An in situ heat treatment system for producing hydrocarbons from a subsurface formation includes a plurality of wellbores in the formation. At least one heater is positioned in at least two of the wellbores. A self-regulating nuclear reactor provides energy to at least one of the heaters to heat the temperature of the formation to temperatures that allow for hydrocarbon production from the formation. A temperature of the self-regulating nuclear reactor is controlled by controlling a pressure of hydrogen supplied to the self-regulating nuclear reactor, and wherein the pressure is regulated based upon formation conditions.