Evaluation of the flow mixing in VVER-1000 downcomer under natural circulation.

Bulatom Conference 28-30 May 2008

V. Lefterov, E. Popov Atoma Consult Ltd

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Reasons for fluid mixing evaluation in a reactor downcomer:

• risk of over-criticality in a local volume of the reactor core by low boron concentration • risk of over-criticality in a local volume of the reactor core by low coolant temperature • risk of local pressurized thermal shock (PTS) on the reactor vessel wall Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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Evaluation of flow mixing – How to?

 large scale transparent model experiments  large scale model experiments, using a lot of sensors in the facility  Residence Time Distribution (RTD) method  three dimensional computational fluid dynamics (CFD) calculations.

Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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CFD model – developed by Atoma Consult Ltd Two mutually independent components:

•

Mesh

– represents the geometrical characteristics of the investigated volume; •

Solver –

implements the mathematical apparatus, applied in the analysis.

4 Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

Investigated scenario Boundary conditions – Main Steam Line Break (MSLB) on loop 2

    Calculations with RELAP5/mod3.3; Conservative initial and boundary conditions; Sensitivity study – failure of EFW and FAIV; Results: FAIV failure creates larger temperature difference (80 o C).

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Initial and boundary conditions Table I – Initial and boundary conditions for the CFD calculations

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Calculation results I

Figure I – Velocity distribution Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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Calculation results II

Figure II – Velocity stream lines Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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Calculation results III

Figure III – Temperature distribution in the downcomer and the core inlet Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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Calculation results IV

Figure IV– Loop 2 flow mixing in the downcomer and the core inlet Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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Calculation results IV

Figure V – “Cold spot” rotation in the core inlet Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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Sensitivity study

Table II – Loop inlet velocities for the sensitivity study Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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Sensitivity study – results I

Figure VI – “Cold spot” rotation in the core inlet for the high velocity case Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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Sensitivity study – results II

Figure VII – “Cold spot” rotation in the core inlet for the medium velocity case Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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Sensitivity study – results III

Figure VIII – “Cold spot” rotation in the core inlet for the forced circulation case Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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Sensitivity study – summary

Table III – Summary on the sensitivity study results Bulatom Conference 28-30 May 2008 Atoma Consult Ltd www.atomaconsult.biz

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Conclusions 1.

  

2.

   

Selected scenario – MSLB on VVER-1000

Boundary conditions – RELAP5/mod3.3; Flow mixing and rotation – CFD calculations; Applied software and models are adequately verified and validated;

Results:

Relatively small core inlet area (~5%) from the quarter fed by the broken loop inlet is with very low (0 - 10 %) mixing coefficient; The flow rotation is independent of the velocities magnitudes at constant “intact/damaged” loops velocity ratio; The flow rotation depends

strongly

velocity ratio; on the “intact/damaged” loops Results can be finally validated against large scale model experiments.

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