Additionally, the extreme sensitivity of modern radiation detectors facilities the use of relatively small sources of radiation, even for studies on full-scale operating units. The makes it possible to carry out measurements without significantly restricting personnel movements around the unit.

Radioisotope investigations fall into two main categories, radiotracer studies and application of sealed sources.

This paper uses case studies from each of these categories to illustrate the wide range of useful information which may be obtained. In most cases, it is difficult, or impossible, to obtain the information in any other case.
Radiotracers can be used to study the flow characteristics of all of the process streams solid catalyst, vaporised feed and steam. A typical radio study involves the deployment at strategic locations around the FCCU of twenty or more radiation detectors. A sharp pulse of radiotracer in a physico-chemical form compatible with the stream to be studied is then injected into the flowing process material. As the pulse of tracer passes each detector, a response is registered. Subsequent analysis of the size, shape and time distribution of the response curves can provide valuable information about the flow characteristics throughout the entire system. The case studies illustrate how information can be obtained about; catalyst and vapour velocities in the riser; problems associated with the riser termination device; flow characteristics (including maldistributed flow) in the reactor and stripper; cyclone efficiencies.

Sealed sources of radiation are generally used to measure the levels, densities and density distributions of catalyst inside relevant parts of the unit. These studies almost invariably involve the use of high-energy gamma-ray sources and are based gamma-ray transmission measurements. Several types of measurement are discussed, gamma-ray density profiling to determine catalyst bed levels in the Stripper vessel; density monitoring in catalyst stand-pipes; multi-diameter; gamma-ray scans and matrix scans to determine cross-sectional density distributions in the Riser.

The paper concludes with a brief description of current trends in the technology and identifies developments which may be expected to make important contributions in the future.