1. Overview of the ASTEC V2.0-rev1 validation
P. Chatelard (IRSN), S. Arndt (GRS), B. Atanasova (INRNE)
G. Bandini (ENEA), A. Bleyer (IRSN), T. Brähler (RUB)
M. Buck (IKE), I. Kljenak (JSI), B. Kujal (UJV)

2. Contents Introduction to the ASTEC validation strategy
Examples of the assessment of some ASTEC V2.0-rev1 modules by SARNET partners
Core degradation module
Containment module
MCCI module
Summary of the ASTEC V2.0-rev1 assessment
Conclusion and perspectives

3. ASTEC context and objectives
IRSN-GRS cooperation since 1996 for development of an integral code
ASTEC (Accident Source Term Evaluation Code) for present/future nuclear water-cooled reactors (PWR, BWR, VVER, CANDU) source term severe accident calculation, from initiating event until radioactive release out of the containment:
Evaluation of source term
PSA level 2 studies (PSA-2)
SA management (SAM) evaluation
Support of experimental programmes
Progressive evolution in recent years towards a state-of-the-art tool for source term calculations:
Most modeling is mechanistic, only sometimes simplified
Repository of knowledge of severe accident phenomenology
 ASTEC = Reference European code in the SARNET network
New series of ASTEC versions (V2 series) since 2009
Mid : V  First V2 validation by several partners (SARNET2 1st period)
Mid : V2.0-rev1  Extended V2 validation by number of partners (SARNET2 2nd period)
End : V2.0-rev2  Validation to be continued in the frame of SARNET2 3rd period

4. ASTEC V2 A new series of versions (1/2)

5. ASTEC V2.0-rev1 validation strategy
Three-tier validation approach (made possible since ASTEC is very modular)
Separate-Effect-Tests focusing on only 1 physical phenomenon
Coupled-Effect-Tests focusing on a set of physical phenomena
Integral tests to check the coupling of physical models and that no essential phenomenon was forgotten or neglected
Very large ASTEC V2 validation matrix covering all SA phenomena and including major (past, on-going) French, German and international exp. Programs (including VVER experiments), such as in particular:
All Phebus FP experiments;
Many ISPs: 27 (BETHSY), 33 (PACTEL), (CORA), 34 (FALCON), 35 (NUPEC), 37 (VANAM), 39 (FARO), 40 (STORM), 41 (ACE-RTF, CAIMAN), 44 (KAEVER), 45 (QUENCH-06), 46 (Phébus-FPT1), 47 (TOSQAN-MISTRA-ThAI), 49 (ThAI-Enaceff);
OECD projects: LHF-OLHF, RASPLAV/MASCA, OECD-CCI;
Circuit experiments: BETHSY, ROSA, PACTEL, LOFT-FP, … as well as the TMI-2 scenario
On-going and future new experiments: LIVE on corium pools, PRELUDE, DEBRIS & PEARL on reflooding, DISCO on DCH, ThAI on containment, EPICUR, CHIP & THAI on iodine, RUSET on ruthenium, VULCANO on MCCI…
The multi-partners validation of ASTEC V2.0-rev1 revision is briefly illustrated in the following through few calculation examples
 3 different ASTEC modules have been selected for that purpose

6. Example of core degradation module assessment Phébus FP bundle experiments
Phébus FPT3 (work performed by ENEA)
Reasonable agreement on thermal behaviour as well as on both oxidation and relocation processes using the new ICARE 2D magma relocation model which is the one recommended by IRSN for plant applications
Bundle temperature at 0.6 m
Total hydrogen production

7. Material distribution at the end of the test
Example of core degradation module assessment Phébus FP debris experiment
Phébus FPT4 (work performed by IKE-Stuttgart)
Very good agreement (all along the transient up to the end of the test) still using the ICARE advanced 2D magma relocation model
Temperatures at the bed centerline
Material distribution at the end of the test

8. Example of containment module assessment KAEVER experiments
KAEVER K123 (test with CsI aerosol in non-saturated atmosphere)
(validation work performed by JSI)
Th. Hydraulics : Very good agreement on pressure & atmosphere temperat.
Aerosols : Very good trend and good order of magnitude for dry aerosols
But results are generally less good for wet aerosols
Dry aerosols concentrations
Pressure evolution

9. Example of containment module assessment MISTRA spray experiments
MISTRA MASP1 (validation work performed by GRS)
Main Th.Hydraulics effects of spray (pressure, atmosphere drops) are well matched by the CPA module from ASTEC V2.0-rev1
But temperature stratification is overestimated by ASTEC
Atmosphere temperatures in radius R4
Sump water level

10. PANDA test-18 (validation work jointly performed by IRSN & INRNE)
Example of containment module assessment PANDA SETH free-plume experiments
PANDA test-18 (validation work jointly performed by IRSN & INRNE)
Reasonable agreement obtained with the CPA module of ASTEC V2.0-rev1 using a refined nodalisation in vertical direction and nodes to model upward plumes  recommended nodalization for plant analyses
Steam concentration along the central axis of DW2
Test configuration for PANDA test n°18

11. Example of H2 combustion module assessment BMC experiments
BMC Ix9 (validation work performed by RUB)
Sensitivity study on the nodalisation scheme using FRONT model in CPA:
Nodalisations A and B : cutting only in horizontal direction (resp. 4 or 8 zones)
Nodalisation C : idem A with an additional cutting in vertical direction
 Nodalisation influences the convection and therefore the burning rate
Overall, ASTEC V2.0-rev1 results are in good agreement to the experiment
Pressure evolution in room R7 where mixture was ignited
Tested nodalisations

12. Example of MCCI module assessment CCI experiments
CCI-5 (validation work performed by UJV)
Quite good results have been achieved with the MEDICIS module of ASTEC V2.0-rev1, using a set of input parameters very consistent with the IRSN recommendations for plant applications
Vertical erosion depth
Final cavity shape

13. Summary of the ASTEC V2.0 assessment
Th.hydraulics in circuits
Good results on SETs and reasonable results on integral tests (including CESAR-to-CATHARE detailed benchmarks on SGTR scenarios)
Core degradation
Good results for both early-phase models (heat-up, H2 production, …) and late phase models (2D relocation, molten pool, corium in lower head, …)
Poor results in case of a reflooding of a degraded core
FP release
Very good results for volatile and semi-volatile FPs and reasonable results (slight underestimation) for the low-volatile FPs
FP transport
Reasonable results on FP transport and chemistry
But the importance of the gas chemistry kinetics has been underlined, in particular with respect to the final ST (for instance, iodine partition at the break)

14. Summary of the ASTEC V2.0 assessment
Containment
Reasonable results on both thermal-hydraulics and aerosols behaviour
Poor results on pool-scrubbing phenomena
DCH
Current models are still too parametric and too geometry-dependent
Iodine and ruthenium chemistry
Modelling at the State of the Art  Global trends are well reproduced
No reason to change the strategy yet adopted for several years in that field which consists in a continuous modelling improvement as a direct feed-back from on-going interpretation of new experiments
MCCI
Basic relevance of the set of models and assumptions
Need for model improvements on coolability aspects

15. Feed-back from the code assessment on code the development process
First step: at short term, the main benefit for SARNET-WP4 partners using ASTEC comes from the periodical release by IRSN and GRS of improved V2.0 versions (revisions or patches)
Last delivered revision : V2.0-rev2 in December 2011
Improvement of the condensation processes in swollen level volumes
Transfer of the COCOSYS model of dry aerosol re-suspension in containment
Improvements of iodine reactions (in particular Ag/I in the sump) …
Next planned revision : V2.0-rev3 to be delivered end of 2012
Extension of the RCS gas chemistry kinetics to the Cs-I-O-H-B-Mo system
Improved model for iodine interaction with paints under irradiation
1st models for corium coolability during MCCI (top cooling and bottom cooling)
Moreover, besides new models, improvements are also expected from the continuous interpretation of the experimental programmes underway or planned in SARNET2, ISTP, OECD or in French frame

16. Towards future ASTEC V2 versions
Second step (medium term): Development of the next generation of ASTEC V2 versions  New models under a new code structure
According to the V2.0-rev1 assessment, main ASTEC modelling efforts shall be spent in priority on the following open modelling issues:
In-vessel SA phase  Reflooding of severely degraded cores
 RCS gas chemistry kinetics
Ex-vessel SA phase  MCCI, pool-scrubbing and DCH
Moreover, besides new physical models or improvements of existing ones, significant evolutions of the general code structure (and in particular of the core degradation module and its coupling to other ASTEC modules) have been identified few years ago at IRSN as a mandatory step to remove some current V2.0 limitations for plant analyses
To answer these requirements, the preparation of the future V2.1 version (future ASTEC major version) has already started at IRSN and GRS

17. End 2013: ASTEC V2.1 version ASTEC V2.1 main features
Integrating most of the SARNET2 knowledge
New CESAR/ICARE coupling (unique t/h in the whole RCS, no more switch after front end phase), including also a 2D extension of the in-core thermal-hydraulics
Full capabilities for shutdown states and air ingress situations after vessel failure (complete Ru behaviour also in RCS and advanced models for fuel oxidation under air atmosphere) and improved capabilities for vessel external cooling
First version of a mechanistic model for reflooding of degraded cores
Extended RCS gas chemistry kinetics (according to available data)
Transfer of the COCOSYS model of DCH
Generalisation of the MDB use (centralized material database) to any ASTEC module
Integrating specific core models for BWR (canisters, sub-channels, …) and CANDU
First version applicable to the major part of Fukushima-Daiichi NPP accidents
First version really applicable to spent fuel pool accidents
Progress towards a “diagnosis” version
Interfacing with atmospheric dispersion tools to enhance capabilities of direct comparison with on-site measurement …

18. Conclusion and perspectives
ASTEC V2 : a reference tool for Gen.II / Gen.III safety analyses
ASTEC is and will remain a repository of the knowledge gained from international R§D, while progressively integrating the feed-back from the interpretations of Fukushima-Daiichi NPP accidents
Axes for future ASTEC modelling improvements beyond V2.1 version are fully consistent with the recently updated SARP ranking
See ERMSAR-2012 paper on severe accidents research priorities
Other long-term objectives
Following-up the ASTEC extension to other reactors
Gen.IV SFR, ITER, …
Progress towards a severe accident simulator

19. Appendices

20. ASTEC nodalisation for MISTRA MASP1 and PANDA Test-18 simulations
PANDA test n°18

21. Sketch of the BMC facility

22. Set of best-estimate MCCI parameters for plant analyses
According to the interpretations with ASTEC of CCI and VULCANO tests with siliceous concrete (CCI-3, CCI5, VULCANO VB-U5), main recommendations for MCCI full scale analyses are :
The use of Bali correlation for convective heat transfer coefficient seems to be appropriate;
The recommended value for the  parameter (Tsol/Tliq interpolation parameter to evaluate the solidification temperature) must range within 0.3 – 0.4;
The proper value for the fraction of radiative power towards concrete above the corium seems to be in the range 0.0 – 0.2;
The slag heat transfer coefficient should be angular dependent. Recommended bottom/lateral values are /1000 W/m2K;
The recommended value of layer volume swelling factor is 1.2