A UF4-fuel, vapor core reactor-MHD generator, closed Rankine cycle power system was initially evaluated as the leading GCR concept within INSPI's research program. Initially, for chemical compatibility with UF4 and for enhanced MHD performance, inert helium was suggested as a buffer gas. The cycle proposed was a hybrid: Rankine for the UF4 and Brayton for the helium. This cycle was the first to be analyzed since abundant thermodynamic data existed. After careful evaluation, it was determined that a complete Rankine-MHD cycle is more promising than the hybrid Rankine-Brayton MHD. In essence, the improvements obtained by the presence of helium in the MHD were significantly reduced by the work needed to compress the returning gas. Also, the UF4-He combination was found to yield a maximum effective radiator temperature of ~1200 K; this results in a radiator area 5-10 times larger than a ‘pure' Rankine cycle. These limitations are removed by the use of a condensing working fluid; however, additional chemistry and materials problems are introduced. To use the ultrahigh temperature fluids produced by the vapor core, the present leading candidate is a complete Rankine cycle for both fuel and working fluid. The current project is analyzing liquid metals and fluorides for a companion working fluid to the UF4. The fluorides LiF, NaF, KF, CsF, RbF and others are presently under consideration; the mixed fluid properties are being compared with performance requirements for the reactor core, MHD, condensing radiator and materials compatibility. Thermodynamic cycles using UF4 fuel-fluoride working fluid mixtures have been determined to satisfy performance requirements established for the baseline selection criteria for these cycles. Such a cycle is presented in the next section. It should be pointed out, however, that helium is being used in the exploratory experimental phase of the program due to its excellent properties as a buffer gas, ease of handling and favorable materials considerations. 2.5 Ultrahigh Temperature Magneto Energy Conversion Magnetic ‘turbines' appear best suited as energy conversion devices for the high temperature fluids produced by the nuclear vapor reactor cores. For these coupled technologies, the material interaction limitations of other energy conversion devices is replaced by a fissioning conducting vapor interacting with external circuits through electric and magnetic fields. Two types of magnetic turbines have been identified: conventional MHD generators and magneto-induction generators. MHD generators are normally DC machines which can operate at modest levels of fluid conductivity (1-100 mho/m) and can be operated in linear or disc flow geometries. Pulsed Electromagnetic Induction (PEI) devices eliminate the need for electrodes at the expense of requiring much higher levels of conductivity. Presently, INSPI's ultrahigh temperature reactor and conversion research program for MWe class power systems has selected MHD conversion as the primary focus because its science and technology are much better known than those of PEI generation; also, a more comprehensive research program can be presently followed for MHD power conversion. However, the scientific and technology base developed for MHD will serve to establish a better base for the potential study of PEI for ‘super' high power, compact devices. MHD generators have the capability to use the UTVRs high temperatures; however, they have very specific technical requirements that impose severe cycle restrictions. The power density in the MHD duct is proportional to the product of conductivity, velocity squared and magnetic field squared (cr»2xB2). Therefore, the enthalpy extraction is very sensitive to the MHD input-output fluid conditions. The vapor core reactor provides a hotter-than-most fluid with potential for adequate
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