Fuel. The Leypunsky Institute of Physics and Power Engineering (part of Rosatom) started up a critical facility to study mixed oxide (MOX) fuel. Tests to validate the neutronic performance of a spectral shift control (VVER-S) reactor core are conducted as part of the Institute’s two-phase experimental program. Preliminary tests of the first phase will make the main-phase research more focused. Experimental programs related to the VVER-S reactor serve as a starting point for the development of MOX-fueled thermal neutron reactors cooled and moderated with water. MOX fuel pellets are made of depleted uranium oxide, and plutonium oxide extracted from spent nuclear fuel of power reactors. VVER-S is a 600 MW pressurized water reactor. What makes VVER-S different is the possibility of shifting the neutron spectrum to control the core reactivity during the fuel burnup by changing the water-fuel ratio, and complete abandonment of standby liquid control systems.