The 2nd meeting of the “Corium and Debris Coolability” Work-Package N°5 was hosted by IVS in Bratislava (Slovakia) from 7th to 8th July 2010. 34 partners from 19 European countries participated at the meeting.

(Attendants to the Bratislava Topical meeting)
The SARNET2 experts were invited to attend the 1st LACOMECO ('Large-scale experiments on core degradation, melt retention and containment behaviour', FP7 frame) User Selection Panel meeting on July 8th in Bratislava, which provided a remarkable source of expertise. To see the full details of the call, please visit: http://www.fzk.de/lacomeco

Reflooding and coolability of a degraded core (coordinated by Jon Birchley, PSI)
Extensive data exist on reflooding of essentially intact geometries. The conclusions can be drawn that intact rods can be successfully quenched up to 2000°C if sufficient water flow exists and non-oxidized metal is not exposed to coolant at high temperature.
In the 2nd current period, investigations shift to reflooding of debris beds. Experimental investigations in the new PRELUDE/PEARL (IRSN) and QUENCH-DEBRIS (KIT) programmes are under way, additionally to the existing DEBRIS-QUENCH one (IKE-Stuttgart). Complementary model developments of ATHLET-CD and ASTEC codes are being performed and will support the analytical and validation studies of the above experiments. Aspects like metallic-ceramic debris simulant materials, size, shape of fragments, heating method, scaling and conditions for reflooding will be analysed.
A Task Group on the experimental activities was initiated with IRSN, IKE, KIT and PSI. A first meeting will take place during the QUENCH workshop at KIT.

Remelting of debris, melt pool formation and coolability (coordinated by Weimin Ma, KTH)
Benchmark calculations are planned using the results of the LIVE-L6 experiment performed at KIT. The following results will be used for the comparison: temperature evolution, heat flux density distribution and crust thickness profile along the vessel wall and the temperature profile at the vessel vertical symmetry axis.
A tentative date to complete the calculation phase is March 31, 2011, which allows sufficient time to compare the data and present the results of the benchmark at a WP5 topical meeting in fall 2011.
For the JPA2 period, 2 new experiments in LIVE with salt and oxide melts are foreseen, with additional supporting tests with the LIVE-slice facility. The DEFOR tests at KTH will focus on debris bed formation under in-vessel conditions. For the analytical efforts, the main emphasis will be to explore limits of coolability in the lower head with the CFD code CONV and to compare the results with ASTEC.

Ex-vessel debris formation and coolability (coordinated by Georg Pohlner, IKE-Stuttgart)
The main focus is on the coolability of melt released from a failed vessel into a water-filled cavity, which includes the issues of debris bed formation by break-up of melt and coolability of debris. Scenario and accident management investigations for these cases include calculations for reactor scenarios in close cooperation with the WP5.4 “Bringing research results into reactor application” (see below).
In JPA2 a joint action including the codes MC3D, CORIUM, DEM, DECOSIM and JEMI, ICARE/CATHARE and MEWA will evaluate the DEFOR and FARO experiments (L-28, L-31), and new DEBRIS, POMECO, STYX and COOLOCE (heap-like debris) experiments.

Bringing research results into reactor application (coordinated by Peter Matejovic, IVS)
Efforts within the 2nd phase will be done firstly on the early and late core degradation phase, core relocation and molten pool formation in the reactor lower head with a special emphasis on the description and modelling of typical operator interventions, and secondly on the way to achieve In-Vessel melt Retention (IVR) by removing the decay heat through the vessel or the Control Rod Guide Tubes into the external coolant.
A necessary condition for prediction is the availability of validated computational tools, capable to cover the above-mentioned phenomena, applicable to different reactor designs (e.g. VVER and BWR) and capable to model SA management actions relevant to IVR, e.g. models for corium relocation in a VVER-440 reactor application using the last version of ASTEC V2 (ICARE module).
In cooperation with the WGAMA of the OECD/NEA/CSNI, the TMI2 alternative benchmark is planned: G. Bandini (ENEA) has prepared meanwhile a new version of the proposal, and IVS and other SARNET2 partners are very interested by this project with the codes ASTEC, MELCOR, ATHLET-CD and MAAP.
Possible investigations should be divided into 3 categories:

• Sequences where reflooding occurs when the core is still intact and no or only local melt formation has already taken place. This is likely if the maximum temperature in the core remains below 2200 K. For such sequences, codes are acknowledged to be in their range of validity.
• Sequences where reflooding starts after a significant melting has occurred within the core. For such sequences, some models used in the codes operate outside of their range of validity and uncertainties in the predictions are larger.

• Sequences with possible corium relocation into the lower plenum and vessel failure. For such sequences, various uncertainties are cumulated in the predicted results, either because of stochastic effects or because of non validated models. Although such sequences are very relevant with respect to safety analysis of new reactors (such as EPR, AP-1000 or APR-1400), they will be studied only if time is available, due to the very large uncertainties that may be expected.

Contact: Alexei Miassoedov alexei.miassoedov@kit.edu