Wayne: What would control this energy output variability if it does exist?
Dr. Herndon: In a nuclear fission reactor, the nuclei of uranium and other actinide elements are caused to fission in a chain reaction, splitting typically into two pieces.
These fission products absorb neutrons and, if left in place, will slow the neutron chain reaction and, ultimately, will shut down the reactor. But the fission products have roughly half the atomic number of the uranium fuel and half the atomic mass. At the pressures that prevail in the deep interiors of planets, density is a function of atomic number and atomic mass. The fission products will therefore be less dense than the uranium fuel and will tend by gravity to migrate radially outward while the uranium fuel re-concentrates inward. One might imagine in the ideal case something of an equilibrium being established. But if, for example, the rate of production of fission products exceeds their rate of removal, the output power of the reactor might be reduced until the fission products have a chance to migrate by gravity away from the reactor zone. Then the power will increase.