Percentile Confidence Level

Y. Liao and S. Guentay

2nd European MELCOR User Group (EMUG) Workshop

Modeling Experience on Disruption of Hotleg

Counter-Current Flow by Thermally-Induced SGTR

2nd EMUG Workshop, NRI, Czech Republic, March 1-2, 2010

Outline

• Introduction to SBO hotleg counter-current flow

• Disruption of hotleg counter-current flow by induced SGTR

• MELCOR calculation results

• Impingement heat transfer for a jet issuing from induced rupture

• Summaries

Introduction to SBO hotleg counter-current flow

SBO severe accident sequence

• Hotleg voided by venting coolant through pressurizer

• Coldleg loop seal plugged with water

• High pressure primary side, dry SG secondary side

Hotleg counter-current natural circulation

• Transfer heat to hot leg, surge line and SG tubes

• Hot flow counter-current to cold flow

• Mixing of hot and cold gas in SG inlet plenum

• Flow recirculation through SG U-tubes

2nd EMUG Workshop, NRI, Czech Republic, March 1-2, 2010

SBO thermally induced SGTR

•SGTR might be induced by thermal

challenge in presence of tube degradation

•Distribution of SG tube degradation exhibits statistical features in defect location and size

•SGTR probability has been analyzed by the PRA technique (NED, 2009)

•Around 2% median probability of tube rupture was predicted for new generation SGs

10^-4 10^-3 10^-2 10^-1

0 10 20 30 40 50 60 70 80 90 100

Mean=0.025, EF=3.8

SGTR Probability

Percentile Confidence Level

Objectives and approach

Suction effects of the rupture flow

• Disrupt hotleg counter-current natural circulation

• Change SG inlet plenum mixing and recirculation flow pattern

Objectives

• To develop modeling experience for post-tube-rupture scenario

• To examine thermal-dynamic properties of the rupture flow and heat transfer

Approach

• MELOCR 1.8.5 applied to Westinghouse power plant

• This is an exercise to set up the modeling technique

• To predict the actual response requires CFD and experimental data

2nd EMUG Workshop, NRI, Czech Republic, March 1-2, 2010

Disruption of hotleg counter-current flow

Scenario description

•Prior to tube rupture

• Hotleg counter-current flow is established

• A degraded tube is induced to creep failure at top of tubesheet

•Post tube rupture

• Rupture flow enlarged by crack opening process

• Flow reversal at hotleg lower part due to suction from SG

• SG tube recirculation caused by suction from ruptured tube

• Gas mixing and mixture pulled into ruptured tube

Assumptions in MELCOR simulation

•Prior to rupture, pairs of flow pathsare used to simulate counter-current flow and SG inlet plenum mixing

•Rupture area develops from 0 to 50% tube cross sectional area in about 1 minute

•When MELCOR predicts flow reversal in the hotleg lower part, each pair of flow pathsare merged into one to avoid unphysical local circulation flow

201

216 190 200190

215241 221242 222

242 222243 223

243 223244 224

228 248228 248

227 247227 247

226 246126 246

235 236 220 201

216 190 200190

215241 221242 222

242 222243 223

243 223244 224

228 248228 248

227 247227 247

226 246126 246

235 236 220 Ruptured tube

2nd EMUG Workshop, NRI, Czech Republic, March 1-2, 2010

MELCOR prediction of post-tube-rupture flow pattern

Hotleg lower part

• Cold gas flow is reversed and replaced by hot gas

SG inlet plenum

• Hotleg gas mixed with recirculating gas

SG tubes

• Recirculating flow caused by suction from both ends of ruptured tube

Ruptured tube

• Incoming hot gas from inlet plenum, cold gas from outlet plenum

201

203

190 200

204 200

190

241242 222242243 223

244 224

228 248

227 247

126 246

224 244 225

245

235 236

235

211 212

Ruptured tube

Flow reversal in hotleg lower part, when rupture area develops to above 40%

tube cross sectional area

203 213 213 203

204

214 ₂₀₄

214

200

224 244 225

245 125 245

235

211 211

212 ₂₁₂

At tube rupture At flow reversal

Upper hotleg is closer to rupture

2nd EMUG Workshop, NRI, Czech Republic, March 1-2, 2010

Rupture flow rate, originated from SG inlet and outlet plenums

201

203

216

213

190 200

204 214

200

190

215241 221242 222

242 222243 223

243 223244 224

228 248228 248

227 247227 247

226 246126 246

224 244 225

245 125 245

235 236 220235

211 212

Ruptured tube

SG tube recirculation flow: a recirculation ratio about 2,

increasing with time due to increase of incoming gas temperature

201

203

216

213

190 200

204 214

200

190

215241 221242 222

242 222243 223

243 223244 224

228 248228 248

227 247227 247

226 246126 246

224 244 225

245 125 245

235 236 220235

211 212

Ruptured tube

At tube rupture

2nd EMUG Workshop, NRI, Czech Republic, March 1-2, 2010

SG inlet plenum mixing, governing rupture flow temperature

At tube rupture

201

203

216

213

190 200

204 214

200

190

215241 221242 222

242 222243 223

243 223244 224

228 248228 248

227 247227 247

226 246126 246

224 244 225

245 125 245

235 236 220235

211 212

Ruptured tube

Gas temperatures:

Temperature jumps caused by tube rupture and disruption of counter-current flow

At tube rupture At disruption of counter-current flow

2nd EMUG Workshop, NRI, Czech Republic, March 1-2, 2010

Brief introduction to jet impingement heat transfer

Impingement of high energy jet issuing from induced rupture may heat up and affect

creep behavior of adjacent tube

Flow structure of impinging jet

•Development of jet boundary

•Gas undergoes expansion and acceleration

•Pressure loss through passing normal shock

•Heat transferred via a viscous boundary layer

Free jet regime Jet boundary

Refleshockcted Intercepting shock ffodntaS kcohs

L

Wall jet regimeImpingement regime Heat transfer

boundary layer

M1>1, M2<1,

p_∞

p1 p₂

0, 0

P T

r

z Me=1, p_e

D

u

∆

Adjacent tube

Ruptured tube

Tw

α θ

Stagnation point

P=pressure T=temperature M=Mach number D=rupture size

L=tube-to-tube distance

A mechanistic jet impingement heat transfer model is developed

•Heat transfer increases with larger pressure ratio and smaller nozzle to surface spacing

•Dependent variables can be provided by MELCOR

•Being validated by experiments and applied to induced SGTR conditions

10³

Nu

pe/p

∞=1.5,z/D=0.5 pe/p_∞=1.5,z/D=0.75 pe/p_∞=3.5,z/D=0.5 pe/p

∞=3.5,z/D=1.0 Test data

Theoretical curve z/D=0.5

z/D=0.75

z/D=1.0

10³ 10⁴

Nu

Pressurized SG Depressurized SG z/D=0.5

0.75

1.0 0.5 0.75 1.0

1/ 3 1/ 2 1/ 4

1/ 2

Pr ( / ) 1

ln( )

( / ) 2

sp L

D RT p

Nu r D p

ν

∞

 

=  

 

Nu slightly decreases with temperature due to density property effect

2nd EMUG Workshop, NRI, Czech Republic, March 1-2, 2010

Summaries

•MELCOR modeling experience was developed for post-tube-rupture scenarios

•Disruption of hotleg counter-current flow is predicted by MELCOR – To occur at rupture area about 40% tube cross sectional area – To alter SG inlet plenum mixing and recirculation

•Validated CFD predictions are needed to determine mixing parameters for MELCOR modeling

•A jet impingement heat transfer model is developed and may be implemented into

MELCOR to evaluate heatup and creep behavior of an adjacent tube impinged by

the rupture flow