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Special Inspection Activities Regarding Cask Loading Misalignment

The Nuclear Regulatory Commission staff is evaluating an incident that occurred August 3, 2018, at the San Onofre Nuclear Generating Station (SONGS) during fuel transfer operations. Plant personnel were transferring a multipurpose canister filled with spent nuclear fuel from the spent fuel pool to the HI-STORM UMAX dry cask storage facility. As the canister was being lowered from the transport cask into the underground storage vault, it became stuck on the gusset or inner ring assembly near the top of the vault's inner liner, known as the "divider shell."

cutaway photo of the divider shell of a multipurpose canister filled with spent nuclear fuel, with arrows pointing to the shield Ring, Gussets and Seismic Restraints inside
Figure 1 – Divider Shell

The plant personnel believed the canister had been fully inserted, because the rigging that supported the canister had gone slack. After a radiation protection technician registered radiation readings higher than expected for a properly loaded cask, the personnel performing the operation realized the problem. They then corrected the canister's alignment and successfully completed the operation.

San Onofre personnel failed to observe (1) that the canister was misaligned, (2) that the canister had made contact with and was resting on the divider shell assembly, and (3) that the lifting devices and equipment – considered "important to safety" because of their role in ensuring safe storage and handling of the waste – were no longer supporting the canister weight. The failure of lifting equipment placed the canister in what the NRC calls "an unanalyzed condition," and created the possibility of a "load drop event," as the canister could have fallen 18 feet into the storage vault if it had slipped off of the inner ring assembly. 

San Onofre personnel implemented appropriate immediate corrective actions. These included restoring the control of the load to the rigging and lifting devices, properly placing the canister in the storage vault, suspending all movement of spent fuel, and initiating an investigation of the event. 

San Onofre informed NRC Region IV staff of the incident on Monday, August 6, 2018, when the licensee provided a courtesy notification and described it as a near-miss or near-hit event.  San Onofre personnel did not report the event as required by regulations. Following prompting by NRC staff, San Onofre submitted an event report (required by 10 CFR 72.75(d)(1)) on September 14, 2018. 

Please see Figure 2 – Isometric View of Downloading Configuration.

The NRC initiated a Special Inspection during the week of September 10, 2018. The Special Inspection team investigated the "near miss" drop event, interviewed personnel involved, and observed equipment operation and preliminary corrective actions put in place to prevent recurrence of the event.   

The following links provide additional information about the event and the NRC's special inspection.

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Page Last Reviewed/Updated Friday, November 13, 2020