- The present work details information (core geometry, material balance, and criticality measurements and calculations) regarding a new benchmark to be introduced to the international community, for dealing with neutronic code validation in the frame of the analysis of severe accidents in fast reactors leading to core degradation and material relocation. This specific benchmark is based on analysis of selected experiments performed at the Schnelle Null-Energie-Anordnung Karlsruhe (SNEAK). Unlike the previously analyzed SNEAK-12A core, which was loaded with enriched metallic uranium fuel, the core analyzed in this study, the SNEAK-12B core, was loaded with plutonium fuel to better represent future fast systems, and the experiments that were considered include fuel relocation and redistribution of structural material. In this paper, the experimental results are analyzed by computational tools such as MCNPX2.7 and Serpent-2.1.29 Monte Carlo codes, and the ERANOS 2.4 system code for deterministic calculations, all based on JEFF-3.1.1 nuclear data libraries. The paper provides a complete and detailed specification for the benchmark problem. Preliminary results of available experimental results ( k eff and axial distribution of reaction rates) are given and additional quantities are presented (such as axial flux distribution). The benchmark offers an excellent opportunity to validate calculation schemes for strongly heterogeneous configurations, in particular the preparation of homogenized self-shielded neutron cross-sections for deterministic core calculations, as well as leakage treatment in locally voided zones. The analyses of SNEAK experiments, presented in this paper, provide grounds for the design of innovative experimental capabilities in severe accident modeling in Zero Power Reactors (ZPR), such as the Zero-power Experimental PHYsics Reactor (ZEPHYR) project led independently by the Commissariat a l’Energie Atomique et aux Energies Alternatives (CEA). This paper is completed by a comprehensive nuclear data sensitivity and uncertainty analysis of the reactivity coefficients and k eff in a companion paper.