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

1. V. Lefterov, E. Popov Atoma Consult Ltd Evaluation of the flow mixing in VVER-1000 downcomer under natural circulation. Atoma Consult Ltd www.atomaconsult.biz

2. Atoma Consult Ltd www.atomaconsult.biz 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

3. Atoma Consult Ltd www.atomaconsult.biz 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.

4. Atoma Consult Ltd www.atomaconsult.biz 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.

5. 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 oC).

6. Atoma Consult Ltd www.atomaconsult.biz Initial and boundary conditions Table I – Initial and boundary conditions for the CFD calculations

7. Atoma Consult Ltd www.atomaconsult.biz Calculation results I Figure I – Velocity distribution

8. Atoma Consult Ltd www.atomaconsult.biz Calculation results II Figure II – Velocity stream lines

9. Atoma Consult Ltd www.atomaconsult.biz Calculation results III Figure III – Temperature distribution in the downcomer and the core inlet

10. Atoma Consult Ltd www.atomaconsult.biz Calculation results IV Figure IV– Loop 2 flow mixing in the downcomer and the core inlet

11. Atoma Consult Ltd www.atomaconsult.biz Calculation results IV Figure V – “Cold spot” rotation in the core inlet

12. Atoma Consult Ltd www.atomaconsult.biz Sensitivity study Table II – Loop inlet velocities for the sensitivity study

13. Atoma Consult Ltd www.atomaconsult.biz Sensitivity study – results I Figure VI – “Cold spot” rotation in the core inlet for the high velocity case

14. Atoma Consult Ltd www.atomaconsult.biz Sensitivity study – results II Figure VII – “Cold spot” rotation in the core inlet for the medium velocity case

15. Atoma Consult Ltd www.atomaconsult.biz Sensitivity study – results III Figure VIII – “Cold spot” rotation in the core inlet for the forced circulation case

16. Atoma Consult Ltd www.atomaconsult.biz Sensitivity study – summary Table III – Summary on the sensitivity study results

17. Atoma Consult Ltd www.atomaconsult.biz Conclusions • 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 on the “intact/damaged” loops velocity ratio; • Results can be finally validated against large scale model experiments.