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The present work details a further investigation of the SNEAK-12A experimental program, which aimed to study material relocation in Sodium Fast Reactors (SFRs) leading to core degradation. The further investigation include sensitivity and uncertainty propagation analysis. In this paper, a comparison is made using two codes, a Monte Carlo based code Serpent 2 and the deterministic system code ERANOS. A sensitivity analysis was made utilizing the two codes, with comparison of two nuclear data libraries (ENDF/B-VII.1 and JEFF-3.1.1). The code-to-code comparison resulted in a very good agreement, while the comparison of libraries showed large discrepancies, manly due to the differences in the sodium cross-section data. The sensitivity analysis, was followed by a complete propagated uncertainty analysis based on the covariance evaluated data available in the COMAC data evaluation. The results of the uncertainties show that there are still large discrepancies linked to the nuclear data. This work is done within the frame work of new core design capacities and new ways of conducting in Zero Power Reactors, such as the ZEPHYR project led independently by CEA.

The present work details a new benchmark to be produced to the International Community, for dealing with neutronics code validation in the frame of SFRs (Sodium Fast Reactors) severe accidents sequences leading to core degradation and material relocation. The benchmark is based on a complete re-analysis of the SNEAK-12A experimental program, using TRIPOLI-4, MCNP and Serpent-2 Monte Carlo codes as reference tools, and the ERANOS system of codes for deterministic calculations, all based on JEFF-3.1.1 nuclear data libraries. The complete material balance is resumed, and the main degradation sequences are detailed. Preliminary results on available experimental results (k eff) are given, and additional local quantities are calculated, such as axial flux distributions, as well as detector responses in function of the distance to the degraded part. The benchmark offers an excellent opportunity to validate calculation schemes for strongly heterogeneous interfaces, in particular the preparation of homogenized and condensed cross sections for deterministic core calculations, as well as leakage treatment in locally very heterogeneous media. This work is made within the frame of new core design capacities and new ways of conducting experiment in Zero Power Reactors, such as the ZEPHYR project led independently by CEA. The present analysis will be completed by a full nuclear data sensitivity and uncertainty analysis of the reactivity coefficients in a companion paper.