In a once-though ion exchange process, the solution to be decontaminated is passed continuously through the ion exchange bed until the bed becomes saturated with the sorbed contaminant. At this point the ion exchange material is replaced. A good example of an operating once through ion exchange plant is the Site Ion Exchange Effluent Plant (SIXEP) at the Sellafield site in the UK. SIXEP (Fig. 3.9) has been in successful hot opera­tion since 1985 and is designed to remove radioactive cesium and strontium from water, routinely purged from the spent nuclear fuel storage basin.

The ion exchange material used is clinoptilolite, a naturally occurring zeolite that is quite specific for cesium and strontium. These species are sufficiently removed by the SIXEP process for the treated basin purge water to meet regulatory requirements for discharge to the ocean. The basin purge water is first filtered in sand filters before the ion exchange process so as to remove suspended solids and sludges that may otherwise block the ion exchange bed. The pressure vessels for both sand filter and ion exchange duties were produced to a common design to maximize operational flexibility.

The basin purge water feed is introduced to the ion exchange vessel top through a diffuser plate to prevent it channeling through the bed. The decontaminated product is collected through a number of stainless steel wedge wire strainers welded into the dished false bottom. Loaded ion exchange material is periodically discharged from the vessel by fluidizing it with water and then discharging it under pressure through the emptying pipe. A lower fluidizing ring is fitted with tangential spray nozzles used to produce a vortex action to assist emptying of the vessel contents, particu­larly the final residue. Two higher rings are fitted with radial spray nozzles to assist with breaking up the bed. It is important to discharge as much loaded ion exchange material as possible since any remaining in the column

will contaminate the product during its subsequent operation after re-filling. Development work on the first SIXEP production vessels enabled the residue remaining in the vessel to be reduced to 0.1%, which was consid­ered well within process requirements.