Assessing the feasibility and consequences of nuclear georeactors in the Earths core mantle boundary

We assess the likelihood and geochemical consequences of the presence of nuclear georeactors in the core mantle boundary region (CMB) between Earths silicate mantle and metallic core. Current geochemical models for the Earths interior predict that U and Th in the CMB are concentrated exclusively in the mineral calcium silicate perovskite (CaPv), leading to predicted concentration levels of approximately 12 ppm combined U and Th, 4.5 Ga ago if CaPv is distributed evenly throughout the CMB. Assuming a similar behaviour for primordial 244Pu provides a considerable flux of neutrons from spontaneous fission. We show that an additional concentration factor of only an order of magnitude is required to both ignite and maintain self sustaining georeactors based on fast fission. Continuously operating georeactors with a power of 5 TW can explain the observed isotopic compositions of helium and xenon in the Earths mantle. Our hypothesis requires the presence of elevated concentrations of U and Th in the CMB, and is amenable to testing by direction sensitive geoneutrino tomography.

💡 Research Summary

The paper investigates whether natural nuclear fission reactors (georeactors) could exist at Earth’s core‑mantle boundary (CMB) and what geochemical signatures such reactors would produce. Recent geochemical evidence (e.g., excess 142Nd) suggests a deep, isolated reservoir that has retained a substantial fraction of the planet’s heat‑producing elements (U, Th, K) for >4.5 Ga. The authors argue that this reservoir most plausibly resides in the lowermost mantle, directly above the liquid outer core.

High‑pressure experiments show that calcium‑silicate perovskite (CaPv), which makes up ~5 % of the CMB, concentrates U and Th 3–4 orders of magnitude more than co‑existing Mg‑perovskite or ferropericlase. Assuming a homogeneous distribution, the initial (4.5 Ga) concentrations in CaPv are estimated at 4.3 ppm U, 7.9 ppm Th, and 23 ppb 244Pu (the latter inferred from its behavior as a large‑radius actinide). These values are far below the ~1 wt % U concentration previously identified as necessary for criticality, but the authors demonstrate that only a modest local enrichment—about an order of magnitude for Pu and a factor of ~20 for the total fissile material—would be sufficient to raise the neutron multiplication factor (k_eff) above unity.

The key role of 244Pu is highlighted: its spontaneous fission rate is ~10³ times higher than that of 238U, providing a substantial neutron source despite its low abundance. In the early Earth, the combined spontaneous fission of 4.3 ppm U and ~19 ppb Pu yields a neutron production rate comparable to the threshold required for a self‑sustaining chain reaction. Once a reactor ignites, fast neutrons are only slowly thermalised (mean free path ≈ 100 m) and are predominantly captured by 238U and 232Th, producing 239Pu and 233U, respectively. These newly formed isotopes are themselves fissile, turning the system into a breeder reactor that can maintain criticality without external fuel supply.

Thermodynamic and dynamical processes at the CMB—centrifugal forces from a faster early Earth rotation, buoyancy‑driven melt segregation, and local density heterogeneities observed seismically (features as small as 30 km)—could plausibly generate the required enrichment factors. The authors estimate that a concentration factor of ~20 would keep the overall CMB subcritical while allowing isolated “hot spots” to operate as 5 TW reactors. Such power output represents 10–15 % of the present surface heat flow (31–44 TW) and is compatible with upper limits derived from KamLAND antineutrino measurements (≤ 18 TW).

Geochemically, a 5 TW reactor operating continuously for 4.5 Ga would release ~7 × 10²⁹ J, producing helium, krypton, and xenon isotopic signatures that match observed mantle values. Specifically, the ³He/⁴He ratio in the atmosphere (1.37 × 10⁻⁶) and the excesses of radiogenic Xe isotopes can be reproduced by binary fission of U and Th (Pu fission is neglected in the model). The model also predicts modest enrichments of fission‑produced trace elements (Se, Mo, Ru, Pd), offering additional testable markers.

Finally, the authors propose direction‑sensitive geoneutrino tomography as the only feasible observational test. Georeactors emit electron antineutrinos with a characteristic energy spectrum; modern large‑volume detectors (e.g., JUNO, Hyper‑Kamiokande) could resolve a CMB‑origin component by exploiting angular information. Detection of such a signal would confirm the existence of deep‑Earth nuclear reactors, while its absence would place stringent limits on CMB enrichment.

In summary, the paper presents a coherent scenario in which modest local enrichment of U, Th, and especially 244Pu within CaPv at the core‑mantle boundary can ignite and sustain natural fast‑neutron reactors. These reactors could account for a non‑negligible fraction of Earth’s internal heat and explain several puzzling noble‑gas isotopic anomalies, and they are testable with forthcoming geoneutrino experiments.