FIELD: nuclear reactors. SUBSTANCE: reactivity is controlled by displacing some portion of fuel between position aiding in chain reaction and that in which it ceases. Fuel portion being displaced is located in central column cavity parallel to direction of gravitational force and placed in annular core; it is, essentially, filler of finely dispersed fuel particles. Portion of main coolant flows through cavity in direction opposing gravitational force and holds fuel particles within reaction zone. When coolant stops flowing in cavity, fuel particles move to position assisting in stopping chain reaction. EFFECT: provision for automating reactivity control in nuclear reactor. 11 cl, 2 dwg
