Experimental measurements of the basic physical properties of the melt NaF-LiF-BeF 2 are presented as validation of the concept of a molten-salt reactor for burning actinides from spent fuel from light-water reactors. Compositions which are characterized by the minimal molar fraction LiF 15-17% and BeF 2 25-27% and meet the special requirements for a fuel salt for the concept under study are found. The melts of the fluorides of three metals have an acceptable melting temperature (<500°C), permit dissolution of actinide and lanthanide trifluorides to molar fraction 2% and higher at 600°C, possess good neutronphysical (even without enrichment with respect to 7 Li) and thermophysical properties, are compatible with nickel-molybdenum alloys to temperature 750°C, are inexpensive and are not strongly activated by neutrons so that they do not present a long-term disposal risk.The handling of spent nuclear fuel and radioactive wastes imposes additional requirements on nuclear power and its infrastructure and technology. Closure of the nuclear fuel cycle makes it desirable to examine a three-component structure of nuclear power where, together with solid-fuel thermal and fast reactors, special reactors with liquid circulating fuel based on metal fluorides for burning plutonium and transplutonium actinides with a degraded composition from spent fuel of solidfuel reactors will operate. The integrated system of conversion of these actinides into molten-salt reactors (reactor + reprocessing unit) can be implemented on the basis of one fuel carrier, bypassing the fabrication and transport state with repeated recycling of the fuel.In this concept of a 2400 MW(t) molten-salt burner reactor with a hollow core, the ternary system NaF-LiF-BeF 2 was chosen as the fuel carrier [1]. The system considered is loaded only with trifluorides of plutonium and transplutonium actinides of spent fuel from light-water reactors and has a high efficiency in burning actinides due to the possibility of operating without additional neutron sources. The fraction of burner reactors of this type required to close the fuel cycle being considered can be about 25%.The optimal neutron spectrum for this reactor is an intermediate spectrum, so that the core does not contain an internal graphite moderator [1]. It is surrounded only with an external reflector, which is made of graphite. In such a system, the specific power density with respect to fuel salt is ~47 W/cm 3 . It is supposed that the inert gases will be automatically removed (in 30 sec) by flushing with helium and noble metals (in 2.5 h) from the core. For the chosen scenarios of the fuel load with

The results of tests of the compatibility of nickel-based alloys and metal-fluoride melts are generalized for molten-salt reactors. The structure and strength characteristics of the alloys Kh80MTYu, KhN80M-VI, and MONIKR in the initial state, after thermomechanical treatment and quenching, exposure for 1200 h to thermal convection loop filled with 58NaF-15LiF-27BeF 2 with heating by 100°C, are investigated. A method for removing impurities from the melt has been developed. The results of corrosion tests performed on samples of nickel-based alloys in a setup with natural circulation with maximum melt temperature up to 700°C and redox potential of the system 1.25-1.33 V relative to a beryllium comparison electrode are presented. It is shown that deep removal of oxidizing impurities from the melt and maintaining a low redox potential give slow uniform corrosion (less than 5 µm/yr) of Kh80MTYu and KhN80M-VI samples. After the tests were completed, none of the alloys studied exhibited any tendency toward intercrystallite absorption.The use of molten salts as working fluids in fourth-generation reactors requires solving certain urgent scientifictechnical problems. The key problem is finding or developing new structural materials which are compatible with molten fluorides under operating conditions. No less important is the problem of developing a technology which ensures the required purity of the circulating melt.The first fluoride-salt melts were developed in Oak Ridge National Laboratory (USA) for molten-salt reactors with circulating fuel, which operate in uranium-thorium and uranium-plutonium cycles [1][2][3][4]. The prototypical structural material for molten-salt reactors became nickel-molybdenum alloy Hastelloy H (Table 1), developed for the experimental MSRE with fuel salt LiF-BeF 2 -ZrF 4 -UF 4 . Experience in operating this reactor showed that fission products (tellurium) give rise to intercrystallite corrosion of the alloy, substantially limiting the service life of the reactor, and for neutron fluence >5·10 20 cm -2 the alloy is subjected to high-temperature radiation embrittlement.

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The extraction of lanthanum, neodymium, and thorium from 73LiF-27BeF 2 , 78LiF-22ThF 4 , 75LiF-5BeF 2 -20ThF 4 , 15LiF-58NaF-27BeF 2 , and 60LiF-40NaF (mol %) fluoride salt melts into liquid bismuth with admixtures of lithium as a reducing agent was studied at 580-750°C. Equilibrium values of their distribution coefficients were measured. Beryllium as distinct from neodymium and lanthanum almost was not extracted into bismuth from salt beryllium containing compositions. A decrease in the mole fraction of LiF in LiF-BeF 2 melts substantially increased the effectiveness of its purification from lanthanides. The LiF-ThF 4 and LiF-BeF 2 -ThF 4 salt mixtures with comparatively high thorium concentrations (≥20-22 mol %) could not be used for effective separation of lanthanides and thorium in a system for extraction processing of fuel salts.

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