Research. Rosatom researchers launched reactor core simulation for the advanced BN-1200M fast neutron reactor using the Fast Neutron Physics Simulator (FNPS). “We have manually assembled over 4,700 fuel rods to have enough critical mass for the reactor criticality to be achieved,” FNPS Director Alexander Zhukov explained. By its size, this critical assembly is the largest of its kind ever assembled either at the FNPS or in the world. The sodium-cooled fast neutron reactor BN-1200M is being developed as part of the Proryv (Russian for Breakthrough) innovative project aimed at piloting the closed nuclear fuel cycle technology.