Abstract
Objective: The aim of this study is to investigate the effectiveness of the expert prediction method for estimating the delay factors and the delay time of the nozzle dam installation and removal tasks. Background: The radiation managers of nuclear power plants have tried to find the delay factors to reduce the delay time of the nozzle dam installation and removal tasks, one of the highest radiation dose tasks performed during the scheduled preventive overhaul period of nuclear power plants. Method: Five experts on nozzle dam installation and removal tasks estimated relative weights for delay factors and the delay time of the task using the analytic hierarchy process (AHP) method. In the experiment using the steam generator water chamber mockup simulator the maximum delay time was measured from the difference between the longest and the shortest performance time of the tasks repeated by the three skilled workers. Results: Some unexpected events during the simulation trial lead to a low statistical correlation between the estimated delay time and the measured delay time of the nozzle dam installation and removal tasks. Expert estimations using the AHP method could not reflect the accidental event such as an unexpected excessively tightening of the bolts on the nozzle dam making the performance time prolonged. Conclusion: The estimates on the delay time using the AHP method were limited in usefulness but the debriefing results showed that the estimates of the delay factors predicted well the real situation of the nozzle dam installation and removals tasks. Application: The AHP method for delay factor estimation might be usefully applied to the other high radiation dose overhaul tasks of nuclear power plants such as refueling, reactor water clean-up, works for decommissioning and so on.