Although crossflow filtration is not a new technique, is it nevertheless one of the more recent applications of filtration used in radioactive processing

environments. Unlike the previous two filtration methods, which produce, in a single pass, solids which are essentially free of the mother liquor, cross­flow filtration produces a concentrated slurry of the solids within the origi­nal mother liquor. Successive additions of water or other process fluid and re-filtration steps are necessary to free the solids from the mother liquor.

Crossflow filtration recycles the solid-liquid slurry at high velocity (typi­cally 4 to 5 m/sec) and moderate pressure through a bundle of several hundred parallel sintered stainless steel, or porous graphite, filter tubes, normally arranged in the manner of a shell and tube heat exchanger. The pore size can range from a few hundredths to a few tenths of a micron, depending on the particle size of the slurry. The tubes are typically 1 to 2 meters long and 5 to 6 mm internal diameter. Sintered stainless steel tubes are welded to the end plates to form the tube bundle, while graphite tubes are sealed with elastomer seals.

During each passage of the slurry through the tube bundle, some of the liquid in the feed slurry permeates the pores in the tube and flows into the “shell side” of the filter unit, which is maintained at a lower pressure than the tube side. Smaller filter tube pore sizes are found to be capable of sus­taining higher specific liquid flowrates (or flux) than larger ones. This is because they are less susceptible to blocking by the solid particles. As the slurry is recirculated through the tube bundle it becomes progressively more concentrated. When the desired concentration is reached the filtration is stopped and the concentrated slurry is mixed with water or other washing agent and the filtration cycle is then repeated. These cycles are repeated until the desired degree of separation of mother liquor from the solids is achieved.

Crossflow filtration has been developed and demonstrated for several nuclear waste processing plants that are under design and construction in the USA. At the Savannah River nuclear site in South Carolina, the Salt Waste Processing Facility will use crossflow filtration to treat high level waste from the tank farms prior to cesium-sodium separation and vitrifica­tion of the highly active constituents in the Defense Waste Processing Facility (Poirer, 2007). The high level waste is first mixed with mono-sodium titanate (MST) to sorb TRU elements in the waste, and then the MST and solids already present in the waste, are concentrated by crossflow filtration. At the Hanford nuclear reservation in Washington, the Waste Treatment Plant also will use crossflow filtration to concentrate high level waste sludge prior to vitrification (e. g. Geeting, 2006 and Peterson, 2007).

A highly successful nuclear application of crossflow filtration that has been in use since the early 1990s is the Enhanced Actinide Recovery Plant (EARP) at the Sellafield nuclear site in northwest England. In EARP, acidic, TRU-contaminated wastes arising from the “Magnox” reprocessing facility, and which also contain significant quantities of iron, are neutralized to cause the iron and TRUs to co-precipitate. The TRU-contaminated floc is sufficiently separated by crossflow filtration from the solution to facilitate discharge of the permeate to sea. In this application a modified form of filter bundle is used, with the floc inlet and outlet, and the permeate outlet positioned all at the same end of the bundle (Fig. 3.6).

Two stages of ultrafiltration are employed in EARP to optimize the pumping requirements. The de-watered floc required for immobilization and disposal is a thixotropic material with a viscosity of approximately 7 poise, which would require special pumps. Therefore, a two-stage ultrafiltra­tion is employed. Up to 90% of the water is removed from the slurry in the first stage so that a standard pump can be employed to recirculate the slurry. De-watering is completed in a second stage on a smaller slurry volume that consequently uses smaller specialized pumps than if a single stage process were employed.

Backwashing and chemical cleaning with nitric acid removes any floc that is fouling the tubes. However, the membranes are expected to have a finite

3.6 a and b Schematic representation and photograph of an EARP ultrafilter module. b Source: Nuclear Decommissioning Authority ("NDA"), copyright: Nuclear Decommissioning Authority ("NDA").

life through incomplete cleaning, mechanical damage, wear to the coating and possible blockage of the pores or the tubes themselves. Therefore, the EARP ultrafilter cartridges are specially designed for remote maintenance and replacement (Fig. 3.7), being contained in type 3 PSCs. A fixed housing is permanently built into the plant below the cell roof and thus within the biological shielding. The filter modules fit within these housings and all inlet and outlet ports and seals are incorporated into one plug unit in the module top. This is sealed to the housing by replaceable O-ring seals. When necessary, the complete module, complete with its O-ring seals, can be withdrawn from the housing into a flask positioned above the cell top, and

a replacement module installed into the housing by reversing the process. In EARP, nine modules of ultrafilters containing 800 membrane tubes are used for primary de-watering while two modules of 500 membrane tubes complete de-watering.