INSTAB
Couplings and instabilities in reactor systems
Markku Puustinen, Jani Laine, Antti Räsänen, Lauri Pyy,
Eetu Kotro, Vesa Tanskanen, Elina Hujala
Lappeenranta University of Technology
SAFIR2018 Interim Seminar, March 23-24, 2017, Espoo
INSTAB – Motivation
Condensation pool is an in-containment heat
sink in BWRs
Steam discharges are received from
Upper dry well
safety/relief valves and blowdown pipes
Water discharge from containment spray and
Residual Heat Removal (RHR) return lines Blowdown pipes
Thermal stratification of the condensation
pool limits the volume of water absorbing
heat
• Full capacity of the pool not used
containment overpressure risk
Wet well
Lower dry well
The INSTAB project studies the stability of
stratification and mixing mechanisms in pool
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INSTAB – Enhancement of safety
Earlier LUT POOLEX and PPOOLEX projects have addressed
the blowdown pipe performance
• direct contact condensation (DCC) leads to instability –
chugging – which causes large dynamic mechanical loads
INSTAB focus in 2015-2016 has been on
• modelling of stratification and mixing in relation to operation
of Safety/Relief Valve (SRV) spargers and Residual Heat
Removal (RHR) nozzles
• supporting the development of the Effective Heat Source
(EHS) and Effective Momentum Source (EMS) models,
originally developed by KTH
• already successfully developed for blowdown pipes
• supporting CFD code modelling of direct contact
condensation situations at LUT and VTT
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INSTAB – PPOOLEX test facility
Height 7.45 m, diameter 2.4 m,
volume 31 m3
• Max pressure 0.5 MPa
• Steam from the nearby PACTEL
facility (1 MW)
Pressure, differential pressure,
temperature, flow, strain and relative
humidity measurements
• kHz range measurements and data
acquisition
Triple high speed camera and stereo
Particle Imaging Velocimetry (PIV)
systems
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INSTAB – Sparger tests: stratification erodes away
Pool water above sparger “mouth” warms up ~uniformly
Simultaneously, the thermocline moves downwards
No chugging, condensation of small steam jets
Complete mixing of the water pool through an erosion process
was achieved in the end
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INSTAB – Erosion is due to large-scale turbulence
Flow field in the vicinity of the
thermocline can be resolved
by PIV
Large eddies exist around the
elevation of the thermocline
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INSTAB – Direct Contact Condensation model
development with NEPTUNE_CFD
CFD modelling of DCC requires that the interfacial area density
between the liquid and vapour phases is resolved
• either by using a very dense computational grid, or
• by applying a special interfacial instability model
Interfacial area density has been modelled in the NEPTUNE_CFD
code with the help of a plausible and simple solution of RayleighTaylor instability (RTI), originally introduced by Pellegrini et al.
• The model seems to perform qualitatively well enough
Simulations of a plexiglass blowdown
pipe case in PPOOLEX have been
done in order to further investigate the
effect of the RTI model on calculation
results of DCC
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INSTAB – Highlights from CFD calculations
The condensation rate is higher and the shapes of fully expanded bubbles are
different in the case with the Rayleigh-Taylor instability model (left) compared to the
case without it (right)
• In general the RTI model seems to give results closer to reality with a low
resolution mesh
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INSTAB – Pressure data interpretation
Frequency in FFT [Hz]
Possible source of frequency
0.5–3.5
chugging frequency
11–12
vessel
41–45
natural frequency of the bubble
80–83
vessel
150
vessel
250–300
bubble/blowdown pipe
SAFIR2018 Interim Seminar
Pressure data from earlier
PPOOLEX tests were fast
Fourier transformed (FFT) and
compared to the bubble
oscillation results of the
NEPTUNE_CFD simulations
It is possible to identify from
the test data frequencies
caused by the blowdown pipe,
test vessel and bubbles
themselves
This helps DCC modellers
focus on reproducing bubble
condensation correctly
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INSTAB – Highlights from RHR nozzle tests
To obtain data for extending the EMS and EHS models to
RHR system nozzles a series of tests was done in
PPOOLEX
The effects of nozzle orientation, T in the pool, injection
water temperature and injection water mass flow rate on
mixing efficiency were studied
• Thermally stratified condition was created by injecting
steam into the pool water via the sparger pipe
• Two regions with clearly different water
temperatures and a narrow thermocline region
between them developed in the pool
Compete mixing was achieved with the horizontal
orientation of the RHR nozzle
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INSTAB – Highlights from RHR nozzle tests
With the vertical orientation of the RHR nozzle mixing was otherwise successful
but incomplete above the nozzle elevation
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INSTAB – CONCLUSIONS
INSTAB tests have generated a large database on
suppression pool phenomena
• Data on stability of pool stratification and
efficiency of pool as heat sink
• Strong contribution to the development of the
EMS and EHS models for blowdown pipes,
SRV spargers and RHR nozzles
DCC modelling in CFD (Fluent, NEPTUNE_CFD, OpenFOAM) evaluated on
the basis of the PPOOLEX tests
• Rayleigh-Taylor instability modelling for the interface is very promising
Closures for the EMS/EHS models: in 2017-2018,
• PPOOLEX spray tests will be conducted
• separate effect tests, where momentum induced by steam injection through
a sparger is measured directly
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Thank you for your attention
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INSTAB – Plans for steam jet testing
In 2017, a separate effect test facility will be
constructed and momentum created by steam
discharge through sparger holes will be measured
directly in order to provide closures for the EMS
model development work for spargers
Flexible junction
Once the EMS and EHS models have been
validated for spargers, they can be implemented
also to other codes than GOTHIC
Force
measurements
Pipe fixed and allowed to
rotate around this point
Steam
injection
~300
~500
Open ceiling
~1000
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