Effect of non-condensable gases on reflux condensation in

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PhD candidate: Filip Janasz

Thesis supervisor: Prof. H.-M. PrasserCo-examiners: Prof. K. Vierow Kirkland, Prof. A. Mityakov

14.05.2019 1

Effect of non-condensable gases on reflux condensation in nuclear steam generator tubes

Filip Janasz PhD Examination

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Presentation outline

Reflux condensation Experimental facility and method Experiment results Numerical

CalculationsConclusions and Outlook


Reflux condensationin context of PWR

• SG U‐tubes are drained of water

• Steam condensation may become a major heat removal path

• Non‐condensable gases ingress is likely to occur. Possible sources: emergency accumulators (N2),oxidizing fuel rods zirconia cladding (H2)

• Loss of coolant accidents (LOCA) ‐ high‐pressure, depending on the reactor design, between 4 and 10 MPa

• Loss of residual heat removal (RHR) system during low power / shutdown conditions, mid‐loop operation ‐ pressures close to containment pressure


Reflux condensationConceptual Model

T

XNC

Dry region

Condensation onset

Liquid film NC gas fraction

Temperature

Gas mixtureCondensateCoolant Coolant

Parameters of interest:

• Temperature distributions

• Liqud film behavior

• Codensation rate

• Pressure

• Coolant temperature increase


Experimental facility and method


PRECISE facility

14.05.2019Filip Janasz PhD Examination 6

Precise Reflux Condensation Investigation Setup


PRECISE facility



PRECISE facilityInstrumentation


Coolant temp. outlet

Coolant temp. inlet

Heating power

Test tube pressure

Coolant mass flow

Coolant pressure

Coolant & outer wall temp. at 4 elevations, 0° and 180°

Heater rod, water and steam temp.


PRECISE facilityInstrumentation

Stainless‐steel test tube, 20 mm inner diameter:

• 11 centerline thermocouples (type K, ⌀ 0.5 mm)• 4 GHFS sensors (locations A, B, C & D) • 4 inner wall temperature thermocouples • 4 outer wall temperature thermocouples • 1 movable film probe (MFP) & thermocouple• 1 fixed part of MFP

In total 20 thermocouples, 4 GHFS sensors & 1 film sensor embedded


Heat flux sensorProperties

• Working temperature up to 1300 K

• Sensitivity up to 0.4 mV/W

• Response time estimated in the range of 10‐9 s

• 0.2 ‐ 0.3 mm sensor thickness

• 95‐100 mm2 area

• <0.1 mm constituent layer thickness


Heat flux sensorOperation basics

senitivity

sensorarea

heatflux ]

• Artificial thermoelectric anisotropy

• Transverse Seebeck thermoelectric effect


Heat flux sensorInstallation

• GHFS mounted in the pocket

• Protected by 0.5 mm of stainless steel

• Pocket filled with electrically

insulation, heat conductive epoxy

4.33∗

. 12 16∗

Groove eroded for GHFS

Centerline thermocoupleguiding tube


Heat flux sensorInstallation

Plug with embedded inner & outer thermocouples Plug laser welded forming a seal


Movable film probe

Fixed electrode array

Traversing thermocouple/ electrode


Movable film probeImplementation


Movable film probePrinciple

Tem

pera

ture

[°C

]

Horizontal position [mm]


Movable film probeSignal relation

TemperatureElectrical signal – low pass filter applied (cutoff frequency 5 )

MFP

vol

tage

resp

onse

[V]

Time [s]

Tem

pera

ture

[°C

]


Experimental results


Steady – state experimentsProcedure

Facility preconditioning:

• Air evacuated from test tube and SG

• Predefined amount of NC gas and water injected

• Heating started

• Set‐point steady‐state conditions achieved within 30 – 90 minutes

• Data acquisition engagedTime [s]

Press [ba

r]

Heating start Steady state reached

Measurement


Steady – state experimentsGas stratification – N2

A B C • Gas mole fraction calculated based on pressure and temperature readings

• Three distinguisible regions

Condensation zone Mixing zone NC gas plug


Steady – state experimentsGas stratification – He

Condensation zone Mixing zone NC gas plug

• For light gas, the decline in temperature is immediate and the profile is sharp

• Heavily stratified gas distribution in the tube

A B C


Steady – state experimentsIncreasing NC gas content

• Increasing amount of N2 expands the plug (6 bar, 20˚C wall ΔT)

• Same behaviour observed for He

Avg. N2 mole fr.


Steady – state experimentsN2 – He mixture compositionNC mixture composition effect on the mixing zone at constant avg. NC mole fraction

N2

He

N2

He

Unstable stratification

Stable stratification


Steady – state experimentsMixture composition effects

• Increasing amount of N2 in NC mixture results in larger mixing zone

• Vertical resolution limited to distance between TC (100 mm)


Steady – state experimentsMixture composition effects

• Increasing amount of N2 in NC mixture results in greater condensation rates

• Higher degree of mixing between steam and NC


Steady – state experimentsReconstructed profiles

Normalized temperature and heat flux vertical profile in the mixing zones, multiple experiments

For all NC mixture compositions, similar temperature / heat flux drop off observed

Saturation temp.

~coolant temp.


Steady – state experimentsHorizontal temp. profiles

Condensation zone

Mixing zone

NC plug zone

• Significant difference between zones

• For all NC mixture compositions horizontal temperature profiles similar

• Slight temperature variation in the mixing zone due to uncertain location in the zone

Tube

wal

l

Tube center


Steady – state experimentsHeat flux

Condensation zone

Mixing zone

NC plug zone

• Similar behaviour for N2 and He, • Significant difference between the

zones• Slight heat flux variation in the

mixing zone due to uncertain location in the zone


Continuous – injection experimentsProcedure

• Facility preconditioned for pure‐steam experiments

• After reaching steady state, valve to NC gas supply tank is opened

• Flow of NC gas due to ΔP 0.2 or 0.5 bar, molar flux of 7.8 x 10‐5 and 22.7 x 10‐5 mol/s

• NC gases mix with steam and are carried upwards

• NC gases accumulate at the top of the tube forming the plug which grows downwards

• Experiments continues until condensation ceases

Test tube& coolant jacket

NC gas tank

Steam generator

NC gas plug

Control valve

NC plug growth


Continuous – injection experimentsSingle TC perspective

• Gradual temperature decrease as the mixing zone passes

• Increase of temperature oscillations towards maximum at the mixing zone centre

• Slow plug expansion mediated by limited gas addition – good temporal resolution


Continuous – injection experimentsTC array perspective

• NC plug growth at 4 bar, 20˚C wall ΔT• 30 x speed (30 FPS video, data captured at 1Hz)

145

140

135

130

125

120

Temp [°C]

145

140

135

130

125

120

Temp [°C]

200 400 600 800 1000 1200

Height [mm]

200 400 600 800 1000 1200

Height [mm]

N2 He


Continuous – injection experimentsPlug growth

• NC mixing zone passes the thermocouple array ‐ spatial resolution not limited

• Velocity of plug growth calculated based on:

o time of temperature decrease observed by thermocouples

o pressure drop in the NC gas supply tank


Continuous – injection experiments

• Plug growth observed with temperature and heat flux measurements

• Mixing zone length calculated based on plug velocity and passage time:

• Mix. zone size for N2 varied between 200 and 300 mm, He ‐ 80‐120 mm

∗


Continuous – injection experiments

Pure N2

N2 & He 1:1 mixture

Pure He


Continuous – injection experimentsFrequency domain - spectrograms

He N21:1 Mixture

Mixing zone arrival


Continuous – injection experimentsFrequency domain – power spectra

Pow

er s

pect

rum

[dB

]

Mixing zone


Continuous – injection experimentsFrequency domain – NC gas impact

5 10 15 20 25 30 35 40 45 50frequency [Hz]

10

20

30

40

50

60

70

80N2 Mix He

0 1 2 3 4 5f [Hz]

0

10

20

30

40

50

60

70

80

Variation in the low end of the spectrum

N2

He


Numerical calculations


Numerical simulationRELAP5/MOD3.4 nodalization


Gas phase recirculation enabled

Numerical simulationNodalization 2.0


Numerical simulationApproaching solution

Evaporation SG

Condensation SG

Condensation Test Tube

Condensation all volumes

Mass flows stabilized at:

• Evaporation flow SG 0.7773 g/s

• Cond. flow SG 0.0001407 g/s

• Cond. flow Test Tube 0.77716 g/s

Mas

sflo

w[k

g/s]

0.777307 g/s


Numerical simulationPure steam

Experiment: • Pressure as controlled variable• Condensation mass flow – free variable

RELAP5 calculation: • Heater power – boundary condition• Pressure – free variable

Result:• Good agreement• Slight pressure overestimation• Minimal impact of nodalization


Numerical simulationNC gases present

Result:• Significant pressure

overestimation• Large impact of

nodalization


Numerical simulationMass balance and distribution

Pure steam With NC gases


Numerical simulationNC disitribution

Nodalization 1

Nodalization 2

Tube center Annulus

• Plug formation as in experiments

• Increased NC content in tube center


Conclusions and outlook


Conclusions• 3D effects in SG tubes impact reflux condensation

• Strong gas stratification is observed

• NC gas molar weight impacts stratification stability

• Unstable stratification increases condensation rates

• Horizontal temperature profiles not affected by NC gas species

• Vertical temperature and heat flux profiles behave similarly

• No dominant frequencies distinguishable in heat flux signal

• Most of the detected heat flux oscillations frequencies below 5 Hz

• RELAP5/MOD3.4 ‐ accurate for pure steam condensation

• Mass conservation problems for NC gas species

• Splitting the calculation domain to enforce recirculation of gas phase – not realistic NC gas distribution


OutlookExperiments:

• Tests with full U‐tube geometry

• Tests with steam/gas flow – through

• Tests with pressure approaching 4 – 10 MPa

Measurement technique:

• Condensate film sampling – gas solubility

• Further developments of the film sensor probe

• Investigate GHFS with higher mV/W sensitivity

Numerical methods:

• Comparison of alternative 1D codes performance (TRACE)

• Observed importance of 3D effects – more advanced numerical analysis (CFD)


Thank you for your attention!

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Effect of non-condensable gases on reflux condensation in