Separation of Plutonium from Chloride Salts is Demonstrated by High-Temperature Vacuum Distillation Method

Figure 1. (left to right) a 1.5 kg (approx.) example of contaminated salt residue. Below left: After treatment using the high-temperature vacuum distillation method, the condensed salt distillate can be discarded as low-level waste. Below right: The plutonium contaminant, now in the form of a concentrated plutonium oxide, will be stored as special nuclear material.

The basis for a vacuum distillation separation is the large difference in vapor pressure between chloride salts and plutonium oxides. Table 1 gives vapor pressures of common pyrochemical salt components at selected temperatures.

Compound	850°C	950°C	1050°C
NaCL	10-0.1	10	10
KCl	100.2	10	10
MgCl2	100.3	101	10
CaCl2	10-3	10-2	10-1
CaF2	10-8	10-6	10-5
Pu	10-8	10-7	10-5
PuCl	10-2	10-1	10-0.1
PuOCl	10-8	10-7	10-5
PuO2	10-16	10-14	10-12

Table 1. The vapor pressure (torr) of pyrochemical salt components at selected temperatures.

Salt	850°C	950°C	1050°C
NaCL	5.3E-10	1.4E-8	2.3E-7
KCi	2.4E-10	6.9E-9	1.2E-7
MgCl2	1.9E-10	5.2E-9	1.2E-8
CaCl2	2.4E-7	3.3E-6	3.1E-5

Table 2. Calculated Pu concentration (ppm) in distillate salt.

The most common residue component in the pyrochemical salt residue is equimolar sodium chloride and potassium chloride salts. This composition is a eutectic mixture with a melting point at 650 °C and was used for that reason. It is obvious from the vapor pressures listed in Table 1 that the best separation can be achieved between plutonium dioxide and the chloride salts. The difference in their vapor pressures is more than twelve orders of magnitude at 1050 °C. Calcium fluoride cannot be separated by distillation because of its low vapor pressure, but it is only a minor constituent and can be left with the plutonium heel

. A separation will not be obtained if plutonium trichloride is present in the system, so the residue salts are first treated by an oxygen sparge process to convert plutonium trichloride to plutonium dioxide or plutonium oxychloride. The discussion that follows will assume that the pyrochemical waste salts have been treated to convert all plutonium and americium species to the dioxide.

The rate of deposition of the various compounds can be calculated based on their vapor pressures. Actual experiments have shown that the rates are much slower than calculated results, sometimes by some orders of magnitude; nevertheless, it appears that all the chloride salts, except calcium chloride, can be distilled at acceptable rates below 900 °C. These same experiments have shown that temperatures above 1200 °C will probably be required to distill calcium chloride at acceptable rates. Those high temperatures will require extensive equipment modification and further developmental work. Since the NaCl-KCl salts constitute the majority of the waste stream, efforts have focused on treating sodium chloride/potassium chloride salts.

Deposition rates can be used to calculate the composition of the distillate salt under ideal conditions. For example, in a system with 100 cm² surface area that contains sodium chloride and plutonium dioxide being processed at 850 °C, the rate of deposition of sodium chloride is 4100 g hr ^-1 while that of plutonium dioxide is 10^-12 g hr. The distillate salt would then contain a weight concen-tration of 10^-9 ppm plutonium. At 1050 °C the distillate salt would have a plutonium concentration of 10^-7 ppm. (All these were calculated under ideal conditions.) Table 2 lists calculated plutonium concentrations at selected temperatures in a specific salt matrix. The americium content must be below 0.03 ppm for LLW, and this requirement can also be met assuming all the americium is present in the system as AmO₂.

The LLW criterion for weapons grade plutonium translates to a plutonium concentration in the salt no more than 1.2 ppm. Therefore, vacuum distillation of a sodium chloride, potassium chloride, or magnesium chloride salt that contains only plutonium dioxide has the potential to produce a distilled salt with a plutonium concentration 10 orders of magnitude below the 100 nCi/g level.

It has been estimated that disposal of stabilized, undistilled salts at the WIPP would be one hundred to one thousand times more costly than disposal of salts treated by the high-temperature vacuum distillation method. A conservative estimate of the disposal cost of the present inventory, stabilized, would be hundreds of million dollars.

Experiments performed with oxidized sodium chloride salts and potassium chloride salts, along with plutonium, have shown that distillation separation is viable: the plutonium content of the salt has been reduced from tens of percent to the ppm and sub-ppm range in the distilled salts. Analysis of the distilled salts showed that approximately one-fourth meet the LLW criterion. The remainder samples showed the same background contamination levels of the glove boxes in which they were handled and processed. Many control experiments were done with initially uncontam-inated salts, and the analyses indicated nearly identical plutonium contamination levels. These results support the conclusion that essentially complete and clean separation of the salts from plutonium oxide can be achieved.

Figure 4. Schematic of the distillation separation appartus. The two process streams are condensed salt distillates (left) and plutonium oxide.

Advantages of Distillation Process

• minimal generation of secondary waste

• compact unit process

• simple, low-cost equipment

• large pyrochemical experience pool

• cost-effective vs. other alternatives