ABSTRACT
Introduction The thermal oxide reprocessing plant (THORP) is a large plant currently being constructed at Sellafield in Cumbria, England, at a cost of approximately £ 1 .3 billion. To give an idea of the size of the plant, it has required over 800 engineering flow diagrams (EFDs), and has over 20,000 digital inputs/outputs for its distributed control system. Since the end of 1 987, in line with heightened awareness in the nuclear power and reprocessing industries of the utility of human factors in enhancing safety and operability , THORP was the subject of a large and resources-intensive human factors (HF) programme (Kirwan, 1989) . This programme was aimed primarily at the safety of the plant, and was run within the Safety and Technical Department at Risley, Cheshire. Task analysis was fundamental to the programme. Various forms of task analysis were utilized, creating a task analysis programme in itself, designed to maximize the usefulness of the interrelations between the task analysis methods used.
