October 25-27, 2005 11th International QUENCH Workshop 1 Top Flooding Experiments and Modeling Estelle Brunet-Thibault (EDF), Serge Marguet (EDF)

october 25-27, 2005 11th International QUENCH Workshop1

Top Flooding Experiments and Modeling

Estelle Brunet-Thibault (EDF), Serge Marguet (EDF)


1

Top Flooding Phenomena


• Hot leg injection

• In BWR quenching ring in upper plenum internals

1.1 Scope of the top flooding physics in PWR

• Condensation in steam generator tubes


1.2 Flooding Patterns

Two quench fronts and two flooding patterns

Co-current flow

• Co-current flow pattern similar to bottom flooding

Counter-current flow

• Liquid single phase • Annular counter-current flow : water

film and dispersed steam

The location of the drying point of the water film is needed to consider the sharp change in the heat transfer between the region of dry and wetted wall and as a consequence to get the steep temperature decrease during quenching.

Bottom inlet water

Lower quench front

Upper quench front

Fuel rods

Single phase liquid

Single phase liquid

Nucleate Boiling

Transition boiling

Film boiling

Inverted annular flow

Dispersed flow

Single phase steam

Water annular downflow with dispersed steam upflow

Liquid film drying out

Top inlet water


1.2 Counter Current Flow Limitation

① ② ③ ④ ⑤ ⑥ ⑦

Water inlet

Water outlet

Steam inlet

Water inlet

Steam inlet

Water inlet

Steam inlet

Water inlet ② ③ ①

The phenomena

Vertical :

Horizontal :


2

Top Flooding Experiments


2.1 ECCHO B – PERICLES (1/3)

Flooding experiments at CEA Grenoble (France – ended 1991)

2 tests facilities (Nuclear technology vol.107)

• ECCHO B 37 rods Non heated shroud

• PERICLES 127 rods Heated shroud

Geometry RSM1.1

• Triangular pitch 12.23 mm

• Hydraulic diameter : 8 mm 33% reduction compared to PWR geometry




Test characteristics

• Pressure : 1, 2, 2.3 and 4 bars• Initial temperature : 300 and 600°C• Injected flow rates : 3.6, 5.4, 8.1 g.s-1.cm-2

• Stainless steel cladding • Length of heated rods : 3.6 m

Test matrix

• 49 bottom flooding tests• 4 top flooding tests• 16 combined injection

flooding tests

Measured variables

• Inner cladding temperature of heated rods• Fluid temperature• Pressure• Water injected flow rates• Electrical power




Findings

• A combined top/bottom injection does not significantly improve the cooling efficiency due to high vapor velocities.

• Counter-current flow rapidly limited

Observations

• The tests are not sufficiently prototypics to have conclusion for power plant applications

Pressure too low The geometry is not a characteristic PWR geometry


2.2 UPTF experiments (1/3)

UPTF Flooding experiments (Germany – ended 1991)

Description (Nuclear Engineering and Design 133)

• Full-scale (1:1) representation of : Upper plenum including internals Downcomer Four connected loops

• Exact representation of : Core barrel including core by-pass Upper end-box and upper part of

fuel element (0,8 m)

• ECC injection into : 4 cold legs 4 hot legs Downcomer at two regions




Description

• Core simulated by means of controlled steam and water injection supplied from external sources

• Reactor coolant pumps and steam generators replaced by simulators• Breaks of variables sizes can be simulated in the hot and in the cold leg

respectivelyTest characteristics

• Primary system pressure : 20 bars• Primary system temperature : 485 K• ECC injection

50 to 600 kg.s-1 for each hot leg injection port 50 to 1100 kg.s-1 for each cold leg injection port

• Onset of flooding at 10.5 bars




Hot leg injection – Findings

• ECC delivery to the core occurscompletely without delay

• Water breakthrough occurs in frontof the injecting hot legs

• Rate and area of water breakthroughincrease with decreasing core simulator steam injection

Observations

• Heterogeneous distribution of steam and water = geometry dependant phenomenon

• Only LOCA scenarios


2.3 PARAMETER (1/2)

Flooding experiments at LUCH Institute (Russia)

Sources:

• Presentation on 8th CEG-SAM meeting “Fuel assembly tests under severe accident conditions” LUCH Institute

Description of the facility

• 19 rods Geometry VVER 18 heated rod, 1 central rod non heated Zr1%Nb cladding UO2 pellets Heated length : 1.275 m

• Tungsten heater elements• Hexahedron shroud• ZrO2 insulation


2.3 PARAMETER (2/2)

Flooding experiments at LUCH Institute (Russia)

Scenario

• Main coolant piping break with simultaneous ECCS failure• Restoring one ECCS channel at the stage of severe accident at

Tclad > 2250K• The core water flooding from top and bottom with total flow rate of

200 kg.s-1

Advantages

• Severe accident scenario Cladding temperature Water flow rate

• Prototypics rods


3

Top Flooding Modeling


3.1 Top flooding modeling in CATHARE (1/2)

General description

• The model assumes a wetted wall with A descending liquid film upstream of the quench front A steep wall temperature gradient in the quench front region A hot dry wall downstream of the quench front

Characteristics (1/2)

• This model takes into account : Nucleate boiling in the descending film upstream of the quench

front Critical heat flux at the quench front Transition boiling immediately downstream of the quench front


3.1 Top flooding modeling in CATHARE (2/2)

Characteristics (2/2)

• This model takes into account also : Heat transfer to droplets sputtered off the film in the quench front region Dispersed flow film boiling and wall-to-vapour heat transfer further

downstream of the quench front

Determination of the quench front velocity

• Application of a local CCFL criterion of the Wallis type at the upper quench front with the critical vapour velocity given by :

L is correlated on the basis of PERICLES and REWETT II experiments

5,0

G

GL

Vcr

gL

v

338,17,6 HDL


3.2 Top flooding modeling in ATHLET-CD

These information are kindly transmitted by GRS (Christine Bals)

ATHLET quench front model

• Determination of the upper quench front velocity Yamanouchi correlation

(only valid for vertical geometries)

Calculation of the Leidenfrost temperature with Schröder-Richter approximation

Validation of this model on FLECHT, FEBA, LOFT, SCTF and CCTF experiments

ATHLET drift flux model

• Determination of the amount of liquid available for top quenching Parameters of this model are influenced by UPTF data concerning CCFL


4

Conclusion


CONCLUSION

The aim of our study

• Elaboration of a severe accident top flooding model integrable in ASTEC and in MAAP4 including:

A model to determine the amount of water available for core quenching

A model to calculate heat transfer between top down flow and upper internals

A model to calculate heat transfer in the core upstream the upper quench front

• Validation of the top flooding model PARAMETER top and bottom flooding experiment PERICLES top and bottom flooding experiments

Documents

October 25-27, 2005 11th International QUENCH Workshop 1 Top Flooding Experiments and Modeling Estelle Brunet-Thibault (EDF), Serge Marguet (EDF)