The Thermal Oxide Reprocessing Plant (THORP) plant at Sellafield includes continuous stirred tanks into which reactants are added to produce a heavy metal precipitate. Stirring of the reaction vessel must be continuous and of the right intensity to ensure the precipitation process proceeds as intended. The tanks are operated with a deep vortex, the presence and depth of which provides valuable information on the performance of the precipitation step.

Access to the tanks is restricted and visual inspection is difficult due to the opaque nature of the tanks contents and the presence of a heating jacket. Therefore a remote method of measuring the vortex depth has been developed based on electrical resistance tomography. The sensor, which is in contact with the tank contents, has been designed to withstand the extreme combined conditions of high chemical reactivity, radiation and abrasion caused by the strongly acidic circulating flow. A Data Acquisition System (DAS) has been designed to cope with the demanding conditions caused by the high conductivity liquor within the tanks and the constraint of a long cable between the DAS to the sensor.

A software user interface has been developed which displays the current vortex image and a historical trend of vortex depth. Images of the vortex are obtained using an image reconstruction algorithm based on linear back projection and further processing of these images provides the measurement of vortex depth. Low and high level thresholds can be set to trigger alarm conditions.