Systematic investigation of non-conformance root cause in prefabrication: a nuclear case study

To help project managers better understand non-conformance in prefabrication projects, this research presents a probabilistic empirical study on non-conformance reports (NCRs), their root cause and frequency of occurrence, as well as their impact on the project cost and time. Data from a completed nuclear project were collected and analyzed, where 1,179 NCRs were raised during the three-year fabrication period, consisting of six distinct modules and a ring girder. Five broad categories are used to distinguish general liabilities of those NCRs (internal vs. external) as well as at which stage along the project the non-conformance occurred (material receipt vs. fabrication). A further breakdown of 33 defect codes from those broad categories reflect the specific root cause of each non-conformance. Probabilistic analysis show that geometric-related non-conformance amongst all defect codes represent over half of all analyzed NCRs. Furthermore, in the interest of understanding the impact of these geometric-related issues on the project, 84 NCRs were sampled for evaluation; they were reviewed and assessed individually by an industry expert who is familiar with the nuclear project in this study, to estimate the cost and time impact of the sampled non-conformance. Based on the estimate, the average cost and time impact per non-conformance are nearly $1,000 and 14.5 man-hours, respectively, accounting for additional resources required to rework, inspect again, and release the assembly. The findings provide project managers with useful implications to identify non-conformance root cause in prefabrication, reduce the risk and impact of these issues, and improve the performance of future projects.