7.1 Requirements

The principal functional requirements of a solvent extraction contactor are as follows:

1. To develop sufficient interfacial area to promote transfer of extractable components between phases

2. To facilitate countercurrent flow of the two phases, without excessive entrainment Additional considerations in selecting contacting equipment are as follows:

3. It should have flexibility to operate under varied conditions of flow ratios and concentra­tions.

4. It should be mechanically dependable and easy to operate and maintain.

5. It should be compact and have low holdup of process materials.

6. Initial cost and operating cost should be low.

The importance of these different factors varies with the application. Although reliability is important for any application, it is particularly important when processing highly radioactive materials, as in reprocessing discharged reactor fuel, where the intense radioactivity makes normal methods of maintenance difficult or impossible. Equipment handling such highly radioactive solutions must be enclosed in massive shielding and operated, and perhaps main­tained, by remote control. If concrete is used as shielding material, thicknesses of 2 to 3 m are often required. To control the cost of such massive shielding, it is important to reduce its bulk, and this means using compact solvent extraction contactors. If vertical column contactors are used, their height should be kept to a minimum. If a horizontal array of mixer-settler contactors is used, the layout of equipment and piping should be compact.

Those applications that require remote operation and maintenance dictate the use of simple, rugged equipment, with a minimum of moving parts and with little tendency to foul, rust, clog, or corrode.

The intense radioactivity associated with the reprocessing of discharge fuel, and the degradation and decomposition of organic solvent when exposed to ionizing radiation, require that the amount of solvent exposed to radiation be kept to a minimum. Also, the length of time that the organic is exposed to radiation should be kept small. This places a premium on compact contacting equipment, with high throughput per unit volume.

Nuclear criticality places special constraints on the size of contacting equipment in fuel reprocessing. This is particularly important when reprocessing highly enriched uranium fuel or for the stripping-scrubbing contactors that separate plutonium from low-enriched uranium, as illustrated in Fig. 4.5. Both 23S U and plutonium fission. As shown by the criticality data in Table 4.11, as little as 760 g of 23SU or 510 g of plutonium can form a critical mass when dispersed at the optimum concentration through a hydrogenous medium, such as a nitric acid solution or organic, with relatively little fission products or nonfissile uranium [A2, Т1]. The sizes of the contactors and other process equipment must be kept small enough to promote neutron leakage and make criticality impossible [C4]. Limiting dimensions may be as small as 14 cm in diameter for a cylindrical column contactor or 4.6 cm in height for an array of mixer-settlers in horizontal slab geometry [A2, Т1]. Larger equipment sizes are acceptable for process operations that do not involve solutions of relatively pure fissile material, such as the extracting-scrubbing contactors that separate the fission products from low-enrichment uranium fuel. The allowable dimensions and throughput of criticality-limited process equipment can be increased by incorporating fixed neutron absorbers, i. e., “poisons,” such as boron or gadolinium, without the equipment.

Contactors with low inventory of process solutions are also important when the material pro­cessed is valuable, such as the plutonium recovered from irradiated fuel. Low inventory is also impor­tant in maintaining a close accountability of the total inventory of fissionable material processed. [10] [11] [12] [13]