Defense-in-depth Keeps the Right Material in Place during SONGS Decommissioning
Back when SONGS was generating clean electricity, the plant and its operating procedures were fortified by “defense-in-depth” – an intentional design where key systems and processes are built upon layers of protection, such that if one failed, others are in place to ensure safety. And while some of these processes rely upon mechanical means, they also leverage the power of human inspection and oversight.
That same principle of defense-in-depth lives on in the work to decommission SONGS, particularly as it relates to radiological safety. While the spent nuclear fuel at SONGS is seal-welded in stainless steel canisters and housed in robust reinforced concrete structures, radiologically contaminated components still remain within the reactor buildings that must be carefully decontaminated, dismantled, packaged, and shipped. We use defense-in-depth to help ensure this work is done safely without impact to the community or the environment.
Recently we saw this in action when a truckload of clean recyclable material was being prepared for off-site shipment. Prior to the load departing the staging area, a standard survey was conducted using a highly sensitive handheld monitor to ensure the load was clean. During the survey radiation levels were detected to be slightly higher than background (radiation from natural sources, like the sun). Per procedure, the truck was turned around and unloaded. Further inspection identified a length of contaminated pipe co-mingled with the clean material. After addressing the issue, and a re-check, the shipment was able to proceed to the next steps in the process—two more check points before being cleared for departure.
Further, action was taken to understand how the pipe wound up in the wrong place. It was determined that the pipe was insufficiently marked with the magenta spray paint that is used to identify contaminated components. As a result, an improvement in our color-coding process was put in place to capture the lesson learned moving forward.
Another example of defense-in-depth involves a recent storage rack shipment from one of the spent fuel pools. In early February, a team member from SCE’s decommissioning contractor, who was monitoring the rack’s shipping preparation, noticed a small amount of water dripping from the waterproof industrial package. The packaging was redone, and all water (less than a pint) was confirmed to be removed from the rack.
This, too, was a good catch early in the process. However, there are three additional steps in the process to identify such an anomaly: a visual inspection, a package certification to ensure the contents are consistent with the type of package, and a final detailed check by the qualified shipper who verifies that the shipment is compliant with Department of Transportation requirements.
With several more rack shipments to be conducted this year, the team has implemented improvements to the process for drying and packaging fuel racks for shipment. In nuclear speak, that means more layers of “defense-in-depth.”