Project Goals

In SFR, a major safety concern is the potential risk of prompt-critical events and of large mechanical energy release to the vessel structures as SFR cores are highly sensitive to any modification of the core configuration (for instance, as a result of fuel melting). Moreover, with sodium as coolant and in case of loss of cooling and/or sodium boiling, the positive reactivity feedback (in some core regions) of the sodium void effect can generate fast power transients with fuel damage and core disruption. A large data base has been gained from past R&D experimental programmes that were dedicated to mixed oxide (UPuO) fuel behaviour all over the world (i.e. CABRI in France, TREAT in USA,…). However new and specific features of future Gen.IV systems (i.e. higher fuel enrichments, up t o 15–20 at%; innovative fuel types other than oxides (carbides,  inclusion of minor actinides, etc..) and the remaining uncertainties make it necessary to keep on investigating before relying on past research to extrapolate its results to the present situation. Severe accidents (also called Core Disruptive Accidents) can be the consequences of various initiating events like unprotected slow power transients, loss of cooling due to blockage of a sub-assembly, unprotected loss of flow.

The current accident simulation capability of SFRs dates back at the 80’s. In addition, the very few tools available lack some flexibility to integrate new physical models, especially for advanced designs. Therefore, based on the importance of considering severe accidents in the design phase of next SFRs, on the limitations of the available computation tools and their intrinsic weaknesses, the JASMIN project aims at developing a new computer code system, named ASTEC-Na, capable in the future of encompassing all the phases of a hypothetical severe accident and based on a robust advanced simulation platform.

The present project concerns only the initiation phase of SFR severe accidents. At the end of the project, ASTEC-Na should be able to:

  1. evaluate the consequences of unprotected accidents with fuel pin failure on materials relocation and primary system loads, due to either reactivity-driven transients, such as sodium voiding, loss of flow or mechanically or thermally-initiated transients, such as the blockage of a coolant in a subassembly,
  2. evaluate the source term produced by the migration of activated fission products inside the reactors and likely to be released to the environment in case of accidental situations.

The project does not address the modelling of the part of the accident that may follow the initiation phase, i.e. the transition phase and the expansion phase, up to the final state of the molten materials (e.g. those located on the debris tray or any new core-catcher design) and the evaluation of the mechanical consequences of the accident on the structures. And it does not address the modelling of sodium fires, sodium-water interactions and sodium-concrete interactions.
ASTEC-Na will be based on the ASTEC code system, jointly developed by IRSN and GRS, extensively validated through European projects (like EVITA, SARNET and SARNET2) and which is now considered as the European reference code for severe accidents in water-cooled NPPs. This code system has already been extended to cover severe accidents in diverse types of nuclear reactors: PWR, VVER, BWR and CANDU (the two latter yet partly for the moment) for Gen.II water-cooled reactors, some Gen.III reactors (like EPR), HTR (partly) and ITER for fusion facilities.

From a software point of view, the development will be based on existing modules of the ASTEC V2 code that will be re-used for ASTEC-Na. Specific physical models will be developed on SFR phenomena, using as a basis the models of existing codes. The outputs of the SP3-6 task ("Evaluation of modelling capabilities of accident scenarios") of the CP-ESFR current FP7 project on the status of code models will be also used as a basis. Hence, the JASMIN project is complementary to the more reactor-engineering focused CP-ESFR project since it will go further in enlarging the simulation capability of severe accidents in SFRs.

The project will entail an extension of the ASTEC domain as a European reference code for Gen.II and Gen.III to the fast reactors domain. This strategy allows getting the maximum benefit from a computation tool already available that has been significantly supported by EC and, at the same time, it will support ESNII to keep the European leading role in nuclear technology by developing a computation tool needed for the indispensable safety assessments and safety enhancements of the fast neutron systems.

In order to assure that this project fits the needs expressed by the ESNII roadmap, discussions will be established and maintained with external "end-users" in order to inform them about the progress of the work and discuss periodically about the adequacy of the code features.