Thorium bismuthides

X.5 Solid thorium bismuthides X.5.1 Phase diagram and structures... 

Status of development. In de eloping a blanket systeni for the LMFR, it has. seemed logical to. select one which is as similar as possible to the core fuel. After ( onsiderable evaluation the principal emphasis has been placed upon a l)ismuth fluid containing thorium bismuthide in the form of very small p.articles. 4 his is commonly called the thorium bismuthide slurry system. 

Chemical composition of thorium bismuthide. 4 he thorium bismuthide intermetallic eompound discussed in thi.s section has the chemical formula ThBi2. This compound is 35.7 w/o thorium. A. second compound, ThaBi4, also can exist and has been obser -ed in alloys containing greater than 50 to 55 w/o thorium. 

Crystal chemistry of thorium bismuthide. ThBi has a tetragonal crystal structure (with oo = 4.942 A, and co = 9.559 A) containing two thorium atoms and two bismuth atoms per unit cell. The density as determined... 

Ordinarily, when thorium bismuthide is prepared at 500°C, very small equiaxed particles (less than 0.5 micron) are formed. These equiaxed particles grow until they reach the average size of 50 to 60 microns, and under certain conditions they can grow to considerably larger dimensions. 

When a 5 to 10 w/o thorium bismuthide slurry is cooled from a temperature of complete solution (above 1000°C), ThBi2 precipitates in the form of platelets having dianieter-to-thickness ratios greater than 50 1. The plane of the platelet is parallel to the 001 plane of the crystal. Platelet diameters up to 1 cm have been observed in alloys cooled at moderate rates. The diameters can be decreased by increasing the cooling rate. 

Other thorium compounds. A. small amount of attention has been directed toward ThC2, ThS, and ThLT slurries in bismuth. However, the major effort is on the thorium bismuthide and thorium-oxide slurries. 

M. E. Seidket, Investigation of Methods for Preparation of Thorium Bismuthide Dispersions in Liquid Bismuth, Final Progress Report, Horizons, Inc., Oct. 31, 1956. 

As is pointed out in Chapter 20, the easiest blanket to handle in the LMFR would be a 10 w/o thorium-bismuthide slurry in bismuth. Chemical processing of this blanket would be very similar to the core processes already described. The major problem consists in transferring the bred uranium and protactinium from the solid thorium bismuthide to the liquid bismuth phase, so that they can then be chemically processed. Two examples of proposed processes are shown in Fig. 22-11, which shows a process that can be used with the fused chloride salt FPS removal process, and in Fig. 24-19, which shows a flowsheet for a process to be used with the fluoride volatility process. 

This stream then goes to column 1 of the fused chloride salt FPS removal system, where all the Th, U, Pa, and FPS are transferred to the ternary chloride salt. Meanwhile, the stripped Bi is returned to dilute more blanket thorium bismuthide. 

The volatility method can be conveniently used to process a thorium bisinuthide blanket. The process must be preceded by a phase separation step which separates the thorium bismuthide solids from the liquid carrier bismuth (Fig. 24-19). The modification of the core liquid process flowsheet is as follows (1) salt effluent from the hydrofluorination step must be stored in order to achieve Pa decay to uranium, and (2) the bismuth liquid is returned to the blanket head end process without the addition of uranium. 

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