Operational Improvements Make the Difference during Download
(This is another in a series of updates on fuel transfer operations at SONGS)
San Onofre Nuclear Generating Station (SONGS) is moving spent nuclear fuel from wet storage in pools to dry storage in robust, stainless steel canisters.
So far, 46 canisters of spent nuclear fuel have been placed into the Holtec dry storage system. All 73 canisters are expected to be stored later this year.
Canister 46 was successfully downloaded Friday night. The process of downloading the canister began Thursday night and continued into early Friday morning. It was safely downloaded Friday night at about 10:40 p.m.
What We Saw
During the download of canister 46, the crew encountered a series of underload signals, which tells them the canister isn’t lowering into its storage module. The signal is an opportunity to stop, adjust, and re-start. The dry storage system has tight tolerances by design. This provides excellent radiation shielding as well as meeting Southern California Edison’s very high seismic design criteria.
In accordance with our procedures, the decision was made to stop and assemble a team to analyze the situation.
Recall, during an 11-month break in fuel transfer operations that ended last July, SCE and our contractor made significant improvements to key aspects of the fuel movement process: new procedures were implemented, new training for the operating crews, and new technology to provide more data for the team monitoring and executing the canister downloads. All of those elements came into play during the downloading of canister 46. Procedures were followed; the crew’s training led to conservative decision-making; and the underload signals provided critical information.
Decision to Download
Following reviews and engineering analysis, including examinations of the storage module, it was determined the canister’s three-inch thick stainless steel base was coming to rest on the interior shield ring. Importantly, the canister was always supported by the Vertical Cask Transporter throughout this process. The underload signal ensures that the slings remain taught and support the canister load at all times. The load monitoring system (added after the Aug. 3, 2018, event) worked as designed, providing early indication to the VCT operator that there was contact with the shield ring, as well as providing concrete data to engineers so they could confirm the canister was never in an unsupported condition.
After the final adjustments were made, the canister was lowered smoothly with no indication of unusual contact.
We continue to review this download and integrate lessons learned into our processes and procedures.