The phenomenology of coolant boiling in the core of a Gen IV Sodium Cooled Fast Reactor (SFR) during an Unprotected Loss Of Flow transient (ULOF), has been the topic of regular exchanges between ROSATOM/IPPE and CEA since 2012. As an outcome, a new common experimental program (HaRIBO, which stands for Ha in aR1 for BOiling) has been proposed to be performed on AR-1 facility in Obninsk.First, the paper reminds key motivations of the program together with connected scientific challenges. These are related to safety enhancements of innovative low void penalty cores that are under consideration by both institutes. Indeed, once boiling onset is reached along the transient, a limit cycle is calculated according to periodic changes from diphasic to monophasic states of the coolant. Incidentally, pins cladding temperature remains close to sodium saturation, instead of excursing up to steel melting temperature. Then, the paper outlines the rationale of the program inline, expected advances compared to former programs, specifics of test section design and its challenging instrumentation as well as tests organization logic are presented. Finally, some outcomes from the thermal-hydraulic preparatory work are reported temperature peaks, which were monitored along preceding tests on a 7-pins bundle, have been analyzed to balance Na superheating concern; simulations of typical tests have been additionally performed to support definition of HaRIBO test matrix as well as location and span of the instrumentation. This work finally presents an ongoing benchmark, providing a first comparison of COREMELT-3D (IPPE)/CATHARE3 (CEA) models for Na boiling.