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218 The properties (especially the melting temperature) of such fuels will. be controlled by the composition of the diluent mixture. Blanket mixtures (and perhaps fuel systems for one-region converters or breeders) will require considerable concentrations of high-melting ThF4. These fluids must, if they are to be compatible with the temperature requirements of large steam turbines and are to retain a reasonable margin of safety between the minimum reactor operating temperature and the freezing point of the fuel, be completely molten at 975’F (525OC). It is clear that the diluent fluoride system must be low melting and must be capable of large depressions of the freezing points of the active fluorides. Simple consideration of the nuclear properties leads one to prefer as diluents the fluorides of Be, Bi, 7Li, Mg, Pb, Zr, Ca, Na, Sn, and Zn (in that order). Equally simple considerations (Table 2) of the stability of diluent fluorides toward reduction by common structuralmetals, however, serve to eliminate BiF3, BF2, SnF2, and ZnF2 from consideration.

No single fluoride c m serve as a useful diluent for the active fluoride. The compound BeF2 is the only stable one listed whose melting point is close to the required level; this compound is too viscous for use in t h e pure state. The very stable and high melting fluorides of the alkaline earths and of yttrium and cerium do not seem to be useful major constituents of low-melting mixtures. Mixtures containing about 10 mole ’$ of alkaline earth fluoride with BeF2 melt below 5OO0C, but the viscosity of such melts is almost certainly too high for serious consideration. Some of the possible combinations of alkali fluorides have suitable freezing points. Equimolar mixtures of LiF and KF melt at 49OoC, and mixtures with 40 mole ’$ LiF and 60 mole ’$ RbF melt at 47OOC. The ternary systems LiF-NaF-KF (see Fig. 1) and LiF-NaF-RbF have lower melting regions than do these binaries. All these systems will dissolve UF4 at concentrations up to several mole ’$ at temperatures below 525OC. They might well prove useful as reactor fuels if no mixtures with more attractive properties were available. ~

Mixtures with useful melting points over relatively wide ranges of composition are available if ZrF4 is a component of the system. Phase relationships in the NaF-ZrF4 system (Fig. 2) show low melting pints over the interval 4.0 to 55 mole ’$ ZrF4. A mixture of UF4 with N a F and ZrF4 served as fuel for the Aircraft Reactor Experiment. The phase diagram3 of this ternary system is shown as Fig. 3. The compounds ZrF4 and UF4 have very similar unit cell parameters and are isomorphous. They form a continuous series of solid solutions with a m i n i m melting point of 765OC for the solution containing 23 mole ’$ UF4. This minimum is responsible for a broad, shallow trough which penetrates the ternary diagram to about the 45 mole ’$ NaF composition. A continuous series of solid solutions without a maximum or a minimum exists between a3NaPoUF4 and 3NaPaZrF4; in this solution series the temperature drops sharply with decreasing ZrF4 concentration. A continuous