Sources of data. Knowledge of distribution equilibria in Purex systems is useful in designing the solvent extraction contactors for a Purex reprocessing plant and in predicting the change in performance of an existing plant when operating conditions are changed. The first experimental measurements were those of Codding et al. [CIO] at the Knolls Atomic Power Laboratory. These were for an aqueous phase containing only water, uranyl nitrate, and nitric acid; an organic phase consisting of a 30 v/o solution of TBP in a commercial solvent (Gulf ВТ or Amsco 123-15); and for a temperature of 25°C. Distribution equilibria were represented graphically as plots of molarities of nitric acid and uranyl nitrate in the organic phase as functions of the corresponding molarities in the aqueous phase. After TBP became generally accepted as the preferred solvent for fuel reprocessing, many additional studies were made of distribution equilibria between aqueous nitric acid and TBP dissolved in a hydrocarbon diluent. These extended the early work to other hydrocarbon diluents, to temperatures other than 25°C, to TBP concentrations between 5 and 100 v/o, and to additional distributed components including plutonium, neptunium, and thorium.

It was found that distribution equilibria are not very sensitive to the composition of the hydrocarbon diluent, provided that it consists mostly of saturated (paraffinic or naphthenic) hydrocarbons containing about 12 carbon atoms per molecule. However, Purex plants now usually specify a synthetic, straight-chain, saturated hydrocarbon made by polymerizing and hydrogenating lower olefins, which contains an average of 12 carbon atoms and is mostly n-dodecane.

The SEPHIS+ computer program was developed by Gronier [G16] for Purex equilibria in 15 v/o TBP. The program was adapted to the conventional 30 percent TBP Purex process by Richardson at Hanford [R7], and was further modified and generalized by Watson and Rainey [W5] at Oak Ridge. The SEPHIS code predicts the equilibrium distribution of uranium, tetravalent plutonium, nitric acid, and water between an aqueous phase containing these components and an organic phase containing TBP at any concentration between 2.5 and 100

tSolvent Extracting Processes Having Interacting Solutes.

v/o, at temperatures between 0 and 70°C. The SEPHIS code may be used for uranium concentrations up to 2 M, Pu(IV) up to 0.2 M, and nitric acid up to about 6 M.

Scotten [S5] at Savannah River developed a similar program, SOL VEX.

Distribution coefficients. The equations used in the SEPHIS code to correlate distribution equilibria are too complex for hand calculation or for graphic representation in a few figures. To provide a semiquantitative basis for stage-to-stage calculation of the separation performance of Purex solvent extraction contactors described in Sec. 4.14, Figs. 10.13 through 10.16 have been plotted from computer printouts from the SEPHIS code kindly provided by Vaughen [VI]. These give distribution coefficients for nitric acid and uranyl nitrate at 40 and 55°C between an aqueous phase and 30 v/o TBP in normal dodecane.

Plutonium and fission-product and other inextractable nitrates are present in significant amounts in contactors HA and HS. Their effects on distribution coefficients of nitric acid and uranyl nitrate may be taken into account approximately by reading distribution coefficients of nitric acid or uranyl nitrate given in these figures at a value on the horizontal, x, axis equal to

the combined molarities of Pu(N03)4 and U0j(N03)2, from the curve whose designated nitric acid molarity equals the sum of the actual HN03 molarity plus the normality of inextractable nitrates. Problem 10.3 requires application of these adjustments.

Distribution coefficients for Pu(IV) may be obtained from Fig. 10.17. This plots the ratio of distribution coefficients of tetravalent plutonium to hexavalent uranium.

(1.0 — 0.0724*u — О. ІЗхрц — 0.0309xH — 0.03lxs)2 298 Г/

In the SEPHIS code [W5] this distribution coefficient ratio DpJDv is evaluated from

where F = volume fraction of TBP in dry solvent *no3′ = total nitrate molarity in aqueous phase

= 2дги + 4xpu + xH + xs (10.2)

Xy = uranyl ion molarity in aqueous phase xpu = molarity of Pu4+ in aqueous phase xh = hydrogen ion molarity in aqueous phase and Xg = molarity of N03" associated with other nitrates in aqueous phase

= *no3 ■ — 2xu — ^Xpu — -*h (10.3)

The coefficients of Xu, xPu, xH, and Xg are so nearly proportional to the charges of the respective cations that this equation may be simplified to

for F = 0.30 (30 v/o TBP). For uranium molarities under 0.5, the difference between Eq. (10.1) and Eq. (10.4) is less than 0.1 percent. Figure 10.17 is a plot of Eq. (10.4) for temperatures of 25, 40, and 55°C.

4.15 Example of Use of Purex Equilibrium Data

Use of these Purex equilibrium charts will be illustrated by calculating the number of equilibrium extracting and scrubbing stages needed in the uranium decontamination unit 2D of the Barnwell Nuclear Fuel Plant, data for which were given in Fig. 10.11 and Tables 10.7 and 10.8.

Table 10.10 gives flow rates and compositions for the streams to and from this unit. Process quantities taken directly from Tables 10.7 and 10.8, or calculated from them in

Moles uranyl nitrate per liter in aqueous phase, x(j Figure 10.16 Distribution coefficient of nitric acid between 30 v/o TBP in hydrocarbon diluent and aqueous uranyl nitrate at 55°C, from SEPHIS code.

equivalent units, are in italics. The remaining quantities in Table 10.10 have been adjusted as stated in the footnotes for two reasons. (1) Component flow rates in some streams have been changed slightly to provide exact material balances. (2) Volume flow rates, which change by a few percent in Fig. 10.11, have been held constant to simplify calculation; compositions were adjusted where necessary to keep component flow rates unchanged.

Figure 10.18 shows the solvent extracting system to be analyzed and the nomenclature to be used. Input and output flow rates and concentrations are from Table 10.10. Three extracting and two scrubbing stages are shown, because the calculation next to be described indicates that between two and three theoretical extracting stages and between one and two scrubbing stages would be sufficient for the specified separation.

In this example, uranium and ruthenium are the key components whose compositions in feed, aqueous waste, and organic extract are specified. Nitric acid concentration in feed is specified, but its distribution between the two product streams must be found by trial, as in the zirconium-hafnium separation example in Sec. 6.5 of Chap. 4. The HN03 concentrations of 0.02 M in organic extract and 1.658 M in aqueous waste were found by trial to require the same number of scrubbing and extracting stages as the specified uranium and zirconium separation, as will now be shown, and hence represent the calculated distribution of nitric acid.

Table 10.11 gives steps in calculating concentrations of nitric acid, uranyl nitrate, and ruthenium as a function of stage number in the extracting section. Starting from the given aqueous concentrations xf, marked with a f, the calculation proceeds through alternative distribution-equilibrium and material-balance calculations. No iterations are required, as the distribution coefficients of uranyl nitrate and nitric acid are available in Figs. 10.13 and 10.14 as functions of the first calculated aqueous concentrations.

0 1 2 3 4 5 Moles N03" per liter in aqueous phase Figure 10.17 Distribution coefficient ratio, tetravalent plutonium to hexavalent uranium in 30 v/o TBP, from SEPHIS code.

Table 10.12 gives the steps in calculating concentrations of these three components in the scrubbing section, starting from the specified composition of the organic extract stream vf, marked with a $. As the distribution coefficients of Figs. 10.13 and 10.14 are given as functions of the to-be-calculated aqueous composition, these must be found by the successive approximation procedure shown in the table. In this, distribution coefficients are assumed, trial aqueous concentrations are calculated, distribution coefficients are obtained from Figs. 10.13 and 10.14, and the process is repeated until calculated distribution coefficients are the same as assumed.

Figure 10.19 is a plot of the concentrations of ruthenium (bottom, circles) and nitric acid (top, squares) versus uranium concentration in the organic streams leaving the designated stages of the extracting section (filled symbols) and scrubbing section (open symbols). The inter­section of the two bottom lines shows that the specified ruthenium-uranium separation would be obtained at a value of n = 2.4 (TV = 2.4 theoretical extracting stages) and m = 2.1 (M = 1.1 theoretical scrubbing stages, because organic stream from stage m flows into scrubbing stage m— 1). The intersection of the two top lines shows that the assumed nitric acid-uranium separation would be obtained at the same values of m=2.1 and л = 2.4 thus establishing that the assumed nitric acid concentrations in aqueous waste and organic extract streams are correct. Concentrations at the intersections of the two curves are the calculated values for the organic stream flowing from the extracting to the scrubbing section.

Stream			In				Out
Feed	Acid	Scrub	Solvent	Total	Waste	Extract	Total
Number, Fig. 10.11	16	17	19	18		20	21
Phase	Aqueous	Aqueous	Aqueous	Organic		Aqueous	Organic
g-mol/liter
HN03	0.86	12	0.01	0		1.658*	0.02“
U02(N03)2	1.431*	0	0	0		0.01864d	0.38
О Ru/liter	0.2675	0	0	0		0.1560*	0.0000174
Liters/h	626	107	340	2305“		1073c	2305
g-mol/h
HN03	538	1284	3	0	1825	1779*	46“	1825
U02(N03)2	896	0	0	0	896	20	876	896
Ci Ru/h	167.4	0	0	0	167.4	167.366	0.0401	167.4

“Quantities in italics evaluated from Tables 10.7 and 10.8. b Adjusted to close material balance.

“Adjusted to keep constant volume flow rate.

d Adjusted to keep mass flow rate in residue the same as evaluated from Table 10.7.

“Nitric acid distribution cannot be specified in advance, but is confirmed by calculation of Tables 10.11 and 10.12.