Correcting the Record: Spent Nuclear Fuel Storage at SONGS
The Samuel Lawrence Foundation based in Del Mar, Calif. released a pair of reports last week that led to two San Diego Union-Tribune articles. The first article, published Jan. 8, focused on an economic study that “what-if’d” the economic consequences of a radiological catastrophe at the San Onofre nuclear plant similar to what happened at Fukushima. The conclusion: $13 trillion in economic losses to Southern California over the next 50 years. That’s a big number for sure. And it's misleading. The fact of the matter is there is no (as in zero) possibility of a radiological catastrophe at SONGS that would affect anything outside the fences of the plant. Southern California Edison related that to the Union-Tribune and asked them to contact the Nuclear Regulatory Commission to verify. They didn’t and the story went to print.
The next day the Union-Tribune contacted the NRC. Here’s what they were told:
“The bottom line is that no credible accident mechanism exists for radioactive material to leave the San Onofre site, given how much the spent fuel has cooled off since the reactors were shut down for the last time,” said Scott Burnell of the U.S. Nuclear Regulatory Commission. “This means there is no basis for examining potential economic effects of an accident beyond the site boundary, never mind out to 10 or 50 miles.”
The reason "no credible accident mechanism exists" is clear. San Onofre is not an operating reactor site. Fukushima was operating at the time of the accident there. Fukushima’s fuel was extremely hot (and couldn't be cooled), several orders of magnitude hotter than SONGS spent fuel is today. The decay heat of SONGS' hottest fuel assembly is roughly equivalent to a hair dryer (about 1500 watts).
The second study released by the Foundation focused on the storage of spent nuclear fuel at SONGS. This study also contained numerous false and misleading statements, supported by opinion, not data or serious engineering analysis.
Let’s take a look at the issues one-by-one:
Sea Level Rise
The authors put a lot of emphasis on future sea level rise and its potential impact on the independent spent fuel storage installation (ISFSI) that holds the Holtec multipurpose canisters. SCE focused on sea level rise as well, and its potential was factored into both the licensing of the ISFSI and its design. Should the ground water rise, the reinforced concrete monolith (3’ thick) and stainless steel liner of the cavity enclosure container will prevent the canisters from becoming wet. Even so, the canisters are qualified for full submergence, but SCE would take action long before being submerged. The paper uses extremely pessimistic projections for sea level rise. However, if sea levels were to start rising at that speed, SCE would still have time to relocate the ISFSI, if necessary.
With respect to the ISFSI, importantly, the California Coastal Commission requires SCE to revisit the location of the facility in the year 2035 to understand the state of sea level rise as well as the status of moving SONGS spent nuclear fuel offsite, whether to a consolidated interim storage site or permanent geologic repository.
An addendum to the CCC staff report offers further clarity:
"Increases in the water table elevation related to sea level rise could potentially lead to intermittent lead to [sic] groundwater contact with the base of the ISFSI toward the end of the proposed 35-year life of the project. However, the design of the ISFSI is such that there are multiple barriers, including the 3-foot thick foundation pad and the steel cavity enclosure container (CEC), between the groundwater and the fuel storage casks, and limited contact with groundwater would not undermine the structural integrity of the ISFSI during the proposed project life." - Addendum to California Coastal Commission Staff Report
Southern California Edison selected the Holtec MPC-37 canister to store the remaining spent nuclear fuel at SONGS. Further, the canister walls at SONGS are thicker than any other stainless steel canisters in the United States because SCE chose to proactively make them 25 percent thicker. They exceed all regulatory requirements.
During a canister downloading event in August, a canister was wedged 18-feet from the bottom of the cavity enclosure container, in an unsupported position. Eventually, the canister was safely downloaded with no impact to anyone’s health or safety.
The SLF has used this event to question the robustness of the canisters, but they do so by using documentation for completely different canisters.
“These canisters are certified for drops of 11 inches,” said Subrata Chakraborty, a University of California San Diego biochemistry professor and co-author of one of the foundation studies. – Study author quoted in the San Diego Union-Tribune citing a study on a different canister than those used at SONGS, Jan. 9, 2019
In one instance, they cite a Holtec Final Safety Analysis Report for the Hi-Storm 100 Cask System. SONGS uses a Holtec UMAX system with the aforementioned MPC-37 casks. The authors also cite NUREG-1864 which again uses a completely different canister than the MPC-37. A non-proprietary drop analysis for the MPC-37 was posted to the SONGS Community website in November and is readily available to the SLF authors. SCE performed a complete and rigorous analysis of the postulated drop and found that the canister would not fail and no radiation would be released. The NRC has reviewed the full analysis and qualified technical experts posed questions which SCE answered satisfactorily.
In summary, the 28 percent chance of failure (crack initiation in a weld, not a through-wall crack) quoted in the SLF paper, again, from NUREG-1864, is for an MPC-68 which has a different design than the MPC-37 used at SONGS, and is not applicable. A similar calculation for an MPC-37 would yield a failure rate of approximately 0%.
A quick word search of the NRC’s final special inspection report for the Aug. 3 canister downloading event finds the term “scratches” used once. Gouge, or gouges, isn’t used at all. So it’s curious why the SLF authors insist on using “gouge” to describe the result of incidental contact that a canister makes with a component called a shield ring as it’s lowered into the cavity enclosure container.
Scratches are normal and expected but SCE performed an analysis to determine the depth and effects of the scratches on the MPCs. The analysis that is outlined in the SLF paper has many errors, the most important being the assumption that up to the full weight of the canister can be applied laterally to scratch the MPC. Because of the geometry, only up to 2 percent of the weight can be applied laterally.
SCE posted an analysis in November to the SONGS Community website that goes into great detail about the potential for scratches to the canister during the downloading process. A paper with additional information is also posted. This second paper explains the “worst case scratch scenario” and how it is arrived at. Here it is in a nutshell:
To calculate the worst case scratch, Holtec assumed an MPC weight about 14 percent above the maximum loaded SONGS MPC; maximized the angle that the MPC could be misaligned based on geometric modeling; and, the wall of the MPC was forced to maintain that angle and that load (about 2,400 lbs.) as it passed across the corner edge of the shield ring inner diameter. In actuality, the angle and load would lessen as the MPC realigned to pass by the shield ring. Still, to further maximize the test scratch, the protective paint layer was assumed to be absent. The result of the bare steel corner with a maximum possible angle and load was a scratch that was 0.010 inches deep and 0.125 inches wide. This is well within the allowable limit for the American Society of Mechanical Engineers code for the 5/8 inch thick stainless steel canister walls of 0.0625 inches deep.
But the canisters also have their own protection against scratches. From the same document as the above section:
The stainless steel on the delivered component has a protective oxide film due to chromium oxide, which was formed in air before shipping. If a new scratch were to occur during downloading, in a non-corrosive environment like ambient air, and at the warmer temperatures seen for a loaded MPC, any scratching of the outer surface repairs itself very quickly.
As SCE has stated, canister inspection is part of its aging management program for the ISFSI. Contrary to the SLF report, multiple nuclear sites have inspected the surface of dry storage canisters stored in their ISFSIs. Several inspections have been performed as a condition of storage license renewal. The technology exists, is currently being used, and will be used in the future at San Onofre to monitor the condition of the canister confinement boundary.
Holtec officials have not, to SCE’s knowledge, stated canisters cannot be inspected, a claim made by the authors of the SLF report.
The above is by no means a complete list of issues with the SLF reports. But we hope it provides a sense of the faulty analysis and misinformation that hinders fair-minded individuals from drawing reasonable conclusions about spent nuclear fuel storage. We encourage readers of this post to visit the Community Engagement Panel section of this website. The CEP is a tremendous resource for factual information on a multitude of subjects related to spent fuel storage and decommissioning. In fact, CEP Chair David Victor provided KPBS with additional factual responses during a wide-ranging interview that aired last week.
The Samuel Lawrence Foundation appears to be working at cross-purposes – with itself. On its website it states under the banner What Do We Want?:
- Stop loading nuclear waste into steel canisters at San Onofre.
- Pressure the federal government to open a permanent disposal site.
Of course, until spent nuclear fuel is loaded into canisters, it can’t be transported offsite. The Holtec MPC-37 canisters used at SONGS are suitable for transportation (multipurpose = storage + transportation).
It would be much more beneficial for all parties to work toward a solution that moves all spent nuclear fuel offsite, such as to a licensed consolidated interim storage site or a permanent federal repository. SCE has asked a team of experts to help make that happen. Read more about them here.