Abelson and Hoover [Al], working in the U. S. Naval Research Laboratory, found that thermal diffusion in UF6 at pressures above the critical (4.6 MPa) resulted in small but measurable enrichment of M5U at the hot wall. Because of the simplicity of thermal diffusion equipment compared with the advanced technology needed for the gaseous diffusion process, the Manhattan District in the United States in 1944-1945 used thermal diffusion of UF6 to raise its MSU content to 0.86 percent, to serve as partially enriched feed for the Y-12 electromagnetic plant. Energy for the S-50 thermal diffusion plant was obtained from steam which later powered the 150-MW electric generating station which drove the compressors of the K-25 gaseous diffusion plant.

The thermal diffusion plant [Al] contained 2100 columns, each with an effective height of

14.6 m. Each column consisted of three concentric tubes, the innermost being made of nickel,

»’Oak Ridge National Laboratory, U. S. AEC, Oak Ridge, Tennessee.

* Mound Laboratory, U. S. AEC, Miamisburg, Ohio.

§ Feed not of normal abundance, contained 1 percent 3He from nuclear reaction.

the middle of copper, and the outer of iron. The inner tube, about 5 cm in diameter, carried condensing steam at temperatures that could be varied from 188 to 286°C. The annular space (about 0.025-cm gap) between nickel and copper was filled with UF6 at a pressure of 6.7 MPa, well above the critical. The outer annular space between copper and iron carried cooling water at 63°C, slightly above the freezing point of UF6. The columns were operated batchwise, with periodic removal of slightly enriched UF6 from a header connected to the top of a group of columns and slightly depleted UF6 from a larger reservoir connected to the bottom. Operation of the complete plant of 2100 columns was affected by frequent leaks and freezeups, so that its performance is less representative than that of tests made in individual columns, which are summarized in Table 14.25.

Their separation performance was characterized by two parameters. Y is In yp/yp, the overall separation between top and bottom when equilibrium is attained at total reflux, ф is a parameter that was inferred from the rate at which product composition at total reflux approached equilibrium. The theory of the time-dependent separation performance of a thermal diffusion column developed by Cohen [C6] and others shows that ф is given by

(14.338)

where Ci and Cs are the parameters in the differential equation for the steady-state separation performance of a countercurrent column:

Operating conditions

tc, estimated, Q = H(1 — 300/T). L = 1460 cm. Y = In (yp/yF) at steady state at total reflux.

Д kg U SWU/yr = (0.238 kg U/352 g UF6) (365 day/yr) (0.80 C? L/4CS) (g UF6 SWU/day) = 0.0494 ф.

dy _ СіУ( — у) P{yp — у)

dz С$ Cs

An equation of the same form (14.181) was derived for the gas centrifuge treated as a countercurrent column. L is the active length of the column, 1460 cm.

The maximum separative capacity, and the power consumed per unit separative

capacity, G/Дпих, given in the last two columns of Table 14.25 have been calculated from Abelson’s parameters Y and ф to permit comparison with the other processes for enriching uranium treated in this chapter. Because the thermal diffusion column operates with constant reflux ratio, its steady-state separation performance as an enricher is given by Eq. (14.237), expressed here in the form

УР=_____________ (P/Ci) + 1______________

yF CP/С,) + exp {- [(P/CO + 1] (CiL/Cs)}

Its separative capacity A, for у < 1, is

A = — P fin —— — + A (14.343)

У yF УР )

With ур/ур from (14.340) and the separative capacity A from (14.343), the maximum value of A at CtL/Cs around 0.6 is found to be

(14.344)

At P/Сі around 1.8, 0.80 is the maximum value of the ideality efficiency Ej for this thermal diffusion column considered as a square enriching cascade.

The next-to-the-last column of Table 14.25 gives maximum values of the separative capacity of this thermal diffusion column if operated at the optimum product rate for each set of the operating conditions given in the first six columns. The last column gives the ratio of the power loss from heat input to separative capacity. The optimum set of operating conditions are those in the third row of Table 14.25, with a UF6 pressure of 6.7 MPa, a steam temperature of 559 K, and an annular spacing of 0.0248 cm. At these conditions this column would have a separative capacity of 2.46 kg uranium SWU/year and would consume heat equivalent to a power loss of 38 kW/(kg uranium SWU/year). The separative capacity of 2.46 kg uranium SWU/year of this thermal diffusion column 1460 cm high may be compared with the centrifuge of Tables 14.15 and 14.16, which had a higher separative capacity of 10 kg uranium SWU/year in a lower height of 335.3 cm. The specific power consumption of 38 kW/(kg uranium SWU/year) may be compared with 0.266 kW/(kg uranium SWU/year) for the U. S. gaseous diffusion plants. The much greater specific power of thermal diffusion was the principal reason that the Manhattan District’s thermal diffusion plant was shut down as soon as the K-25 gaseous diffusion plant began operation.

Although its very poor power utilization compared with gaseous diffusion and the gas centrifuge precludes use of thermal diffusion for large-scale uranium isotope separation, the simplicity of the equipment, the absence of moving parts, and the large separation attainable in
a convenient height have led to its use for small-scale separation of many isotopes, as suggested by Table 14.24.