Bounding Parameters of a Theoretical Deep-Earth Uranium Fission Reactor

More than 50 years ago it was postulated that a naturally occurring nuclear reactor was possible if it started 2 billion years ago. The subsequent discovery of the natural reactor at Oklo confirmed that it is possible for a nuclear fission reactor to naturally form and cycle on and off over extended periods of time. The hypothesis of a naturally occurring reactor was extended to include the possibility of significant quantities of uranium aggregating inside the molten core of Earth due to gravity during its formation. When sufficient quantities of uranium accumulate, a self-sustaining fission reactor could form, which would fluctuate in power as uranium fissions and fission products are produced. Lighter elements would migrate out of the reactor region, and heavier elements would coalesce due to gravity. In this technical note, SCALE, a state-of-the-art nuclear engineering computer code system developed at Oak Ridge National Laboratory, was used to investigate this hypothesis. The analysis indicates that the overall operational parameters of a postulated nuclear fission reactor located in the inner core of Earth must fall within a relatively narrow band in order to still be operating today. If the overall power level were too low, the reactor would not breed sufficient fissile material, and the average enrichment would drop below the level required to form a self-sustained fast reactor. If the power level were too high, the reactor would have burned itself out well before the present day. The objective of this technical note is to provide calculations that support an existing geo-reactor and the operating parameters that would govern a deep-Earth reactor and allow it to still be operating today, 4.5 billion years after Earth was formed. To help bound the possible power range, a simplified, one-dimensional, homogeneous, deep-Earth reactor having a steady-state fission power is simulated over geologic time. Power levels and start times are varied. The simulations show that if the reactor were still operating today, it would have an overall lifetime average operating fission power of <3 TW. Analyses show that both instantaneous and cumulative ³He/⁴He ratios are a function of fission power, ²³⁵U/²³⁸U ratio, total uranium mass, and geo-reactor starting time. The calculated ³He/⁴He ratios are consistent with those observed in nature.