Numerical simulation is used to study the possibility of long-term chain nuclear reactions deep inside the Earth over 4 billion years. The active layer (the natural nuclear reactor operating on fast neutrons in lakes) can form when uranium oxides or carbides precipitate from a liquid layer onto the Earth's solid interior core. A mechanism of uranium concentration in the Earth's core is studied, experiments are performed, and it is shown that a nuclear chain reaction with breeding of fissioning nuclides could have occurred in such a layer. The basic neutron-physical characteristics of such a natural nuclear reactor are calculated. It most likely operates in a pulsed mode. The critical condition for the duration of the reaction is the power level. It is found that for certain optimal power this process in the Earth's core can last for more than 4 billion years up to the present time.The Earth emits more heat (~45 TW) than it could as a result of only cooling [1]. Additional heat (~40%) is released as a result of the decay of radionuclides. A substantial quantity of radioactive elements is found not only in the Earth's crust but also in the lower mantle [2]. The heat flux from the Earth's core due to cooling and crystallization of its material ranges from 5 to 10 TW [3]. It is known that because of the differences in the half-lives of 235 U and 238 U the fraction of the former nuclide decreases with time, and billions of years ago uranium contained more 235 U [4]. It follows from the present-day concentration of uranium in pitchblende that about 2 billion years ago and earlier, when the 235 U fraction exceeded 3%, a spontaneous fission chain reaction could have occurred in rich uranium deposits. In 1972, traces of a natural nuclear reactor were discovered in West Africa in a uranium deposit at Oklo (Gabon) [5]. Later about 20 reactor zones were found in the same region. Investigations of other locations of actinide concentration and operation of natural nuclear reactors, not only in the Earth's crust but also in the Earth's core, have been started [3,6,7].The yield of fission products, which migrate from the chain reaction zone and reach the Earth's surface, is analyzed to determine the characteristics of natural nuclear reactors. The ratio 3 He/ 4 He is instructive [3]. Approximately 1 of 10,000 fissions of actinides occurs not into two but three fragments, one of which is tritium, which after undergoing β decay converts into 3 He. Thus, the ratio 3 He/ 4 He in volcanic lava in Hawaii is almost 40 times higher than its value in the Earth's atmosphere. Data confirming these facts in other locations (actually over the entire Earth) exist [8]. Other fission products of the actinides from a natural nuclear reactor, which have reached volcanic lava, are also being studied. Electron antineutrinos, which arise as a result of the β decay of the fission products from fissioning of heavy nuclei during a nuclear chain reaction [9], can be anoth-

and Power Engineering named after A. I. Leypunsky, Obninsk, RussiaThe possibility of continuous nuclear fission chain reactions during four gigayears up to the present in the interior of the Earth is studied. Natural fast reactor in the form of lakes could be formed as a result of precipitation of uranium from the liquid layer on the solid Earth core. Mechanism of uranium concentration in the Earth core is described. In such layers a chain nuclear reaction with new fissile nuclides breeding could proceed. Some characteristics of georeactor have been calculated. Its operation in a pulse mode as it was in case of a natural nuclear reactor in Oklo (Gabon) is most probable.

Abstract. Technique of evaluation of multiplying and reactivity characteristics of fast reactor operating in the mode of multiple refueling is presented. We describe the calculation model of the vertical section of the reactor. Calculation validations of the possibility of correct application of methods and models are given. Results on the isotopic composition, mass feed, and changes in the reactivity of the reactor in closed fuel cycle are obtained. Recommendations for choosing perspective fuel compositions for further research are proposed.

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