From the very beginning, leaders of the fast-neutron reactor development program had safety concerns. Reactivity safety was studied theoretically using a number of criticality experiments under different scenarios such as refueling, transition from subcritical to supercritical, and the effectiveness and safety of the control rods. These findings were subsequently supported by practical experience with the BOR-60, BN-350 and BN-600. Additionally, special experiments were performed to study fires resulting from sodium leaks into the air (which can be effectively suppressed) and into the water in steam generators.

A more significant problem centered on the construction of the steam generator. Two types of steam generators were tested, water in pipe surrounded by sodium (straight-type) and sodium in pipe surrounded by water (reverse-type). Experience acquired from both types revealed that the reverse-type is safer and led to the idea of including an intermediate sodium heat-transfer loop between the radioactive sodium primary coolant and the steam generator.