Nuclear Power Plants with Circulating Uranium Hexafluoride Based Fuel. Results of Investigations of Fluid Dynamics and Heat Transfer. Applications, Challenges, and Prospects. An Overview

The propositions of the development of reactor systems using UF₆ as the nuclear fuel were put forward in USSR and USA already in the fifties of the last century, while from the beginning of the seventies the UF₆-based reactors have become considered as the power sources for spaceborne nuclear power plants (NPP). The application of UF₆ circulating in the closed NPP contourmakes it possible to realize the potential advantages of the flow-through design due to the gaseous fuel mobility compared with the existing NPPs with solid cores. At different plant designs, reactor arrangements, and special flow organization in fuel elements the power range from hundreds of kilowatts to tens megawatts can be realized. The areas of application of power plants with circulating UF₆ can, in particular, include the spaceborne NPPs of wide power range for electric and plasma rocket engines used in the manned flight to Mars, the reactor-laser with direct pumping of gaseous laser mixtures with nuclear fission products, and the on-ground nuclear electric power plants of new generation with high performance with respect to the fuel cycle and safety. On the basis of an analysis of the results of investigations performed in USSR and USA to the end of the nineties of the last century and presented in this overview it can be concluded that from the standpoint of the physics of working processes and constructional materials resistant in the UF₆ environment, there are no insurmountable obstacles for developing the NPPs with circulating UF₆. The list of problems, whose solution can favor the further development of this line of research, if it would be implemented, is formulated. In Section 1 the fuel element designs are described and the techniques of flow organization in them are experimentally validated. In Section 2 the computational and experimental support of the programs is presented. It includes the calculations of the fluid dynamics and heat and mass transfer, the thermal and transport properties of uranium hexafluoride, the special properties of uranium hexafluoride as a working body, and the uranium hexafluoride effect on the constructionalmaterials. In Section 3 the projects of spaceborne power plants of closed and open type on a wide power range are reviewed. In Section 4 one of the promising lines in the field of the nuclear energy use, namely, the reactor-laser development, is presented. Section 5 overviews the on-ground plants, where, apart from the electric power plants, of interest are transport power plants, high-temperature engineering systems, in particular, for hydrogen production, the plants for producing high neutron fluxes, and some others. Section 6 presents considerations concerning the realization of rig reactor experiments with uranium hexafluoride circulating in the reactor core, whose criticity is fully ensured by the gaseous UF₆. These experiments could be the final stage in corroborating the power source of the new type both under space and on-ground conditions. Section 7 contains an analysis of the available publications on the state of the art of investigations in the United States, which allows one to suppose that in USA a vast program of studies on the use of nuclear reactors with circulating UF₆ in the space and on-ground energetics is being systematically performed. In Section 8 some problems which need to be solved in developing the power plants with circulating uranium hexafluoride are listed; they require a vast amount of scientific research. The work on the determination of the design of the power plants with circulating UF₆ and hydrodynamic processes occurring in them were conducted in the Keldysh Research Center (before 1976 the Institute of Thermal Processes) under the supervision of A.A. Pavel’ev, who was the author of many ideas in the field of hydrodynamic stability and turbulence realized in experimental setups and computationmethods. In memory of his invaluable contribution into this line of research the authors dedicate to him this overview.