Seeing inside Sellafield's reactors

Perhaps more than any other, the nuclear industry has a great interest in non-invasive process monitoring techniques.

A recent example is at BNFL's Sellafield site where, following a successful demonstration, the company has decided to trial electrical resistance tomography (ERT) to monitor a unit within the Thorp fuel reprocessing plant. The operation takes place in a continuous stirred tank into which reactants are added to produce a heavy metal precipitate. Stirring of the reaction vessel is vital - it must be continuous and of the right intensity. But because of the non-transparent nature of the vessel contents, operators could not see exactly what was going on.

Other forms of monitoring proved impractical, so BNFL decided to try ERT.

In general, ERT involves placing arrays of electrodes (typically 8 to 16) around the inner circumference of a process vessel. The electrodes are in contact with the process fluid but do not disturb the flow pattern.

Applying current through successive pairs of electrodes generates a series of measurable voltage difference signals, which can be turned into a 3D image of the vessel contents using specially written algorithms.

The system BNFL has installed, however, uses a new method in which the sensors are actually fitted to a probe that can be inserted as a baffle into the mixing vessel.

This Optomix probe (shown above in its full scale version) is the latest product from ERT developer Industrial Tomography Systems and was developed in collaboration with the University of Leeds.