A method for controlling the total power output of a nuclear reactor, comprising: providing a reactor core that includes a fuel assembly, the fuel assembly including a plurality of fuel tubes; providing a reflector zone that surrounds the reactor core; providing a housing having a cavity, adjacent to the fuel tubes; allowing movement of a moderator into and out of the cavity of the housing at a lower portion of the housing; characterized in that a plurality of housings, each having a cavity, is provided adjacent to the fuel tubes, wherein movement of a moderator into and out of each cavity of the plurality of housings is allowed at a lower portion of the plurality of housings; moderator in a gaseous state is confined in each cavity of the plurality of housings at an upper portion of the plurality of housings; wherein the moderator moves from at least one of the plurality of housings to the reflector zone through the lower portion of the at least one of the plurality of housings, and the moderator moves from the reflector zone to the at least one of the plurality of housings through the lower portion of the at least one of the plurality of housings and further characterized by removing an amount of heat from the moderator in the cavity of the at least one housing that is substantially equal to an amount of heat imparted by conduction of heat, neutrons, or gamma radiation from the fuel assembly into the moderator in the cavity by means of a moderator coolant inflow tube including one or more holes located in the sides of the moderator coolant inflow tube configured to allow relatively cooler fluid moderator to flow into the cavity of the housing to mix with fluid moderator already disposed in the cavity. providing a reactor core that includes a fuel assembly, the fuel assembly including a plurality of fuel tubes; providing a reflector zone that surrounds the reactor core; providing a housing having a cavity, adjacent to the fuel tubes; allowing movement of a moderator into and out of the cavity of the housing at a lower portion of the housing; characterized in that a plurality of housings, each having a cavity, is provided adjacent to the fuel tubes, wherein movement of a moderator into and out of each cavity of the plurality of housings is allowed at a lower portion of the plurality of housings; moderator in a gaseous state is confined in each cavity of the plurality of housings at an upper portion of the plurality of housings; wherein the moderator moves from at least one of the plurality of housings to the reflector zone through the lower portion of the at least one of the plurality of housings, and the moderator moves from the reflector zone to the at least one of the plurality of housings through the lower portion of the at least one of the plurality of housings and further characterized by removing an amount of heat from the moderator in the cavity of the at least one housing that is substantially equal to an amount of heat imparted by conduction of heat, neutrons, or gamma radiation from the fuel assembly into the moderator in the cavity by means of a moderator coolant inflow tube including one or more holes located in the sides of the moderator coolant inflow tube configured to allow relatively cooler fluid moderator to flow into the cavity of the housing to mix with fluid moderator already disposed in the cavity.