A step-by-step analysis approach was proposed:

1. Compile a state-of-the-art report which reviewed past and recent studies performed in the framework of sodium fast reactor and build a bibliographic repository which would stress core transient behaviours as a function of fuel characteristics (oxide, carbide, nitride and metal).
2. Perform a parametric study based on two different core sizes: large size core (3600 MW thermal) and medium size core (1500-2500 MW thermal). For both cores sizes three types of fuel were proposed: oxide, carbide and metal. The comparative study was aimed at identifying the advantages and drawbacks for each concept based on nominal performances and global safety parameters:
   • Neutronics characterisation of global parameters (k-eff, power and flux distributions, void effect, Doppler, etc.)
   • Feedback coefficient evaluation, discussion and agreement on corresponding calculation methodology
   The group provided initial core descriptions for both size cores. All the input data were provided to the WPRS community and each contributor performed step-by-step core analysis and benchmark comparisons using best-estimate calculations.
3. Based on the results obtained in the previous step, transient calculations were performed on a few selected cases for the principal unprotected transients (unprotected transient overpower (UTOP), unprotected loss of flow (ULOF), unprotected loss of heat sink (ULOHS) and the core behaviours characterised using a matrix classification.
4. Synthesis of the whole work into a final report including recommendations to improve safety and future work toward severe accidents and minor actinides management.

The data were mainly geometry and materials definitions that enable neutronics characterisation of the cores for both equilibrium beginning and end of cycle states. No thermal-hydraulic data were provided.