effects
• Temp distribution in the ECT doesnt predict
  properly
• Heat transfer is reduced very slightly in the
  heat exchanger

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SUMMARY

• The realistic calculations for the natural
  circulation of the VISTA facility is performed to
  find thermal hydraulic characteristics in the
  PRHRS and capability of the MARS code to predict
  single-, two-phase natural circulation
• The PRHRS accomplishes well its functions in
  removing the transferred heat from the primary
  side in the SG as long as the Hx is submerged the
  water in the ECT.
• Natural circulation of the VISTA facility depend
  on
• Latent heat in the reactor vessel
• Friction and form loss of the geometry
• Heat transfer at the SG and the heat exchanger
• Result of MARS code calculation
• Calculate reasonably the natural circulation flow
  rate
• Under-predicts heat transfer at the SG and the
  heat exchanger
• Over-predicts the primary SG outlet temperature
• Over-predicts the heat exchanger outlet
  temprature
• Appears a periodic oscillation during the two
  phase natural circulation

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SUMMARY

• Find from this study
• Natural circulation flow rate is around 10 of
  the initial flow for the integral reactor
• The local boiling is occurred at the top of the
  heat exchanger
• Dominant heat transfer is boiling and
  condensation for the steam generator and heat
  exchanger under natural circulation condition,
  respectively
• Accurate model of the heat loss and heat capacity
  for the primary system is important for the
  natural circulation