On November 3, 2008, an unexpected drift of the last three of 177 control rods occurred at the Dresden Unit 3 boiling water reactor. The root cause of the control rod drift was the manner in which the hydraulic control units (HCUs) were isolated during the outage. The U.S. Nuclear Regulatory Commission (NRC) Office of Nuclear Regulatory Research (RES) performed a demonstration study of inadvertent control blade drift using RES-sponsored nuclear analysis tools. The smallest margin to recriticality was determined by calculating the control rod worths at each core state using the core simulator PARCS/PATHS and an innovative algorithm to identify the highest worth combination of rods. This study did not try to evaluate any correlation between drifting rods that may occur in a real plant due to the actual physical configuration of the system. The purpose of the analysis was to demonstrate the tools that could be used to analyze the situation if that information is known.

For the current purpose of this demonstration, Edwin Hatch Unit 1 Cycle 3 (H1C3) was selected as the reference core and cycle. Based on the results of these calculations, it was possible to determine the fraction of rod groups that would produce criticality consequences in each of these scenarios. The results confirmed several aspects of conventional thinking, such as the most reactive point being the beginning of the cycle at the coldest conditions. Further, with a single blade drifting out of the core, the analysis results confirm that shutdown margin is maintained. It was found that a small population (about 1%) of drift scenarios with two rods produced criticality consequences according to our best-estimate-plus-uncertainty method, while this fraction increases to about 3.5% for three rods and about 14% for four rods. The results of the study have confirmed the adequacy of the NRC control rod drift analysis methodology; however, the results are not generically applicable and apply only to H1C3.