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Yangli Chen, Huimin Zhang, Walter Villanueva, Weimin Ma, and Sevostian

Bechta

Division of Nuclear Power Safety Royal Institute of Technology (KTH)

Stockholm, Sweden

Study on the Nodalization Effect of MELCOR for Simulation of Nordic BWR

EMUG, Switzerland, April 3-5, 2019

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2

• Motivation

• Features of Nordic BWR

• MELCOR models

• Simulation results

• Concluding remarks and perspectives

Outline

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• Analysis of current SAMG of Nordic BWRs Motivation

• Employ the cavity (lower drywell) flooding as a SAM measure to promote melt

fragmentation and quenching, and

formation of a coolable debris bed on the drywell floor (ex-vessel coolability).

• MELCOR provides the initial and boudary condition for a coupled calculation

coupled calculation

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4

• Thermal power: 3900WMth

• Vessel diameter: 6.4m

• Small containment

• Forest of penetrations

Design features of a Nordic BWR

Volume: 1/5 of that of PWR

Inerted with N2 for H2 risk

Pressure supression with wetwell (condensation pool)

(5)

• MELCOR 2.2.9541 is used for the integral simulation of the whole plant.

• A 2D axisymmetric geometry is used to model the RPV.

• A hemisphereical shape is used to model the lower head.

• Penetrations failure deactivated.

• Scenarios: Station Blackout (SBO);

SBO combined with LBLOCA.

MELCOR Modelling

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6

Three core meshing shemes

Coarse mesh 6rings X 18levels

Medium mesh 15rings X 46levels

Fine mesh

21rings X 60 levels

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Three core meshing shemes

CVH

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8

MELCOR calculation results

• Accident progression

• Calculation matrix

Station Blackout (SBO)

SBO with large break LOCA at steamline with area of 0.1m2

SBO SBO+LOCA 6-ring SBO-6 LOCA-6 15-ring SBO-15 LOCA-15 21-ring SBO-21 LOCA-21

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Invessel Accident Progression

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10

Invessel Accident Progression

• SBO

6-ring

15-ring

21-ring

1h 2h 3h 4h 5h

Vessel failure time

6h7min

6h35min

6h42min

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Invessel Accident Progression

6-ring

15-ring

21-ring

1h 2h 3h 4h 5h

Vessel failure time

• SBO

6h7min

6h35min

6h42min

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12

H

2

generation

• SBO • LOCA

H 2 Mass (kg)

Reference:

Y. Chen, H. Zhang, W. Villanueva, W. Ma, and S. Bechta, ‘A sensitivity study of MELCOR nodalization for simulation of in-vessel severe accident progression in a boiling water reactor’, Nuclear Engineering and Design, vol. 343, pp. 22–37, 2019.

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Fine TH nodalization for the core

• More axial levels

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14

• Accident progression

Results

Main event Corse nodes Fine nodes

Initial accident 0 0

Downcommer low water level signal 0.30h 0.32h

ADS activation 0.47h 0.49h

Gap release 0.76h 0.81h

Core support plate failure 1.44h 2.20h

Vessel failure 6.07h 6.52h

Containment venting 10.75h 10.91h

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• CV Water level Results

CV100

CV103 CV104 CV105 CV106

• CV Temperature

Core plate failure

5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

-0.5 0 0.5 1 1.5

Axial power profile

Core height (m)

Power factor

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16

Results

Core plate failure

• H 2 generation

 Fine TH nodalization leads to little more H2 generation.

 H2 from Zr oxidation is similar.

 Difference comes from stainless steel oxidation which is intense at plate failure of fine TH node case.

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Concluding Remarks

• A previous study discusses the effect of core nodalization on the in-vessel progression of a Nordic BWR

• A continuous study taking the TH nodalization into account

 Three meshing schemes and two accident scenarios considered.

 Main events during the accident progression is slightly delayed in finer mesh.

 H2 generation is scenatio-based.

 The TH nodalization for the 6-ring core mesh case is refined axially.

 Main events is also slightly delayed with finer TH nodalization, especially the core plate failure time.

 The power distribution affects the water level and CV temperature for finer case.

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18

• Refining the TH nodalization for the 15-ring core mesh case is tried, but the calculation time step decreases to 10

-4

s.

• The TH nodalization seems not influtiential regarding the in- vessel corium behaviour, therefore for our study interest, it may be not necessary to have finer TH nodalization for the core part.

Perspectives

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This research is supported by:

SSM (Sweden) ENSI (Switzerland)

Acknowledgment

Referenzen

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