1. ERMSAR 2012, Cologne March 21 – 23, 2012 Experimental and computational studies of the coolability of heap-like and cylindrical debris beds E. Takasuo, S. Holmström, T. Kinnunen, P.H. Pankakoski, V. Hovi, M. Ilvonen (VTT Technical Research Centre of Finland) S. Rahman, M. Bürger, M. Buck, G. Pohlner (Institute of Nuclear Technology and Energy Systems, University of Stuttgart)

2. ERMSAR 2012, Cologne March 21 – 23, 2012 Introduction Ex-vessel debris coolability is a key issue at the Finnish and Swedish BWRs – Melt pours from the RPV to a deep water pool (flooded lower drywell of the containment) – Porous debris bed is formed as a result of melt solidification, fragmentation and settling of the particles Coolability mainly depends on the debris bed configuration – Can be highly complex: porosity, particle size and morphology, overall geometry  Depends on the melt discharge scenario and interactions with the pool 2

3. ERMSAR 2012, Cologne March 21 – 23, 2012 Scope of the present studies Experimental studies – Measurements of dryout heat flux in two representative debris bed geometries Simulations – Prediction of dryout heat flux in the experiments (in an already established debris bed) code validation – Quenching analysis during debris bed formation in a plant scale scenario 3

4. ERMSAR 2012, Cologne March 21 – 23, 2012 Experimental activities at VTT The COOLOCE (Coolability of Cone) test apparatus replaced the STYX facility in 2009 – Conical (heap-like) and cylindrical test beds – Test series have been run for both geometries for a range of pressure Objective is to clarify the effect of geometry (lateral flooding vs. height) and provide new data for simulation code validation 4 STYX with downcomers (2008)

5. ERMSAR 2012, Cologne March 21 – 23, 2012 Conical test bed The heating arrangement (electrical resistance heaters) and thermocouples The conical particle bed filled with particles (spherical ceramic beads, ø 0.8 - 1.0 mm) held in shape by a net

6. ERMSAR 2012, Cologne March 21 – 23, 2012 Cylindrical test bed The heating arrangement (electrical resistance heaters) and thermocouples The cylindrical particle bed filled with particles (spherical ceramic beads, ø 0.8 - 1.0 mm), porosity ~ 38%

7. ERMSAR 2012, Cologne March 21 – 23, 2012 Experimental results It was found that the coolability of the conical debris bed is improved by 50%-60% compared to the cylindrical bed in case the beds are equal in height However, if the beds have equal radius and volume (flat- shaped cylinder) the coolability of the conical bed is poorer by about 50% – The effect of the increased height (and thermal loading near the tip of the cone) is greater than the effect of lateral flooding 7

8. ERMSAR 2012, Cologne March 21 – 23, 2012 MEWA and JEMI codes MEWA 2D and JEMI are developed by IKE (University of Stuttgart) specifically for severe accident analysis The different stages of melt and debris coolability can be evaluated with the coupling of MEWA to JEMI – Melt breakup and jet quenching – Particle settling – Initial quenching of the debris bed – Fully quenched debris bed coolability 8

9. ERMSAR 2012, Cologne March 21 – 23, 2012 Comparison of experiments and MEWA simulations 9

10. ERMSAR 2012, Cologne March 21 – 23, 2012 MEWA results: Cone and tall cylinder 10 Particle temperature in post-dryout conditions

11. ERMSAR 2012, Cologne March 21 – 23, 2012 MEWA results: Cone and flat-shaped cylinder The beds have equal radius, i.e. the scaling corresponds to reactor scenarios 11

12. ERMSAR 2012, Cologne March 21 – 23, 2012 Debris quenching simulations Quenching of initially hot debris was modeled by MEWA – Simultaneous settling and quenching of hot particles that form a conical bed in the water pool – Initial particle temperatures from jet breakup and particle mixing calculations with JEMI – More realistic compared to earlier approaches that deal with already established, initially hot debris bed – Postulated accident with a melt mass of 185 tons with the discharge rate of 0.157 m 3 /s 12

13. ERMSAR 2012, Cologne March 21 – 23, 2012 JEMI/MEWA simulation results Simulations with realistic initial conditions – Cooling is supported by quenching of the lateral region during settling – Cool down of particles is observed when they reside at the surface of the debris bed – The quenching is fast enough so that the heat-up due to decay heat does not yield temperatures beyond 2000 K – The cooling of the upper parts by gas flow is effective because of the fast quenching of the lower parts Simulations with uniform initial temperature – Quenching of the lower bed regions occurs slower – The cooling of upper regions by gas flow is not effective due to slow water infiltration in the lower parts – Melting temperature (>2800 K) is reached in large parts of the bed already after 2800 s 13

14. ERMSAR 2012, Cologne March 21 – 23, 2012 14 Quenching during build-up

15. ERMSAR 2012, Cologne March 21 – 23, 2012 15 Quenching during build-up

16. ERMSAR 2012, Cologne March 21 – 23, 2012 16 Quenching during build-up

17. ERMSAR 2012, Cologne March 21 – 23, 2012 17 Quenching during build-up

18. ERMSAR 2012, Cologne March 21 – 23, 2012 18 Quenching during build-up

19. ERMSAR 2012, Cologne March 21 – 23, 2012 19 Quenching during build-up

20. ERMSAR 2012, Cologne March 21 – 23, 2012 20 Quenching during build-up

21. ERMSAR 2012, Cologne March 21 – 23, 2012 21 Quenching during build-up

22. ERMSAR 2012, Cologne March 21 – 23, 2012 22 Quenching of an established bed

23. ERMSAR 2012, Cologne March 21 – 23, 2012 23 Quenching of an established bed

24. ERMSAR 2012, Cologne March 21 – 23, 2012 24 Quenching of an established bed

25. ERMSAR 2012, Cologne March 21 – 23, 2012 25 Quenching of an established bed

26. ERMSAR 2012, Cologne March 21 – 23, 2012 26 Quenching of an established bed

27. ERMSAR 2012, Cologne March 21 – 23, 2012 MEWA simulations: Maximum temperature 27 CASE1: Quenching during build-upCASE2: Uniform initial temperature

28. ERMSAR 2012, Cologne March 21 – 23, 2012 Possibilities of 3D modeling 3D approach facilitates the modeling of complex geometries and/or internal inhomogeneity, pool model may be included Demonstration calculations of the dryout behavior of an established debris bed have been conducted by PORFLO – Multi-purpose 2-phase 3D solver developed at VTT – Models suitable for porous beds have been included The code is capable of capturing the main processes of debris bed dryout – Development is still on-going, suitability for coolability prediction has to be verified for various cases 28

29. ERMSAR 2012, Cologne March 21 – 23, 2012 PORFLO results Liquid saturation in post-dryout conditions for the conical and cylindrical beds 29

30. ERMSAR 2012, Cologne March 21 – 23, 2012 Summary New experimental data of the effect of lateral flooding and debris bed height on coolability has been obtained – Poorer coolability for the conical bed due to greater height The MEWA simulation results agree very well with the measured dryout power Preliminary 3D calculations with PORFLO suggest that full 3D approach could be feasible for coolability analyses Simulations with quenching during debris bed build-up suggest improved coolability margins compared to cases with already established hot debris bed 30