Our earth is still a young planet with substantial heat sources that are characterised by volcanic activity and earthquakes generated by the movement of tectonic plates. The theory of plate tectonics successfully explains various geological phenomena that occur on the continents and in the oceans of Earth, but the driving force behind plate motion has not been entirely resolved. Regarding origin of the heat, the current consensus is that the flow of heat from Earth’s interior to the surface comes from two main sources: radiogenic heat and primordial heat. Primordial heat, which was generated during initial formation of Earth, is the kinetic energy transferred to Earth by external impacts of comets and meteorites and the subsequent effects: gravity-driven accretion, friction caused by differentiation of Earth’s mantle structure (sinking of heavy elements like Fe, rising light elements like Si) and latent heat of crystallization released as the core solidified1.

Since Kuroda2 first proposed that natural fission reactors were operating on Earth around two billion years ago, much attention has been focused on nuclear energy as the driving force of plate motion. Herndon3 asserted the feasibility of planetocentric nuclear reactors and developed the concept extensively. Because there is very little U in iron meteorites, however, a nuclear reactor in Earth’s core or on other terrestrial planets seems unlikely4. Meijer and van Westrenen5 reported nuclear fission of U and Th as heat generation sources at the mantle boundary within Earth’s core, based on the distribution of an isotope of Nd in rocks6. Bao7 noted that there are many heat producing elements (U and Th) in a calcium perovskite reservoir at the base of the mantle.

In 2005 and 2007, scientists at the Kamioka Liquid-Scintillator Antineutrino Detector(KamLAND)8 and Borexino9 detected signals of antineutrinos, (i.e., “geoneutrinos”) produced inside Earth, respectively. Neutrinos, very light subatomic particles, are generated by the nuclear fission and decay of radioactive elements as well as by nuclear fusion that occurs in the sun and stars. Because fission occurs on the timescale of a fraction of a microsecond, it is necessary for heat generation to include a chain reaction of the nuclear decay of atoms in rocks and minerals with high concentrations of radioactive decaying atoms. As such, alpha and beta decay can supply heat on timescales comparable to the age of Earth. The KamLAND Collaboration reported that heat from radioactive decay of radiogenic isotopes such as 238U and 232Th contributes about half, 21 TW, of Earth’s total heat flux (44.2 ± 1.0 TW) and that Earth’s primordial heat supply has not yet been exhausted10.

However, there are four unanswered questions regarding the decay of such radioactive isotopes. The first question asks why a large nuclear mass emission from radioactive elements primarily concentrated in the shallow crust would not lead to the death of many living things. Although it is believed that the radiogenic heat production rate cannot cause damage to living things, we have seen an example of spontaneous ignition due to high enough concentrations of radioactive elements in crustal rocks at Oklo in Gabon, Africa11. In the case of spontaneous ignition, the emission products from radioactive elements would be distributed through active volcanoes and the movement of mountain ranges. Indeed, we do not suffer from natural radioactive pollution. The second question regards the amount of Pb that exists in Earth’s crust. The KamLAND Collaboration10 reported emission of by two reactions, 238U → 206Pb + 8α + 6e− + 6 + 51.47 MeV and 232Th → 208Pb + 6α + 4e− + 4 + 42.7 MeV after six- and four-times of β decay, respectively. If these reactions are responsible for the continuing heat generation in the crust, a large amount of Pb would be included in natural rocks and ores. However, the concentration of Pb in the crust is only 12.5 ppm12. The third question addresses the heat to thermal imbalance: the estimated slope of temperature change from the core to the crust is negatively linear. The linear slope can be explained by heat generation in Earth’s inner core only. If the estimated heat contributions from the mantle (10 TW) and the crust (7.9 TW)10 are correct, the temperature curve must have two peaks, one in the crust (6–40 km) and one in the mantle (410–2900 km) (Supplementary Information 1). The inhomogeneity of surface heat flow in the crust could be derived from geological disturbance in pressure-less region. As for the fourth question, if radioactive decay has also been occurring on Venus, which is Earth’s sister planet with similar size and composition, we should observe plate tectonics as a result of the carbonate magma-ocean. Plate tectonics, however, are not evident on Venus13. Thus, these facts put severe constraints on the possibility that radiogenic heat production in the crust and the mantle are producing .

Alternatively, we consider inductively the possibility of nuclear fusion, which does not create harmful radioactive waste but generates a large amount of heat. Because an increase in paraeomagnetic magnitude between 2.7–2.1 billion years indicates the nucleation of the inner liquid-core14, nuclear fusion would have started around 2.2 billion years ago15. Our hypothesis may explain why plate tectonics exist on Earth but not on other terrestrial planets, such as Mercury, Venus, Mars, and Earth’s moon. Furthermore, another example of nuclear fusion in Earth’s interior is that the origin of N in Earth’s atmosphere is interpreted to be the result of endothermic nuclear transmutation16,17.