European Validation of ASTEC-V1 through the EVITA project

H.-J. Allelein, K.Neu (1), J.P. Van Dorsselaere, N. Trégourès (2), M.K. Koch, M. Bendiab (3), L.Kubisova (4).
EUROSAFE, 25-26 Novembre, Paris

The European Validation of the Integral Code ASTEC (EVITA) involves 19 partners from eight European countries plus JRC. It started in February 2000 and ended in July 2003. The main objective is to distribute the severe accident integral code Accident Source Term Evaluation Code (ASTEC), jointly developed by “Institut de Radioprotection et de Sûreté Nucléaire” (IRSN, France) and “Gesellschaft für Anlagen- und Reaktorsicherheit” (GRS, Germany), to European partners in order to apply the validation strategy issued from the VASA project (4th European Community Framework Programme).

Severe accident management (SAM) measures are currently being developed and implemented at Nuclear Power Plants (NPP) worldwide in order to prevent or to mitigate severe accidents. This needs a deep understanding of processes leading to severe accidents and of phenomena related to them. As greater account of severe accident measures is taken in the regulation of plants, there will be the need to show a greater degree of validation of codes and a better understanding of uncertainties and their impact on plant evaluations.

The EVITA evaluation of ASTEC code capabilities and the corresponding feedback towards the code development is an important step towards the intention to provide end-users (like utilities, vendors and licensing authorities) with a well-validated European integral code for the simulation of severe accidents in NPPs.
ASTEC versions V0 and V1 were installed successfully on the partners' platforms. The extensive portability check concluded that the ASTEC user should not fear portability effects. One of the partners’ conclusions was that the level of ASTEC models was near the state of the art in most domains. Of course understanding and thus adequate modelling is still missing, like in all other codes, in some domains: reflooding of a degraded core, MCCI, iodine behaviour in RCS, etc… Obviously recommendations were made to continue efforts of validation and plant application. The new version V1 allows simulating complete scenarios including the front-end phase. Both developing organisations, GRS and IRSN, which will continue to assure the code maintenance beyond EVITA, ensure that the EVITA outcomes will be respected for the future ASTEC development. Some of the above needed improvements are already foreseen, as well as the extension to Boiling Water Reactors.

Further recommendations are given to improve:

• Management of input decks: tools for automatic check, standard inputs for generic plant applications,
• Visualisation / post-processing (remark: actually the GRS visualisation tool ATLAS will be coupled to the next code version),
• Users’ support, including: continuous training courses (not only for beginners, but for experienced users too), wish for faster response of the ASTEC Maintenance team, more complete and detailed code documentation.

The plant applications with the first version of ASTEC V1 showed that the code is still not so robust that a Bulletin Notes Techniques et Publications DPAM N°1 janvier 2004 66/100 sequence is calculated up to a foreseen end. Suggestions are made especially for increasing the robustness of the coupling of the two new modules CESAR-DIVA that calculate the circuit thermal hydraulics and the core degradation, so that a significant progress is expected in short time. With respect to computing time, the EVITA users - researchers, licensing authorities and industry - accepted the definition elaborated in the VASA project as target, that a full sequence calculation (incl. postprocessing) should not need more than 12 hours.

The progress since the beginning of the project where only a preliminary code version was available is important. The actual version allows to simulate the entire sequence of events during a severe accident. Besides, a first level of validation was attained successfully within the project. As EVITA has very successfully made the first step into the intention to provide end-users (like utilities, vendors and licensing authorities) with a well validated European integral code for the simulation of severe accidents in NPPs, the EVITA partners strongly recommend to continue validation, benchmarking and application of ASTEC. This work is foreseen in SARNET (Severe Accident Research Network) in the 6th Framework Programme where ASTEC will play a key role as the reference European integral code.

(1) : GRS, Köln (Germany)
(2) : IRSN, Cadarache (France)
(3) : Ruhr-Uni-Bochum/LEE (Germany)
(4) : UJD SR, Bratislava (Slovak Rep.)